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EMC Question for 400Hz Power System

Photo by Ethan McArthur on Unsplash

11 May 2020

EMC Question for 400Hz Power System

Question:

I would appreciate it if you could provide your view / share your expertise on the EMC/EMI protection requirements between the  400Hz Ground Power Systems and the various ICT/ELV installations within an Aircraft.

Answer:

An interesting and important question!
And much too big an issue to be able to deal with comprehensively, in a blog post.

The (so-called) ‘Ground Structure’

The big problem is the use of the word 'Ground' (or 'Earth') - because it can be used in very special and specific ways, and also in very general ways - like a sort of jargon.

We can never really understand what anyone else really means when they use the words 'Ground' or ‘Earth’, unless we spend some time discussing its detailed physical structure with them – and then we often discover that they are not very clear on what they mean, themselves!

All my working life in the electronics industry (coming up on 50 years) this confusion over what individuals really mean when they use the words like 'Ground' or 'Grounding' (or 'Earth' or 'Earthing') has cost companies huge amounts of wasted time and money, and is still costing them.

It is usual to electrically connect all the different items of metalwork together for safety reasons. By using multi-point connections to create a gridded or meshed ‘ground structure’ safety can be improved, and the RF impedance reduced – making it possible to relax the requirements for EMC mitigation (filtering, shielding, ESD/transient/Surge suppression, galvanic isolation, etc.) between the different items of equipment sharing that ‘ground structure’.

How to do this is described in practical detail in my material on “EMC for Systems and Installations”:

All the above describes buildings and sites, but the material is easily extended to cover vehicles of all/any types, land, marine, subsea, air (fixed wing or rotorcraft), space, etc.

Electromagnetic (EM) Mitigation by Galvanic Isolation
(i.e. protection by filtering, shielding, surge suppression, galvanic isolation, etc.)

At one extreme, typical of some military equipment/system design companies, the designers of a unit will not trust what anyone else might understand about 'grounding', so whilst the metal enclosure of their unit is directly electrically connected to the ‘grounding structure’, they galvanically isolate all power inputs and outputs, and all analogue and digital I/Os that enter or leave their unit. 

Then, whatever has or has not been done by the rest of the system or installation outside their unit – they simply don't care, as long as they have designed their galvanic isolators and other EM mitigation to cope with the worst-case CM noises, transients and surges.

Some go as far as galvanically-isolating (or ‘floating’) all their electronics inside their metal enclosures. They appear to do this because they don’t understand how to take advantage of the fact that the Skin Effect causes RF currents to flow along surfaces (see: https://www.emcstandards.co.uk/skin-effect-and-surface-currents).

Unfortunately, the large amounts of stray capacitance between the ‘floating’ internal electronics and the inside surfaces of their metal enclosures and cable shields AC-couples them together, above some frequency, in an uncontrolled way. This creates many EMC problems that can need a lot of extra space, weight, cost and time to overcome, but hey – the government can afford it! 

Anyway, galvanic isolators must be rated for the highest transient/surge voltages that can occur, making them larger, more costly, and heavier. Modern RF switching technologies can make even 4kV-rated digital isolators very tiny, at the price of significantly increased RF noise emissions – so they need more filtering and shielding, and the benefits of their smaller size, cost, and weight might not be as good as had been expected.

I know of at least one large and important government project that has been delayed for years because of the unexpectedly high levels of noise being emitted by modern micro-miniaturised digital isolators. It is quite normal in 2020 for these tiny digital isolators to emit excessive noise all the way up to 3GHz.

EM Mitigation by other methods
(i.e. by filtering, shielding, surge suppression, etc.)

To design effective EM mitigation we need to understand:

i) the EM threats to our items of equipment from outside, and their internal EM weaknesses (susceptibilities);

ii) the EM threats from inside our items (their emissions) and the external emission limits that must be complied with.

The aviation industry in general is very good at specifying the threats and susceptibilities we need to know, in the form of EMC test standards, but I always recommend not relying solely on specifications provided by others.

After all, if our equipment fails to work as intended, proving that the problem was caused solely by our customer’s inadequate EMC specification is a very costly exercise, and even if we win the case it can leave us heavily in debt! Better to employ a little caution from the first, to reduce some very serious financial risks.

The use of CM chokes goes a long way towards overcoming the EMC problems caused by having a poor ‘grounding structure’ between systems in an installation, or between different units in a system.

So, my mental starting point for any design is always start with putting each item of equipment in a shielded metal box, shielding every signal cable (and correctly terminating the shields at both ends), plus filtering every single input and output, including power, with at least a common-mode (CM) choke-based filter, plus ESD suppression for all digital I/Os.

If the EMC requirements allow, I can then remove the cable shielding and/or replace the CM chokes in the filters with some differential-mode (DM) ferrite beads. ESD suppression is always required for all digital I/Os, although some digital I/O devices say they have them inbuilt – although I worry about how rugged they really are.

But if the EMC requirements are tougher than my initial design can easily cope with, I can use double-shielded cable (with both shield layers correctly terminated at both ends), and/or add a second stage of either CM or DM filtering.

Filters can be very useful for suppressing transients and surges, they are not restricted to use just on continuous RF threats. However, some applications can suffer powerful low-frequency noises, transients, surges, etc., that cannot be dealt with cost-effectively by filtering alone, so we need to combine their filters with powerful transient/surge suppressor devices, such as MOVs, GDTs, SADs, etc.

Isolating transformers are very effective CM low-frequency noise/transient/surge suppressors, and can easily be combined with DM surge suppressor devices to give excellent protection against all manner of nasty EM threats, even the effects of lightning strike – as long as the break-over voltage of the transformer is high enough.

Isolating transformers are often specified for AC-DC power converters for safety reasons, where they also provide useful suppression of low-frequency noises/transients and surges when combined with DM surge suppressor devices. Isolating transformers can also be made with one or more interwinding shields, to improve the frequency range of their suppression.

Signals/data can often use isolating transformers too (like Ethernet always has), but – as I mentioned above – designers these days are likely to be seduced into using cheap, small, lightweight micro-miniaturised digital isolators – only finding out about their severe noise emissions problems very late in the project when even small changes can cause a lot of added cost and delay.

EM Zoning (sometimes called Segregation)

To make EM mitigation techniques work as well as they can – so as not to waste time/money/space/weight – it is necessary to understand (and apply!) what we call ‘EM Zoning’ or ‘Segregation’, first standardised by IEC TR 61000-5-6:2002 “EMC Installation and mitigation guidelines - Mitigation of external EM influences”.  

Incorrect EM Zoning means that ‘crosstalk’ or ‘leakage’ tends to defeat the EM protection purposes of galvanic isolation, filtering, shielding, ESD/transient/surge suppression, etc.

If good EM Zoning is not built-in from the start, the necessary redesign to apply it so that adequate EM mitigation is achieved, is usually difficult, costly and time-consuming. I have had to fix many such designs during the last 30 years!

However, I have heard of projects that have failed because the cost/time of the necessary redesign has been too high – usually only after a lot of good money has been thrown at it. Often, such failures are caused by the designers not understanding how to do EM Zoning properly, so they just stagger from one bad design iteration to another until their management pulls the plug.

Every manufacturer wants to make good profits without exposing themselves to too much financial risk, which makes it very important indeed these days to ensure that the initial design has good EMC built-in. For why, see https://www.emcstandards.co.uk/re-spinning-a-design-re-spin-is-jar.

Where to find the details on EM Zoning and EM Mitigation

For the past few years, all my courses posted at https://www.emcstandards.co.uk/online-training have been based on the EM Zoning approach, and they teach well-proven design techniques developed over 40 years to (almost) guarantee – up to 24GHz so far – the quickest and lowest-cost design/development projects that exceed their functional specifications and pass all of their EMC tests on their first production prototypes.

The resulting equipment should also have the lowest overall cost of manufacture, hence the greatest profitability, with the lowest financial risks.

Module 1 “The Physical Basis of SI, PI and EMC” describes the principles of EM Zoning and mitigation, without practical details, and is posted at https://www.emcstandards.co.uk/the-physical-basis-of-si-pi-and-emc.

EM Zoning specifically, is covered in Module 1’s sections 16, 17 and 18 – and an earlier version of it is available as a free Webinar from https://www.emcstandards.co.uk/go-emgineering-si-pi-emc-webinar-1d (you have to register as a Member to get it, but registration is free and quick).

My other modules are all very practical indeed! They describe how to choose the types of mitigation; and how to dimension them according to the ‘threat levels’ and the immunity of the devices and cables to be protected:

  • Module 2: cables, cable shielding and connectors
  • Module 3: filtering
  • Module 4: enclosure shielding
  • Module 9: suppression of electro-mechanical devices
  • Module 11: suppression of ESD
  • Module 12: suppression of transients and surges on AC and DC power, and analogue and digital I/Os
  • Module 15: low-frequency power quality issues, such as dips, dropouts, interruptions, waveform distortions, LF CM noises, etc.
  • Note that the practical details on creating good ‘grounding structures’ and other EMC issues for systems and installations, is covered by the links on “EMC for Systems and Installations” provided on the first page of this blog article.

Earlier versions of all the above training material – before I had developed the EM Zoning approach quite so well – were published in my 2010 textbook “EMC Design Techniques”, ISBN 978-0-9555118-4-4 available from https://www.emcstandards.co.uk/emc-design-techniques.

Free versions of the articles I wrote in 2006-2009 before developing them into the 2010 textbook above, are posted at https://www.emcstandards.co.uk/design-techniques-for-emc-2006-9-series1.

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I hope this helps!

All the very best, Keith Armstrong

Photo by Ethan McArthur on Unsplash

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