SWA and damp.

[La Poste]

Greetings.

My neighbour has a barn that needs doing but also this.
He has just had a major extension done, a lot of work and a bit of it sub standard.
He didn't realise at the time that the guy living next door to him IE me could have probably done it better. :stuart:
Anyway he has had a lot of garden lights fitted.
A 1.5mm SWA cable runs from the house down to the bottom of his garden.
It terminates in a splash proof adaptable box and then feeds various lighting circuits.

There is a stream at the bottom of his garden and the sparky who fitted all the lights down there probably did
not realise the area was prone to flooding, I mean it gets completely submerged.
This outside circuit is wired to the fuse board inside the house and is attached to an RCD that also supplies a lot of other circuits.
Of course whenever he tries to switch his outside lights on half of his house goes dark.

I was having a look today and testing, I isolated the SWA that feeds the lights and started to measure it.
The IR results I got for L-E and N-E were about 9 Mohms but when I measured the L-N I got 0 MOhms.
I then transferred to Ohms and got a reading of 10.7 K ohms which then crept up to 20 K ohms.
You can see by my pictures.

The cable looked dry at both ends, maybe just a bit damp but I wondering why it would be that my readings are so low between L-N.
Is this just damp or has the cable been damaged in some way by all the gardening work that has gone on on top
of it.

I also went to see another guy who wanted outside lights fitted, a different customer.
He told me that his existing supply, a 2.5 SWA had a buried joint in the ground, a torpedo joint.
I measured that cable and found the IR between L-N started at 0.30 Mohms and then gradually crept up to 2.5 Mohmns.
I am not sure whether to condemn this cable and lay a new one.

So my question is really what is acceptable for SWA and how much do they vary due to damp.
Once we get a bit of juice running through them will this IR figure increase dramatically?
It's one of those areas that are grey, not black or white and maybe with a bit more experience
I could make an educated guess but right now I am not sure.

Any comments welcome.
Thanks.

 

[Inteificio]

That was my guess, woot!

Thanks Lenny.

 

[leckyant]

ringer
I think a sheath test is to test wether the SWA is at ground zero ohms. If it is,the outer plastic is punchered

 

[oldtimer]

I still think there is a joint kicking about here so what ever the poster finds mind tell us

 

[leckyant]

So consequently if you do a 'in res' or a low reading ohms test on the SWA to earth and its a low reading. The whole cable is liable to be ingressed with moisture

 

[leckyant]

Which if it is............
Thee inner cores are liable to be at a low insulation resistance

 

[Engineer54]

Courtesy of NEXANS cable.


SHEATH INTEGRITY TEST


A sheath integrity test (e.g. 1000 V minimum insulation resistance tester) applied between the
outer-most metallic layer and earth can identify after-installation damage to the non-metallic outer
sheath.
The measured value should be read after application of the voltage for 1 minute. Ideally the
measured value should be corrected for temperature to a standard value at 20°C if correction
factors are available. A rough guide is that the insulation resistance decreases to one half of the
value for a 10°C rise in temperature. The cable temperature should be recorded along with the
measured values.
Measured values of Insulation Resistance for the sheath should be greater than calculated
values. Calculated values for new cable range from 1.5 MO/km to 4.0 MO/km @ 20°C for PVC
sheaths and from 120 MO/km to 300 MO/km @ 20°C for PE sheaths. Values are highest for
small cables & thick sheaths and lowest for large cables & thin sheaths. (Factory tests show that
measured values are up to an order of magnitude greater than the calculated values.)
Earth the screens after an Insulation Resistance Test on a sheath for at least 5 minutes before
handling or performing other tests.

Lenny,

That's the official version of a sheath integraty test, but that's more to do with long, to very long runs. Anything around a 100 metres or so, far better to use a low ohm meter and basically take any reading shown, will indicate a sheath failure. You can always substantiate the failure by using a 1000V megger, but i rarely find it necessary to go to the lengths you have described. In fact when i do use a megger i generally use the lower voltage settings.


I tend to only use the official method on newly laid MV SWA multi core cables, but then use a 5KV megger. Even then, i tend to use a low ohm test before hand, on shorter runs just to give me an indication that there could be a problem.

Using a 1000V megger on LV SWA cables can give false readings, especially when the sheath isn't an overall thickness and has, lets say thin spots. A small leakage will then be recorded even though there is no actual break in the sheath.

In conclusion, if the guy's here want to use megger /IR to conduct a sheath integrity test on relatively short SWA runs, as will generally be found in Domestic situations, then better to use 100V setting if they have, or stick to a good low ohm meter...

 

[leckyant]

So having said that,in a perfect world,just **** the old cable off.n puter new in.

 

[leckyant]

If anyone once wants to to say I'm wrong Please do!! I will always suck mi grannys eggs
.

 

[oldtimer]

If anyone once wants to to say I'm wrong Please do!! I will always suck mi grannys eggs
.

No this is good advice my think is yep ok there is a hole in the sheath but as for moisture getting in to the 2 pvc cables within does not ad up ok then what well a damaged cable I get that or someone does this on the cheap and got some scrap cable off a job and chucked 2 bits in with a JB
Also has the poster measured the insulation and resistance between the cores and earth at both ends I ask because when looking for a cable to earth fault in a building and the cable is run in metal trunking the end with the lower resistance is the end I work back from ie the closer the fault to one end the lower the resistance

 

[Engineer54]

So having said that,in a perfect world,just **** the old cable off.n puter new in.

Either that, or don't use/rely on the SWA as your CPC because the SWA ain't gonna be lasting to long. I've known a good foot or more of steel wire armour completely corroded away in less than 18 months. Depends very much in the chemical make up of the soil in which it's direct buried!!! When all said and done, there are very good reasons why there is a cable trench backfill sequence, and why you should be using soft sand for cable lay and and top fill before back-filling top soil. I know full well, that most domestic direct buried cables don't see any sand at all. How many times have you and others here, cleared the trench floor of stones etc, or sifted your backfill material of debris and stones ?? Not many i think!!!

 

[La Poste]

Thanks for the replies.

I will be doing a bit more testing in due course.

Basically the cable got laid, then it got compacted down by the new garden that was put on top of it and then it got completely flooded, it's sure had a bit of abuse.

 

[trev]

Please tell me you're not going to ignore that terrible IR result you thought was not so important

 

[Guest112]

so basically how do you perform the sheath test? mentioned with normal megger like an insulation test between the armourings and literally other probe on the sheath of the cable to test the sheaths insulation/integrity? Please explain ive pulled in and made off a lot of armoured cables for my company and tbh never seen/heard of this done before so im assuming its to check older cables or like poster has mentioned when the grounds been messed around with after laying one in. THANKS

 

[La Poste]

Please tell me you're not going to ignore that terrible IR result you thought was not so important

Don't worry I've had my daft moment, it's been and gone.

The only clue I can add at this stage is the fact the resistance of the cable between L-N was lower at one end than the other.
With my Megger meter to measure resistance on the low scale the meter clicks inside like it is making some sort of physical connection with in itself.
When I measured from one end of the cable the meter kept clicking like it was finding it hard to estsblish a connection between line and neutral to measure.
When I put the meter onto the Mohm range the reading again became zero.

When I measured at the other end of the cable the low resistance reading is the value shown in the picture attached and again on the Mohm range the reading became zero.

I want to get my head around this sheath test also because I am going to try to do it.
The cable is in the garden next door so it's easy for me to pop round there and check although I am also a bit unsure of how to conduct this test.

I will also have a look around to see if there is any evidence of underground joints, you know bits of cable that may have been attached to the SWA whilst on its journey to the bottom of the garden.

 

[Engineer54]

NO it's not to do with checking old cables, though it can be used to do so. Sheath testing is normally primarily made to New cables before final gland termination to enclosures.

Testing is performed between the SWA, to either a spike or more likely, proven earthed metalwork etc. Obviously the SWA must not be in contact with any earthed metalwork during the the testing period.

You may well have pulled in many cables in the past without undertaking sheath testing, So you'll never know if any of those cables SWA's are now corroding away or not. Any large project with project supervision, that requires work signing off before payments are released will most certainly be conducting such tests on direct buried cables.

One thing is for sure, roughly handled cables, bad pulling techniques, lack of attention to back filling procedures, and the chances are you will get a failed sheath test guaranteed!! Which is going to bad news where installations are relying on the SWA as it's primary source of earthing/CPC.