Poor IR for SWA

[PeatBog]

I have a customer that want's a shed supply fitted. Cable was installed when patio was installed 8 years ago. Cut end of cable left unterminated and open to elements. Before I quoted I decided to test the cable. I cut off 30cm of cable before strpping.

I found poor IR results:

Core 1 & Core 2 = > 299 MOhms

Core 2 & Core 3 = > 299 MOhms

Core 1 & Core 3 = 2.7 MOhms

Core 3 & Armour = 0.5 MOhms

Core 1 & Armour = > 299 MOhms

Core 2 & Armour = > 299 MOhms

Could this just be water penetration via cut end or does it sound like more mechanical cable damage?

Should I cut another metre off the cable and try again or put a load on it to see if it "dries off"? Any suggestions.

 

[matt.leung]

how long is the cable, and how much spare do u have to terminate.....?

 

[Andy™]

most likely its water, and probably near the ends. although i have seen one case where the SWA acted like a hose.... needless to say, IR on everything was 0.01 at best

 

[kme]

WOAH !!!!!!

Basic maths here:

If L3-CPC = 0.5 meg

and L1-L3 = 2.7 meg

How could L1-CPC possibly equal 300 meg+????????

It SHOULD be 3.2 meg.

Therefore:

My opinion would be that the insulation of the individual cores was damp when you tested, and you read the dampness acrross the outside of the cores, prob. near the end. Dry the ends, and re-test - poss at 1KV if you can (you`ll likely hear the arcing)

 

[Evans Electric]

I presume its an SWA and yes , left out they can soak water up like anything, the old capillary action, it may be scrap TBH !!.

Deke

 

[Robin Spark]

I presume its an SWA and yes , left out they can soak water up like anything, the old capillary action, it may be scrap TBH !!.Deke

Dont you think he will get away with using cable and having core 3 as cpc?

Then the worst IR value will be 2.7MOhms which will be ok.

Ps also wonderring can OP confirm if both ends of the cable were left outside and did you split the conductors apart before testing?

 

[green-hornet]

Water will enter any unprotected cable and travel by means of cappilary action deep into the core.

This water ingress will have a detrimental effect on any ir test.

Personnaly I would not touch it with a barge pole never mind trying to throw 240 volts down it.

 

[Northern Electric]

I totally agree with Green Hornet here,

The cable has obviously deteriorated through being left open ended for so long. Put a price in to replace the circuit.

 

[batty]

I think it is more likely cable has been damaged when it was installed. If it was water ingress why would all cores not megger out the same. Anyway I don't think I would be happy connecting it.

Batty

 

[green-hornet]

I think it is more likely cable has been damaged when it was installed. If it was water ingress why would all cores not megger out the same. Anyway I don't think I would be happy connecting it.Batty

One possible explanation could be the manufacturing process ie one cores insulation is not as tight as the others.

Another could be that when the water entered the cable it will always follow gravity and find the lowest least resistive path.

I think we are all in aggrement that the cable should not be used and replaced for a sound substitute.

 

[Evans Electric]

Few years back I over - ordered and was left with 12 mtrs of 185mm meter tails, Stupid boy!! Left it outdoors where we keep our scrap cable ready for the tatman, out of the blue a mains job cropped up requiring 185mm tails. Take the coil to the job , by mid afternoon there was a large pool of water , having seeped out of the end. Very tempted to try draining it and use it, as I was saving a fortune, but the thought of water starting to seep into the 400 amp meter box convinced me not to.

Deke

 

[green-hornet]

All I can say to that Deke is

Applaud Smiley

O)

Guiness DrinkGuiness DrinkGuiness Drink

 

[Zeespark]

I have just done an IR test on some SWA cable thats been left unterminated to my garden shed, both ends left outdoors for the last 5 years. The results came back as >999Mohms on all 3 cores.

The problem that you face is more than likely a manufacturing fault rather than an environmental one.

AndyGuiness Drink

 

[PC Electrics]

I agree with GH.

It's my opinion that the cable is likely to be full of water along its entire length thanks to capillary action. Water also brings with it lots of chemicals which cause and accelerate corrosion, so whilst the ends might 'look' okay the swa could be rotted out in the middle. The white powder you see on the swa is zinc oxide caused by the (sacrifical) corrosion of the zinc to ZnO. Like ferrous oxide, ZnO takes up much more volume than the base Zn . So if there's an area of severe corrosion somewhere along the length the outer casing will have been stressed and maybe split (letting in more water) and the inner cable squeezed possibly splitting the insulation leading to low IR.

I'd say it's scrap.

 

[stator-electrical]

You should definitely not use this length of cable.

As previously stated by many others - any moisture around the open ends of the cable will be drawn in resulting eventually in rotting of the insulation.

There is always a temptation to go ahead in these situations as most of us know that once terminated and switched on, it would most likely appear to be ok (no visible arcing or tripping of protective devices) but the truth is that you could be putting people and property in danger!

 

[green-hornet]

I agree with GH.It's my opinion that the cable is likely to be full of water along its entire length thanks to capillary action. Water also brings with it lots of chemicals which cause and accelerate corrosion, so whilst the ends might 'look' okay the swa could be rotted out in the middle. The white powder you see on the swa is zinc oxide caused by the (sacrifical) corrosion of the zinc to ZnO. Like ferrous oxide, ZnO takes up much more volume than the base Zn . So if there's an area of severe corrosion somewhere along the length the outer casing will have been stressed and maybe split (letting in more water) and the inner cable squeezed possibly splitting the insulation leading to low IR.

I'd say it's scrap.

We now have a chemist as well as a vet:x

 

[Robin Spark]

I think you all might be missing the point on this one but until Peatbog replies then we wont know for sure.

Has Peatbog properly checked and made good both ends of the swa cable? He says he stripped the cut end but arent both ends cut in reality? If the one end has been crushed and not cut cleanly then he is going to have IR problems especially if coupled with a bit of damp.

 

[green-hornet]

I think you all might be missing the point on this one but until Peatbog replies then we wont know for sure.Has Peatbog properly checked and made good both ends of the swa cable? He says he stripped the cut end but arent both ends cut in reality? If the one end has been crushed and not cut cleanly then he is going to have IR problems especially if coupled with a bit of damp.

No peatbog has not replied yet, so there is a lot of posts to read when he does get back.

I think you are right Robin if the cable is damaged by crushing at any point coupled with water ingress, it is safe to say it is goosed.

 

[PC Electrics]

I think you all might be missing the point on this one but until Peatbog replies then we wont know for sure.Has Peatbog properly checked and made good both ends of the swa cable? He says he stripped the cut end but arent both ends cut in reality? If the one end has been crushed and not cut cleanly then he is going to have IR problems especially if coupled with a bit of damp.

Robin makes a valid point, but personally I prefer to think that colleagues on here who have posted results, have done the testing properly.

On that basis, the cable's had it.

Actually, GH, I have an O Level (remember them?) in Chemistry, but also a BSc (Hons) in Mechanical Engineering. That and 24 years as an automotive R&D Engineer exposed me to plenty of opportunity to observe corrosion at work. But to be fair, corrosion is the simple combining of a base metal and oxygen - not especially complex.

So, feel free to ask about problems with your car or van - I might be able to help!

 

[green-hornet]

I remember the "O" levels, thats the time where you really had to know your stuff!

I did O level biology and one of my studies was oxydisation on natural gasses like methane and hydrocarbons.

Long time ago 1970's but I still remember the research.

I remember a percentage burnout of hydrocarbons on natural gasses increased methane by about 4% to 16% in higher hydrocarbons.

However I have lost all knowledge of what this actually means to me today.