High Ze - DNO added an RCD!

[Kevin]

We've had a couple of jobs like this lately, where we have reported a high Ze reading back to the DNO. They have subsequently come and fitted a 100mA RCD between the meter and the CU.

I would have thought they would need to find the reason for the high values instead. Surely an RCD is used a supplementary protection, not primary. The job I looked at was a TNC-S system, so logically thinking, wouldn't a high Ze value possibly indicate a problem with the neutral?

Also, obviously the problem of the high Ze remains, so Zs readings may well be over the accepted limits..then how am I supposed to complete a test certificate??

many thanks

K

 

[IQ Electrical]

Your Zs readings are covered by the addition of the RCD (max 500 Ohms).

I would expect the 100mA RCD to be a temporary measure in the interim while investigative work is carried out.

 

[Kevin]

Thanks for the reply.

So you would be happy to sign a test cerificate, or a periodic if the Zs readfings were too high, as long as there was RCD protection? Or would you contact the DNO, and establish if they were coming back to rectify the problem first ?

K

 

[Deleted member 26818]

I doubt the DNO will actually conduct any further investigation.
Your problem, will be in the future when the RCD plays up, who will be responsible for replacing the RCD?

 

[IQ Electrical]

On the basis of the RCD, your Zs readings will comply because your disconnection times are met by that device.

Your PIR will show how compliance was achieved.

The DNO normally use an RCD as a temporary measure when Ze is slightly above the agreed maximum.

What was Ze out of interest?

 

[IQ Electrical]

I doubt the DNO will actually conduct any further investigation.
Your problem, will be in the future when the RCD plays up, who will be responsible for replacing the RCD?

How is that his problem?

As long as the DNO supplied and installed device is noted on your 'comments' section then you 've done exactly what would be expected of you.

 

[Kevin]

The one job I went to it was 1.11

My thoughts though, are that there could well be an issue with the incoming neutral, as it's TNC-S.

Not sure how we would complete an installation certificate, if one was ever required, would a comment on the existing installation suffice ?

 

[IQ Electrical]

Yes 'following our report of an exceeded max Ze reading, the DNO supplied and installed a 100mA RCD. Our certification was carried out with this device in place'

 

[JD6400]

If i were you i would try to find out if your DNO are planning to come back in the near future to sort out the fault . If not i would look at it as a latent fault and swap it to a stake . It could easily be a poor neutral joint thats on the way out , depending on your bonds could be bad mojo in that house !

 

[Kevin]

Nice one, thanks.

Like Spin said, I have my doubts the DNO will be back, so I want to make sure we're covered.

K

 

[Deleted member 26818]

How is that his problem?

No of course, it will be the client's problem.

I've seen this in the past, and the hassle getting the DNO to accept that the RCD was their equipment was unbelievable.

 

[Kevin]

If i were you i would try to find out if your DNO are planning to come back in the near future to sort out the fault . If not i would look at it as a latent fault and swap it to a stake . It could easily be a poor neutral joint thats on the way out , depending on your bonds could be bad mojo in that house !

Could be, tenant was also complaining of power fluctuations, though no evidence of it whilst I was there of course. Have now passed it back to the DNO.

 

[AlanWirral]

Sounds funny to me? After initial testing (pre board change) unless the Zs's were well within the limits i'd be thinking about walking away until the problem was solved by the DNO.

Like said though if the results are fine and you have noted the problem then it's all good.

 

[Amp David]

I doubt the DNO will actually conduct any further investigation.
Your problem, will be in the future when the RCD plays up, who will be responsible for replacing the RCD?

Good point as the DNO won't want to know as its after the meter.

SOunds like a kop out to me!

 

[malcolmsanford]

It may not be a neutral problem at all, after all a PME system is just a glorified TT system. They may have problems with the stakes/spikes they had fitted, there may have been construction work further upstream that meant stakes were removed or having to be resited.

I go along with IQ here I think this is something temporary. I can not imagine the DNO just fitting an RCD and walking away. There may be a failing neutral but I bet you they are trying to find what it is and how to fix it.