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Tripping an MCB to isolate a circuit.

Postby chris_on_tour2002 » Sat Aug 06, 2011 12:22 pm


I am trying to isolate a circuit to move a socket in a floorbox. The MCBs at the consumer unit are labelled up but there are I have a choice of four to choose from for the circuit I need to isolate.

Now of course one could ordinarily just flick all switches one by one until I hit the right one and then remove the cartridge from the board to be certain that it's dead. Problem is, however, that one of the circuits powers a server and if I take it out I would not be very popular.

I tried using a socket tester with an RCD trip tester but the it won't trip. Silly question perhaps but are these trip testers likely to trip the MCB first or the RCD?

In any case the trip tester is not tripping anything, I've used two testers, one is telling me that there is a fault with the earth volts (fault >50v) and another just reports a fault no trip.

I'm using the plug-in type, are they any good for this? And is there any other safe way of determining which circuit I need to isolate at the board without just switching it all off?

Cheers for any advice,

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Postby ericmark » Sat Aug 06, 2011 8:23 pm

Although it is common to isolate circuits by switching the MCB this is really only permitted with a TN-C-S earth system as with a TT earth system the neutral may not be at earth potential. Even a TN-S may be a problem. As an electrician we will often just use the MCB but then treat the circuit as if we were working on it live.

As to a RCD tester they will not trip a MCB they would normally be used 6 times on each circuit twice to show it will not trip, Twice to show it will trip, and twice to show it trips in the required time. Reason for twice is one is with pos half cycle the other with neg half cycle. A plug in tester like the Socket & See SOK36 Professional Socket Tester, does a test similar to pressing the test button on a RCD and does not really do a full test on a RCD.

In the past I have worked out which MCB supplies what by using a clamp on ammeter and putting a load on and off the circuit but to do this requires one often to go into a live board and health and safety rules mean this is often not permitted. Again we break rules and it is something which has come up many times. Some companies consider testing as not working other consider testing as working and the HSE will not allow us to work on live circuits.

When you say "one of the circuits powers a server" I assume you are at work and therefore you would have to be an electrician to do this work under HSE rules? I have had the can't turn of server argument before and I had to point out I was not asking permission to turn it off that was going to happen all I was doing was asking what time would suit best. If the server is really important it will be one a UPS if not on a UPS then the IT department clearly consider it's not important.

Although in theory one does not have to modify an installation to conform with new regulations, one still does need to do a risk assessment and show one paper either why it is not going to be up-graded or show it is ear marked for up-grading at some future date. As an in house electrician we often don't follow the rules as to inspection and testing and paperwork keeping but if anything goes wrong the management are very quick to point the blame.

I would advise you to point out to management that 514.1.1 requires labels and you will need to isolate items in order to comply. If you are not employed as an electrician likely your not covered by insurance. Although I was a qualified electrician and had worked for the company as such, when I moved sites I was employed as a mechanic to allow me to be paid more. However when the electrical engineer looked into it he realised the insurance problem and with all electrical work I had to officially assist another electrician even though in real terms he was helping me.

Yes a boss may turn a blind eye to some one doing work for which they are not employed. But if anything goes wrong its "I don't know why he was doing it" when questioned by HSE. You don't sound as if your an electrician if your not employed as an electrician don't touch.

At home can't see why one would be worried about a server?
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