DIY Doctor

Eric and Friends I could do with your help

Postby Tim Wood » Wed Oct 15, 2008 5:15 pm

Hello Eric and friends.
My continuing interest in electrical theory has thrown up a question or two that perhaps you could spare the time to answer.
I have a friend who likes to think he is a up and coming Del Boy Trotter he does house clearance work.Now the last time I was round at his place I noticed this Test meter he had just got hold of.I showed a bit interest in it and then I had the hard sell.He thinks it's worth hundreds,to me it's worth nothing.
To cut a long story sort I had it for a few days .It's a Metrel Z Loop Tester,just plugs into the socket that suits me.Had a look at the manual so lets get some testing done!I have a TN-S system with 80 A. RCD. breakers are all "B" type,the average loop readings in the kitchen ring,main ring,and garage ring were 0.33 "O's" and the amperes was 740.The average line reading
were a bit lower at 0.29 "O's" amperes at 860.My shed readings was not so good, loop 0.60 amperes 400.The supply comes from C/unit to a 16 type "B" mcb, a 25 A. RCD on outside wall before it goes underground to a small consumer unit which has a 63 A. RCD in the shed.
I know it looks like a bit over the top and the wrong order of mcb and rcb's which it probable is.But if I was to turn everything on in the shed it wouldn't make 10 amperes.And thats 20 meters of 6mm cable feeding it.Its all been fine for the last 6 years,what do you think Eric?
One more small point you could help me with.On this Tester you can also check the loop resistance= R l/15 mA.The reading I got from this test was 27.4 from the house and 27.7 from the shed.I have not been able to find out much about this test,so are the figures good or bad?
Thanks for you time to any one still here.

Regards Tim wood
Tim Wood
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Postby ericmark » Thu Oct 16, 2008 6:23 am

The 16th Edition test set, as it is often called, requires interpretation. The C&G course 2391 is normally around 36 hours spread over 12 weeks. When testing a house we will measure:-
Earth Loop impedance which also give one of the Prospective Fault currents a second fault current is also measured using neutral on the main incomer.
We measure the earth leakage trip and this is done 6 times twice at half to show it does not trip, twice to show it does trip, and twice to measure time at 5 times tripping value. The double testing is as one is done on positive cycle and one on negative.
We also measure the insulation resistance and the R1 + R2 and R1 + Rn values.
We use all this to calculate volt drop, and if a overload device will trip on time. Although lectures may use complex numbers to explain it we don’t use them in calculations but never the less I would not like to do it without my PC sticking all the reading into an excel spread sheet gives me most of the results without too much hassle I think there may be some slide rule type devices but I don’t use them.
If everything is OK great but if there are faults in the system under test these test instruments can produce some dangerous voltages.
So if I take shed reading ELI = 0.60 ohms which will give 383 amps which your meter has calculated for you at 400 amps which would be correct if one used 240 volt instead of 230 volt. A B16 MCB will require 16 x 5 = 80 amps to trip or 230/80 = 2.875 ohms ELI which is what the table 41.3 on page 49 shows. So you are well within the required figure. As to RCD in the shed the 63A is greater than the 16A supply so that’s OK but once remote from the main incomer then we can get volt drop and RCD’s require voltage to work so to ensure fail safe we would normally use an active RCD where remote from main incomer. At the consumer unit we use a passive type as very little chance of volt drop and we want it to hold in on a power cut and once supply is returned to allow fridges, freezers etc. to auto restart without needing to reset RCD.
I would say either leave it alone or learn how to use it by going to a technical college you could buy Guidance notes 3 from IET which explains a lot but you will not find much on the net it is something people expect to get paid for when training and all you are likely to find are tasters.

Postby Tim Wood » Thu Oct 16, 2008 9:30 am

Thanks for your reply Eric.At 74 my collage days are long passed.My interests these days are centred around things that do not cost me any money.And I know not to poke one's fingers in places they are not wanted.Any comments on the loop resistance values?As regards the tester my friend has it for sale at £125 any takers? (joke).

Regards Tim
Tim Wood
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Postby ericmark » Fri Oct 17, 2008 5:14 am

The loop values at the consumer unit are called Ze normally around 0.35 for a TN-C-S or PME system.
The loop values at socket called Zs and we subtract Ze from this to give R1 + R2 reading
MCB's come in type B, C, and D and their magnet part will trip at 5, 10, and 20 times rated thermal overload setting.
So a B16 MCB will trip at 5 x 16 = 80 amps using ohms law 230 volts divided by 80 amps = 2.87 ohms so if R1 + R2 exceeded 2.87 ohms a direct short would not trip the magnetic part of the MCB so that is the limit.
A D6 MCB would be 20 x 6 = 120 amp 230/120 = 1.9 ohms max.
There are tables with all these values but easy enough to calculate.
A MCB has two devices built into one the thermal bit takes time to warm up and will allow start currents to flow but not fast enough to fully protect so also a magnet bit which works in milli seconds but it is easy to use too much cable for on a short circuit for enough current to flow which is why we do the loop test.

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