Please help me with college coursework

[andrewhubbard]

Hi im a 1st year apprentice and in need of some help. i have a question in my coursework and it says this (word for word) heres the question:
when testing between phase and neutral tails at the c.u. a reading of 0.6 M ohms was observed - describe the procedure you would adopt following this reading.

 

[WayneL]

Hi im a 1st year apprentice and in need of some help. i have a question in my coursework and it says this (word for word) heres the question:
when testing between phase and neutral tails at the c.u. a reading of 0.6 M ohms was observed - describe the procedure you would adopt following this reading.

Hi Andrew,

That is an IR test figure - and it's too low

You'ld need to find the offending circuit/s

You would switch each breaker off one by one (leaving them off), re-testing L - N each time until the

fault clears - the reading will be a lot higher and the last breaker that you switched will be a faulty

circuit.

To ensure it is only one circuit that has a fault, you should then switch each breaker back on, one by

one, checking the reading hasn't dropped again each time.

If the readings remain the same with all the other circuits switched back on, then the circuit you initially

switched off is your faulty one.

You would then proceed to find the fault within the circuit.


Hope this helps

 

[MarcT]

Hi Andrew,
In addition to what Wayne said, you might find that each of the circuits connected to the CU has a low but acceptable IR value, the sum of which (parallel resistors) adds up to the figure given, in which case you would record the IR value for each circuit.

 

[andrewhubbard]

i would like to thank you for you help, this has helped me out no end. thanks again drew

oh just one other thing what is the lowest result you can have from this test before it would appear as there is something wrong

oh just one other thing what is the lowest result you can have from this test before it would appear as there is something wrong

 

[WayneL]

i would like to thank you for you help, this has helped me out no end. thanks again drew

oh just one other thing what is the lowest result you can have from this test before it would appear as there is something wrong

1Mohm at 500V test

I personally would look into it further if it were under 2Mohm testing

at the incommers.

The alternative to what I described in my first post is to test each circuit

individually - breakers open (off) and test on outgoing side of each and the neutral bar to

find the faulty circuit/s

 

[Sparky Ninja]

To add on to the great advise already given, you may want to also consider to check if all loads have been removed from the circuit ie, was the circuit properly prepped for the test.

 

[Jason Smith]

Hi mate i see you have learnt to post a new post !! just another bit of learning for you, you will sometimes come across situations where you cant remove equpment that will badly effect a IR test, i.e low voltage lighting, ceilin g fans etc. In this case you would test at 240 volts as you then wouldnt blow them up or damage delicate electronics. What we do is join the phase and neutral together, and test this union between earth, and we can do that at 500 volts as no potential exists between the phase and neutral. reading will be greatly inproved if there is no genuine fault, other than equpment causing the poor results.