live dish washer

[Geoff]

I would be grateful for some advise regarding a fitted dish washer. I was in the process of removing it from its kitchen cabinet when I had an electric shock on touching both the washer's metal interior and the stainless steel sink. I checked which was live and using one of those cicuit testing screw drivers it was the dishwasher. Neither the circuit breaker or the main RCD tripped. I have had a good look at the wiring in the dishwasher and can find no obvious fault. I have also tested the diswasher regularly and it has not been live again. I assume the shock was caused by me shorting the circuit between the then "live" dishwasher and the sink which I again assume is connected to earth via the cold water pipe. Can anybody offer any advice as to what could have caused the fault as I would have thought that if the dish washer was faulty it would be "live" on a regular basis.

[Colin Wilson]

Was it in a particular place in the cycle at the time ?

[BigWallop]

Is the sink earthed ? Some sinks and tap combinations are insulated from each other by rubber washers sandwiched between the tap body and the sink itself.

Is the socket that the dishwasher is connected to earthed properly ? TURN THE POWER OFF at the mains. Open the socket and check that all the wiring is in the correct places and fully tightened.

Was there any other appliance plugged in or running while you got the shock ? It could be that another appliance has a fault or low resistance and it caused the earth to rise slightly and give you a tingle from the potential difference.

Do you have any low voltage lighting or other type of light fitting that are connected to the ring circuit ? Where they switched on at the time ?

[romic]

I had a tingle (not a full blown shock) from my washing machine in the past. When I checked, it turned out that there was a loose earth in the socket. Securing the earth solved the problem. I hadn't got a RCD fitted, so don't know if there was sufficient leakage to trip one or not. It certainly didn't blow the fuse when the earth was reconnected and the machine run again.

Roger (my reader sometimes loses mail/newsgroup messages

- if you think you should have had a reply/comment, please e-mail me again. Ta!)

[Pete C]

You need to check out the dishwasher earth, the supply earth and the earth bonding of the sink...

Is the dishwasher cord damaged or has been trapped or under stress in any way? Is the earth on the dishwasher casing and it's plug secure, and is there continuity between them? Is there continuity between the earth on the socket, the sink and the supply electrical earth?

cheers, Pete.

[Matt Beard]

The RCD won't trip on very small leakages - say enough to give you a bit of a nip or even enough to hurt but not kill. Especially if it is a 100mA whole-house jobby. (For the circuit breaker to trip with you being the path from live to earth you would be talking enough damage to need to use dental records to identify the remains - assuming they could pry your fillings out of the ceiling!)

Having said that, for the case to be live and the RCD / MCB not blow there is something very wrong. The case should be connected direcly to the "earth" in the plug and this should be at exactly the same potential as the sink (assuming that it is correctly bonded). Either you have two faults within the dishwasher (earth disconnected and a short to case) or the wiring to the socket used by the dishwasher is faulty.

Whatever happens I wouldn't use the dishwasher or even attempt to remove it again until someone with good electrical knowledge has checked it out.

[Geoff]

Thanks for all the replies they are extremely helpful.

We have only recently moved into the house and the wiring although all PVC and also with a modern consumer unit is pretty old. There is on an odd collection of circuits with one single socket on its own circuit! Also, there is no earth in the lighting circuits and there appears to be no earth bonding for any of the metal objects in the house, including the sink next to the dish washer. This will have to be rectified as I have recently purchased an electric shower.

I thought there was a problem with the RCD as several times it has tripped when I was doing work to individual circuits. I had assumed that as I had popped the circuit breaker for the circuit then that ciruit was comletely "dead". However from reading this news group I now understand that, if you short the earth and neutral then the RCD will trip. I stand to be corrected!

With regard the problem the supply cord did initially appear to be trapped at the back of the washer, but on inspection the cord was not damaged, but it had some black "tar" like subsidence. This had attached itself to the wire from the back of the washer. I assume that this was caused by the heat of the dish washer softening the materail and then attaching itself to the cable where it came into contact with the cable.

I can't recall whether there were any other appliances running at the time although I dont think there were. As to the "shock" it was more of a jolt. The machine was not actually on at the time.

Notwithstanding a faulty dishwasher, I would suggest by your replies that the problem would appear to lie with the earth circuit either in the dish washer itself or the socket or circuit it is plugged into.

[Andy Hall]

That's permissible as long as the MCB is no more than 20A. If you have a split consumer unit with RCD, this circuit could be upstream of it and supply a freezer, for example.

In the early days of PVC wiring, that was not a requirement. I believe earthing of lighting circuits became a requirement in the 70s.

It should be done anyway.

Another possibility is if there is a radio interfence suppressor employing capacitors in the dishwasher. WHere these are used, the appliance needs to be grounded or there can be an effect of the chassis acquiring a voltage level at about half of mains but with no real current behind it. This can explain a non sustained jolt such as you experienced.

I would suggest getting a simple continutiy tester/digital meter. They are pretty inexpensive. You can then do an earth continutiy test from the dishwasher chassis all the way back to the main earth terminal near the consumer unit.

Mains testing screwdrivers do not really give reliable results one way or the other.

.andy

To email, substitute .nospam with .gl

[Christian McArdle]

It is normal to have a dedicated circuit without RCD protection for the fridge/freezer. This ensures that an earth RCD trip elsewhere in the house doesn't ruin all your food.

The kitchen sink does not need to be bonded. Indeed, it is better to insulate it by using plastic supply pipes if possible. That way, it doesn't act as an earth, so you don't get a shock through it.

Yes, you should install supplementary bonding in the bathroom. However, again it is safer to use plastic pipework. If all plastic pipework is used, then the baths and radiators don't need bonding either. The electric shower DOES need cross bonding.

Nope. The MCBs do not fully isolate the circuit, they just cut the power. Personally, I always isolate the whole thing at the main switch (unless it is an DP RCBO circuit, but these are rare).

Prepare to be corrected! Depending on the type of your earthing arrangements, a neutral/earth short is unlikely to draw enough current to blow the RCD. However, it may provide a sneaky backdoor for the neutral current from a different RCD protected circuit to bypass the RCD and set it off. Such an effect may be dependent on the current usage of that circuit and might only manifest when a beefy appliance is turned on.

It could easily just be a faulty earth, rather than a more serious fault. Obviously, it must be fixed.

Christian.

[Pete C]

Hmmm, I would have thought that if a kettle or radio lead fell into a metal kitchen sink it would be better off if it was earthed.

cheers, Pete.

[Christian McArdle]

I'd still prefer insulated and with all sockets on an RCD. Even if the non-earthed sink doesn't provide sufficient earth leakage to blow the RCD, it is easy enough to just turn off the appliance before retrieval.

Besides, you should be using kettle leads. It is much safer to use a cordless kettle with a short lead to the base that won't even reach the sink.

Christian.