IR testing problem

[soulman]

Hi, I was doing some tests prior to installing a fan on a lighting circuit. IR readings L-E were dead short & N-E were a dead short, yet the lighting circuit works o.k. I have had this problem with energy saving lamps, when removed the circuit was o.k but this was giving 0.04Mohms at 500v not a dead short. Any ideas??

Cheers

 

[Octopus]

Aerial amp?

 

[soulman]

Cheers murdoch, i didn't realise an aerial amp would give a dead short LN-E.

 

[Marvo]

A low IR reading with a mega tester doesn't necessarily constitute a 'dead short'.

The L-E & N-E low impedance could be a combination of a load still being connected to the circuit on test and the neutral still being connected in the CU. If the N was disconnected then a borrowed or crossed neutral on that circuit would give the same result.

 

[Deleted member 9648]

If theres a N-E fault and no RCD the circuit will work fine,but if lamps are left in circuit when you IR test you'll get a L-E fault reading through the lamps as well.

 

[ian.settle1]

No HF fluorescents in the circuit as they have filter circuits across the mains and earth

 

[chrish_1977]

Dimmers? Outside security pir floodlight?

 

[soulman]

Cheers Marvo & wirepuller thats very useful advice, i have come across pir's & dimmers with ln-e i find i get readings around 0.04Mohms but i had a dead short 0.00Mohms

 

[SJD]

but i had a dead short 0.00Mohms

Why not switch your MFT to continuity, and measure the "short" in Ohms, when it then likely won't read zero, and might give more of a clue as to what is responsible.