Unexplained MCB trips on outbuilding feed

2 years ago I put electrics into our garage. It's connected as
follows:
Main House TN-S 100A supply split with Henley Block ->
Small CU with DP switch + 20A Type B MCB ->
100m of 10mm SWA ->
Garage CU with DP switch + 2 RCBOs (16A & 6A)
The first third of the SWA is buried under tarmac & grass. The last
two thirds are above round, running through rough scrub land
(brambles, small trees, rhododendron etc). The intention is to bury
this in the next couple of years when I get around to clearing the
land. The earth at the head end is connected to the SWA armour. This
is not connected to anything at the garage end; I've fitted an earth
rod at that end.
Recently (the last 4 months or so), the 20A MCB at the house end has
been tripping. It doesn't trip immediately; it may be a couple of
hours or a couple of days after reset that it trips. I've done various
tests, but the most poignant ones are:
- With SWA disconnected at the head end, it doesn't seem to trip
- With SWA disconnected at garage end it does trip
- With SWA disconnected at both ends, using a simple DMM on highest
resistance mode, I measure open circuit between all conductors
So, it looks like the problem is an intermittent short in the SWA
somewhere, I guess caused by moisture getting into a damaged area.
I've examined the portion of cable that's still above ground which
seems fine. The ground near the buried section has not been disturbed
since installation. My question is where do I go from here? Any other
tests I can do?
Reply to
Bodgit
What do you mean by SWA disconnected at garage end? Are you just disconnecting the armoured?
A megger test would be the next step, however I am tempeted to agree with your diagnosis.
Cheers
Reply to
ARWadsworth
No; disconnecting the two cores - the armour is not connected at the garage end.
Mmmm. Is there any way I might be able to locate the problem in the cable without digging the whole lot up?
Reply to
Bodgit
Interesting one this... ;-)
An insulation resistance test on the SWA would be my next test - between each core and also each core to armour.
With it all wired up, and no load switched on in the outbuilding, it would also be interesting to stick a clamp meter round one of the tails into the small CU at the head end to see if there is any current being drawn.
(I take it the outbuilding CU is plastic?)
Reply to
John Rumm
without digging the whole lot up?
No, not unless you cut the cable in half. Given where its buried that might be a good option, then at least there's a 50/50 chance you wont need to dig up the tarmac etc.
Another possibility that has sometimes been done in the past is simply to swap the L&N conductors over, so the poor insulation one then has near zero voltage between it and the earthed armour. I'm not clear whether that would be part p compliant if you taped the conductors to indicate their new roles, probably not. Quite a good chance of it working though.
Both of those leave you with a cable that's failed on one conductor, so is liable to on the other too. I'm assuming both ends are clean & dry.
If those 2 options fail, its either a new cable or, less likely, convert the feed to something the cable can work with. If you put in a new cable, don't forget to include some cat 5 for possible future pc, alarm, etc.
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Reply to
Tabby
You appear to have a TT system in the garage then using RCBOs as earth fault protection? You need to use a proper insulation resistance tester (A Megger in usual parlance) but don't discount something in the garage putting an intermittent high current load (stalled freezer motor?) on the circuit since you don't say much in your question about what's plugged into the garage system. I would be surprised if a cable fault sufficent to trip a 20A MCB would not rapidly establish itself as permanent rather than transient.
Reply to
cynic
Not with any test equipent I have ever owned or used. Although I might be tempeted to try a 24V dc supply (or higher) and measure any current drawn
I would be double checking the cable at the glands and begging a test meter off someone before dividing the cable into two.
Reply to
ARWadsworth
He said that it still trips with SWA disconnected at the garage end, so that would rule out loads in the garage...
Reply to
AlanD
I had considered that, however the OP says that when he disconnects the 2 cores at the garage end (the armoured sheath is not connected at that end) the 20A MCB still trips.
I agree that it will get worse.
Reply to
ARWadsworth
Or don't! Walk a couple of race horses up and down the run of the cable. Where they fall over is the place to start digging:-)
Reply to
ARWadsworth
If it turns out there is a hard short, then you may find a network (as in ethernet) cable tester will help. Many have a capacitive cable length measurement capability that might give you the length to short[1]. Since they won't be calibrated for SWA, the length will be wrong, but measure from both ends the ratio should be right.
[1] or it might just refuse to work into a short
Failing that you are into TDR type kit or decent storage scope and a pulse generator.
Reply to
John Rumm
I would want to know if there is current flowing into the SWA in this circumstance. If there is not it could indicate a MCB with a faulty amplifier, and its just getting tripped by noise pickup on the SWA (acting like a 100m aerial). Swapping the head end MCB would be a cheap enough "test".
Reply to
John Rumm
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Thanks for all the replies.I'll take John's suggestion and replace the MCB first. It's a Contactum CU which seems a bit cheap and nasty to me. If that doesn't fix it I'll do an insulation test and if that doesn't work I'll be chopping the cable. Knowing my luck the problem will be right under the tarmac!
Cheers, Dave.
Reply to
Bodgit

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