Why does this shock me?

[Sunbeam]

This afternoon I turned off a switched fused spur supplying a 3kw water heater and got a packet off the switch. This is an MK dpdt thing with non-metallic front and pattress. Plastic for want of a better word. Supply is from a 16A mcb via a 60A rccb. All items and and cabling are new within the last 2 months. All are located within an unheated outdoor, but waterproof, building. Condensation may be a problem but there was no sign of any water in, on or around the offending spur. I put a tester to the plastic casing and it showed mains voltage over most of the surface. Any ideas why I copped a packet and also why the rccb didn't trip? Many thanks Sunbeam

[Christian McArdle]

What earthing system is used for this outbuilding? Perhaps the house earth has been inappropriately exported, or there isn't an earth at all.

If there is any distance from the house, the building should have a local TT earthing system with earth rod.

In any case, perform an earth loop impedence test and ensure it is reasonably low.

Christian.

[andrewpreece]

Replace the isolation switch with another one and see if the problem persists; sounds like a funny one so faultfinding by substitution is a good place to start. Post back if you do it and let us know whether it has any effect.

Andy.

[Tony Williams]

Any chance that a live wire is being touched by a front panel fixing screw?

[Dave Plowman (News)]

The obvious answer is that it's been coated with something conductive. Moisture on its own won't make plastic live.

[Sunbeam]

Thanks Christian,

the earth is good. New 50m run of 10mm2 SWR to the junction box which is

0.5m from the incoming (electricity company) 3 ph meter. Tested when installed a couple of months back Whether the earth be good or otherwise, why did I cop a packet from a totally plastic encased switched fused spur?

Sunbeam.

[Sunbeam]

Thanks Tony,

that was the 1st thing I checked. The screws are not live but the rocker switch and an area extending approx.12mm around it are showing full voltage. Have checked the connections within and they are all faultless.

Sunbeam.

[Sunbeam]

That would seem most odd. This is a brand new fitting from a well known & hopefully reputable manufacturer. It has been installed straight from the box, correctly and with no modifications. The fitting has definitely not been coated in any way other than by the manufacturer or the atmosphere.

Perhaps so, but it is the only reasonable alternative that I can think of at present.

A feeling that I also enjoy.

Sunbeam.

[Ed Sirett]

This would imply not only the falt you describe but also that the earthing is suspect since the screws are earthed via inset washers in the fitting.

[Tony Williams]

It is as though the metal bracket the rocker switch pivots on has a voltage on it. A bit of surface dust and condensation providing a conductive path to the outside. Sounds like an iffy switch.

Perhaps this is a situation where you might consider a high current pull switch instead, for future safety.

[Christian McArdle]

50m from the house with an exported earth rings a few alarm bells, although I doubt you'd ever get enough potential difference between it and local earth to cause a shock, unless your house is built in a "difficult" location, such as on a granite clifftop.

Christian.

[Christian McArdle]

Have you heard of the bathtub reliability curve? Always suspect new components before ones that have been proven by a few year's service.

Christian.

[John Stumbles]

Very true. I've had failures on brand-new bathtubs, but ones that've been in for a while generally seem to keep on going. :-)

[Christian McArdle]

But after 10 years or so, they do rust away at an increased rate.

Christian.

[Christian McArdle]

P.S. Was it a bathtub that you were replacing in Talfourd Avenue yesterday?

Christian.

[Aidan]

It sounds like something that the manufacturers need to be advised of. There are counterfeit products around. Keep hold of the evidence though. Replace it asap.

[Pete C]

Hi,

Sounds like the there is a fault with the earthing of the switch. Is there a plate behind the front which earths the two screw holes? If this is floating it could allow some capacitative coupling or tracking (through damp) of mains voltages.

If connected and there is a break in the earthing elsewhere then capacitative coupling from live could allow it to float at a high voltage, especially if the wiring is old.

If all the earthing of the switch and its supply is in order, it could be defective plastic that can absorb water through being porous. Try a different make switch so to be sure it is not from the same batch as the original.

cheers, Pete.

[richard]

Well I'm curious as to what the "voltage" has been measured with. Not one of those lightstick things by anychance, that show a voltage if you scratch your arse the wrong way? I know he had a shock, but damp hands damp fitting might havr caused that, or static discharge to the metal screws.

[John Stumbles]

Nope - that's about the one thing that's still OK in that house :-)

Gas valve on ancient floor-standing boiler packed up on Friday, No immersion heater either ... brrr!