Cooker circuit fault. Live cpc!

[phil333]

Hi guys.

We have been brought into a domestic installation to complete remedial works. The previous electrician who worked for a builder was not very competent in his work and also could not sign off any of his work. He wired a new extension which included a kitchen. The council has been bought in to complete a certificate but has flagged up remedial works to be completed before he completes his test.

We have changed his new split load board to a new amd3 split load board, and have come across a fault.

The cooker circuit has a live cpc (floating voltage) but doesn't trip the MCB or the RCD.

When the dp cooker switch is off the voltage between the cooker cpc and the MET is 110V, when you turn on the dp switch it becomes 230V. The scary thing is, that when you connect this cpc to the MET the voltage drops to 23V but doesn't trip anything!

All the above tests have been complete when all other circuits are off. Polarity is correct in the cooker switch, insulation resistance test shows 299 to each core from the CU to the cooker outlet. No continuity between any cores either. r1+r2 readings are 0.11ohms, zs (when cooker switch is on) is 0.23ohms. RCD times are within the time limit.

Note : green gunk has been found in the conductor of the cooker circuit, which I have heard can cause issues...

Has anyone had this issue before as it's boggling my brain. The circuit has been isolated until the problem has been resolved.

Many thanks

 

[PEG]

I did suggest that in post #4

...Yeah,fair play,but it was so good,i named it twice :stooge_curly:

 

[Midwest]

Measured with a multimeter, that is what you would expect with a floating CPC in T+E. It's sandwiched between L & N and capacitive leakage will take it to a voltage somewhere midwayish. Not indicative of a fault.

Also not specifically indicative of a fault, just means that the cooker leaks much more to L than N. Even sound elements do tend to leak, test the cooker separately.

Not scary at all, but wrong. If the CPC is connected to the MET, the voltage between them is by definition zero, so I don't know what the 23V represents. I wouldn't expect anything to trip.

I'm not saying there isn't a fault but if you're testing with a Hi-Z meter, everything you describe sounds normal to me.

Bow to your superior knowledge Lucien, but can't understand how OP's MFT is conducting Zs & RCD tests, with all those ghost voltages knocking about, or is that not the case with all digital MFT? Would it be an idea for the OP to use an analogue meter to check voltages?

 

[DPG]

It will only be digital multimeters that show this problem (due to the very high input impedance which allows them to take measurements on very sensititive circuits). MFTs, even though digital, should not show this problem. Some people interchange multimeter and MFT though, which can confuse things Daz

 

[HappyHippyDad]

Bow to your superior knowledge Lucien, but can't understand how OP's MFT is conducting Zs & RCD tests, with all those ghost voltages knocking about, or is that not the case with all digital MFT? Would it be an idea for the OP to use an analogue meter to check voltages?

Thats what I thought Midwest, but after using an analogue meter to check for voltage and reading the responses from the thread below (see posts 21 and 22) I feel happier with the drummond.

http://www.electriciansforums.co.uk...-odd-request-regarding-phantom-voltage-2.html

 

[egg67]

Think as the OP had not got back to us the green stuff is more the problem, this might sound silly but Who you goner call

 

[drzsta]

My Megger MFT will not allow me to do continuity testing if it detects ghost voltage.

Job today and a lighting circuit done in pyro but no earth continuity. MFT was telling me 100v between pyro sheath and MET.
Stuck my fluke indicator on it and was showing 12v. Get a bit of a tingle when you touch the two :yes: