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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)
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:
- Are the existing leakage requirements sufficient to prevent a hazard?
- If so, are they being effectively applied and policed?
- If not, what is an acceptable requirement?
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,
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.