IR testing

[boycie]

When IR testing on lighting circuits i keep coming across dead shorts between L+E and N+E on the same circuit. I then proceed to investigate and the faults clear (maybe a screwed neutral or something ). however im scratching my head as to why im getting dead shorts between L+E when the mcb holds in fine Electrical Trainee syndrome). any advice on this would be fantastic !

 

[Marvo]

When you say 'dead shorts' can you give further info? Are you seeing a dead short with the megger at 250/500/1000v or are you seeing a dead short with a normal tester on ohms scale? Once the fault has cleared what reading are you getting on the circuit with the megger and at what test voltage?

 

[trev]

Is your MFT calibrated? I often get dodgy readings when my calibration is due

 

[Plonker 3]

Have you discoonected all loads from the circuit, alos have you disconnected the neutral at the DB before testing?

 

[guest119]

Neutral still connected at db and a load connected.
Bell transformer etc?

 

[Deleted member 72666]

If your reading is 0.1 Meg then it's not a 'dead short' which is 100k and is the reason the mcb doesn't trip. But like others have pointed out is that you've probably haven't disconnected all loads.

 

[guest119]

Yea, dead short.
I always got a slap for saying that.
Had to say zero zero zero, representing 0.00.
Or as the 2391 lecturer used to say, a fault of negligible resistance exists.

Boydy

 

[telectrix]

dis the N in the DB and then IR test.

 

[oldtimer]

dis the N in the DB and then IR test.

This is basic stuff my friend so if you are not getting it you may need to go back to basic training and yep disconnect the neutral

 

[Octopus]

Bell transformer? Extractor fan?

More investigation me thinks!

 

[boycie]

the readings i am getting on my completely disconnected from the board cable is 0.00Mohms (dead short). I am expecting to get >299Mohms when clear at 500v as its a new install. My tester was calibrated 2 weeks ago. any reasons im picking up dead shorts between L+E ? TA

 

[SJD]

Umm, as already mentioned, 0.00 MOhms is not necessarily a dead short, have you tried measuring on the MFT's continuity setting?

 

[Plonker 3]

Have you thought you might actually have a fault somewhere?

Have you IR tested individual legs of cable to try and find it?