bathroom anti mist mirror with leds

If you can feel a shock then there could be something very wrong - so be careful.


I would normally expect LED systems - especially in a bathroom - to be SELV - so earthing of that side should be neither necessary or indeed desireable (earthing things introduces a potential path for a shock current to earth so is better avoided where possible).


The anti-mist header might be mains powered though - I'd hope it was protected by double/reinforced insuation, but an earthed system could be possible. It could be that that's giving the problem.


The reading with a digital meter isn't particularly conclusive - such meters can show readings up to a significant fraction of mains voltage without there being a real problem if there is a large impedance in the path somewhere. The typical explanation is capacitive coupling where bits of metal can pick up a significant voltage from nearby insulated conductors (say a de-misting pad) but the current that could flow in such circumstances is far too low (fractions of a mA) to pose any kind of shock hazard. In this case if a shock has been felt there could be more going on - but at this stage it's hard to tell whether the meter reading is a red flag or just a red herring.


What was being touched when the shock was felt? For a current to flow though someone it normally need to flow both into and out of someone's body - i.e. you need at least two points of contact - so what else was being touched at the same time? There's always the possibility that the 'other thing' had some dangerous voltage on it and the mirror was the innocent party just providing a path back to earth (or N). You can get very odd effects like this when most or all of the entire installation isn't correctly earthed - most things are hovvering around at a hazardous voltage but people notice shocks when they touch things that are actually earthed. I'm presuming the shock was the "buzz" or "tingle" of a mains shock, rather than the "pin-prick" of a shock from static electricity.


  - Andy.

I have seen demister designs that are essentially a meander line of deposited resistance element printed onto a flexible film, stuck on the rear of the mirror like a big sticking plaster. The resistor is then across the mains. Such thing produce a few tens of picofarads of capacitance per square inch  between mains and the mirror glass. If it is one  of those, do not be tempted to try and peel it off to look  - the result is a no longer silvered mirror.

Depending how it is made, the mirror "silvering" may be conductive or not. The one thing it will not be is real silver, sadly.  If it is metal, then there is an effective  second plate to the accidental capacitor to the heater, and depending on the area, the impedance between it and live may be low enough for a tingle. Worse if the sticky back plastic has a nick in it, the mirror metal may be live.

 So does the voltage go away when the supply to the demister is removed - can you switch lights and demister separately ?


As noted already, when investigating possible shocks.  always assume the worst until proven otherwise - switches may be in the neutral, the  earthing of the house may be high Z, L-N polarity may be reversed. Probably not any of those, but keep one hand behind you while you do a quick sanity test to verify before wading in and assuming that flicking the light switch makes it truly safe to investigate.

Assuming it is the mirror that is a bit live, and it's supply and switching  etc is fine, maybe take some pics of the internal wiring - we may be able to help better if we can see it.

I had the same thing on a snake breeders numerous racks that housed 6 snakes each. The heat mats that kept the snakes warm, exactly the same as on a mirror demister, were charging the frame giving a small shock. I did a bit of research into it and its really common in the snake breeding world. Bonding them to earth to reduce their potential would get rid of this, but unfortunately he had too many racks and when they were all bonded they would trip the RCD. I seemed to remember it was induced voltage by the heaters when I looked into it. The effect was like a static discharge, once touched it would be fine for ages. I made him a makeshift discharger to touch the metal of the rack to the floor (they had plastic wheels) from a bit of 10mm earth and that sorted it. So probably the same on the mirror, not sure how you would sort it out though. If the voltage is constantly present, doesn't go to zero once touched and felt a shock, then there must be something more to it. The plastic covering the heaters goes brittle over time an breaks down

Thanks for all the feedback chaps. I think as things are, finding a more suitable/different mirror may be the best option.


Thanks for the quick detailed responses.