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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

 

 

 

a picture of the pin side of a UK BS1363 mains plug

Checking British Mains Plug License Number

BSI License Number

BSI has a facility for checking the license number of a BS 1363-1 UK mains plug.

picture of form on bsi website

Hopefully the number you have entered is genuine and you are presented with some search results:

list of text results from a bsi search

You can then click on the line you are interested in and check the details of the plug against the sample you have.

list of information for lj01 plug from previous search

 

ASTA License Number

ASTA numbers are assigned by Intertek and can be checked using their online tool.

screenshot of asta website

Search results give the brand and manufacturer. The hard part now is tying this up with the sample in front of you!

 

LVD Voltage Limits are RMS + Thoughts on Marginal Voltages

The LVD a.c. voltage limits are defined in terms of rms. From 2014/35/EU

This Directive shall apply to electrical equipment designed for use with a voltage rating of between 50 and 1 000 V for alternating current and between 75 and 1 500 V for direct current

So that’s 50Vrms and 1000Vrms.

 

Example

A customer was asking about a low current (sub 1mA) 40Vac rms source and if the LVD applied.

This voltage is less than the 50Vrms threshold mentioned above so would be technically exempt from the LVD. Unless of course the equipment contained a radio module in which case the RED makes the LVD applies with no lower voltage limit.

I think that there is still a risk that needs to be assessed here.

Just because you are exempt from the directive doesn’t mean you are exempt from making your product as safe as possible.

 

Looking at the main table from 62368-1 for categorising shock risk, 40Vrms/50Hz means it is an ES2 hazard. The voltage source would have to be under this limit for normal operation, abnormal operation (e.g. blocked vent, stalled motor, controls set incorrectly) and single fault (open/short circuit) conditions.

 

This means, even though the LVD is not strictly not applicable, it would be wise to put in a Basic Safeguard between the user and the exposed voltage.

Additional: The provisions of the General Product Safety Directive (2001/95/EC) would apply to any product falling outside of any specific safety standard. The Harmonised Standards for this Directive include EN 60065 and EN 60950-1. Since both of these have been superseded by EN 62368-1 it would be reasonable to use this standard instead.
Thanks to Charlie Blackham from Sulis Consultants for the tip.

This safeguard could be an enclosure, insulation, an interlock or barrier.

Instructions or PPE aren’t sufficient as they are considered supplementary safeguards.

 

But what about the current limits?

That’s just considering the voltage source purely from a voltage perspective. If it can’t drive enough current into a 2000 ohm load for more than 2 seconds to form a hazard then that might change the classification.

This current is measured using one of the appropriate networks from EN 60990 such as the one below

 

 

But I know what I’m doing…

The requirement for safeguards depends on if you classify the user as a normal person or an instructed person

Skilled person > instructed person > normal person

3.3.8.1 instructed person
person instructed or supervised by a skilled person as to energy sources and who can
responsibly use
equipment safeguards and precautionary safeguards with respect to those
energy sources

3.3.8.2
ordinary person
person who is neither a skilled person nor an instructed person

3.3.8.3
skilled person
person with relevant education or experience to enable him or her to identify hazards and to
take appropriate actions to reduce the risks of injury to themselves and others

The level of safeguard required between the user and the ES2 hazard is defined in EN 62368-1

For a normal person we must use a basic safeguard

But for an instructed person we may use a precautionary safeguard

A Precautionary safegard (defined in 0.5.5.3) could take the form of instructions or training, but the addition of warning stickers, PPE could also be considered part of this.

 

Summary

This is why I like EN 62368-1 over some of the older safety standards (I’m looking at you, 60950)

Rather than a prescriptive “thou shalt use 2.5mm clearance or be smote verily” it helps and guides you through all the steps into understanding why or how a requirement is derived.

Also the companion EN 62368-2 explanatory document contains even more background and context. I wouldn’t recommend applying -1 without having -2 to hand.

Stay SAFE kids.

 

Off The Shelf and Non-Compliant Power Supplies (from Amazon)

A customer had purchased some power supplies from Amazon UK to get started with the development on their product. And why not? There are lots of cheap products available and everyone has a budget to meet. The chances are that they’ll get damaged, lost or broken anyway.

They were happy with the (perceived) quality of the PSU so approached the manufacturer directly for bulk pricing for volume production. However, the Amazon sample made it’s way to Unit 3 Compliance for EMC pre-compliance testing is where the fun began…

infographic comparing two power supplies

Externally, the only way to tell the difference between the compliant and non-compliant versions is a slight difference in the length of the barrel connector and a slightly different shape of strain relief grommet.

These devices are being marketed as the same device on the outside and yet are completely different on the inside!

I’ve not been able to subsequently find this exact power supply on Amazon but there are similar looking variants still available.

 

A Real Problem

Crucially, it’s not just EMC that is being sacrificed. This “race to the bottom” of extracting every last penny from products has more serious consequences.

More dangerously for consumers, electrical safety is also being compromised as shown in this study from Electrical Safety First on Apple chargers.

At a previous employer, an inspection was performed on 50 power supplies (again, bought from Amazon) that one of the project teams had purchased for powering various development platforms within the company. This revealed some serious safety problems (creepage and clearance) resulting in the entire batch being quarantined and scrapped for recycling.

Another aspect to consider – if the manufacturer has two different, almost indistinguishable products then how does your supply chain guarantee that you will receive the correct one? What is to stop the manufacturer from swapping out the more expensive compliant power supply halfway through production?

The principle of caveat emptor still applies. Disingenuous product markings are being used to falsely indicate compliance.

 

What To Do?

The obvious way round this is only to buy small quantity power supplies from trusted suppliers. I know from working with other customers that suppliers like RS and Farnell / Element 14 take compliance seriously. Buying from these sources is more expensive financially but what price do you put on your own safety?

If you are relying on buying a pre-approved power supply always ask for the EMC and safety test reports and the Declaration of Conformity. A supplier who cannot readily supply these readily should be disregarded.

Compare the details in the reports with the physical sample in front of you. Especially for safety reports, photos of the unit are generally included, inside and out. Look for any differences between the two.

Differences in EMC performance are not obvious. The only way to be sure of the quoted performance is to perform some quick tests, conducted and radiated emissions being the two main ones.

 

How We Can Help.

Here at Unit 3 Compliance we can give you some peace of mind that your power supply isn’t going to cause you any issues. Some of the things we do include:

  • Provide full EMC testing for all off the shelf products
  • Electrical safety analysis and testing
  • Help you understand the technical and financial compromises between units
  • We can review test reports and compare to physical samples with an experienced eye
  • Every incoming customer power supply is given a HiPot test as standard to help catch any problems

Please get in touch to reduce your stress levels.

 

Busy, and a Birthday

It has been a very busy few months at Unit 3 Compliance; it feels like the chamber turntable hasn’t stopped spinning. There has been a wide range of products through the door from prosthetics to video wall controllers, from high spec IoT products to motion sensors, from lighting power supplies to RF amplifiers. I really love the variety of work!

I’ve also had some safety assessment work to carry out on which is always interesting. Disassembling mains transformers to measure the creepage distances inside is fascinating, getting out the angle grinder to hack the laminations apart just adds to the fun.

There have also been a fair amount of design reviews and general consulting work in between. To be able to work with customers right at the start of the project is invaluable as it sets them on the right path without having to find problems further down the line.

I’ve found lots of interesting nuggets of EMC information during this process that I’m looking forward to sharing with you in some future blog posts once I get time to sit down and write them up.

I managed to escape down to Lincoln to speak at the Open Source Hardware User Group oshcamp18 meetup on the subject of EMC testing. The delegates came up with lots of good questions at the end and the audience participation (see below slide) of the talk went down well. Higher! Lower!

play your compliance cards right!

Good to see reconnect with some old contacts and make some new ones. The other talks were very interesting also, lots of good work going on in the open source hardware field at the moment.

Lastly, and it snuck past without me spotting it, Unit 3 Compliance had it’s first birthday. It’s been a whole year since I got the keys to the unit. In that time, and with lots of help, it has gone from this:

the empty unit

Via this:

To this:

And finally this:

 

Here’s to the next 20 years of compliance, I hope to see you on the way.

 

 

Case Study: AC Mains Input EMC and Safety Troubleshooting

Many of the customers I deal with are technically savvy and extremely good at designing innovative and clever devices. I’m always learning something new every time I get a different product through the door. Unfortunately it isn’t practical or possible to be good at everything and EMC expertise, especially when it comes to fault finding and problem solving, can be hard to come by. This is where I come in.

I’ve been helping a good customer on a product that they’ve been working with that had some EMC troubles on a prototype design. It had originally been taken to a different test lab where they had performed a mains conducted emissions measurement showing a clear failure at low frequencies. There were a couple of other hard copy scans supplied where a capacitor value had been adjusted to try and improve the emissions but with no effect.

In need of some expertise, they got in touch.

Mains Conducted Emissions Testing

I received the product and quickly set it up in our screened room to perform some EN 55014-1 conducted emissions measurements. Below you can see the first scan result, showing a failure of up to 10dB on the Quasi Peak detector. There’s clearly some room for improvement so let’s analyse the problem and see what we can do.

mains conducted emissions - before

Our starting point for the improvement work

Lower frequency mains conducted emissions are not uncommon and are usually caused by differential mode voltage noise. This is generated by current flowing through the impedance presented by the primary side bulk decoupling and switching circuit. The switching frequencies of the power supply controller are usually in the 30 kHz to 250 kHz range putting it (and it’s harmonics) right in this lower frequency (sub 1MHz) range for this test.

Improving differential mode noise can be done in a number of ways. Removing the noise at source is the approach I advocate, in this case this can be achieved by reducing the impedance of the rectified mains bulk decoupling capacitor. A review of the BOM showed that the units had been built with some general purpose electrolytic capacitors with a relatively high impedance. So the first thing that I did was to swap out these parts for ones from the Nichicon PW series of low impedance electrolytic capacitors.

after fitting low impedance bulk decoupling

Changing the electrolytics to a low impedance variety

The result: a big improvement on the QP measurements, bringing some of them down by around 10dB. The improvement on the Average detector readings was less pronounced, especially around 550 kHz where only a 3dB improvement was registered. It is likely that the HF impedance of the decoupling capacitor is still a problem. One option is to apply a suitably rated high frequency decoupling capacitor in parallel with the bulk decoupling capacitor. The other option is to improve the filtering on the AC mains input to prevent the noise from escaping back down the line.

Filtering for differential mode noise can be provided in several ways. The most common method is to make an LC filter from the leakage inductance of a common mode choke paired with a Class X safety capacitor between Live and Neutral. The leakage inductance is in the tens of micro-Henries whereas the common mode inductance is often a couple of magnitudes larger up in the tens of milli-Henries. Simplistically (there are other effects to consider) a 10uH leakage inductance paired with a 470nF capacitor will roll off frequencies above 100 kHz. Well, let’s try that!

now with added class X cap

Now with an additional 470nF Class X capacitor soldered across the mains input terminals

Performance is improved by around 5dB across a wide range of frequencies; indeed the improvement can be seen up to 15 MHz. This leaves a margin of around 2dB to the average limit line which is perhaps a bit close for comfort and I would generally recommend looking at a little more filtering to bring this down a bit further to allow for variations in production and tolerance of components. Options for further improvements could include a second Class X capacitor to form a pi filter but because of the low impedance of the differential mode noise this approach might not be as effective. Adding some inductance to form an LC filter with the bulk decoupling capacitor is another approach.

However this proved the case to the customer for a PCB redesign to make space for the larger bulk decoupling capacitors and at least one Class X capacitor.

Surge and Safety

Following on from this work, at the customers request, I carried out a full suite of EMC tests on the product to EN 55014-1 (emissions) and 55014-2 (immunity). One thing that I noticed was the sound of an electrical breakdown during the application of a differential mode surge test. Taking off the outer casing, I managed to catch the below arc on camera during a 1kV surge event.

Arcing caught on camera

Snap, crackle and pop.

The arc appeared around the resistor; desoldering and removing it from the PCB showed a couple of points where there was arcing between the resistor body and the trace running underneath it.

Arcind damage to the PCb to surface

Arcing evidence on the PCB

This problem has occurred because the resistor R1 is in series with the Live phase and the trace underneath is connected to the Neutral phase. When mounted flush to the PCB normally, the resistor has only its outer insulation between live and neutral. Reviewing the relevant electrical safety standard for the product requires a minimum clearance (air gap) for basic and functional insulation is 1.5mm. This can be achieved by standing the resistor up on spacers to keep it away from the PCB but then it starts to approach VDR1 and Q4 meaning a considered manufacturing approach is required. This was another incentive for redesigning the PCB.

The take-away lesson from this finding is to consider the Z axis / third dimension when reviewing a PCB as it can be easy to see things purely in two dimensions!

I hope you found this case study useful and that it has given you some tools with which you can improve your designs.

If you need some EMC fault finding expertise then get in touch: I’d be happy to help and I love a good challenge!