insulation resistance of a 3ph circuit in 2391

[bluesky]

Hi guys, another silly question, how to measure the insulation resistance in a 3phase circuite? shall i do it in 2 steps?

1: between cu and isolator/starter.
2:between isolator/starter and 3ph socket

cheers

 

[brucegenerator]

Turn off power.Simple installations should be checked as a whole.test to earth from all live conductors including neutral connected together.or each live seperatly to earth.when you have vulnerable circuits i.e.. dimmers etc.i think.

 

[Sintra]

Also have to test between phases.

 

[bluesky]

hi, i am still bit in dark, when i test continuity of a 3 phase circuit i should test seperately between cu and isolator; isolator and socket, then add up, but when testing insulation why should i test it as a whole?

please enlighten me!

 

[philbluespark]

is there any reason why you cant test between cdu and socket with isolator closed? (for continuity)

 

[WayneL]

Hi Bluesky,

You don't want to be doing it circuit by circuit - you'll be there 'til the following day

The procedure is simple.

Sub board main-switch open.

Motor isolator open.

Dimmer bypassed.

FCU bypassed.

All other switches closed.

Three phase board - all MCBs closed. Main switch open.

Select 500v range and perform ten point test.

i.e.

Place probe 1 on earth bar and probe 2 onto - L1.....L2.....L3.....N (4 tests)

Place probe 1 on neutral bar and probe 2 onto - L1.....L2.....L3 (3 Tests)

Test L1 - L2, L1 - L3, L2 - L3 (3 Tests)

NOTE: L1, L2 & L3 are tested on installation side of 3-phase main switch.

Then:

Test from motor isolator to motor (or 3 phase socket)

Test single-phase sub-board circuits individually (due to RCD) - should be two circuits - don't forget to throw the two-way switch to test your strappers.

Job Done

Don't think I've forgot anything.

PS
The 2391 board will be TN-S, if you had a TN-C-S supply you woulld have to remove the neutral test link otherwise you would get continuity between neutral and earth.

 

[bluesky]

thanks waynel, your answer is just like sunshine...

 

[gordon344]

Good answer, but what is the FCU?

 

[ian.settle1]

Good answer, but what is the FCU?

Fused Connection Unit is the correct terminology otherwords a fused spur

 

[Risteard]

The 2391 board will be TN-S, if you had a TN-C-S supply you woulld have to remove the neutral test link otherwise you would get continuity between neutral and earth.

You need to break the neutral link on any TN system - TN-S included - for the purposes of an insulation resistance test.

 

[WayneL]

You need to break the neutral link on any TN system - TN-S included - for the purposes of an insulation resistance test.

Hi Risteard,

I understand your reasoning, but I didn't on my 2391 and the assessor didn't say anything.

We weren't told to do it during the course, either.

Ironically, I got >299 M Ohms on my N-E test, but I do know why you're saying you should disconnect.

I can't understand why it didn't show up the fact that they are connected on the test. Any ideas?


It's alright, it's just come to me.

The 2391 board had a seperate 'main isolator' which was probably 4-pole.

When the assessor opened this, it will have disconnected the neutral as well

 

[Risteard]

The 2391 board had a seperate 'main isolator' which was probably 4-pole.

When the assessor opened this, it will have disconnected the neutral as well

You beat me to it. I suspect that is what has happened. When I did it we certainly had to break the link.

 

[Scotchydog]

im doing am2 on tncs do i have to remove the main neutral out of bar or take each one out and on my lighting circuit do i only flick the switches on whichever phase its on thanks

 

[Deleted member 26818]

im doing am2 on tncs do i have to remove the main neutral out of bar or take each one out and on my lighting circuit do i only flick the switches on whichever phase its on thanks

On single phase installations, the main switch will be double pole.
As such there is no requirement to disconnect the N-E link.

 

[i=p/u]

just link all phases together and test to neutral and earth , save bit a time,, ive just read something like this so not 100% ok i am 100% sure

 

[Scotchydog]

On single phase installations, the main switch will be double pole.
As such there is no requirement to disconnect the N-E link.

Do you reckon it will be the same with 3 phase board if not whats the best way to go

 

[Scotchydog]

Cant i just remove the main earth if i get a reading between N-E

 

[Deleted member 26818]

Do you reckon it will be the same with 3 phase board if not whats the best way to go

Many 3phase installations have main switches that just isolate the phases.
If you don't disconnect the neutral, you will be testing it back to the supply.

 

[Scotchydog]

Many 3phase installations have main switches that just isolate the phases.
If you don't disconnect the neutral, you will be testing it back to the supply.

The neutral on that circuit or like the main neutral to the bar?

 

[Deleted member 26818]

The neutral on that circuit or like the main neutral to the bar?

It depends on how you are testing.
If you are disconnecting the conductors at the CU to test an individual circuit, then it would not matter.
However if you were to conduct a global IR test, you would have to verify that the neutral is disconnected, or you would be testing the IR of the network supply neutral.