testing tips and advice

[iamelectric]

testing is all new to me so i have a few questions to ask , so i know what to do in future if there are any surprises in store

1- when testing an old board:
some cables IR test 6 Mohms , above the min 2 but is this cause for concern or normal ?

2- both rcds tripping at once , how do i solve it , IR between Ns find the circuit sharing the neutrals and put them on the same rcd side?

3- IR test is 0.00 between cables a short , and whats most likely when its 0.02 ?

4- what if you cant find what a cable is supplying?

5-how do you do the relevant tests on a lighting circuit that has no cpc at switch or pendant ?

6- what sort of things effect testing , if there is something still plugged in that you have missed , say you are getting 0.02 MOhms L-N but nothing is tripping , is this something plugged in somewhere , or if its a reading like that N-E and nothing is tripping , i have read threads to do with smoke alarms effectin testing and dampness

thanks for any help

 

[soulman]

I found testing really difficult at first. The best advice i could give, is try to understand why you are testing & what the expected results should be. If you are IR testing an existing installation alway test LN-E & soft test first.
Q1, If you conducting a global test you may get a result of 6Mohms, the circuit may stay around 6Mohms for many years.
I installed a replacement shower for a customer (like for like) IR reading was 10Mohms, I IR tested a few more circuits LN-E as i thought that reading was quite low & found other reading's >300Mohms. I found on further investigation that the shower cable was melted together in the loft where someone had joined the cable. 10Mohm had passed, but just didn't feel right. Once corrected shower circuit >300Mohms.
Here is something i was never taught at college or in the 2391, something i discovered a few months ago.
TN system type C circuit breaker, ELI too high, circuit design was poor very long run. Solution, change for a type B circuit breaker all is fine. Someone suggested placing an RCD in circuit which would give a maximum 1667ohm instead of the type B circuit breaker. I was also taught that either changing the protection to a type B or putting an RCD in circuit would solve the problem. But never taught, you must take into account the L-N impedance in the event of a L-N short, the circuit must still be able to disconnect quickly enough.

I built myself a test rig as i struggled so much, test a friend's house or you own house.

 

[trev]

TN system type C circuit breaker, ELI too high, circuit design was poor very long run. Solution, change for a type B circuit breaker all is fine. Someone suggested placing an RCD in circuit which would give a maximum 1667ohm

Both extremely poor solutions! The type C MCB would most probably be there for a reason, unless of course that this was another example of circuit design by an idiot. The other "solution" doesn't even warrant discussion.

 

[SPARTYKUS]

Rcd should only be there as additional protection on a TN system (sure you knew that).

 

[Octopus]

testing is all new to me so i have a few questions to ask , so i know what to do in future if there are any surprises in store

1- when testing an old board:
some cables IR test 6 Mohms , above the min 2 but is this cause for concern or normal ?

2- both rcds tripping at once , how do i solve it , IR between Ns find the circuit sharing the neutrals and put them on the same rcd side?

3- IR test is 0.00 between cables a short , and whats most likely when its 0.02 ?

4- what if you cant find what a cable is supplying?

5-how do you do the relevant tests on a lighting circuit that has no cpc at switch or pendant ?

6- what sort of things effect testing , if there is something still plugged in that you have missed , say you are getting 0.02 MOhms L-N but nothing is tripping , is this something plugged in somewhere , or if its a reading like that N-E and nothing is tripping , i have read threads to do with smoke alarms effectin testing and dampness

thanks for any help

Normally means that something is still connected/plugged in.

 

[trev]

Get your hands on guidance note 3 mate and read it till there's no print left.
1 It really depends, if you have loads of time you could investigate but as it's an old board you're most likely doing an EICR. In that case it's your job to inspect, test and report.
2 A classic example of a borrowed neutral, ideally find the borrowed one and rectify it. It's usually on the stairs lights, putting both lighting circuits on the same RCD isn't ideal. Your pre change tests should have identified the problem and you should have booked time in to put it right really
3 Something is plugged in or there's a neon on the circuit. As an example, Glade plug ins alledgedly have a resistance of 0.57.
4 If the CU is labelled wrongly or not at all switch things off and see what isn't working, obviously in some situations then extreme care must be taken and people should be told of it before it happens. If you can't find something still, leave ity off and wait for the call "My --- isn't working"
5 r1, record on the sheet and note on the CU that class 1 fittings must not be used.
6 Damp plaster, something plugged in, insulation breaking down. All kind of things can affect your results mate

 

[pennywise]

Breaks in ring final circuits on one conductor or just a missed link, disconnect one of the legs from the effected pair ie CPC or neutral etc put it in block in the board leave the other one connected, unplug everything!, then liven it up, go around with a plug in tester until you find one socket without a fault like a missing neutral etc and a nearby socket with the fault you've created, this should allow you to pinpoint a break quicker without having to take the plates off on every socket searching for a lose cable . Bit screwed if its been wired in a star method from a JB but it at lest narrows down the area to be looked in

 

[iamelectric]

thanks i done the courses and read GN3 but they dont really help you with fault finding , really need somemore experience , thanks for the replys

 

[SPARTYKUS]

If you were 490 miles closer I could have, for a fee, arranged your house to be transformed into a real live test rig full of actual faults

 

[Amp David]

Keep your practice board and keep practicing on it.

Types his reply, then sits back and waits for the barrage of abuse for not being helpful enough HA HA!!!

 

[iamelectric]

the practice board is not much good as my wiring of it is faultless lol

 

[iamelectric]

i will just have to keep asking questions for now , what about pressure faults , what test would show this up , or a socket screw clamping down on a N or L ?

 

[trev]

Isnn't Flying Sausage up Glasgow way? I'm sure he could turn your house into a test rig

 

[davethsparky]

A few years ago I did did a course where I learned the answers to these kinds of questions and all sorts of useful things about fault finding. One of the very useful things I learned was how watching the movement of the needle in an IR meter is as important as the value it settles at. The needle will move differently if there is damp in the circuit to if there is a fault of specific resistance or a dead short.
Of course if you use one of these fancy digital jobbies for fault finding you are denied this sometimes very useful feature.

The course I took was quite a long one, weighing in at 3 years long, think that's probably why it seems to have gone out of fashion and shorter courses are preferred. The course was taught by an experienced electrician and I think the title of the course was something like 'apprenticeship'.

 

[Sparkie30]

The course was taught by an experienced electrician and I think the title of the course was something like 'apprenticeship'.

not long ago i came across this experienced electrician who went through his 'apprenticeship' but was useless git to say the least.