Insulation test L to E long length of SWA

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sidekick

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• Feb 28, 2024

• #1

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 ?

 

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• Feb 28, 2024

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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?

 

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• Feb 28, 2024

• #3

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.

 

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• Feb 28, 2024

• #4

sidekick said:

Insulation test live to earth = 0.7 ohms.

Click to expand...

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?

 

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• Feb 28, 2024

• #5

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).

 

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• Feb 28, 2024

• #6

Also is the earth conductor above separate from the armour?

 

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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.

 

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• Feb 28, 2024

• #8

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

 

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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?

Click to expand...

Underground and laid about 20 plus years ago.

 

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• Feb 28, 2024

• #10

So is it 0.70 ohms or 0.70 Mohm?

 

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• Feb 28, 2024

• #11

westward10 said:

So is it 0.70 ohms or 0.70 Mohm?

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

 

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• #12

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

Click to expand...

megaohms

 

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• #13

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

 

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• #14

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).

Click to expand...

2 core swa and armour used as earth

 

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• #15

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

 

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• #16

westward10 said:

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

Click to expand...

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

 

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• #17

James said:

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

Click to expand...

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

 

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• #18

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

 

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• #19

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

 

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• Feb 29, 2024

• #20

brianmoooore said:

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

Click to expand...

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)