This article started with a simple question: what is the correct washer to use to secure a ring crimp terminal on a threaded bolt?
I have seen either spring/split washers or internal/external star washers being used.
I asked on LinkedIn and found some good advice, some received wisdom and “we’ve always done it this way”, but not much in the way of citeable standards or references from technical authorities.
Goals of Earthing / Bonding System
The fundamental goals of the electrical fixings in the protective earthing / bonding system are:
Provide (at assembly and maintain during use) a low resistance, potentially high current (tens/hundreds of Amps depending on supply) electrical contact
Not corrode or loosen under the normal environmental operating conditions to the point where the resistance goes out of specification
In reality there are many factors that one could worry about:
Common Safety Standard Review
A review of the more frequently used safety standards for electronic products yields the following clauses.
“parts fixed by means of screws or nuts provided with self-locking washers or other means of locking are not liable to become loose or detached
NOTE Spring washers and the like can provide satisfactory locking”
“Screw connections shall be secured against loosening”
“The contact pressure required for a bonding connection shall not be capable of being reduced by deformation of materials forming part of the connection”
“Screws and nuts that make a mechanical connection between different parts of the appliance shall be secured against loosening if they also make electrical connections or connections providing earthing continuity. This requirement does not apply to screws in the earthing circuit if at least two screws are used for the connection or if an alternative earthing circuit is provided.
NOTE 1 Spring washers, lock washers and crown type locks as part of the screw head are means that may provide satisfactory security.”
“27.2 The clamping means of earthing terminals shall be adequately secured against accidental loosening.”
Clamping means of earthing terminals for external conductors shall be adequately locked against accidental loosening.
“parts fixed by screws or nuts provided with a locking washer are regarded as not liable to become loose”
(note clause not specifically related to earthing)
Typical Locking Fixings
The below image shows a variety of locking methods that I would consider acceptable for this purpose.
The standards are not prescriptive about the type of locking washer to be used.
Spring washers, lock washers and threaded fastener locking features are all valid approaches.
No washer is also an acceptable method provided there is a locking nut of some kind of suitable locking adhesive used.
Two independent fixings are considered to be acceptable in some standards.
Testing Testing Testing
In all cases, conformity with the standard is checked by inspection and/or appropriate testing. Testing is key.
Testing the protective earthing / bonding system includes measuring the resistance and/or measuring the current handling capability of the connections.
If you are the manufacturer and wanting to use a non standard fixing method then it may be acceptable. Any non-standard or atypical methods would need adding to the product compliance risk assessment.
The testing specified in the standard is the bare minimum and additional testing may be required to demonstrate that everything is indeed safe. Testing could include extended high humidity environmental testing to check for corrosion and representative vibration testing to make sure that loosening does not occur in use.
Of course, the simplest way is to just use a standard washer to reduce arguments.
Like all simple questions, there is a surprising amount of depth and possible considerations, including:
Corrosion, plating, passivation, surface oxide layers, dissimilar metals. This is a book in of itself!
Considering the current path. Using a locking washer with a small surface contact area in the primary current path can increase the resistance. Aiming for a larger surface area with a good quality connection would be optimum
Surface preparation: clearing paint, anodising, rust, or oxidation.
Minimum fixing size. Some standards call up a minimum 4mm diameter and number of threads engaged for certain types of screw fixings. This is not universal across all standards. If in doubt, selecting all threaded hardware to be at least M4 in diameter seems like a sensible option.
I would be very interested to hear of studies, standards, procedures, reports… indeed any published material that covers this topic of washers and fasteners specifically for electrical connection.
References & Links
My original question posted on LinkedIn asking about washers
https://www.unit3compliance.co.uk/wordpress/wp-content/uploads/2022/04/protective-earthing-point-stud.png8441187James Pawsonhttps://www.unit3compliance.co.uk/wordpress/wp-content/uploads/2017/01/unit3compliance_400x400.pngJames Pawson2022-04-30 10:38:202022-04-30 10:43:11Spring or Star Washers for Earthing Stud?
A short post prompted by a (summarised) request from a customer:
We’d like to test to the following standards for our CE/UKCA marking
– EN 61326-1 (Class B emissions, Industrial immunity) – EN 61000-6-2 (Industrial Level Immunity) – EN 61000-6-3 (Class B Emissions)
This customer is very compliance conscious, as their products end up in all kinds of harsh and hazardous environments where they are protecting the health and safety (and lives in many cases) of their customers.
As such, it is understandable that they want to “throw the kitchen sink” at the EMC performance. Selecting Class B emissions and industrial immunity is a great way of demonstrating the robustness of your product in a wide range of electromagnetic environments.
The selection of the relevant harmonised standards is the responsibility of the manufacturer. When the manufacturer chooses to apply harmonised standards he shall select them in the following precedence order:
Product-specific (family) standards are those written by ESO’s taking into account the environment, operating and loading conditions of the equipment and are considered the best to demonstrate to compliance to the Directive.
An example of a product specific standard would be EN 61326-2-6 “Electrical equipment for measurement, control and laboratory use – EMC requirements – Part 2-6: Particular requirements – In vitro diagnostic (IVD) medical equipment (IEC 61326-2-6:2012)”
These product specific standards often refer back to the root family standard, EN 61326-1 in this case.
Only if the manufacturer’s equipment does not fall into a product standard should the generic standards be applied.
5.2 Generic harmonised standards vs product specific harmonised standard
A manufacturer which has the intention to apply a harmonised standard for the conformity assessment of its products, has to apply in priority the product specific harmonised standard and only if this one is not available, the generic one, in order to benefit of presumption of conformity with the essential requirements of the RED.
Applying Multiple Standards
There are cases where applying several different Harmonised Standards could be the correct thing to do.
For example, if the equipment is a piece of measurement equipment that incorporates a lot of IT functionality (networking, data storage, PC control) then the manufacturer could decide to assess against EN 61326-1 for laboratory equipment and against EN 55032 for IT equipment. Both standards would appear in the test report and on the DoC.
Check Annex ZZ
One of the commonly overlooked Annexes (Annecies? Annecii?) is this one at the start of the standard. This details what Essential Requirements from the Directive are being covered by the standard.
Important: not all standards cover all Essential Requirements. You must check Annex ZZ carefully against them.
If you end up needing to apply more than one Harmonised Standard to a product to cover all of the Essential Requirements then you should state this on your Declaration of Conformity.
Presumption of Conformity
Remember that using Harmonised Standards (or Designated Standards for UKCA) gives you a “Presumption of Conformity” without further requirement to demonstrate compliance with the relevant directives/laws.
“Ultimately, the presumption of conformity is no more than a reversal of the burden of proof. This means that a product complying with the relevant [harmonised] standards may be challenged, for example by the market surveillance authority, only if actual evidence can be produced that the manufacturer has violated the requirements of the directives.”
Annex ZZ of a Harmonised Standard is your friend when it comes to understanding this link between the standards and the directives.
When the DoC Doesn’t Quite Cover It
This example of EN 61326-1 illustrates one of the problems of applying a Harmonised Standard that has multiple levels within it.
In this case, the EMC performance of equipment complying with EN 61326-1 could fall into one of six distinct categories.
Class A (industrial)
Class B (domestic)
Controlled (shielded and filtered environment)
Industrial (heavy machinery)
On the face of it, a product tested to Class A / Controlled (poor EMC performance) can’t be distinguished from one that has passsed Class B/Industrial limits (excellent EMC performance).
What to do?
The way I suggest overcoming this and informing the end user a little more clearly about the performance of the product is to explicitly state in the DoC what levels the product was assessed against during any testing.
This equipment was assessed against the following Harmonised Standards:
– EN 61326-1:2013 “Electrical equipment for measurement, control and laboratory use – EMC requirements – Part 1: General requirements” (Class B emissions, Industrial Immunity)
I hope you enjoyed this short dive into standards land. It’s a nice place to visit but you wouldn’t want to live there!
https://www.unit3compliance.co.uk/wordpress/wp-content/uploads/2021/01/en61326-1-title-block.png4521053James Pawsonhttps://www.unit3compliance.co.uk/wordpress/wp-content/uploads/2017/01/unit3compliance_400x400.pngJames Pawson2021-01-28 09:08:072022-03-17 11:26:04Choosing EMC/Radio Standards for CE/UKCA - Generic vs Specific
It’s a little known fact that Brian Wilson from the Beach boys was an environmental test engineer at Big Corp before pop stardom called him away from his first career.
He quickly realised that the hit parade wasn’t ready for ballads about Bellcore or ditties about humidity and switched to songs about the West Coast, cars and surfing. What could have been eh?
What is true however, is that whatever product it is that we make, at some point we have to release it from our development lab and into the big bad world. This raises the question: how well is it going to survive?
Why Perform Vibration Tests Anyway?
As we know, there are known knowns, known unknowns, unknown knowns and unknown unknowns…. y’ know?
“But James,” you cry, “surely I could just send my product via DHL / UPS / least favourite courier service (delete as appropriate) and achieve the same effect as testing?!”.
Well that’s very true, but I’m sure it’s going to be quite difficult to calibrate Dave the delivery driver, Vinnie the Van, and Winston the warehouse man for a consistent acceleration profile.
Testing is a simulation of what can happen in the big bad world. Like all simulations, it is a sensible average of many different situations. I’m not sure how these standards were originally derived but I’m sure more thought went into them than just kicking a development sample around the car park.
(If not actual thought, then at least a lot of tea and biscuits at the committee meetings, and that’s good enough for me)
Testing it primarily about mitigating risks, both of known risks and unknown risks. Whilst you might not be able to envisage a situation where your product is subject to a 1g, 30Hz vibration, real life might have other plans.
Primarily we are trying to quantify our knowns and unknowns.
Common Questions, Regular Requests
A version of one of these questions crops up every month or so.
“I want to do vibration testing, is there a standard that I can use?”
“What level of vibration do I need?”
“Do I need to do a random vibration test or is a swept test OK?”
“How do I simulate something being driven around in the back of a van?”
Vibration standards are quite often customer driven with a defined procurement specification. How close those specifications are to real life conditions depends on how much research that organisation has done or if they just pulled the numbers out of a hat. Often there is no way to get at the data or decisions that led to those choices and one must take them at face value.
Sometimes serious research goes into these levels, with car manufacturers employing multichannel data recorders and taking the latest model out for a test drive whilst getting tangled in accelerometer cables (safely I’m sure). This ends up being proprietary data so good luck getting your greasy mitts on it unless you happen to be working for them.
For the rest of us without a big budget how do we even make a start? Just go an buy every single BSI standard on vibration testing? Oh wait, we said without a big budget…
In the absence of any customer specification to work to, a good place to start in determining required levels and profiles is with the ETSI Environmental Engineering series of standards.
It’s a Splendid Smorgasbord of Shaky Situations
(or a Buffet of Battering if you will…)
The root standard ETSI EN 300 019-1-0 v2.1.2 (most recent at the time of writing) gives an introduction to the “Environmental conditions and environmental tests for telecommunications equipment”. Since this could easily be used to describe most modern electronic equipment then it is widely applicable.
This incredibly useful series of standards are provided for download free of charge, a much more welcome approach to the usual way that standards are sold at high prices.
Not only that but they don’t just cover vibration. They cover the expected environment in terms of
Temperature and rate of change of temperature
Solar and Heat radiation
Moving air speed (wind)
Weather conditions (driving rain, icing, etc)
Biological conditions (mold, rats, animals)
Concentrations of chemicals
Vibration and shock levels
That’s a pretty comprehensive list! The root standard contains a useful table that enables easy selection of a sub part appropriate for your product.
Each Class has two standards that relate to it:
Part 1 (suffix -1-x) specifies the expected environmental conditions for the situations (storage, transportation, in use) and locations (underground, on a ship, etc)
Part 2 (suffix -2-x) specifies the recommended test levels and methods/standards for each class.
This table has quick links to each of the Part 1 and Part 2 standards for the various classes for reference:
Part 1 – Environment Definition
Part 2 – Test Specification and Methods
Root Standard with background and general definitions
“Yes Robin, but how do we apply them to our (bat) product?”
Let’s invent an imaginary product.
The Monitor-o-Matic 9000 is a battery powered environmental monitor for indoor and outdoor locations. It gets periodically transported around site in the back of a van (maybe not always in its nice Peli case, naughty naughty)
In this instance we could reasonably apply:
Storage – the equipment is often kept in the van when not in use which can get pretty hot and cold. So we’ll apply “Class 1.2 Weather protected, not temperature controlled storage locations”
Transportation – with a bumpy van, and the M-o-M 9000 not always stored in its box means we’ll pick the harshest tests of “Class 2.3 Public transportation”
Usage – Non-Weather Protected Locations could be characterised as locations “…where transmitted vibrations are experienced from machines or passing vehicles. Higher level shocks may be experienced e.g. from adjacent machines.” We’ll choose “Class 4.1E: Non-weather protected locations – extended”
So lets pull out only the vibration requirements from each of these (bear in mind they also call up temperature, humidity, chemical resistance and other factors)
Unsurprisingly (some might say shockingly), transportation forms the biggest risk of vibration and shock to the product. We’ve now got some test levels to work with.
Note that some of the standards don’s account for the product being dropped from height. The storage standard does have a static load test simulating things being stored on top of the product.
Similarly the transportation test has the same static load requirements but adds free fall, toppling and rolling into the mix.
Questions to Ask Yourself
We normally ask ourselves what the expected use conditions are for a product. As with anything like this, we must also ask “what are the foreseeable misuse conditions?“.
Confession: I’ve Been Seeing Other Standards
This is just a simple guide to using the ETSI standards to make sensible design decisions in the absence of any other good information. However, many standards codify vibration and shock requirements, making the decisions a little easier.
EN 54 for Fire Alarm components has some quite challenging requirements for alarm components. In one shock test I performed for a customer on a smoke alarm the unit pinged straight off it’s base at the lowest shock level, necessitating some further work on the mounting (it subsequently passed no problems).
Military, Automotive and Aerospace sectors will definitely have their own vibration requirements and will commonly be clearly specified as part of the procurement specification.
Anyway, I hope you find this useful. Get in touch if you need some further guidance or need to ask any questions.
https://www.unit3compliance.co.uk/wordpress/wp-content/uploads/2019/01/300-019-1-0-environmental-and-vibration-table.png12361262James Pawsonhttps://www.unit3compliance.co.uk/wordpress/wp-content/uploads/2017/01/unit3compliance_400x400.pngJames Pawson2020-11-03 22:28:482020-11-03 22:35:16How Do I Choose Vibration Tests For My Product?
Because of the United Kingdom leaving the EU, the CE Mark will no longer be recognised as demonstrating conformity with UK legislation.
Instead the CE Mark will be replaced by the UKCA mark (UK Conformity Assessed) which will be required to sell your products in the UK. This mark can coexist with the CE mark on the same label.
The transition period starts this coming January 2021 and UKCA marks become mandatory for the UK on 1 Jan 2022.
Whilst it sounds like a year in enough time to get everything in order think back to university and how much time you had to finish your dissertation – am I right? Start sooner rather than later, especially if you have multiple products.
This applies to goods sold (“placed on the market” to use the correct term) in England, Scotland and Wales. Northern Ireland will still require CE marking due to the Irish border.
How can we help?
Preparation of UK Declaration of Conformity
Updating your Technical Documentation to meet the new requirements
EMC or safety testing to meet the technical standards required
Action Stations for UKCA
You will need to create a new “UK Declaration of Conformity” similar to the EU Declaration of Conformity (which you will still need for CE marking). Contact me if you need a template. If you’ve been a customer and we’ve performed CE marking testing for you then we’ll be sending out UK DoC templates for your products before the end of this year.
The EU Technical Documentation that I’m sure you keep up to date for all your products will need an additional section with references to the UK Statutory Instruments (equivalent to the Directives) and Designated Standards. Let me know if you need some help with this.
Add the UKCA mark to your product label. You can find image files on the gov.uk website. It must be at least 5mm high.
It can be applied as a temporary label until 1 January 2023 after which it must be “permanently attached” in the same fashion as you currently apply the CE mark.
The product, or documentation where this is not possible, must have the manufacturer’s name and UK address shown. If the manufacturer is outside the UK, this must be the importer’s address.
UK Manufacturers Selling to EU
You are now a “3rd country” and will need an EU Sales Office (assuming you don’t already have one) whose address and contact details will need to go on the EU Declaration of Conformity. Various companies offer an “EU Authorised Representative Service” which can be found with a little searching.
If you use a UK based Notified Body, they will probably have already been in touch to discuss what is happening with your compliance certification. If not, get in touch with them sharpish and ask about your compliance status.
1st January 2021
UKCA becomes valid and can be placed on electrical / electronic products to demonstrate conformity with UK legislation.
CE mark enters transition period but is still valid for 12 months.
This transition period applies if you currently self declare CE compliance using an EU Declaration of Conformity (the vast majority of products do this).
1st January 2022
CE mark ceases to be valid in the UK.
UKCA mark becomes mandatory.
The EU directives relating to CE marking are already UK law. SI 2019 No. 696 will modify the below SIs (and more) to add UKCA marking and change the terminology. All compliance documentation must refer to these Statutory Instruments instead of the EU Directives.
Harmonised Standards become Designated Standards and use the BS prefix (e.g. BS EN, BS ETSI EN). No list of Designated Standards is available yet, this is likely going to be published around 1 Jan 2021 where the list gets transposed from existing standards.
Most standards change at a slow pace so we’ll have to wait and see how quickly changes to the IEC, CENELEC and ETSI standards filter through to the UK standards list. Certainly no massive changes in technical requirements will happen overnight.
“Do you do certification or just pre-testing?”
“Can you certify our products?”
“Can you do EMC testing even though you aren’t accredited?”
The concept of “certification” is an interesting and, judging by these real customer enquiries that we’ve received, a confusing aspect of EMC testing.
The short answer to the questions above is “no, you do, in a way, most of the time, for most things” but like most short answers it isn’t particularly helpful.
To help clear this up, lets have a quick look at declaration vs certification for EMC testing to CE marking (EMCD & RED), UKCA Marking, and “FCC” CFR 47 Part 15B, lab accreditation and the operating philosophy of Unit 3 Compliance.
Unit 3 Compliance test results are valid for a wide range of regulatory approvals, including CE marking, UKCA marking, and FCC
In the context of the CE Mark, there is no such thing as a ‘CE certificate’ or a ‘CE certification’ process. Same applies for UKCA.
You (the manufacturer) “self certifies”; or rather you legally Declare your product to be compliant with the EU Directives or UK SIs
UKAS Accredited Laboratory testing is not mandatory for CE, UKCA and many FCC tests.
For the USA (FCC) certification does exist but it depends on the product. Many products are exempt from certification.
Unit 3 Compliance Test Results Validity
Unit 3 Compliance can be used for testing?
CE Marking &
EMC or Radio Equipment Directive
Unintentional Radiators (Part 15B)
Unintentional Radiators with FCC Approved Radio Module
Intentional Radiators (Part 15C)
Accredited laboratory required for final test
When CE marking for selling products in the EU, most electronic products are going to be covered by either the EMC Directive (2014/30/EU) or the Radio Equipment Directive (2014/53/EU). The latter refers to the wording of the EMC Directive anyway.
In all cases, the manufacturer “self certifies” by assessing the product (usually to a Harmonised Standard) and then producing and signing a Declaration of Conformity (a legal document) to confirm that their product meets the Essential Requirements of the Directives in question.
Note that the directive requires the manufacturer to “assess” the product. It doesn’t specifically require testing of a product. However, by testing the product to Harmonised Standards, you gain a “Presumption of Conformity” to the requirements of the Directive.
However, testing is the best way to determine performance; EMC behaviour is largely dictated by parasitic components that are not generally present on the design documents.
It is then up to the manufacturer to ensure that all future products remain compliant through control of production.
Try searching either of these Directives for the following:
and you will find that these words are only used in relation to a Notified Body (NB) or an EU Type Examination Certificate provided by such a body. This approach is only mandatory for a narrow range of products or applications (e.g. where no Harmonised Standard exists for the Radio part of the equipment).
Similarly, there is no requirement to use an ISO 17025 accredited laboratory for any of the assessment activities. Accreditation is managed in the UK by UKAS and as such are sometimes referred to as “UKAS accredited laboratories”. This also includes testing submitted to a Notified Body to support an EU Type Examination Certificate process.
There is no “certification” of products for CE marking
Using an accredited laboratory is not mandatory for CE marking
Whilst not strictly required, testing is definitely the best way to determine EMC performance
EU laws have been transposed into UK laws so the same requirements for CE marking apply to UKCA.
When seeking to comply with the “FCC” requirements of CFR 47 Part 15 for sale of products into the USA, we need to consider the type of product we are making and fit it into one of these categories.
Unintentional Radiators are products that can generate RF energy but are not designed to radiate it. Essentially, a product that does not contain a radio (like Bluetooth or Wi-Fi). Examples would be a power supply, a desktop PC, etc. (defined in 15.13 (z))
Intentional Radiators (defined in 15.13 (o)) are products that intentionally emit RF (e.g. mobile phone, Wi-Fi router)
A complicating factor are Radio modules that have undergone a Modular Approval process (15.212). This is an easy way to add radio functionality to your product. These have already been reviewed by a Telecommunications Certification Body (TCB) and approved by the FCC.
Provided an approved module is installed into your equipment in line with the OEM instructions then your responsibilities as manufacturer are to verify that the combination of Unintentional Radiator and Radio Module do not infringe any radiated emissions limits.
15.101 shows the paths available (SDoC or Certification) for different types of Unintentional Radiator.
2.906 Self Declaration of Conformity can take place in any test laboratory whereas 2.907 Certification has to take place in an FCC registered laboratory (must me nationally accredited to ISO 17025). In all cases the provisions of 2.948 measurement facilities apply.
Accreditation of laboratories is a slightly different subject. Accreditation is a method by which the test procedures of a test laboratory are verified by an independent 3rd party (e.g. UKAS in the UK) to be compliant with ISO 17025.
Similar to ISO 9001, ISO 17025 is a quality management system that demonstrates a laboratory is operated to a certain standard. 17025 also extends this quality system to the tests being carried out where the individual test procedures and personnel are checked by an external assessor.
This is useful to demonstrate competence of the lab to their customers. It also demonstrates (but does not guarantee) the quality of test results have met a certain agreed basic standard. Some manufacturers choose to always use accredited laboratories for their testing for a variety of reasons e.g. their quality policy might dictate it.
At Unit 3 Compliance, we choose not to be an accredited laboratory.
Accreditation costs a lot of time and money in fees, inspections and internal paperwork. This cost ultimately gets passed on to the customer. By remaining un-accredited we can keep our fees around 33% less than an accredited laboratory.
Many accredited labs subscribe to a business model of employing multiple technicians to perform the day to day testing whilst retaining a couple of engineers for consultancy and compliance paperwork. The operation can end up as a bit of a sausage factory – seeking to have a full calendar of testing and turning the handle as quickly as possible.
The fallout from this is that test reports often take a a back seat and are delivered weeks after the testing has been completed and in the event of problems you might not have time in the relevant test area to perform diagnosis of the problem before you are hurried out for the next customers’ scheduled test to take place.
Most people at the test lab have been working in that environment for most of their working lives. This makes them very capable at performing the tests but their lack of experience with product design means the staff are frequently not as capable of knowing how to change the design to fix EMC problems.
You might get informal suggestions of “try and improve the shielding” or “you need a ferrite on that” but beyond that the likelihood of getting good quality problem solving advice is low.
I certainly don’t want to tar all accredited labs with the same brush. There are good labs and good engineers out there. However with some labs it can be pot luck whether you get Technician A (interested, helpful, keen, knowledgeable) or Technician B (uninterested, jobsworth, clock watching).
Whilst many accredited labs do have experienced personnel on site, getting access to them in a time or cost sensitive manner is often hard. Because of the requirements of accreditation and the need for impartiality, many labs run their consultancy services as a separate division within the company. Sometimes they aren’t even in the same building as the test lab! Inevitably these services have to be accessed outside of the test cycle leading to delays.
How we operate
Unit 3 Compliance is not a sausage factory. Our motivation is doing interesting work and solving challenging problems for people who care about their products.
We are significantly cheaper than an accredited lab, putting EMC testing within the budget of startups and smaller businesses. It also makes it more economical for medium to larger companies to run ongoing quality control checks, product cost down exercises and experiments.
We have a strong product design background, particularly in design for EMC. We can suggest, trial and optimise EMC fixes during the test process rather than send you back to base to figure it out for yourself. These fixes take into account the nature, volume and cost of the product – there’s not one fix that is suitable for all applications.
First time EMC pass rates are generally low. Of all the products that I’ve tested, less than 20% have passed first time. Many of those passed because we reviewed their design first from an EMC perspective and made suggestions for improving the design.
Because we have a strong background in fixing EMC problems and not just testing, we can resolve your EMC problems faster than anyone else. This is not an idle boast but something we genuinely believe. Every problem we fix makes us faster and better next time and this compounding experience is available to you.
We turn every test session into a miniature EMC class, explaining the tests, why we perform things the way we do and how it sits into the larger framework of standards, directives and compliance. We work hard to acquire our experience and love to share it with our customers.
https://www.unit3compliance.co.uk/wordpress/wp-content/uploads/2017/08/certified.png288526James Pawsonhttps://www.unit3compliance.co.uk/wordpress/wp-content/uploads/2017/01/unit3compliance_400x400.pngJames Pawson2020-09-23 10:36:492021-03-26 12:20:50"Do You Do Certification?"
Like most long articles, this started off as a short one. It all stemmed from a customer question:
“We had some issues using a LED driver that could not cope with load dump and volt spikes. Do you have any provisional tests that could determine the circuit reliability? It doesn’t have to be to [ISO 7637-2]”
The ISO 7637-2 standard defines automotive conducted transient test pulses on vehicle power lines (12V or 24V). It is called up by standards including:
UNECE Regulation 10.06 for E-marking
EN 50498 (aftermarket automotive equipment)
ISO 13766-1 (earth-moving and building construction machinery)
I don’t have an ISO 7637-2 pulse generator (edit: I do now!). Automotive surge generators are less commonly found in many EMC test labs due to their more specialised nature.
Systems are available to hire; budget for €/£1000/week for a generator that will cover Pulses 1, 2 and 3. They are also available to buy new; expect to pay around €/£15k. If you need to cover pulse 4 then this will increase the costs yet again, mostly for the bipolar amplifier.
But, like most EMC test labs, I do have an IEC 61000-4-4 (EFT) and IEC 61000-4-5 (Surge) generator capable of 1.2/50us and 10/700us pulses.
Question: Could I use the IEC generator to simulate the surge pulses from the ISO generator?
This question comes with caveats:
The aim here is pre-compliance / confidence testing with the tools available. Not to replace the ISO 7637-2 tests entirely.
We are only looking at the potentially destructive Pulses 1, 2a, 3a and 3b.
Unit 3 Compliance can perform pre-compliance and full CE Marking testing to EN 50498. We can also perform pre-compliance testing for many of the R10 tests for E marked products.
IEC 10/700 pulse generator can be used as a close substitution for a 12V system
For a 24V system the 10/700 pulse is not as good a match. Follow the flowchart to select the test compromise and set the surge voltage based on the values in the tables.
ISO Pulse 2a
Not a good match, recommend a compromise between current and energy as shown in these tables
ISO Pulse 3a, 3b
IEC EFT generator is a good match and can be substituted for ISO pulse 3a and 3b
Pulse Parameter Comparison
Comparing the pulse widths and impedances against each other gives a mixed picture.
For Pulse 1, neither waveform is a great match with both of the ISO pulses having a longer pulse width than the 10/700 generator. Whilst the 24V bus pulse has a much higher impedance, this could be corrected with an additional series resistor in the IEC generator output.
For Pulse 2a, the 1.2/50us IEC generator appears to be an excellent match.
For Pulse 3a and 3b, the 5/50ns EFT generator is pretty close but the width of the ISO pulse is three times bigger.
However, as we shall see below, this approach is incorrect as it does not tell the whole story.
Pulse Width Definition
The problem comes from how the pulse widths are defined in the standards. Let’s take the comparison between ISO Pulse 1 to IEC 10/700 comparison as an example.
e can see that the ISO pulse width is defined at the 10% crossing point, whereas the IEC pulse width is defined at the 50% crossing point.
This is not helpful.
How do we compare a ISO 1000us @ 10% with a IEC 700us @ 50% waveform?
Open Circuit Ideal Waveform Comparison
I found some information over on the PSCAD website that showed the equation for the waveshape (from IEC 61000-4-5)…
…along with some Matlab optimised coefficients for alpha, beta and k.
From the PSCAD website “Standard Surge Waveforms” https://www.pscad.com/webhelp/Master_Library_Models/CSMF/Surge_Generators/Wavelet_Transformation_(WT).htm
ISO 7637-2:2011 gives the equation for the falling edge only of the pulse waveform. It also states that “The influence of the rise time is not taken into account (tr << td), which is allowed for all pulses specified in this part of ISO 7637”
I also modified the equation for the IEC waveshape equation to take into account the generator and load impedances by taking the first term of the ISO equation and adding it to the start of the IEC equation.
A required surge voltage of 1V was used for simple direct comparison.
For Pulse 1 we can see that the 10/700 IEC waveform is actually a really good match for Pulse 1 for a 12V bus.
The same cannot be said for the 24V bus requirement. Some further thinking is required here.
The 55 ohm impedance for the 24V version of the pulse is the 15 ohm 10/700 generator natural impedance with a series 40R resistor in addition.
Despite Pulse 2 looking like a good comparison initially, the modelling shows that it is actually a very poor match.
For Pulse 3, the IEC EFT generator is a very good match and should be able to be used without any issue
Dealing With Pulse 1 (24V) and Pulse 2a
How could we go about compensating for the poor match between Pulse 1 (24V) and 10/700 IEC and between Pulse 2a and 1.2/50 IEC?
We need to ask ourselves: are we more interested in the peak voltage & current or pulse energy?
To answer this, first we need to understand the power input design of the Equipment Under Test (EUT)
EUT Design Assessment
It is useful to establish the following EUT design parameters:
Is there a discrete reverse protection diode? What is the Vrrm and Trr rating (reverse recovery time) of this part?
What is the maximum clamping voltage of the TVS diode and can the downstream circuitry survive this voltage?
It is important to remember that Pulse 1 is a negative going pulse caused by the disconnection of a large inductive load in parallel on the vehicle power bus. If the EUT has a reverse protection diode fitted then it’s Vrrm and Trr will change the effect of the test on the EUT.
It is also important to test at full current consumption if a reverse recovery diode is present as this will affect recovery time and therefore surge performance.
EUT Surge Suppression
The assumption is that we are testing an EUT that contains some basic low voltage electronics of some kind. The extension of this assumption is that it has some kind of surge suppression component connected across the power inputs.
This could be a Metal Oxide Varistor (MOV) or a Transient Voltage Suppression Device (TVS). These have a non-linear impedance with voltage and will restrict or “clamp” the input voltage to a defined level. Perhaps a component like a SMBJ26CA-TR.
This clamping voltage is dictated by the impedance of the part when conducting. This would be a diode-like VI curve for a TVS or the current-dependant resistor of a MOV.
Peak current is dictated by available peak voltage and generator impedance. So we need to be interested in the peak current to ensure that the correct clamping voltage is met.
Also, because the MOV or TVS absorbs some of the pulse energy internally, these components will have a datasheet rating for pulse energy. Exceeding this could cause significant damage to the part and affect its capability to handle future surges.
Pulse 1 Peak Voltage & Current or Pulse Energy?
Our main tools for adjusting an IEC pulse to suit an ISO pulse are:
The surge generator has an easily adjustable peak voltage through the control panel or software so this is the main method that will be used.
The Peak voltage is a significant consideration if the system has the reverse protection diode but the compromise test will depend on it’s voltage rating.
I’ve produced a flowchart to help selection of the right test level for using IEC 10/700 instead of ISO Pulse 1
Pulse 1 Best Compromise Voltage
I ran some more simulations in Geogebra adjusting the ratio between the IEC and ISO peak voltages and tabulated the results.
The best compromise is to minimise the total difference between current and voltage when expressed as ratios. This works out at a V_iec or around 0.6 * V_iso.
This yields the following test voltages, peak currents and pulse energies for the different severity levels.
It is interesting that the series impedance for the 24V version of ISO Pulse 1 is up at 50 ohms. This higher impedance implies that the surge expected in such a system would be induced from a parallel adjacent cable in a wiring loom rather than something directly connected to the ignition switch / inductive load circuit directly.
Pulse 2a Best Compromise Voltage
Same approach as for Pulse 1
Test Practicalities & Further Compromises
Pulse 1 Power Disconnection
The waveform for Pulse 1 shows a synchronised disconnection from the DC supply and application of the surge voltage. Since this is not easily done without
It is the surge pulse that will cause the damage rather than the momentary disconnection of voltage therefore, for these compromise tests, this is being ignored.
Coupling/Decoupling Network Requirements
The CDN inside the IEC test generator for mains coupling is adequate for the task of decoupling but the options inside my KeyTek ECAT test generator preclude the coupling of the 10/700 waveform. Instead, some creative front panel wiring with banana plugs will be required.
Since this CDN is designed for decoupling of surge and EFT impulses from the mains, I’m sure it will adequately protect the 12V linear power supply being used and also prevent the power connection from unduly affecting the test.
In may case, input is through a 16A IEC mains plug/socket but it is easy to make an adaptor. Output is via a BS1363 socket or, more convieniently, 4mm banana plugs.
This took way longer to research and write that I was hoping. Something in the order of three days of work was spent going backwards and forwards, thinking about it whilst doing DIY at home (nearly painting the cat as a result) and half listening to Tiger King on the TV.
I’m quite pleased with the result and I hope this eventually proves useful to someone.
https://www.unit3compliance.co.uk/wordpress/wp-content/uploads/2020/06/iso-7637-2-pulse-1-vs-iec-61000-4-5-waveform-comparison.png8861931James Pawsonhttps://www.unit3compliance.co.uk/wordpress/wp-content/uploads/2017/01/unit3compliance_400x400.pngJames Pawson2020-06-29 19:36:022020-10-19 08:10:37IEC Surge/EFT Generators for ISO 7637-2 Automotive Pre-Compliance
“EMC Testing (TBC): Must comply with IEC 60601-1-2:2014, Medical electrical equipment — Part 1-2: General requirements for basic safety and essential performance — Collateral Standard: Electromagnetic disturbances — Requirements and tests”
So lets take a look at a the EMC tests that might be required for a typical Rapidly Manufactured Ventilator project.
This is with the view of meeting the Essential Performance / Basic Safety requirements of EN 60601 whilst addressing the highest risk items first. This is prioritising speed of testing instead of performing a belt and braces, test everything approach that would be the common approach for Medical Devices.
These ventilators are going to be used in a hospital / clinical care environment under medical supervised use and not in a home environment.
60601-1-2 classifies a hospital as a Class A emissions environment for Radiated and Conducted emissions. This means that less time needs to be spent fighting to get the emissions below Class B.
Risk items to radiated emissions could include any brushed DC pump motors as these are notoriously noisy. Ferrite cores may be required around motor cables to mitigate this noise.
Following the design guidelines further down this article for any PCBs is recommended and will greatly assist with reducing EMC radiated emissions.
Most RMVs will be using an off the shelf power supply already approved to EN 60601 for Safety and EMC. AC Power conducted emissions should therefore look after itself and won’t be a significant worry for testing.
For Harmonic Distortion and Flicker, there is an interesting note in EN 60601-1-2 in Annex A
“It is assumed that ME EQUIPMENT and ME SYSTEMS used in hospitals (and large clinics) are not connected to the PUBLIC MAINS NETWORK.”
If this is the case, then Harmonics and Flicker requirements and tests need not apply as these only relate to the public mains network.
Most ventilator systems have no external electrical ports apart from the power supply. They are mostly self contained units. This greatly speeds up and simplifies the testing, and reduces the risk of problems with Signal Input / Signal Output Ports (SIP/SOP Ports in the standard, analogous to a Signal Port from other EMC standards).
Mains borne interference (EFT, Surge, Conducted RF) should be handled by the EN 60601 pre-approved power supply without issue. It will still need checking but ultimately the risk is low.
Dips and Interrupts and the hold up time of the power supply is something that would need considering at the Risk Analysis level to derive the correct immunity criteria for each of the individual tests.
If this needs improving then selecting a slightly larger power supply than nominally required could help. More likely, additional bulk decoupling on the main power rail (e.g. 1000uF) will help maintain the system DC voltage under these conditions.
Immunity Performance Criteria
Caveat: This section is me thinking aloud as I have no domain specific knowledge for Risk Management and Medical Devices. I’m trying to approach this from a common sense perspective to aid anyone working on a RMV.
The function of the EMC Immunity tests for Medical Devices is to ensure that the Essential Functions continue to operate and that Basic Safety condition is maintained.
Normally the immunity performance criteria would be based on the type of EM phenomena being simulated in the test. This is normally Criteria A for continuous phenomena (radiated or conducted RF immunity) or Criteria B for momentary phenomena (ESD, EFT, Surge, Dips/Interrupts). Criteria C only tends to crop up for longer duration power interruptions.
In the case of a Medical Device, maintaining the Essential Performance is the key parameter. If a momentary EM phenomena causes this to happen then this is a major problem.
Therefore immunity performance criteria must be considered for the key function of the device as well as the duration of the EM phenomena.
Based on this thought process, a sensible starting point for the immunity criteria is:
Assume criteria A (unaffected performance) for:
Key function of assisting patient breathing for all tests. This includes momentary EM phenomena tests = ESD, EFT, Surge, Dips/Interrupts.
Non-critical functions under continuous EM phenomena tests = Radiated and Conducted RF Immunity
Assume Criteria B for:
Key function performance for this means that there should be a function in the RVM firmware that remembers its last current operating state and settings and that it starts up in that state from a power cycle. This creates a requirement for programmable non-volatile memory (some kind of EEPROM) in the RVM.
All momentary EM phenomena tests for non-critical functions e.g. display readout may temporarily distort or flicker so long as it recovers
Assume Criteria C for:
Non-critical functions from momentary power loss e.g. screen/display readout or setting
There are two big risks to the immunity performance: Radiated RF Immunity and ESD.
Radiated RF Immunity
The basic requirement for radiated RF immunity is a flat 3V/m from 80MHz to 2.7GHz. So far so good, this is a fairly easy test to meet.
Now the bad news. Table 9 gives a list of spot frequencies to be tested to simulate close range exposure to common wireless technology standards. The table is summarised here:
Test Level (V/m)
18 Hz pulse, 50%
FM +/- 5kHz dev. 1kHz sine
710, 745, 780
217 Hz pulse, 50%
810, 870, 930
18 Hz pulse, 50%
1720, 1845, 1970
217 Hz pulse, 50%
217 Hz pulse, 50%
5240, 5500, 5785
217 Hz pulse, 50%
As you can see, this has testing up to 28V/m, a significantly higher field strength than 3V/m!
Risks to the EUT
This test loves to mess with analogue sensors. In the case of ventilators, the pressure sensors used frequently have an analogue output to a DAC on the CPU. This presents two risk areas:
Demodulation of noise inside the pressure sensor amplifier. This takes the small transducer signals and amplifies it up to the output voltage. Noise demodulated here would cause the carrier to be superimposed on the pressure readings.
The input of the ADC could be susceptible to noise picked up on the analogue voltage from the pressure sensor, even if the pressure sensor itself is unaffected. This will affect the readings.
Since the airflow and pressure sensors are a key component to the operation of the ventilator, these must be protected at all cost.
Design ideas to mitigate this interference include
Keep traces/connection as short as possible between sensors and ADC
If you can mount them all on the same circuit board then do so
This circuit board will have one layer dedicated to a solid ground plane fill over the entire plane. All ground pins
Cables = antennas that are good at receiving the interference. Minimise use of cables where possible.
Figure out what your minimum bandwith requirements for airflow are and filter the signal appropriately. You probably won’t need to sample the airflow faster than 10kHz so put a low pass filter right next to the ADC input. Something like a 4k7 and a 1nF will give you a 3dB of 34kHz. This will reduce the risk of RF noise being demodulated by the ADC input.
Decouple the supply lines to the pressure sensor well
Add a small filter to the pressure sensor input, perhaps another RC filter as shown above. This will help prevent the pressure sensor from being affected by the test.
It is possible that the pressure sensor will be directly affected by the radiated noise picked up by the sensor body itself and not by the traces. It would be prudent to provide a PCB footprint for a shielding can near the sensor. I have seen this effect on gas sensors in the past.
Assuming that the advice above is followed, the risk to the EUT is manageable.
One of the interesting features of Radiated RF Immunity testing is that of the Problem Band where most issues occur.
Most of the time, the problem band is in the 100MHz to 300MHz area (I’ll cover this in more detail in a future article). Cables tend to be the best antennae at these frequencies and, hopefully, our ventilator only has one cable of interest – the AC power cable. This has plenty of filtering for conducted emissions reduction which should handle this noise.
Probably the two biggest problem frequencies from the spot frequencies above are going to be 385 MHz and 450 MHz.
Then we are into the realms of direct pickup on internal signal cables and PCB traces at higher frequencies. If we’ve laid out our PCB well as highlighted above (short analogue traces, filtering, good ground plane, shielding provision) then this will help mitigate our risks.
The levels of ESD testing are almost twice that of the regular EMC standards with a requirement for 8kV contact and 15kV air discharges.
ESD is very good at upsetting digital systems and it has a particular fondness for edge triggered pins e.g. reset lines and interrupts.
If the reset line for the CPU controlling the RVM is shared with other digital circuit blocks or supervisory controllers then an RC low pass filter at the input to the CPU is highly recommended. This helps prevents unwanted resets.
Checking can be implemented in the Interrupt Service Routine to ensure that an interrupt condition actually exists, effectively de-bouncing the input.
Thankfully the Ingress Protection requirements for the RVM of IP22 and the requirement to provide flat, easily cleanable surfaces will probably dictate the use of some kind sealed membrane keyswitch panel. These have good ESD immunity as no direct contact discharge can take place on an switch where the plastic covering remains in place.
Whatever user interface technology the RVM employs, this will be a key risk area for ESD. If this is on a separate PCB to the main controller, all interfaces will need some kind of filtering. A small capacitor to ground on each of the lines that goes to the keypad would be a good idea. 0603, 100pF usually works well here.
Lastly on the mechanical design, keeping the electronics well away from the enclosure seams will also reduce the risks of creepage of any discharge into the circuit board.
Summary Test Plan
Radiated Emissions, Class A, 30MHz to 1GHz (EN 55011)
Mains Conducted Emissions, Class A, 150kHz to 30MHz (EN 55011)
Text in bold is highlighted as a risk item.
ESD, (EN 61000-4-2), 8kV contact, 2/4/8/15kV air. Test to connectors as well.
Radiated RF Immunity (EN 61000-4-3)
80MHz to 2.7GHz @ 3V/m
Various spot frequencies at up to 27V/m
EFT (EN 61000-4-4), AC Mains Port, 2kV
Surge (EN 61000-4-5), AC Mains Port, 1kV line-to-line, 2kV line-to-ground
Conducted RF Immunity (EN 61000-4-6), AC Mains Port, 3V/m (6V/m in ISM bands)
Dips and Interrupts (EN 61000-4-11), AC Mains Port, various
Not only has this article identified key EMC risks to Rapidly Manufactured Ventilators but also provided some design guidelines to dealing with the problems that might arise.
Some of the guidelines within might be useful to anyone designing a Medical Device. We haven’t covered the requirements for Patient Coupled Ports or SIP/SOP ports from an EMC perspective as they aren’t of too critical a concern for an RVM.
We can see how looking at the standard and pulling out the required tests can help us understand the risks involved in the design.
Experience of knowing how the tests will typically affect the EUT is the key to unlocking good design practices. In my case, this comes from having worked on many designs with problems and the learning that comes from fixing the issues that crop up.
Remember EMC test success comes from good EMC design. For a time critical RVM there is one chance to get it right – no do-overs!
I hope you found this article useful. See you when all this has calmed down. All the best, James.
https://www.unit3compliance.co.uk/wordpress/wp-content/uploads/2020/04/BIPAP.jpg7981024James Pawsonhttps://www.unit3compliance.co.uk/wordpress/wp-content/uploads/2017/01/unit3compliance_400x400.pngJames Pawson2020-04-18 15:55:522020-04-18 16:05:00Ventilator Projects and EMC Testing (EN 60601-1-2:2014)
1. CE marking probably won’t be necessary for UK for temporary ventilators 2. Still need to apply to MHRA for permission to use in clinical setting 3. This will likely involve the supply of test data to MHRA 4. The requirement for EMC testing is currently To Be Confirmed 5. U3C will give any ventilator design a pro bono EMC design review
In the current Covid-19 pandemic there are many project teams around the world coming up with designs for ventilators. The potential is for many such devices to be required.
The problem that this article identifies is that for normal medical devices, the length of time taken to get products approved is defined in terms of months. This is especially true for ventilators which are, according to several sources, a Class IIb device requiring a robust quality system and Notified Body approval.
The current requirement is for devices to be delivered within weeks.
The procedure covered under the “Exceptional use of non-CE marked medical devices” involves applying to the MHRA directly for a pass to supply this product for use without CE marking. This request will have to include data that shows compliance with the test criteria in Appendix B of the MHRA specification.
Page 11: “3. When the current emergency has passed these devices will NOT be usable for routine care unless they have been CE marked through the Medical Device Regulations. The device must display a prominent indelible label to this effect.“
Page 24: “EMC Testing (TBC): Must comply with IEC 60601-1-2:2014, Medical electrical equipment — Part 1-2: General requirements for basic safety and essential performance — Collateral Standard: Electromagnetic disturbances — Requirements and tests”
Note that the requirement for EMC testing remains TBC = To Be Confirmed. I strongly feel that at least some testing should take place, but functionality of the ventilator has to come first.
Next article will be a breakdown of the EMC test requirements called up in EN 60601-1-2:2014. I’ll also highlight some risk areas that designers of these ventilators will need to to bear in mind.
If anyone has any need for pro bono EMC design review services for ventilator projects or needs some EMC testing turning around quickly then get in touch.