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.

James · Feb 28, 2024

westward10 said:
That'll be Meg ohm then
I had a college teacher who said mega ohms I cringed every time he said it.

[ElectriciansForums.net] Insulation test L to E long length of SWA

Hate to say it Westward, but mega Ohm is the correct way of defining it.

ferg · Feb 28, 2024

James said:
View attachment 113647
Hate to say it Westward, but mega Ohm is the correct way of defining it.

To be fair to Westy it wasn't adopted until 1873 which is probably after he went to college.

sidekick · Feb 28, 2024

I should also say that this swa cable has a 63a mcb at the front end and is randomly tripping ?

brianmoooore · Feb 28, 2024

Is that with whatever's at the far end switched off?

sidekick · Feb 29, 2024

brianmoooore said:
Is that with whatever's at the far end switched off?

No tripping when sub main switched off as that was only briefly while testing .
Live and earth disconnected at front end (neutral still connected) and mains switch and all breakers off at back end to test. Random tripping was when all on (2 to 3 times in last 24 hrs)

pc1966 · Feb 29, 2024

sidekick said:
I should also say that this swa cable has a 63a mcb at the front end and is randomly tripping ?

If it randomly trips with the load-end of the SWA isolated then it confirm the IR results that the cable is damaged somewhere.

Most likely you see your 0.7 M ohm result after it has gone "Bang!" internally and you go to test it, then after time water creeps back in and it goes bang once more, etc.

There are techniques to help locate where along the cable a fault it, but some depend on it being a low and stable short (so you can see the relative resistance to a fault from the cable ends to guess approximately the ratio of cable to it), and others require expensive time-domain reflection sort of equipment (to look at the waves bouncing around just after it clears a fault). Has there been any civil works, digging of gardens, etc, recently? That might point to a likely location.

But it could just be poorly installed cable without sand/fine gravel around the cable resulting in a stone crushing it over time.

R-fur · Feb 29, 2024

sidekick said:
Underground and laid about 20 plus years ago.

You say it is about 100m long, walk the route if you know it and look for possible disturbances, like cultivated ground, new fence posts etc. A common method of localising is to dig down half way and cut and test the cable. Then repeat until the fault is located. But be careful to make sure that you cut the correct cable. You have to risk assess, are there other cables in the area? Can they be isolated while you cut? It just a matter of patience and thinking. I know some people on here don't like the idea of joints, but done correctly they should be as good as the cable. I come from a DNO background and have used this method many times.

pc1966 · Feb 29, 2024

R-fur said:
I know some people on here don't like the idea of joints, but done correctly they should be as good as the cable.

100% agree that if you can find the damage it is not too difficult to fit proper resin joints and they are good for decades underground.

If you can find the location!

davesparks · Feb 29, 2024

westward10 said:
That'll be Meg ohm then
I had a college teacher who said mega ohms I cringed every time he said it.

The standard prefix for million in the SI system in accordance with international standards is mega.

timhoward · Feb 29, 2024

pc1966 said:
There are techniques to help locate where along the cable a fault it, but some depend on it being a low and stable short (so you can see the relative resistance to a fault from the cable ends to guess approximately the ratio of cable to it), and others require expensive time-domain reflection sort of equipment (to look at the waves bouncing around just after it clears a fault). Has there been any civil works, digging of gardens, etc, recently? That might point to a likely location.

^^ This is the bottom line.
Have a good look for any recent activity.
If that fails, you might find an older sparks locally that still has a wind up megger that does Varley loop tests, or someone that has a Megger Bridge, or an even older person who has a Post Office box! This all requires
1) you know the route
2) there is one undamaged core
3) the fault to earth has a stable resistance (as @pc1966 said)

If that fails you are into hiring a TDR- again you need to know the route for this to help, and therefore extra cable tracing tools to determine the path might be needed.

On jobs like this that I've done it tends to end up either
-easily finding the reason (shiny new fence post) or
-identifying roughly which section of the cable is faulty without finding the reason, making cuts, establishing the remainder is fine, and running a new section.