Spring or Star Washers for Earthing Stud?

Introduction

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:

some considerations for protective earthing conductor connection to metalwork

 

Common Safety Standard Review

A review of the more frequently used safety standards for electronic products yields the following clauses.

Standard Clause Clause Text
EN 62368:2014 4.6.1 “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”
EN 61010-1:2010 6.5.2.2 c) “Screw connections shall be secured against loosening”
EN 61010-1:2013 6.5.2.3 k) “The contact pressure required for a bonding connection shall not be capable of being reduced by deformation of materials forming part of the connection”
EN 60335-1:2012 28.4 “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.”
EN 60335-1:2013 27.2 “27.2 The clamping means of earthing terminals shall be adequately secured against accidental loosening.”
EN 60730-1:2016 9.3.6 Clamping means of earthing terminals for external conductors shall be adequately locked against accidental loosening.
EN 60730-1:2017 11.2.2 “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.

a table showing locking nuts and washers

Conclusions

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.

Selecting suitable environmental test levels for your product can be based on your experience as the manufacturer with typical operating environments, or perhaps using the ETSI EN 300 019 environmental engineering standards.

Of course, the simplest way is to just use a standard washer to reduce arguments.

 

 

Not Covered

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.

 

Your Thoughts

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

  1. My original question posted on LinkedIn asking about washers
  2. NASA fastener design manual, page 10 has details of locking mechanisms
  3. The always amusing and informative AvE
  4. NordLock brand washers under the Junker test

 

 

 

comparison of weee crossed out bin logos with and without bar (simple graphic)

WEEE Symbol With Or Without Bar?

To Bar, or not to Bar, that is the question:
Whether ’tis nobler in the regulations to suffer
The affixing of a line under ones bin of marking,
Or to add the date of manufacture to ones label…

Hamlet, Act 3, Scene 1 (an early draft)

 

Two symbols walk into a bar…

The “crossed out bin” logo of the WEEE Directive 2012/19/EU is a common sight on electronic equipment to denote that these should be separated from regular waste for recycling.

This is commonly seen both with and without the solid black bar underneath the bin. But which is correct?

Unsurprisingly the answer is “it depends” (I swear I’m capable of giving an answer that doesn’t begin with this phrase, honest)

comparison of weee crossed out bin logos with and without bar (simple graphic)

Directive check

Article 14

Information for users

4. With a view to minimising the disposal of WEEE as unsorted municipal waste and to facilitating its separate collection, Member States shall ensure that producers appropriately mark — preferably in accordance with the European standard EN 50419 (25) — EEE placed on the market with the symbol shown in Annex IX. In exceptional cases, where this is necessary because of the size or the function of the product, the symbol shall be printed on the packaging, on the instructions for use and on the warranty of the EEE.

Article 15

Information for treatment facilities

2.   In order to enable the date upon which the EEE was placed on the market to be determined unequivocally, Member States shall ensure that a mark on the EEE specifies that the latter was placed on the market after 13 August 2005. Preferably, the European Standard EN 50419 shall be applied for this purpose.

 

Seems pretty clear – some kind of mark should be applied to denote manufacture after 2005 (that’s a loooooong time ago now) and the PREFERENCE is to use the methods given in EN 50419.

 

EN 50419 breakdown

Summarised, clause 4 (marking) of EN 50419:

 

4.1 Requirements of marking

a) Identify the producer of the WEEE (brand name, trademark, company name/number). Name to be consistent with name used to register WEEE producer with the authorities

b) equipment [placed] on the market after 13 Aug 2005 shall be identified by EITHER

1) the date of manufacture in text form

2) marking as shown in Figure 1 of the standard

4.2 Design of the marking shall consist of a solid bar with dimensions shown in Figure 1 (shown below)

 

EN 50491 figure 1 bin and bar dimensions

 

Summary

It’s pretty simple, either

  • Apply the bin only logo (Option A) but make sure you have date of manufacture clearly shown on the product label
  • Apply the bin-and-bar logo (Option B)

 

 

 

Graphical Guide to EMC: Near Field Probing (free eBook)

 

Download “The Graphical Guide to EMC: Near Field Probing” eBook here (40MB)

 

I have a love / hate relationship with textbooks.

They are thick, have lots of words, make me feel clever, and stop my bookshelf from floating away. They often have the one thing that you are looking for.

On the other hand, they have far too many (big!) words, too many equations with no context or explanation. I find it very difficult to sit, read and quickly gain an intuitive understanding.

 

I prefer to communicate with pictures. This is why my presentations are image heavy and text light. I’ve sat through far too many “PowerPoint Karaoke” sessions where the presenter reads the words on the slide.

Also I love the format of cartoons and graphic novels but you rarely see them outside of the fiction sphere. I’ve recently been thinking about what a combination of a graphic novel and a text book would look like.

 

With the recent acquisition of an e-ink tablet with drawing stylus to replace my 74 different notebooks and notepads I started sketching out some ideas for a guide to using near field probes. A subject that I’m often asked about and is complementary to our free Pocket Probe Set that we give away at shows and to customers.

One thing turned into another and once I started drawing I couldn’t stop. You can download the full eBook from the link at the top of this page or by clicking here.

 

 

I’ve released this under the Creative Commons Attribution-ShareAlike 4.0 International (CC BY-SA 4.0) license. This means you can share or adapt this work but you must provide a credit / link back to the original source (here). Any adapted work must be shared with the same licence terms.

 

I’d be interested to hear your feedback on the format and content of this mini eBook – please get in touch and let me know. If there is a positive response then more content may follow.

Thanks and all the best

James

 

 

self interference demo USB3 and 2.4GHz

2.4GHz Intra-System (or Self/Platform) Interference Demonstration

In this blog we are going to take a short look at noise and interference in the 2.4GHz band. Our example victim is a Zigbee controller and the sources are nearby USB3.0 devices and Wi-Fi sources.

 

Background

One of our customers makes these rather useful USB Zigbee Coordinator sticks, frequently used for controlling smart home or IoT devices like light bulbs.

These devices operate at 2.4GHz, a very crowded frequency band with Wi-Fi, Bluetooth and Zigbee all fighting for a narrow, congested slice of spectrum.

One of the common issues faced by users of this band is that of intra-system interference, sometimes referred to as “self” or “platform” interference. This is where components in the same system interfere with each other, primarily due to their proximity.

[Note: The counterpart to intra-system (within the system) in this context would be inter-system interference (between separate systems), which is what the conventional EMC test regime of radiated and conducted emissions and immunity seek to characterise.]

This common problem is something that our customer knows all too well from helping their clients integrate these Zigbee products into the end application.

So, during a recent visit to our lab for some testing on a related product, we spent some time investigating this noise on a typical setup.

 

Demonstration Setup

The setup in the below image is common to many users with a Raspberry Pi Model B and lots of stuff plugged in to the USB ports. In this case, a Zigbee adaptor (black case) and an USB3.0 SSD in close proximity.

These parts, including the spectrum analyser, is part of the customers in-house electronics development laboratory.

 

self interference demo USB3 and 2.4GHz

 

The effects of USB3.0 on the 2.4GHz spectrum are well known. A good example is this 2012 paper from Intel which

For this demo, we used a near field capacitive probe and a 2.4GHz antenna to measure noise in the 2.4GHz to 2.5GHz band local to the Raspberry Pi.

This demonstrated the degradation of the noise floor with various levels of system activity including

  • Measurement of system noise floor
  • Presence of a USB3.0 SSD running a large file transfer using the dd Linux command
  • Activation of the Raspberry Pi internal Wi-Fi

The below image shows three traces under these different conditions.

 

spectrum of 2.4GHz band showing ambient noise, SSD noise and Wi-Fi emission

 

Experiment Conclusions

The conclusions we can draw about the in-band noise are:

  • Noise from the SSD raises the noise floor by approximately 10-20dB (a factor of x10 to x100)
  • The Wi-Fi transmission from the Pi is 40dB above the local noise floor. This will mask any received Zigbee signals from a remote transmitter.

 

In-Band vs Out-of-Band Sensitivity

Well designed radio systems are generally very robust to out-of-band interference i.e. anything outside of the narrow radio band that it is tuned to. For instance, a Zigbee radio system set to channel 20 (2.450GHz) will reject anything below 2.445GHz and above 2.455GHz.

 

Intra System Interference Diagnosis

Advice on diagnosing these issues is mostly outside the scope of this short blog. Differences in systems, components and ambient noise levels makes it impractical to offer guidance for all situations. However, some generic problem solving pointers are presented below.

A systematic approach to isolating the problem is required.

One of the primary rules of problem solving is to change only one thing at once and observe the effects.

In EMC terms, it is possible to change several things at once without realising it. Cable position, the specific port that a device is plugged into, location of nearby equipment and cables, even how firmly a connector is tightened will all make small differences that stack up. (Don’t use anything other than a torque spanner on those SMA connectors though!)

Another key rule is if you think something has made a difference, reverse the change and see if the problem re-occurs. Unless you can achieve consistency then you might be changing something else unintentionally, or the problem is caused by something outside of what you are changing.

Correlating the problem against time can help. Does it happen when something else happens (other devices on, or off, or switching, certain configurations, times of day, etc.) This can give clues.

Lastly, we should be looking for a significant step change in improvement to identify the issue. Phrases like “I think it made a bit of a difference but I’m not sure” indicates that we are dancing around the issue and not getting to the heart of it.

Ultimately, for a detailed understanding, the spectrum analyser is a key tool in gaining a proper grasp of this issue.

 

Solutions

The solutions to the problem are simple yet sometimes difficult – a technical balance needs to be struck.

Use of Ethernet rather than Wi-Fi on the Raspberry Pi.

It is not practicable to synchronise transmission from the Raspberry Pi Wi-Fi with that of the Zigbee stick. The simplest way of ensuring the Wi-Fi does not interrupt the Zigbee transmissions is to disable the Wi-Fi and provide network connectivity via Ethernet instead.

Depending on the installation this might not always be practicable but it certainly is more reliable.

 

Separation of components

Moving the antenna away from the noise source is usually the best way to achieve increased performance.

In this instance, placing the module at the end of a USB cable and away from other electronic items is a good start.

Another option that is not as ideal: a good quality SMA extension cable could be used to extend the antenna away from the problem area. This introduces loss into the RF channel, reducing signal quality.  Measurements made in our lab on a cheap extension cable from RS show a power reduction of 6.5dB at 2.4GHz for a 5m cable. This equates to a ratio of around 0.25 meaning we are broadcasting and receiving a quarter of the power we were before.

Also, it is still possible for the noise to couple onto the nearby module even without the antenna attached meaning the problem does not get entirely resolved.

 

Better quality components

Sourcing a bunch of cheap-as-possible parts from Amazon or eBay is likely to bring problems.

Using devices from big name manufacturers and buying from reputable sources helps. But, even reputable components are designed to a price point and can still cause problems if the other points in this blog are not taken into account.

USB cables can be a big source of the problem. Unshielded back shells (the part between cable screen and connector body) compromise the shielding to the point where their performance at high frequencies is equivalent to an unshielded cable.

The only way to tell if a cable is good quality is to perform an autopsy on the ends and check on the cable shielding

Remember that Pawson’s Law of Cable Quality states that the EMC performance is inversely proportional to the physical appearance. Braided covers, shiny plating, metal connector bodies, transparent mouldings etc are all indications of money spent on the OUTSIDE of the cable. EMC quality comes from the INSIDE and is not visible.

shiny usb cable vs boring usb cable

 

 

Hope this was useful! See you soon.

James

 

 

 

V2 pocket EMC debug probe PCB - near field probe set - board front

Pocket EMC Debug Probe V2

This is a guide for the assembly and use of the Version 2 “Pocket EMC Debug Probe” from Unit 3 Compliance.

Download our free eBook on Near Field Probing

 

V2 pocket EMC debug probe PCB - near field probe set - board front V2 pocket EMC debug probe PCB - near field probe set - board rear

 

Assembly Guide

Components required:

0805 resistors for R1 = 470R and R2 = 10k, gives 450 ohm parallel combination. This is required for the 10:1 into a 50 ohm input.

0805 capacitor C1 470pF to 10nF, C0G/NPO dielectric, 50V. For the AC coupling of the signal.

Sourcing SMA edge mount female connectors (RS, eBay)

Recommendations for the probing pin (socket strip or a bit of wire)

Suitable ferrite cores for the current transformer

90 degree options for the B-field loop probe and E-field capacitive probe (on E-field probe snap off – scrape copper on each side of slot before you snap off the end to enable soldering)

 

A Bag of Water.

This is a very useful analogy to use when considering an EMC emissions problem, particularly true for radiated emissions in the (often problematic) 30MHz to 1GHz band.

 

Lets get squeezing.

Many of you will have experienced this before. Making a change to an emitting structure inside the equipment by changing the electrical connection between two points results in some emissions going down and some going up.

radiated emissions plot

Then you make another change and this has the opposite effect.

This is like squeezing our bag of water. We can move the water around in the bag much like we can emissions around in the spectrum. The harder we press down in one area, the more it pops up in another.

Emission goes up.

Emission goes down.

 

Reducing the volume

But unless we reduce the amount of water in the bag we will nearly always have a problem. The water is incompressible and it just finds new places to appear.

To achieve this in an EMC context we need to reduce the overall energy in the system.

This could be achieved either by keeping the energy controlled on a PCB away from the radiating structure or by adding lossy components (filters, ferrites, etc) to reduce the amount of energy coupling into the radiating structure.

Changing grounding and bonding within a system without reducing the energy is going to be an exercise in frustration and probably wasted time. Better to address the problem at source where possible.

 

Caveats inbound

There will always be a requirement for us to have to try and achieve the goal of “shaping” our bag of water to fit the radiated emissions limits.

A good example is a manufacturer that has already built a production run of units and needs a quick fix to get them onto the market.

Whilst this is often achievable, there are often significant rework / modification costs involved.

There is also the question of repeatability and consistency. If small changes in bonding of parts can make a large difference to emissions, how can you guarantee that each unit will be compliant? Testing multiple samples can help. As can having good production inspection points during the manufacturing process.

But common mode noise is a slippery customer and these kind of fixes should only ever be considered as temporary pending design changes to address the root cause of the issue.

 

A small plug.

Help is available.

We are really good at this kind of work

We’ve been through the cycle many many times with many many different products.

Using Unit 3 Compliance to help with your emissions problems gets you access to our years of accumulated experience.

Our on site test lab allows us to have a rapid cycle time between analysis of a problem on the bench, developing a fix, and testing in the chamber.

 

Hope this was interesting!

James

test report extract showing all tests passed

Thoughts: First Time Pass Rate

This question popped up on one of the email lists that I’m a member of.

“Calling all labs – In your experience how many products pass the Unintentional Emissions test first time?”

I thought I’d share my response:

 

Speaking from my own experience. Over the last four years of running a consultancy, pre-compliance and low cost test EMC laboratory I would (very roughly) estimate that around:

  • 50% of products pass their desired radiated emissions limits without any modification
  • 33% or less pass all of the applicable tests first time without modification

 

The major caveats and notes here are that

  • These figures are for customers products where the EMC performance is not known before testing. We do a lot of work helping people solve existing EMC problems but we are not counting this in these figures.

 

  • Most of my customers are smaller businesses that can’t afford to employ an engineer to just look after compliance. That job role is either split amongst several people or the engineer in question has to look after quality, manufacturing, sustaining, thermal, system, and everything else. Speaking as someone who has designed many products and systems in the past, trying to design for functionality whilst simultaneously considering best EMC performance is HARD.

 

  • The products that pass first time generally fall into one of three categories
    • Products that we have design reviewed before the design was finalised
    • Retests of products that have already been through our lab once
    • Products that are very simple in nature

 

  • Our hit-rate at being able to solve our customers problems is around 90-95%

 

  • The “ones that got away” where we were unable to help deliver a compliant include
    • No action taken: Products where it was deemed by the manufacturer not economically feasible to modify the product (e.g. product going end of life)
    • No further communications from the manufacturer so we don’t get to find out what happened next (no news is good news, right?)

 

I would echo the sentiments of others on this thread regarding the need to design in compliance from the start.

One of the problems with the field of compliance is that it is too often “learned through experience in industry” and not explicitly taught. When it is taught at academic level it is often a surface treatment with a theoretical look at shielding or maybe crosstalk with no other practical context or background.

 

The split between industry and academia is one of the possible causes. Yes, there are exceptions to this but they primarily remain exceptions. I had discussions with a local university about some guest lectures on compliance and the theme of the response was “it doesn’t really fit into any of our modules” and “we can’t have it as an optional lecture as none of the students will attend”.

The number of times I hear “oh, thanks for that. No one has every explained it that clearly before” is worrying!

 

That’s my rant, hope it was useful

James

 

 

 

Thoughts on In House Pre-Compliance Test Equipment

From an email sent to a customer who asked for some feedback on their list of proposed EMC pre-compliance test equipment.

 

“On the subject of equipment, sounds like you’ve identified a nice little pre-compliance setup there! I agree that investing in equipment is a much better long term view than just hiring some in (for the common tests at least).

Com-Power stuff is very good, I use some of their kit myself. The TekBox stuff is very reasonably priced for pre-compliance and again, I use some of their equipment in my test setups. If you are making conducted emissions measurements with a LISN you’d be wise to put a limiter in series with your spectrum analyser input to prevent costly damage. Armoured cable isn’t strictly necessary as it can be difficult to handle, just regular good quality coax is fine.

As a substitute for radiated measurements you can use a current probe around cables as these tend to be excellent emitters of radiated noise. It helps if you already know what the problematic frequencies are. Again, Tekbox make some very reasonably priced probes.

I would also consider the Signal Hound SA44B or BB60C (I have one and I really like it) spectrum analysers as an alternative to the Tektronix one. There’s quite an in depth review of the BB60C here. Their software is easy to use and crucially is free with an EMC pre-compliance option.

If you need test equipment support, I usually talk to Joy Torres at Instruments 4 Engineers in Stockport. She is very helpful and can often get you access to good prices. joyt@instruments4engineers.com. I know she represents Tek, Com-Power and Signal Hound.

The main downsides of making your own emissions measurements is the amount of ambient noise from other electronics, radio sources, reflections off nearby surfaces, etc. It is good for “is A better than B” testing, but it doesn’t really get you to a “but does it pass?” kind of answer. This takes some practice to get right and to get to know your equipment.

The question you could ask yourself is “do I have both the capital and the time to invest in this solution?”. I’ve talked to other companies about their setting up of pre-compliance facilities in house and their struggles tend to be:

  • Engineers lack time to work on the EMC project aspects as well as their regular project work
  • A lot of up front time required to learn the variables of the test setups and standards
  • How to make useful measurements and interpret the results
  • How to match the results from this testing to predicted test lab pass/fail results (spoiler: it is very tricky)
  • EMC knowledge is not well shared amongst employees or the engineer who has the knowledge is on holiday / off sick / unavailable / working on something else

None of these obstacles are insurmountable with good planning and management  🙂  but it is worth going in with eyes open.

In my experience, the best weapon in the EMC armoury is not a spectrum analyser, nor an antenna, not even a full test lab. It is a design review. When we design something we define its EMC characteristics. Getting this up-front bit right is the key to shorter design cycles, fewer prototype runs, reduced time to market and much less stress. Working together to catch the problems before they become problems gives experience in how to design for EMC and the lessons learned can be carried forward from project to project. We’ve helped many customers this way and we’d be happy to help out on your future product ideas.

Similarly, if you want to run quick checks on equipment that needs equipment that you don’t have then we are happy for you to send us the kit via courier and to run the tests on your behalf. I appreciate that our office isn’t exactly nextdoor so anything we can do to help minimise disruption to you and your team would be our pleasure.”

 

Hope some of this is useful

All the best

James

 

EMC Certification is not just a rubber stamp by the test lab!

“Do You Do Certification?”

“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.

In summary:

  1. Unit 3 Compliance test results are valid for a wide range of regulatory approvals, including CE marking, UKCA marking, and FCC
  2. 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.
  3. You (the manufacturer) “self certifies”; or rather you legally Declare your product to be compliant with the EU Directives or UK SIs
  4. UKAS Accredited Laboratory testing is not mandatory for CE, UKCA and many FCC tests.
  5. 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

Regulatory Regime

Product Type

Unit 3 Compliance can be used for testing?

CE Marking &
UKCA Marking

EMC or Radio Equipment Directive

Yes

FCC

Unintentional Radiators (Part 15B)

Yes

FCC Unintentional Radiators with FCC Approved Radio Module

Yes

FCC

Intentional Radiators (Part 15C)

Pre-compliance only.

Accredited laboratory required for final test

 

 

CE Marking

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:

  • certificate
  • certification
  • accredited
  • accreditation

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.

In summary:

  • 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

 

UKCA

EU laws have been transposed into UK laws so the same requirements for CE marking apply to UKCA.

 

FCC

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

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.

 

comparison of emc lab capabilities

 

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.

If you’d like to benefit from this then get in touch.

 

 

 

References

 

ESD Latch Up Behaviour in Diodes Inc. Power Switch Parts

A new customer came to me with their product that was having problems during testing at another laboratory. There were radiated emissions problems (mostly solved with improvements to the ground plane scheme on the PCB) and a very interesting (and challenging) ESD problem which I’ll cover in this blog.

Here was the device exhibiting the problem, a Diodes Inc AP22802AW5-7 “power distribution load switch”. Input VBAT from a stick of AA batteries, SW_PWR from a rotary switch, and output to the rest of the circuit.

Problem outline

The ESD problem was described by the customer:

The EUT stopped working when 4kV contact discharges were applied on discharge point shown. I removed the batteries and I put them [in] again and there was not any response from the sample (no otuput and the green LED remained OFF).

[A second sample] was then tested with the same result, although this time not on the first discharge

Upon inspection both devices had failed due to the load switch (AP22802AW5-7Diodes), with one failing open and one failing short and both becoming very warm.

ESD diode placed on input and output of load switch (with no effect)

ESD diodes placed on all [discharge points] (with no effect)

ESD diode places on VCC close to pullup resistors for [discharge points] with no effect

First thing first was to get the product set up on the ESD table (with a bit of added blur to protect the innocent).

It was very easy to re-create the problem observed at the original test lab with the second contact discharge to the EUT exposed contact point causing the unit to shut down.

In each case, the power switch was failing low resistance from IN to GND. The initial theory was that the device was being damaged by the high voltage punching through the silicon layers leaving a conductive path.

 

Eliminate the possible

I made a series of experiments to determine the coupling path into the problematic device. Working on the principle that, because of the 15cm distance between discharge point and problem device, that conduction might have been the problem.

  • Capacitors on Vin and EN
  • plus disconnect EN line
  • plus ferrite beads and capacitors on Vin, Vout and EN
  • plus local TVS diodes on pins of device
  • plus ferrite beads in series with [EUT input] lines

Whilst none of these experiments were successful they certainly helped eliminate conduction as the coupling path.

Because of the very high frequency content of the ESD pulse, capacitive coupling is likely going to be the dominant coupling method. Whilst it could couple into the device directly, there was more opportunity for the pulse to couple into the traces connected to the device first. Filtering the inputs eliminates two coupling possibilities

 

Change of sample

The PCB was starting to get a bit tired from the repeated hot air SMT de-soldering and re-soldering so I swapped to another supplied sample. To be able to operate the unit out of the casing I swapped to a linear DC bench supply instead of the AA batteries.

This proved to be an interesting mode as it allowed me to kill the power quickly. The next set of experiments were in an attempt to reduce the effect of capacitive coupling to the problem device.

  • Improved ground stitching / connection
  • Changing supply voltage
  • Indirect HCP discharge – not to EUT but to the Horizontal Coupling Plane albeit with a vertical ESD gun to increase capacitive coupling to EUT.
  • Reduction of coupling into Vin terminal by removing components and copper
  • Addition of copper foil shield over the top of the device

 

Failure mode discovery

Setting the current limit on the DC supply to a fairly low value (about 20% higher than nominal current draw) was a good idea.

When applying the ESD strikes the supply went into foldback as the EUT power input went low resistance. I discovered that quickly turning off the power and then turning it back on effectively reset the failure mode of the device. This proved to be repeatable over several discharges: zap – foldback – power cycle – EUT OK.

What silicon component behaves like this? A thyristor.

This is a phenomena known as “latch up” where the parasitic thyristor structure present in the CMOS process fires due to over voltage… such as an ESD strike for instance!

Because the device is only small the power dissipation caused by the battery short circuit current is enough to “pop” the device through overheating.

 

Out of circuit testing

Whilst it doesn’t get used very often, my Sony Tektronix 370 curve tracer is perfect for testing components like this.

(not mine, picture From CAE Online)

Here’s the VI curve of an undamaged device. It’s a bipolar voltage between VIN and GND. On the left of centre is the standard forward biased body diode. On the right is the reverse biased breakdown of around 8V.

Now for a damaged device. In this case the current changes quickly for a small applied voltage and there is no non-linear characteristic. Essentially, a short circuit.

Turning up the maximum voltage that the curve tracer can apply and dialling down the series impedance allowed me to simulate the over voltage fault condition and create a latch up condition. This latch up wasn’t permanent due to the bipolar sine wave nature of the curve tracer applied voltage.

However turning up the voltage enough to cause excess power dissipation inside the device did result in the same failure mode using the curve tracer.

 

Summary

I have never encountered a device that is this unusually sensitive to ESD events before. A nearby 2kV discharge on the PCB top layer ground plane was enough to cause the latch up condition.

I noted in the report to the customer that this device had been changed to “not recommend for new designs” by Diodes Inc. I wonder if they identified this condition in the device and withdrew it for that reason.

The customer resolved the issue by replacing the device with a different part and we all lived happily ever after.

The end.