Flooded Ducting and Low IR so

Energetic · Feb 18, 2016

Morning all,

Pulled up a couple of string cables from a section of ducting that has been flooded this week. IR tests are low and indicate a problem. Inverters RCMU's are not liking it.

No visible damage or problems with the cables so just wondering if its possible that they can absorb water over time which would lead to this failure and also make re-running the strings in the same way pointless. The water table there is high(as it probably is in most places atm) but i would guess ducting is likely to fill up every winter and may not even fully dry out in summer after this winter.

BruceB · Feb 18, 2016

Re: Flloded Ducting and Low RIso

If you look at the standards and manufacturers specs you will find that T&E and SWA are NOT designed to be underwater permanently because they do absorb water. You need a more specialised cable. Google pond cable for starters, although if we are talking string cables here you might need to watch insulation levels with 'domestic' pond cable.

PS:
Try HO7RN-F

HandySparks · Feb 18, 2016

Re: Flloded Ducting and Low RIso

What do you do (if anything) to stop ducting containing SWA cable filling up with water for long periods?

Poke a few drain holes in the lowest part of the duct? Cap any upward-facing exposed duct end? If so, what with? I've seen expanding PU foam used, but it's a pain to use in small quantities and, being thermally insulating, may compromise the current carrying capacity of the cable.

davesparks · Feb 18, 2016

Re: Flloded Ducting and Low RIso

HandySparks said:
What do you do (if anything) to stop ducting containing SWA cable filling up with water for long periods?

Poke a few drain holes in the lowest part of the duct? Cap any upward-facing exposed duct end? If so, what with? I've seen expanding PU foam used, but it's a pain to use in small quantities and, being thermally insulating, may compromise the current carrying capacity of the cable.

You use proprietary duct seals like the inflatable ones or cut to size ones or silicone or whatever other product you can find to do the job.
All duct terminations should be sealed to prevent water, rodents and General rubbish getting in there.

SolarCity · Feb 19, 2016

I had no idea that SWA would absorb water. So I imagine this can be quite an issue when burying cables with a permanently high water table?

davesparks · Feb 19, 2016

I'm pretty sure SWA doesn't absorb water! But it does fill up with water if it has the slightest bit of damage to the sheath,

BruceB · Feb 19, 2016

SWA is designed for direct burial, but not permanent immersion. In most situations a bit of earth leakage is not critical, but in the OP's case it was.

You may be interested in the following extracted from Appendix Q of BS 5467:

Q.3.5
Cables specified in this British Standard are not specifically designed for use:
a) as self-supporting aerial cables;
b) as submarine cable or for laying in water-logged conditions;
c) where subsidence is likely, unless special precautions are taken to minimize damage;
d) where any exposure to excessive heat is involved;
e) where the oversheath is subjected to a voltage test after installation.

Note point B.

The problem is generally nicks etc in the sheath which allows the armouring to become saturated which eventually rusts and can force metallic particles through the bedding and insulation to the conductors as it expands.

That is the PVC SWA standard and I had that extract to hand. Not checked the other standards, XPLE SWA and T&E, but I would expect similar.

davesparks · Feb 19, 2016

BruceB said:
SWA is designed for direct burial, but not permanent immersion. In most situations a bit of earth leakage is not critical, but in the OP's case it was.

You may be interested in the following extracted from Appendix Q of BS 5467:

Q.3.5
Cables specified in this British Standard are not specifically designed for use:
a) as self-supporting aerial cables;
b) as submarine cable or for laying in water-logged conditions;
c) where subsidence is likely, unless special precautions are taken to minimize damage;
d) where any exposure to excessive heat is involved;
e) where the oversheath is subjected to a voltage test after installation.

Note point B.

The problem is generally nicks etc in the sheath which allows the armouring to become saturated which eventually rusts and can force metallic particles through the bedding and insulation to the conductors as it expands.

That is the PVC SWA standard and I had that extract to hand. Not checked the other standards, XPLE SWA and T&E, but I would expect similar.

The most interesting point about that is that it appears to be saying that the cable is not suitable for having a sheath test carried out. I wonder how they expect the integrity of the sheath to be confirms without it?

LeeH · Feb 19, 2016

I would be very surprised if XPLE SWA would have issues submersed in water if the sheath was intact.

BruceB · Feb 19, 2016

I have checked now: the standard I quoted, BS 5467, is for xple insulation, pvc bedding and pvc sheath.

davesparks · Feb 19, 2016

LeeH said:
I would be very surprised if XPLE SWA would have issues submersed in water if the sheath was intact.

Why? The XLPE is only the insulation, the sheath is still the same PVC material?

Energetic · Feb 22, 2016

Thanks all for the input. Very interesting and i believe the reason why the string cables have failed. Time will tell on this one as i suspect they will fail again if this is the case.

On the SWA, i would never have thought it would be in any danger until this thread started. Took this on site today, run by the customers local spark, was a bit of an eyeopener. Also this is not water, its the lovely juice that runs out of a dairy...

LeeH · Feb 25, 2016

davesparks said:
Why? The XLPE is only the insulation, the sheath is still the same PVC material?

Always thought it was the outside too Dave.

Spoon · Feb 25, 2016

It does specify that the XLPE is the Insulation and the Inner & Outer Sheaths are PVC in the specs I've seen.

LeeH · Feb 25, 2016

Either way, I'm not in the camp that thinks SWA is sponge like.

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