Odd Fault on Ring Final

[markadams]

Hi,

Went to look at a job today where the customer reported the RCD intermittently tripping.

I did an RCD Trip test and it failed to trip on x1, x5 or ramp test, the mechanical button tripped it.

After a bit of further testing I discovered that the Kitchen RFC had an IR reading of 0.00 meg ohn between N-CPC when disconnected.

I split the ring in the middle and tested one side at >500 Meg ohm and the other side at 0.00 Meg Ohm. I disconnected another socket on the faulty side and the readings on both sides gave >500 so I tested between the two points and still got >500 ???

I reconnected the sockets and tested again and strangely the ring now tested >500 and the RCD tripped when testing??

I don't like this as something caused it but I cant find out what now as it is testing fine.

I noticed that despite it being a new RCD the readings were very high 121 at x1 and 46 and x5, I did a Zs test and got 76 (way over for a PME system), I had a look at the service head and it was a new PME head but had NO earthing cable connected. I will get the customer to contact the DNO.

 

[Octopus]

At one of the sockets you took off, could the fixing screw have nicked a neutral?

 

[markadams]

I couldn't see any signs of that.

 

[Specialist]

If it's anything like around here, the DNO will tell you to supply & fit a length of 16mm earth yourself.

 

[Wilko]

As you've not got a proper installation earth that would be my first priority - perhaps DNO thought this was a TT site? The 76Ohm result seems to suggest there's a rod somewhere or the local gas and water are still in metal pipes ?
I think you must have disturbed the RFC fault ... personally I'd take that last outlet apart again and go CSI on it

 

[davesparks]

The logical conclusion is that the fault was at or near the second socket that you removed and therefore must have disturbed the fault.

Yes the RCD tripping time at x5 is high, what have you done about it?

 

[markadams]

The logical conclusion is that the fault was at or near the second socket that you removed and therefore must have disturbed the fault.

Yes the RCD tripping time at x5 is high, what have you done about it?

I agree but I can find nothing wrong with it, I wiggled the wires and checked it with a fine toothcomb but nothing.

The property is empty at the moment so I have not done anything about the RCD, it is a brand new RCD as I had one on the van and swapped it out as a quick initial test.

 

[David M]

Priority has to be sorting out the main earth. As for the N-CP fault if the IR test was done properly and the test results accurate then my guess is there was a fault that you inadvertently disturbed, else you've missed something.
Regardless whether the property is empty, it is your duty to ensure that the installation is safe, by either isolation or rectification if it is required.

 

[markadams]

I agree David, which is why I changed the RCD (again) which is now giving readings within the regs. As for the Main Earth, I have reported this problem to the customer as they need to contact the DNO (which has been done now).

I will await a response once the DNO has attended.

 

[markadams]

The 75 ohm Zs reading is coming from the water pipe (copper).

 

[marc8]

Give an option to make a TT if no joy with DNO.

 

[richy3333]

I noticed that despite it being a new RCD the readings were very high 121 at x1 and 46 and x5, I did a Zs test and got 76 (way over for a PME system), I had a look at the service head and it was a new PME head but had NO earthing cable connected. I will get the customer to contact the DNO.

Can you clarify... Did you undertake a Zs of the RFC or did you undertake a Ze? Which loop impedance, internal or external?

It sounds like a Ze and your quote was just a typo? If there's no earth then as others have said it must be coming from the bonding? Was there any?

Is it a new installation or a new cutout as if so surely you just need to connect your earth cable (of appropriate size) from the cutout to your MET?

 

[markadams]

Can you clarify... Did you undertake a Zs of the RFC or did you undertake a Ze? Which loop impedance, internal or external?

It sounds like a Ze and your quote was just a typo? If there's no earth then as others have said it must be coming from the bonding? Was there any?

Is it a new installation or a new cutout as if so surely you just need to connect your earth cable (of appropriate size) from the cutout to your MET?

I took a Zs on the RFC, I also took a Ze on the Earth/Bonding conductors which was 70.4.

Yes all I need to do is connect a 16mm conductor to the met from the cutout but it is sealed so I wanted to let them do it as I have also asked the customer to get a DP isolation switch installed.

 

[HappyHippyDad]

I think I'd keep checking out the ring as with a ZS of 76ohms and a Ze of 70.4ohms it looks like the R1+R2 may be a tad high! If you only look one reading for the Zs then I'd do it again as sometimes the results can vary depending on the connection you have made with your probes or clips.