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Capacitive Coupling in Domestic Portable ELV/SELV Equipment
1 Posts
I am trying to learn more about members' experience relevant to ELV and SELV equipment/appliances around the domestic environment.  I am also seeking ti find out if the IET has any formal document or guidance for designers or users of such devices - e.g. Apple iPads with metal backs, metal cased portable lamps and the like.  There is quite a bit of discussion about it in various forums outside of ours (and a bit in some of our Wiring Forums, where some members clearly know chapter and verse about causation!).

I have just bought a new metal cases LED reading lamp and experienced a familiar 'vibration to my fingers' when touched or rubbed gently.  This effect has been occasionally familiar to me over the years but I never fully understood what caused it until now!  I communicated with the lamp's designer and manufacturer and he had not encountered it, and was amazed when I told him I measured 79v ac between the case and domestic earth.  I now know this is a capacitive coupling voltage due to the fact that in a lot of such appliances. it has passed tests as ELV/SELV, having no fixed earth.  However, I was curious as to why the designer did not know that this voltage existed on his lamp, when plugged in to the domestic socket - but he assured me the lamp had passed all the required tests and accorded with the relevant BS.

This feeling to the human body or 'effect' seems to range from not noticeable, to strangely frightening for people who don't understand its cause and being involved deeply in electrical safety (and a former member of the IET Health and Safety Policy Advisory Group) in my past career, I felt that consumers should be informed formally about it when designers know it exists.  Also, testing may need to identify a standard for measures to inform or design out. the effect.

My general research about this resulted in the only formal document I could find, from the Lighting Industry Association, which has an short 'Technical Statement' on the effect but only to state that this is a non-injurious sensation. (https://www.thelia.org.uk/store/viewproduct.aspx?id=16854987)

To that end, I will appreciate any feedback on further work suggestions or general experiences.

Thanks, George Wedgwood MIET CEng CFIOSH (Retired)
3 Replies
Andy Millar
1732 Posts
Hi George,

When you say: "I felt that consumers should be informed formally about it when designers know it exists.  Also, testing may need to identify a standard for measures to inform or design out. the effect."
I suppose my thought is, what exactly is "it"? I'm sure the power supply manufacturers would push back with the question - is there actually a safety issue here? I've come across this effect quite a few times, and I certainly agree that it seems a bit wrong for the d.c. supply to drift towards the mid point of the incoming A.C. supply (which tends to happen with typical EMC protection arrangements). But that's different from saying there is actually a risk of harm. It's some time since I looked at the specs but from memory there are already limits on the allowed leakage current to protect against this. It can be almost eliminated, as is implemented in medical rated converters, but there's a considerable difference in cost.

So I'd suggest it needs more of a multi step approach:
  1. Are the existing leakage requirements sufficient to prevent a hazard?
  2. If so, are they being effectively applied and policed?
  3. If not, what is an acceptable requirement?
 I will admit that I've always considered this a lower issue than other risks from poorly designed converters: the EMC issues (which are becoming a right pain), and the fire risk (which as with the current leakage risk I don't know how much of a problem it actually is, but I've certainly seen a few where I've been very unhappy with how they've run hot). But I do think you're right that mains leakage issue does need to, at the absolute minimum, be kept under constant review.

Switch mode power supplies are wonderful devices for many reasons, but balancing effective EMC control against mains leakage has always been a pain of a problem...

Interesting point, good to see it being raised,


Simon Barker
769 Posts
The capacitive coupling between the AC and DC sides of a switched mode power supply is usually not an accident.  They deliberately put a class Y capacitor between the input and the output to reduce EMC problems.

2663 Posts
Oh, if only one pole of the incoming mains was at near earth potential and we knew which one it was, we could decouple all out lines to that one, rather than to the mid point or the negative of the DC bus.
Unfortunately, in Europe at least we have to treat live and neutral as equals, and that has been the case for some time.
Actually the Y capacitor can be greatly reduced or even eliminated, though at the price a slightly larger transformer. Consider the conceptual images of an SMPS below. The red arrows show the unwanted current path for a PELV connection (grounded output). Where class II operation and or a non grounded output is required, then the same 300V p-p square wave in series with the inter-winding capacitance now appears between input and output. So the earth path is reinstated by a capacitor value chosen to be near open circuit at 50Hz, and large enough to do something at the switching frequency.
One solution is an isolating transformer at 50Hz, forcing the HF path to be a short local loop. However there are few cases where this really helps as the usual  intention of the SMPS is to eliminate the size weight and losses of a transformer .
The lower pic shows a better fix, with an inter-winding foil, that  may be earthed, neutralled, or connected to the DC bus, that intercepts the current in the inter-winding capacitance, and returns it whence it came without involving the secondary circuits
Such designs are far quieter for EMC as well. Sadly commercial pressure to keep the transformer cheap tends to drive the design in  the other way.  Once you have a few hundred volts p-p with edge ristimes on the quick bits of perhaps 50ns, so many volts/nsec it is very hard to get the EMC genie back in the bottle without large Ls and Cs.


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