Electric Shock!

[Lancs Lad]

What the heck just happened?

I bought a new Bush DAB radio from Argos yesterday, and it's on my kitchen window sill. It's powered from a 6 volt transfomer plug.

I was adjusting the aerial, and my other hand was touching the stainless steel sink. I got a very small, but very definite, shock from the sink. I actually did it again, a few more times, on purpose, just to make sure that it really happened! And I got a little belt every time.

There's absolutely no moisture involved, nothing is wet - so what's going on?

Anybody have any ideas? Do radio aerials actually carry electrical current?

Allowable leakage through the Y caps in the PSU, you are moist enough even when dry to get a tingle. One solution is to earth the PSU output, but that opens another can of worms.

[Cobaltblue]

Sounds like the usual capacitive " leakage" you get from modern smps wall warts.

Do you have an rcd for your sockets?

When you say shock is it just a tingle?

I get them all the time but it's possible that it's more serious so you need to find out how much current is involved .

The isolation on some wall warts leaves a lot to be desired.

Cheers

Mike T

[G8HQP Dave]

Probably leakage current from EMC filter capacitors in the power supply. If so, it is a good sign - your power supply actually contains these capacitors; many do not.

[paulsherwin]

If it's just a tingle then it's likely to be the class Y caps in the wall wart. This is considered acceptable nowadays.

If you received a serious belt then this indicates a fault which should be investigated, though this would normally cause an RCD breaker to trip (assuming one is in use).

[Lancs Lad]

Not a shock, I suppose. Just more like the 'zap' you sometimes get from the car door - if you know what I mean! Like static electricity. It wasn't a continuous thing, I couldn't still feel it if I kept touching the sink. Just a quick zap.

It was very odd!

[Herald1360]

Get a meter on it and see what's going on.....

[GMB]

I would be seriously surprised to see a Y-class capacitor involved anywhere in a modern PSU as the output is usually not earthed.

The usual problem I have seen many times is that thanks to the non-earthed output there is capacitance to the mains through the PSU's transformer (whether analogue or switched mode) and it is this inter-winding capacitance that is enough to be noticed by yourself or a neon screwdriver.

I really hate this modern trend that a neon screwdriver will light up on a thing that is rated as "safe".

It is a shame that the mains earth is not used for an electrostatic inter-winding screen to kill this problem but I have a suspicion that the regulations have conspired to mean that it is now easier to have no earth connection at all on these designs.

Even double insulated SMPS have a class Y twixt input and output for EMI reduction. Well the better ones anyway.

[paulsherwin]

You can often measure some serious volts between the metalwork and earth of an unearthed appliance powered by a wall wart, though the available current is very low so it isn't dangerous. You can reassure yourself that nothing really bad is happening by earthing the aerial (with a length of wire, not your body). If your house RCD doesn't trip (or a plug in RCD if you have ancient house wiring like mine) then you can relax.

[AJSmith625]

I get the same with my Intempo DAB radio but its more of a continuous tingle. As others have said, permissible capacitive leakage in the power supply.

Alan.

[Lancs Lad]

Thankyou, all, for your very expert replies - much appreciated

Yes, Paul, my household wiring is pretty ancient too. I still have rewireable fuses, and only one ring main for all the sockets in the house!

[mark2collection]

Just measured my Pure DAB.

From earth to exposed metalwork on the set (the aerial), it's half mains.

Mark

[russell_w_b]

Was the measurement made with a DMM (Hi-Z) or an AVO or similar?

[mark2collection]

Russell, it was indeed a modern DMM.

Using my Avo (MKVI), the reading's 35 volts AC.

Just my making contact with the -ve output of the PSU is enough to 'drag' the reading down.

Mark

[russell_w_b]

Originally Posted by mark2collection

Just my making contact with the -ve output of the PSU is enough to 'drag' the reading down.

That's the trouble with high impedance instruments - they're not really suitable for measuring low-impedance source derived voltages! If you live within the vicinity of a radio transmitter (say), you only have to wave the probe around with the other probe earthed to read an a.c. voltage of, perhaps tens of volts. Holding a DMM probe on a car battery and the other probe in the hand whilst leaning against the bodywork will give you a reading of 12V d.c. too.

35V is still rather high, mind. I checked my Apple wall-wart between earth and the pins and measured 63V a.c. with a DMM and about 3V on the 100V range of an AVO 8 VII. Nothing on the lower resistance 10V and 3V ranges. I earthed the wall-wart output through the meter. No RCD trip; 0.08mA measured.

With regard to the orig. post... When risk-assessing the effects of an electric shock, death from electrocution through casual contact with a phase conductor is certainly not always going to happen. What is more likely is that the shock will cause a secondary action, like falling from a ladder caused by a sudden hand movement as a result of the shock (or even tingle), or bashing one's hand on a sharp edge. If that were to happen through casually brushing against the unearthed metalwork of a CE and safety marked domestic appliance there may be questions asked.

Lancs Lad... Can you get someone else to do what you did and gauge the reaction? Try not to tell them what it might be for fear of influencing them. It may be that you are unduly sensitive to a.c. voltages. When I rest the inside of my wrists on the corners of my (earthed) photographic light box I can feel a very slight tingle, but I have to work at it.

[peterpixel]

Have you checked that the sink unit is at earth potential? I've had many tingles over the years but things came to ahead when I decided to have a air to water heat pump installed.After 16 months(true) of "installation" I downloaded an installation manual from Panasonic and read it.I then corrected the position of the pump outside which required some plumbing alterations.After cutting the pipes I received more than a tingle.I knew that the pipework was bonded to the main earth which was a incoming lead water main.Checking between the water main and an earth spike I measured 100v at about 30mA and assumed the water main had assumed a coating of insulation.With the addition of a length of heavy earth cable and an earth spike I cured the problem.The worrying thing was that at least three electricians had worked on this install and not one had noticed this,one had even installed a new consumer unit.The point I am making is that maybe it's possibly the sink that is at a raised potential,not likely but worth checking.May all your tingles be little ones.Peter.

[GMB]

Even double insulated SMPS have a class Y twixt input and output for EMI reduction.

Maybe for a fully double insulated device, but surely that is illegal for a device where a human can contact the low voltage supply unless the device is certified to medical standard?

Y-class are rated for connection between live an a safety earth - the point being that in the unlikely event of a failure you still have the earth to keep the voltage down while the fuse has a think about it.

[julie_m]

Class Y capacitors are specifically designed never to fail low-impedance.

[GMB]

Class Y capacitors are specifically designed never to fail low-impedance.

I have seen this said many times. It is sort of true but not in the way that is being understood by most people I suspect.

X and Y class capacitors are much the same in the materials used to construct them. They have a "self healing" aspect so that impulse overloads blow holes in the metal film plates but not usually in such a way as to damage the dielectric. Y class capacitors are made with a different interleaving of the plates which results in a structure that is sort of equivalent to having two capacitors in series - so if one half fails it still looks like a capacitor (of higher value - that's a useful clue).

To quote one manufacturer: "Y capacitors are used in applications where failure of the capacitor could lead to the danger of electrical shock to the user, if the ground connection is lost."
Note that: they have to be normally used with a ground connection.

X and Y class capacitors are actually tested by subjecting them to big transients. Y class a tested to about twice the voltages of X class, but as they are effectively two capacitors in series that means the dielectrics are tested much the same. They have to take several big hits and then live on for a further longish period.

So Y class are capable of doing anything that X class can do - like catching fire. They are not as different as I suspect people believe. They just take a lot longer to do it. I am pretty sure they do not count as "double insulated" since the types of the two insulators are required to be different.