Minor electrical works cert


Postby Basil20 » Sat Jan 10, 2009 9:34 pm

Hi guys
i am a joiner and just completed loft conversion for customer,
the loft already had ring main & light circuit, i added 2 dbl sockets to the ring main, and a few downlights to light circuit
BCO said i need to fill in a minor electrical installation works cert,
i have a multimeter tester, but not sure on how to test for earth fault loop impedance and earth continuity and also insulation resistance for ring main and light circuit,
some advise would be greatly appriciated, thanks
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Basil20
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Postby ericmark » Sun Jan 11, 2009 2:43 pm

Simple answer with a multi-meter you can’t. You could hire a test set to do the tests but using the earth loop impedance tester can cause some danger and I would not be happy to instruct a non electrician on this forum on how to use the meter.
The form you need can be downloaded from http://www.theiet.org/publishing/wiring ... /index.cfm PART 3 Essential Tests will be the problem where a 500 volt insulation tester, and RCD tester is required the Earth fault loop impedance can be calculated either from length of cable used in ring main or from the R1 + R2 readings. A “mega” will normally have both insulation and a low ohm meter built into same machine.
What I am not sure on is why the BCO wants you to do these tests? I would not have thought the work you have done requires notifying under Part P and even if it did then the LABC are responsible for site safety not you!
I have heard before how some councils try to back heal their responsibility.
I would think you have three options.
1) Pay an electrician to test it.
2) Test it with multi-meter and give LABC the minor works cert minus the RCD tripping times and cross fingers!
3) Tell LABC that’s their job and quote the part in the Part P document.

So what is wrong with multi-meter:-
The regulations 612.3.2 state that you need an insulation resistance of 1 MΩ at 500 V and 612.2.1 continuity using a supply of between 4 to 24V with a short-circuit current of not less than 200 mA.
However multi-meters do have a high range my AVO Mk8 I think uses a 12 volt battery but not sure. And for low range a 1.5 volt battery and no way does it use 200ma. But with power off at main switch opening any socket on a ring main you can measure the resistance between both lines, both neutrals (which should be the same value) and both earths from this you can work out the max Earth fault loop impedance for whole circuit. Ze Earth fault loop impedance at incomer is found by enquiry i.e. for PME (TN-C-S) supply taken as 0.35Ω (If TT supply you will need customer to dig out last PIR to find resistance of earth rod which I can see being a problem) If for example the resistance between both lines was 0.5Ω and the earth measured 0.8Ω since ring main at centre to consumer unit will be half those figures per cable and since two cables ¼ of those figures so total of 0.32 add the 0.35 to this that means Zs is 0.68Ω on a 32A MCB type B you need 5 times 32A for the circuit to trip within the permitted time so a 160 amp needs to flow using ohms law R = V/I so 230/160 = 1.4375Ω maximum allowed for it to trip on time so it would pass.
For lights maths is easier but measuring is harder as you need to open consumer unit and remove feed wire from MCB and connect to earth bar to be able at furthest light to measure R1 + R2 i.e. resistance between earth wire and live wire. Then of course replace. In this case since no ring just add the R1 + R2 value to the Ze valve for example 1.2 + 0.35 = 1.55Ω again assuming B6 MCB then 6 x 5 = 35 amp and 230/35 = 7.7Ω so well within max allowed.

The insulation resistance is direct measurement.

Or of course you could completely cheat and guess on figures and just hand in sheet. I will assume you have read the Part P document there is a link on projects section? Page 11 section 1.24 is interesting bit. I would say reading that you don’t need to test. But since I am not an ordinary person under the regulations I do have to provide documentation so I can’t get out of testing. Please do tell us what you do and how you get on. As electricians we don’t get to know what Joe Public has to go though and it is interesting to find out.

Eric
ericmark


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