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.

 

[diditrain]

Not trying to tell you how to suck eggs, but were all of the loads disconnected before performing the insulation resistance test

 

[alli]

Just a thought, I've had a few problems with garden lights, some where down to when the SWA was terminated and the inner sheath had been cut, whoever did it accidently knicked/ringed the conductor sheaths which after several years caused a short.

Also have all the sockets been found, we had a buried socket the other day which nobody knew off or had forgotton about. Every day it rained the lights would knock out the RCBO. Could there be a T in the cable with a hidden light?

 

[oldtimer]

Not trying to tell you how to suck eggs, but were all of the loads disconnected before performing the insulation resistance test

Or if it is a big cable run is there a joint box from the house to the jubction box plus as diditrain said is there something else connected ? although the 9 Meg L-E N-E could point to a junction or termination box or light fitting with damp in it . It can be easy to miss something because you look at it how you would wire it intead of the guy who actually did it plus ask the customer questions like when you got this put in did you have an option to put another light in at a future date ? the answer may be o yea I rememeber there was a junction box fitted.............

 

[La Poste]

Food for thought thanks.

I had a look at the inner cables of the SWA for nicks but I might have missed one.

I shall ask my neighbour whether he has any more lights tomorrow.

I definitely disconnected all known loads and had both ends of the cable in free air not connected to anything.

Funny thing is when I measured from one end of the cable the house end I got the readings that you can see in the picture IE 20 K ohms but when I measured at the other end near the stream I got greater than 99 K Ohms, the meter sounded like it was having difficulty in reading the ohms range.
When I transferred back to the M ohms range the reading once again became zero.

So one end of the cable has a much lower resistance reading between L-N than the other.

Some thing very strange is going on inside that cable.

 

[Guest112]

break the circuit down test it in sections you should find the fault

 

[Engineer54]

One of the biggest problems with SWA installations is the actual laying of the cable. Far too often they will be laid on the floor of a dug trench, that probably isn't deep enough, where there are sharp stones present, then to cap it off,they are back filled with the unsifted removed soil that also contains sharp stones and other sharp debris. And finally that soil compacted around the cable. The chances of a sharp stone or stones penetrating the sheath and passing between the armour strands into the area of the conductors is NOT uncommon. It doesn't take that long at all, before the cable will start degrading due to moisture ingress.

One reason why any direct buried SWA cable should always be subjected to a sheath test before any final connections are made...

 

[La Poste]

I'm thinking that most L-N resistances for lights with bulbs in are what 80 Ohms across the bulb filament?
So why would 20K Ohms resistance between L-N be a problem, it's not.

L-E or N-E are the important readings and they are 8.0 Mohms respectively so really this cable must be OK to supply lights.

The RCD tripping problem must be further on down the line.

Thanks.

 

[Engineer54]

I'm thinking that most L-N resistances for lights with bulbs in are what 80 Ohms across the bulb filament?
So why would 20K Ohms resistance between L-N be a problem, it's not.

L-E or N-E are the important readings and they are 8.0 Mohms respectively so really this cable must be OK to supply lights.

The RCD tripping problem must be further on down the line.

Thanks.

Perhaps we should rewrite the minimum IR requirements, if we were to take any notice of that daft statement then!!!

 

[ringer]

One reason why any direct buried SWA cable should always be subjected to a sheath test before any final connections are made...

What is a sheath test, E54?

 

[Inteificio]

What is a sheath test, E54?

Glad I am not the only clueless one! Could be a useful test for me to learn.

 

[La Poste]

Perhaps we should rewrite the minimum IR requirements, if we were to take any notice of that daft statement then!!!

That's two daft statement in one day, I had a good day yesterday. :stuart:

I would also love to know what a sheath test is, what is it?

 

[trev]

Not wanting to start a bunfight here but I have to agree with E54 here. There are minimum acceptable IR results which we as electricians must adhere to. If they are not satisfied then we must find out why and to say that L/N readings are less important than L/CPC or N/CPC is in my opinion ludicrous.
If something went wrong with the installation and your test results schedule ended up in front of a judge what would you think the legal opinion would be of walking away from an IR reading like that?

 

[mpspark]

I'd also like to know how to carry out a sheath test please, E54.

 

[Guest123]

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.