IR testing in domestic dwelling

[An Buachaillín Bán]

Hi all,

Testing question for you.

I'm currently testing an existing domestic installation. The wiring is about 20-25 years old.
We have rewired some of the older circuits and added newer circuits too. During my insulation resistance test, over two-thirds of my results were coming out at 500MOhms. These circuits had new cabling and older cabling on them.
I know its unusual to get such high (perfect readings) like that, so my question is, would the fact that I'm testing through older cables with newer cables bring the IR reading up? The only other circuits that are lower than the 500 are circuits with only the older cabling on them. I have switches on for lighting circuits, no loads connected that would alter results

Any advice would be greatly appreciated.

 

[ferg]

Resistances will normally be lower than the lowest section of cable being tested.
Some older cabling does still measure out perfectly so by reducing the amount of older wiring you are increasing the probability of getting perfect readings.

I would expect new wiring to measure off the scale unless damp from plaster etc was an issue.

 

[littlespark]

Is this L to N readings?
Disconnected loads, but what about electronic dimmer switches, usb charger sockets, smoke detectors?

Might be worth splitting circuits down and locating what stretch of cable is low

 

[An Buachaillín Bán]

Is this L to N readings?
Disconnected loads, but what about electronic dimmer switches, usb charger sockets, smoke detectors?

Might be worth splitting circuits down and locating what stretch of cable is low

Thanks for the reply. It's live cables to CPC, ie L+N to E.

 

[An Buachaillín Bán]

Resistances will normally be lower than the lowest section of cable being tested.
Some older cabling does still measure out perfectly so by reducing the amount of older wiring you are increasing the probability of getting perfect readings.

I would expect new wiring to measure off the scale unless damp from plaster etc was an issue.

Thanks for the reply Ferg. I pretty confident I'm testing correctly, but just wanted a 2nd opinion. Thanks

 

[bennie]

It's not uncommon to get perfect results on an old installation. Cables lasts a lot longer than people think. I've tested housing estates that haven't been touched for 50 years and the majority had perfect results and the ones that didn't had the dreaded cable cancer.

 

[timhoward]

I generally try the clips together to make sure I get zero before proceeding.

On some testers it’s easier to make daft mistakes than others. E.g Metrel uses the brown and blue leads for IR and the brown and green leads for continuity. I occasionally forget to swap over! Megger is much easier as the same two do nearly everything.

 

[oscar21]

My tester goes up to 2000M ohms and even on brand new wiring it rarely hits that target, sometimes it will show something random like 1746 or 1838, I think it just picks any old number and displays it. Even some new circuits measure fairly low though, I had 150 or so on a brand new lighting circuit recently but it had just been plastered and there was condensation everywhere.

I also think some LED batton fittings or the discos are leaking a fair bit to earth under test to reduce the values a fair bit - yes I know you are supposed to disconnect everything like this to test properly but if L-E are ok and N-E are ok then L-N must be ok seen as it has the earth in between the two of them and if you've wired it wrong you will soon know when you energise it all.

Saying nall that though, does it really matter what the reading is in the grand sceme of things 150M ohms is still 150 million ohms resistance, you won't get much current flowing through that.

Google says

Current (I) = 1.5333333333333E-6 ampere (A)

Power (P) = 0.00035266666666667 watt (W)