Too Many Earths?

[wingcoax]

Got called to a neighbors property the other day, They had an EICR a couple of months ago and a new kitchen installed last week. The problem is that when they plug in the hob ignition the main RCD trips(literally just the earth pin on the plug) it turns out that the gas pipe is copper solid pipe. Therefore the gas supply is earthed via bonding so the hob is double earthed with the plug. the socket is ok when used for anything else and ok on the hob when earth wire temporarily disconnected. Can anyone shed any light on it?

 

[westie101]

i would be inclined to check the earth & polarity of the socket you are using.

Looking at this from another view by plugging the cooker, which is bonded to earth, into the socket you are tripping the RCD so the fault could well lie in the socket!!

 

[333rocky333]

id be inclined to measure and see if there a voltage betwen the socket earth and the plug earth pin

 

[wingcoax]

Westie, i have changed the socket and checked the connections ,so has the engineer from the company who supplied the hob. Polarity and earth are fine.

 

[OwainDIYer]

Is the cooker socket also new? Has it worked before with anything else?

Check which neutral bar in the CU it's connected to - it might be on the wrong bar for the RCD.

 

[EFLImpudence]

All gas pipes will be bonded so that's nothing to do with it.

IR the plug - 250V - continuity may be enough.

Is there an oven under the hob?
Is it on the same circuit and RCD?

 

[Lectrician]

The RCD trips as soon as the earth pin makes contact with the socket earth? The live and neutral do not get near?

 

[bhm1712]

have you measured voltage between the oven earth pin and socket earth pin? have you actually considered that the oven may actually be faulty?

 

[wingcoax]

New ring installed when kitchen fitted. socket in question is spurred off double socket above work top. As stated the engineer from hob suppliers has been on site,and cleared doubts on faulty hob. all sockets on ring work on other appliances, no voltage between earth pin and socket earth.

 

[EFLImpudence]

The RCD trips as soon as the earth pin makes contact with the socket earth? The live and neutral do not get near?

Yes - but presumably it would not do that if the cpc was not connected to anything at the other end.
It doesn't trip when L & N plugged in with cpc disconnected.

So there must be a path from the cpc to somewhere there shouldn't be.

 

[viewer]

What happens if you use an extension lead cooker to another socket?

 

[wingcoax]

Tried plugging into socket in lounge, everything fine.

 

[EFLImpudence]

Is the lounge on the same RCD?

If not, try another socket in the kitchen.

 

[bernardgreen]

Assuming it is a standard RCD that has no earth or CPC connections then there has to be an un-balance between Live and Neutral currents in the RCD for it to trip.

Therefore connecting the earth lead from the hob to the CPC in the socket MUST be affecting either the Live or Neutral current at the RCD.

Therefore there has to be a sneak circuit between one or more of the earths and either of the Live and Neutrals.

A insultation resistance check using DC will not find sneak circuits that include capacitive paths and these capacitive sneak circuits, between Live and an " earth" or CPC can produce enough un-balance to trip an RCD.

Are there any connections between the hob's casing and any other double insulated ( un-earthed ) electrical appliance casings? Cooker hood for example. Connecting the hob's earth would then provide a path from that appliance's casing to earth. If that appliance casing was floating between Live and Neutral ( capacitive coupling ) then that could be the path that creates the un-balance that trips the RCD.

 

[securespark]

You have double-checked the effectiveness of all PEB's?

Disconnect the ends and measure resistance. You're looking for less than 0.05 Ohms.