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Do I need to EMC test a pre-approved power supply? – EMC Explained

One of the most common questions we get asked when we send an EMC Test Plan / quotation to our customers is along the lines of:

 

“Our equipment is powered from a pre-approved CE marked power supply so we don’t need to do any AC mains EMC testing… right?”

 

If a power supply has already been EMC tested (if it has a CE or UKCA mark you would hope that this was the case) then it is a fair question – why should we retest it?

Adding AC mains specific tests into the EMC Test Plan adds time and therefore cost, something that some of our customers would like to avoid. For smaller businesses, the cost of assessment for EMC might be one of the largest external costs incurred on a project.

The main assumption driving this question is that EMC emissions – the noise that is coming out of the power supply and either back onto the AC mains or radiated from the power supply – is the only EMC problem we have to worry about. It’s the main one, but not the only one.

The pre-approved power supply will have been tested for immunity, but only the immunity performance of the power supply itself, not the equipment that it is powering.

Some noise will get through the power supply and into the equipment being powered. How does your product respond to this noise?

Also, how low are the AC mains conducted emissions from the power supply? Have you seen a test report? How reputable is the vendor?

Testing is the most reliable way to find out.

 

Our Recommendations

We generally recommend to our customers that they perform all of the applicable tests to the product.

(What, a test lab recommending testing? I’m shocked!).

Firstly, the tests are called up in the EMC standards, and for CE/UKCA marking, testing to a Harmonised Standard gets you a “Presumption of Conformity” to the requirements of the Directives – a pass without any further Risk Assessment or justification on your part.

Deciding not to perform the testing puts the responsibility on you to assess the remaining EMC risks. If you needed us to do this assessment for you or advise on it, the cost of a few hours of consultancy time would be equivalent to just doing the tests in the first place.

Secondly, EMC performance is often dictated by parasitic capacitances and inductances, component values that are not on the datasheet or intentionally designed into the product. Even knowing their magnitude does not give a good understanding of how they will interact. Testing allows us to measure their interaction under standardized conditions.

 

 

Risk Assessment Factors

As discussed above, our recommendation is always to perform testing on applicable ports, the AC mains port included.

If you are worried about costs or time taken for testing, then you might decide to omit some of the specific tests. The below table outlines some of the factors you may wish to consider when making this decision.

The more items that apply from the Risk Increasing Factors column, the less strong your argument becomes for not carrying out testing.

 

Risk Reducing Factors Risk Increasing Factors
Class II power supply (un-earthed)

 

Class I power supply (earthed)

Especially if the DC negative of the power supply output is connected to Protective Earth in the system.

Power supply comes from reputable vendor (e.g. Meanwell, XP Power, Recom, Traco, TDK Lambda, Puls, etc) Power supply comes from cheap or from far east supplier
Power supply external to product Power supply internal to product
No analogue or sensitive circuitry Analogue circuitry e.g. audio, 0-10V I/O, 4-20mA I/O

Sensitive, low level signals e.g. thermocouple, RTD

No other long (>3m) cables connected to equipment One or more long (>3m) cables connected to equipment
Main use in Basic (residential, commercial) EM environment Flexible use, could be used in Light Industrial or Industrial EM environments

 

If you are at all unsure then you should test the AC mains port with your intended production power supply.

For the ultimate in performance, or if the equipment is for flexible use (could be powered from an AC/DC supply or from a distributed DC power supply) then we would recommend treating the DC power input to your product as a signal port with a length greater than 3m.

This would then call up Conducted RF Immunity (EN 61000-4-6) and Electrical Fast Transient (EFT, EN 61000-4-4) testing to the power port at the appropriate levels for the end EM environment (e.g. Basic or Industrial)

One step further would be to apply line-to-line and line-to-earth surges to the DC input, assuming that the design already contains a transient surge voltage suppressing element like a TVS diode or an MOV.

Let’s take a look at some of the technical justification behind the selection of these items.

 

AC Mains Port vs DC Power Port

If you typically derive your equipment power from an AC mains power supply, then it is unlikely that you will fall under the DC Power Port classification.

The term DC Power Port in EMC terms means a very specific classification of port. We discuss this in some length in this article.

 

Power supplies do not always meet the regulations

A scenario that we have experienced on several occasions: the power supplies that end up with our customers or in our test lab are not the same as the ones in the manufacturer supplied EMC test report.

 

Another customer had similar problems on  power supply that they had received samples of in that the EMC performance varied wildly. In this case the clue was that the weight of the two samples was significantly different.

 

 

These power supplies were almost identical on the outside but significantly different on the inside. Same manufacturer and model number, different components. Imagine the conversation:

“I’d like to order some HM-A132 power supplies please”

“Certainly sir, which ones?”

“Erm…”

 

This is mostly related to cheaper power supplies sourced from China. We often see significantly different results to those shown in the manufacturer test report.

The worrying thing is if changes like this are being made on the basis of EMC, what changes are being made that affect Electrical Safety that are going unchecked? We can check that for you as well.

 

Cable Routing

If your power supply is integrated into your equipment then there is the possibility of noise on the AC mains cable coupling onto other nearby cables.

It is also possible for noise to couple (both to and from) components connected to the AC mains and internal system components. This could be an emissions (noise getting out) or an immunity (noise getting in) risk.

 

This is particularly likely if you are using slotted trunking and mixing AC mains cabling in with other cables.

 

 

 

This is less important for an external power supply like a laptop type charger or a plug top power supply as the AC mains cable remains outside of the equipment enclosure.

 

 

Power Supply Common Mode Impedance

Electrical noise inevitably gets coupled onto the AC mains bus. Normally this noise is coupled onto the AC mains Common Mode. This means all the lines together in relation to a high frequency “ground” reference plane.

The noise current through the power supply and equipment will flow something like this:

The noise reaching the equipment will have been attenuated by the Common Mode impedance of the power supply and the currents diverted through the parasitic capacitance of the power supply relative to the HF ground reference plane used in the tests.

Crucially, some noise still gets through to the power supply and will flow through the product. The magnitude of this current can be estimated or measured but relies on electrical parameters that are not on the power supply datasheet.

It is this noise current that we are interested in. How does it affect your product? The only way to find out is to perform testing.

 

Class I vs Class II Power Supply CM Impedance

The construction of a typical switch mode AC/DC power supply is broadly similar across a wide range of topologies. One of the main EMC variations results from if the power supply is Class I (earthed) or Class II (unearthed).

 

Class II

A Class II power supply relies on Double or Reinforced insulation between Live parts and user accessible secondary low voltage parts for Electrical Safety. There is no connection to Protective Earth. This kind of power supply is usually identifiable by:

  • the square-in-a-square double insulation symbol (IEC 60417 symbol # 5172)
  • a plastic earth pin on a UK mains plug (technical name is an ISOD or Insulated Shutter Opening Device)
  • An IEC C8 “figure-8” AC mains inlet socket with just two pins

 

Looking at the typical internal structure of a Class II AC/DC SMPS we can see that the components providing Common Mode noise attenuation are

  • the inductive common mode filter (Lcm)
  • the components across the safety isolation barrier, transformer Tx and class Y capacitor Cy

The value of parasitic parallel capacitance of the choke or transformer (or wanted series capacitance of Cy) will reduce the impedance ( Xc = 1 / [ 2 * pi * f * C ] ) and allow more noise current to flow at higher frequencies.

This capacitance is usually a low value to prevent too high a touch / leakage current to flow which would compromise Electrical Safety.

However, at EMC frequencies of MHz and higher this presents a much lower impedance allowing noise currents to flow through the cable.

 

block diagram of a class II power supply showing EMC immunity noise current through the power supply

 

Because current always flows in a loop, and because current always returns to the source, to close this common mode current loop we need to have return currents flowing. We usually think of these coupling capacitively onto a nearby metallic element like a nearby metal structure.

In the test lab we simulate this with a nearby metal plate but in real life this could take a number of forms (building steelwork, conductive cable trays, other wiring).

 

 

Class I (Or Class II with Functional Earth) Power Supplies

With a Class I power supply, the Protective Earth is connected to accessible metalwork for Electrical Safety reasons (prevention of electric shock). Basic insulation (or higher) is required between the live parts and user accessible secondary parts.

Possibly the protective Earth is also connected to DC negative somewhere in the system as well.

A Class II with Functional Earth power supply is similar from an EMC point of view but very different from an Electrical Safety point of view. In this case, the Earth is connected for functional reasons (reducing noise or EMC emissions) but the power supply still relies on Double or Reinforced insulation for safety.

This isn’t a very common power supply topology choice, so I was surprised to see it marked on my laptop charger power supply.

 

 

In both cases, when we apply common mode noise to the AC mains input (L+N+E) then the Protective Earth conductor allows the noise to bypass the common mode impedance of the power supply. It is for this reason that we view the use of a Class I earthed power supply as a higher EMC risk for immunity reasons.

 

block diagram of a class I power supply showing EMC immunity noise current through the power supply

 

 

How this noise couples into the rest of the equipment, its magnitude, and how it affects it depends massively on the construction of the equipment. Again, testing is the best way to determine this.

 

Conclusion

Power supplies and the equipment they power are not perfect and can have varying EMC performance depending on how you connect them and how the equipment is designed.

It isn’t always easy to estimate how likely EMC issues are, even for experienced engineers and problem like us at Unit 3 Compliance. It is for this reason that we would always recommend testing to characterize the unknown EMC performance.

If you do decide to omit some testing, then the Risk Reducing or Increasing Factors above should help with that decision.

Again, we hope that this guide was useful to you in some way. Get in touch with us if you have any thoughts, questions, observations, or (obviously) a need for EMC or Electrical Safety testing.

All the best!

 

 

 

meanwell power supply comparative radiated emissions

Meanwell Power Supply Radiated Emissions Investigation

We’ve been doing pre-compliance scans on a customer’s product and helping them overcome some interesting radiated emissions problems (spoiler, 3rd party display module. Again.)

Now that it is back in the lab for it’s final EMC measurements we suddenly found that we were measuring an extra 10dB of noise at 170MHz.

Hang on… it was passing during the pre-compliance measurements last month… what is going on?

During the pre-compliance measurements, we only saw this peak when we were powering the EUT from the provided open frame mains power supply. So we removed it for some investigations.

Using our Tekbox TBCP2-750 current probe and Signal Hound BB60C spectrum analyser, we measured all of the cables connected to the power supply.

 

On the AC mains supply:

current probe on ac mains input

On the DC power output cables:

current probe on dc power output

and on the 12V auxiliary fan power cable

current probe on 12v aux fan output

 

What we measured was a bit of a surprise:

meanwell power supply comparative radiated emissions

 

Of all the cables we expected to have a problem with, the low power 12V fan cable was not our first candidate. It looked to be carrying the most noise at 170MHz so we did what every good EMC engineer does – put a ferrite on it!

 

 

Now it meets Class A with a 5dB margin at that frequency.

Upon further investigation, a second fan had appeared inside the equipment since our pre-compliance measurements. The engineer had mentioned improving the temperatures within the product but we hadn’t opened it up to verify if any changes had been made.

The cable routing for the new fan was undefined, allowing it to lie across the power supply, or next to other components depending on how it was assembled. This appeared to be the cause of variability that we had observed in our testing.

 

Takeaways

One of the key rules of EMC troubleshooting is to change only one thing at once, and be careful that you are only changing one thing. Reassembling the unit with different fan cable position accounted for some of the variability in emissions performance.

Don’t assume that just because you are using a pre-approved component that it will automatically pass when integrated into your system. Having worked with many Meanwell power supplies of all different flavours over the years this is only the second time we’ve had any significant issues with one.

 

Bye for now,

James

 

 

 

I Will Happily Spend Your Equipment Budget…

 

 

Customers sometimes ask me “what EMC pre-compliance equipment should we buy?”

My reply is that I’m more than happy to help them spend your equipment budget!

Here’s an email that I sent to a customer recently when they were asking for feedback on some test equipment that had been proposed to them

 

A good spectrum analyser is pretty indispensable when it comes to wrestling with EMC issues. The Siglent ones (available from I4E and Telonic) are pretty damn good for the money, I’d buy one if I was in the market.

Near field probes can be used to narrow down the emissions source pretty effectively. Either the Tekbox ones from Telonic or the Beehive ones from Farnell are pretty good.

I have just written a free ebook on the subject of near field probing which might be of interest.

Another good addition is a current probe like the one I brought during my visit. This lets you easily characterise emissions on cables. Add an attenuator set to protect the spectrum analyser input.

Challenges with on site pre-compliance measurements

  • Dealing with background noise (near field probes and current probes are pretty resistant)
  • Relating levels it to a formal measurement standard (not possible)
  • Interpreting the results and figuring out what to do about it (expert level!)

If you decide to go down the road of getting your own equipment I’d be happy to come up and do a day with you running through measurement setups, tips and tricks if you think that would be helpful.

Alternatively, if you want to make some antenna based on-site radiated measurements I can come up and do a day with you with our spectrum analyser and portable antennae.

 

 

 

 

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

 

You can download our free eBook on Near Field Probing here

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 (and slept 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 + 5/8 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 our free eBook on Near Field Probing 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!

Thanks and happy probing!

James

 

 

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

Pocket EMC Debug Probe V2

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

 

EUT Monitoring Hardware

For Equipment Under Test (EUT) monitoring during EMC tests we’ve adapted a Digilent Analog Discovery 2, added an input filter board, and enclosed in a nice case from Lincoln Binns.

This connects to the (rapidly evolving) Monitor-o-Matic 8000 software mentioned in a previous post.

This adaptor and software is going to be used this week during testing of a piece of industrial equipment. This test adaptor will be monitoring both 4-20mA and relay outputs (using the in built power supplies to generate the voltage on one pin of the relay contacts and the digital inputs to monitor the other pin).

 

Analog Discovery 2

The AD2 is a very versatile piece of kit with a good balance of analogue and digital input and output for a reasonable price. It includes

  • 2 x 14-bit, 30MHz differential scope channels
  • 2 x 14-bit, 10MHz waveform generators
  • 16 x digital logic I/O with pattern generation and logic analyser
  • 2 x programmable power supplies

 

Future Plans

We’re going to be monitoring how well this device performs for monitoring during testing. We may need to add extra filtering beyond that already fitted such as optical isolation for the digital inputs.

We may also look at fitting a battery and a USB to fibre optic converter for fully isolated measurements in a variety of EMC environments.

 

 

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.

 

 

 

Schaffner/Teseq NSG 5500 test system

New Automotive Test Capabilities ISO 7637-2

The best day is new equipment day 🙂

We are continuing to invest in our test capabilities. As such, the Unit 3 Compliance EMC test laboratory has just acquired a Schaffner (Teseq) NSG 5500 automotive surge/EFT test generator.

Schaffner NSG 5500 test systemWith this, we now have the capability to test your equipment to the ISO 7637-2 standard for automotive conducted transients.

The NSG 5500 will generate the ISO pulses 1, 2a, 3a and 3b, along with the Load Dump and Clamped Load Dump pulses 5a and 5b.

This gives us the capability to support your automotive product development to these standards:

  • EN 50498:2010 – Aftermarket electronics for vehicles – full testing for CE marking
  • CISPR 25 for non Immunity Related Function EUTs
  • UNECE R10.06 (pre-compliance)
  • ISO 13766-1:2018 Earth Moving Machinery (pre-compliance)
  • ISO 7637-2:2011 automotive conducted transients
  • ISO 16750-2:2012 automotive electrical loads (part)

 

Footnote:

Timing is a curious thing. Like two buses arriving simultaneously after a long wait I find things tend to cluster up. This acquisition occurred not long after publishing this blog post on how to test to the automotive standards without an automotive surge generator.

Compromise EFT Test Setup

When the customer supplied cable isn’t long enough to fit inside the standard EFT/B capacitive clamp what do you do?

One answer, for pre-compliance testing, is to make your own clamp from aluminium foil cut to length and separated from the GRP by expanded foam blocks.

The capacitive clamp is not a sophisticated piece of test equipment and a close compromise can be achieved quickly with commonly available lab materials.

Details of a compromise EFT test setup using aluminium foil and foam blocks.

Making sure there is good contact to the GRP from the generator is important which is partly achieved by taping the cable down with some conductive adhesive aluminium tape.

Overall area of the injection plate is reduced by 25% from the standard capacitive clamp plate area. Therefore the injection voltage was increased by 25% to compensate for the reduced capacitance.

Safety warning: don’t touch the foil when the generator is running!

Obviously not good enough for exact testing to the standard but it is within the spirit of the test and will give some useful information.