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Topic Title: Supply details & rcd test question
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Created On: 21 July 2012 10:59 AM
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 21 July 2012 10:59 AM
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Just after some advise following a T&I on a small office block.
It consists of 4 offices and the landlords area, we were only testing the 3 offices rented by our clients.
There is a 1 3 phase TNCS supply. A single phase db in each office and 3phase db in the landlords area each db has its own meter and separate tails into the large electric boards enclosure. I assume there are separate electric board cut outs inside the enclosure for each

So, i will be doing a separate electrical installation condition report for each DB rather than 1 with continuation sheets for lot. But do I put 1 or 5 as the number of supply's to the building. I would be tempted to put 1 as there is only 1 incoming supply, or should it be 5 as there are 5seperate cut outs& meters.

3 lighting circuits all protected by 61009 10A 30ma rcbos, 2 of which (on rcd test) gave aprox 300ms (x1) >500ms (x5) on test at end of line. IR tests were fine (with limitation noted on L-N) so was Zs.
When I disconnected the circuit and tested the rcbo it gave 30ms/12ms, so no problem with rcbo.
I have been told that if the rcbo tests ok with no load then it is ok, but I don't agree as under normal operating conditions it fails the trip time?
 21 July 2012 12:21 PM
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--assuming you are talking about a multihead, (ryefield),
4 offices and 1 L/L = 5 supplies.

 21 July 2012 12:23 PM
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i always took it that number of supplies was to each installation and was referring to a back up generator ? so would say 1


Specialised Subject. The Bleedin Obvious. John Cleese
 21 July 2012 12:38 PM
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Hi Pik,

1 - from your description it sounds like a separate report is needed for each office.

2 - RCD testers only simulate a current imbalance rather than causing an actual fault, so some kinds of connected equipment with capacitors and the like can cause the slowing down of timed readings as they continue to in effect add a little extra power back into the circuit after the device has operated.

Sometimes it can save time to test an rcd at a remote point, if it passes then theres no problem, but if it seems slow then testing again at the device with cables disconnected reveals that the RCD times are met then I would say that the RCD has met the requirements of the regulations.

 22 July 2012 06:53 AM
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Once a circuit is energised and supplying a real world load then its potential may not be removed by a RCD as quickly as we would think .
You have very little control over this.
All you can do is check the RCD is within spec (on an unenergised circuit).

If you are conducting a separate PIR/EICR for each office on its own then can you not count each one as one installation having one supply therefore one Ze (or Zdb if you like)? so long as you make it crystal clear to future users.

Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik

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