Combined L and N to Earth insulation resistance test

[lynnieboy]

Can some one please explain the theory behind the L and N combined to Earth insulation resistance test. I get the fact you do it to eliminate damaging sensitive equipment but what is the science behind it. I am new on the forum so would appreciate the feed back.

 

[m4tty]

By testing live and neutral together To earth you bypass the load.

 

[sparkytim]

+1

The L to N short prevents ( or rather minimises it to a safe level) any current passing between them at any other point i.e thru equipment that may still be in circuit, which @ 500 Volts could damage it.

 

[SLIPSHOD & SLAPDASH]

But it wont show faults between the L & N. Hopefully a L to N fault will operate the CPD.

 

[springcrocus]

Keep in mind, you would only do this on an existing installation (PIR etc.) to save disconnecting everything. On a new install, you should do the full set of IR tests (L-E, L-N, N-E) before connecting anything.

 

[binky]

Hopefully a L to N fault will operate the CPD.

Precisely, so on an existing circuit CPD would have tripped if fault existed, but a fault is unlikely to be present if there is no short to earth, which as an un-insulated conductor should be first port of call for any leaky leccy.