Electric Zap !!!

[mdrandall]

I've called it a zap because I don't believe it's full mains.

In my utility room I have a wall mounted boiler, washing machine, tumble dryer and a metal sink. Today I lent over to remove the clothes from the washing machine and steadied myself by holding onto the sink. I got an electric shock!

Now I'm not an electrician but have a degree in electronics so I understand a small bit of the theory. However, I can't fathom this out.

If I use a digital multimeter, it show about 170 volts ac between the sink and any metalwork on the machines or their earth leads. However, if I switch off the boiler supply at the fused spur switch on the wall, the voltage disappears (as does the zap 'cos I tried it!).

The sink itself does not appear to be earthed (i.e. no continuity between it and the earth leads), although it is all metal with metal pipework feeding metal taps.

Any ideas why the sink could be affected by the boiler being switched on please?

The house is 6 months old so I am contacting the builder tomorrow. I have previously asked him about a slightly noisy telephone line which reduced my dial-up internet connection speed also when the boiler is on. We did not resolve this problem as I got ADSL instead.

Thanks

 

[pdcelec]

Sounds like someone has put a nail/screw through the cable from the FCU for the boiler. It sounds like it has not completely broken any cables but enough to cause problems. Is there any voltage on the boiler casing or the pipework feeding it?

If you take the front off the spur for the boiler and disconnect the outgoing side, then turn the spur switch on is the problem still there or does it go away. This would for sure tell you that the outgoing cable is damaged.

 

[TeesdaleSpark]

Also it could be a live to earth fault on the boiler.

Is there an earth cable connected to the water pipe somewhere near where it comes into the house? If there is then there should be no potential between the sink and any other earth - assuming good electrical connection between water pipe and sink.

The first thing I would check is that the element has a fault to earth. Sounds like it could be travelling down the water pipe to the sink and if there is no earth either on the water pipe or the boiler then when you short across with your arms you are making the circuit.

However all what I have said could be *******s. You need a decent electrician to come out and check it over for you.

Regarding your telephone line probably best to talk to BT about that. Are the telephone cables running near power cables?

Did you get an installation certificate for the electrics? everything should have been tested and any problem found then.

 

[mdrandall]

Thanks Gents,

I'll look further. But it seems that all the earths in the area i.e. nearby twin 13amp, earth in light switch, earth in FCU for washing machine etc are all linked together and give a good continuity reading. In other words they are all at 170 volts from the sink with the boiler FCU switched on.

The earth for the house is under the kitchen sink. I have tried pluging an extension lead into one of the kitchen 13 amp sockets, extending the cable into the utility area and buzzing throught the earths and it all seems good. So I am really confused as to how the utility sink is getting some volts unless as suggested the boiler itself is conducting through to the house pipes somehow.

I have also tried the airing cupboard where the pump and Megaflow live. All the pipes I tested seemed to have good continuity with the earth in a local 13 amp socket and the earth in the central heating terminal block box.

There is only about 1 foot from the boiler FCU to the boiler itself so I doubt that there is a screw through the cable but I will check.

The boiler case and it FCU earth are all connected to the other earths in the house.

The reason I mentioned the phone is that various extension sockets are run through the house and the interference is only there when the boiler is powered up. I thought it may be a clue.

I have also checked my neighbours who have an identical house and there is no volts on their utility sink.

 

[mdrandall]

OK, I seem to have tracked down the problem.

When I first moved into the house, I replaced the existing Danfoss RMT230 room thermostat (3 -wire) with my old Landis&Gyr Chronogyr Rev10 (2-wire).

After reading a few posts on this forum, I just cutoff the neutral wire and insulated. Then I attached the red permanent live to one side of the new thermostat and the yellow switched live to the other.

This system has worked fine for 9 months without any problems although its quite possible I haven't noticed the volts on the sink before.

I have refitted the original Danfoss RMT230 complete with the neutral and the problem has gone away.

However, I would really like to use the Landis electronic as it allows various temperature settings throughout the day.

Is there a correct way of terminating the neutral in the thermostat?

Thanks

 

[andy]

OK, I seem to have tracked down the problem.

When I first moved into the house, I replaced the existing Danfoss RMT230 room thermostat (3 -wire) with my old Landis&Gyr Chronogyr Rev10 (2-wire).

After reading a few posts on this forum, I just cutoff the neutral wire and insulated. Then I attached the red permanent live to one side of the new thermostat and the yellow switched live to the other.

This system has worked fine for 9 months without any problems although its quite possible I haven't noticed the volts on the sink before.

I have refitted the original Danfoss RMT230 complete with the neutral and the problem has gone away.

However, I would really like to use the Landis electronic as it allows various temperature settings throughout the day.

Is there a correct way of terminating the neutral in the thermostat?

Thanks

the neutral may have nothing to do with this. what might be the problem is that the stat is leaking to the backplate, which should be earthed. this is then going into the boiler thru the cable. then from the boiler case to the output pipes. is the boiler protected by an RCD? id imagine it would have tripped a few times by now if you did have 1

 

[mdrandall]

No there is not an RCD for the boiler. Also the thermostat does not have a backplate, just a hole in the platerboard. There is an earth wire but it is not connected (nowhere on the RMT230 to secure it).

Still can't see why connecting the neutral to the RMT230 perhaet or offset bit fixes the leakage through the pipes????

 

[andy]

No there is not an RCD for the boiler. Also the thermostat does not have a backplate, just a hole in the platerboard. There is an earth wire but it is not connected (nowhere on the RMT230 to secure it).

Still can't see why connecting the neutral to the RMT230 perhaet or offset bit fixes the leakage through the pipes????

i doubt connectin the neutral will do anything. try a different stat

 

[mdrandall]

Thanks for all your info. It seems that leaving the neutral wire floating allows the utility area sink to get a few volts from somewhere. Defeats me as to where from.

Unfortunately the Landis electronic room stat only has two connections as it's just a relay. There is nowhere to hook up the floating neutral or an earth

 

[securespark]

Have you tried disconnecting the neutral from the other end??

 

[Adam_151]

Applying Occam's razor here, is it possible that the digital room stat was activated, but when you swapped it for the old one, that had a different setting and wasn't activated (this first assumption is blown out the water by the current weather though), now if we assume the sink isn't very well earthed due to pfte tape on the plumbing joints (quit common I suspect), and the screw holding the sink up goes through the switched live (its happened before)

 

[mdrandall]

I tested the volts on the sink with the stat commanding heat or not for both old and new stats. Again there is always about 170 volts ac according to my multimeter between the sink and earth if the neutral 3rd wire is left open circuit so I don't think it has anything to do with the stats themselves.

I will try leaving the old 3-wire stat in place but disconnecting the neutral and see if the volts return.

I will aso try the idea of disconnecting the neutral wire at the boiler end assuming I can find the correct end.

Thanks once again for all your ideas...........

 

[mapj1]

There is another possibility, which would not show up on buzzing extension lead earths from the dining room or whatever. While rare, we have had it here before - It is not impossible that the house wiring earths while all connected together, are not at the same potential as terra firma earth, at least not when some leaky appliance (in this case suspect the boiler or something in that spur). This could be because the Main Earth Terminal (MET) somewhere near the company fuse and meter doesn't actually go anywhere - due to a defective main earth perhaps (it should also cross bond to your plumbing pipes.) To be sure if it is the sink that is live, or the housewiring that is floating, you may wish to measure relative to a temporary true earth (a garden fork stuck in the ground and a bucket of salty water thrown over it, or a length of old pipe hammered in will give enough for a high impedance meter to chew on.)
Don't leave it - this could be the early warning of something much more unpleasant. Is your incoming supply PME or TNS (I assume not TT as an RCD would have tripped by now.)