How can you clear a fault but have no power

[captaincaveman]

Hi all.

Big old house with upstairs lights tripping. Barely any access in attic due to old beams etc.

Insulation resistance fault of 0.34 Live to Earth
Disconnected all conductors at assumed first light fitting.
IR fault at CCU now >10MO
I re-energise at the CCU so I can see if any other lights are now working so I can narrow down the fault.

BUT, I test for power at the conductors i've disconnected at the first fitting and there is no power there!
How is that possible? If the fault is before the fitting then it would still fail the IR test, surely

p.s. I suspect rodents but don't know yet

 

[captaincaveman]

.... Unless the CPC is broken between the CCU and first fitting and then the Line is broken after the CPC break and touching the CPC going forward and earthing via an alternative path??
But then Why am I not getting voltage at the cpc?

 

[littlespark]

Old house, old wiring?

Is it joint box, or loop in (3 plate) wiring?

Beams or not, I expect you’re going to need to go up into the attic.

Are rodents likely? Fields nearby? Existing evidence?

 

[captaincaveman]

Old house, old wiring?

Is it joint box, or loop in (3 plate) wiring?

Beams or not, I expect you’re going to need to go up into the attic.

Are rodents likely? Fields nearby? Existing evidence?

At this "first" fitting it's loop in. The rest appear to be joint boxes and old par 38 light fittings.
It is reasonably old wiring but not ancient.
it's near wildlife / golf course and although i haven't seen droppings i have come across gnawed insulation

 

[freddo]

Where are you measuring voltage from and to when there is no power with the wires apart? I assume when you got there the lighting circuit trips instantly?

When fault finding insulation faults I usually aim to disconnect at the middle of the circuit not right at the start, otherwise you could be there all day if the fault is at the end! Then keep splitting it down halving the faulty section each time. What happened when you reconnected everything?

 

[mainline]

Where are you measuring voltage from and to when there is no power with the wires apart? I assume when you got there the lighting circuit trips instantly?

When fault finding insulation faults I usually aim to disconnect at the middle of the circuit not right at the start, otherwise you could be there all day if the fault is at the end! Then keep splitting it down halving the faulty section each time. What happened when you reconnected everything?

I would have thought he's sorted it by now, if he hasn't then that's going to be a hell of an invoice.

Pity he hasn't returned to say what the fault was.

 

[freddo]

I would have thought he's sorted it by now, if he hasn't then that's going to be a hell of an invoice.

Pity he hasn't returned to say what the fault was.

Oh dear, I missed the date!!!