Insulation test L to E long length of SWA

sidekick · Feb 28, 2024

Insulation test live to earth = 0.7 ohms.
This is a long length of buried swa cable (approx 100m) from mains in to submain.
Test done with live and earth disconected at main and with switch off at sub main.
Question - is this an accurate reading or am I misding something.
Its a big job to replace if I've got this wrong ?

R-fur · Feb 28, 2024

I would say that that cable is not suitable and should be replaced or repaired. What is the route? if it is underground is there any sign of excavations on the route?

brianmoooore · Feb 28, 2024

If this is a genuine reading, then the cable is severely damaged, and must be repaired, replaced, or taken out of service.
Why would L+N to earth give any better reading?
Chances are that the cable is damaged at one point, and similar measurements taken at the other end of the cable should give some indication of where.

Avo Mk8 · Feb 28, 2024

sidekick said:
Insulation test live to earth = 0.7 ohms.

I wouldn't expect an Insulation test on a MFT to be able to resolve a fraction of an ohm.
Are you genuinely measuring ohms between L & E, or are the units Megohms for example?

pc1966 · Feb 28, 2024

Also the CPC should be connected to earth during any IR tests.

Here (if SWA) it is very unlikely to make a difference, but more generally with stuff like T&E you can have a live nicked by a screw/nail and it is leaking to the wall, etc, but not to the CPC that still has the insulation separating them (and from wall, etc).

pc1966 · Feb 28, 2024

Also is the earth conductor above separate from the armour?

brianmoooore · Feb 28, 2024

I'm sorry, but if someone doesn't know the difference between ohms and megaohms, then they probably shouldn't be using a MFT in the first place.

westward10 · Feb 28, 2024

0.70 ohm between live earth you will have a 300A+ flowing.

sidekick · Feb 28, 2024

R-fur said:
I would say that that cable is not suitable and should be replaced or repaired. What is the route? if it is underground is there any sign of excavations on the route?

Underground and laid about 20 plus years ago.

westward10 · Feb 28, 2024

So is it 0.70 ohms or 0.70 Mohm?

MDJ · Feb 28, 2024

westward10 said:
So is it 0.70 ohms or 0.70 Mohm?

very good question and I was just about to aske the exact same question, hopefully the OP will come back and answer it

sidekick · Feb 28, 2024

MDJ said:
very good question and I was just about to aske the exact same question, hopefully the OP will come back and answer it

megaohms

westward10 · Feb 28, 2024

That'll be Meg ohm then

I had a college teacher who said mega ohms I cringed every time he said it.

sidekick · Feb 28, 2024

pc1966 said:
Also the CPC should be connected to earth during any IR tests.

Here (if SWA) it is very unlikely to make a difference, but more generally with stuff like T&E you can have a live nicked by a screw/nail and it is leaking to the wall, etc, but not to the CPC that still has the insulation separating them (and from wall, etc).

2 core swa and armour used as earth

westward10 · Feb 28, 2024

Disconnect the line and neutral at both ends to remove any possible external influences switching off can't be guaranteed.

Insulation test L to E long length of SWA

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