Fault on MI cables on outside ligting circuit

[rolyberkin]

I may be being a numpty but here goes, I had three hours today to sort this out and had to leave it disconnected in the end as had other urgent committments.

Client has an outside light which is tripping RCD, this is fed via a fused switch from a rfc this circuit also feeds an adjacent fuse switch which supplies a number of pond pumps, all connections are made in a metal junction box from which 5 x swa cables are connected, two of which feed switch and provide 3 x returns to then go onto feed 3 x seperate 3 core swa cables which in turn then are connected externally to 3 x 2 core MI cables to feed 3 x external lights.

With me so far.............?

The insulation resistance of external light feeds are low, 1.10Mohm 1.12 Mohm and the third shows a fault bertween connected live and neutral and earth as I cacall about 0.5ohm I think a downpipe has leaked near the end of the MI cable and possibly soaked the end.

I disconnected all lights with a view to return later in the week to rectify and then the pond circuit started to trip the RCD which before had been running fine, the pond circuit shares an earth with the lighting circuits via a metal junction box but all the neutrals were all disconnected, I cannot understand why the RCD is now tripping a different circuit which was previously fine uless the RCD is on it's way out? The circuit is fine until the pond pump and UV filters are energised.

Any ideas before I go back especially with regards to the MI cable, if I strip back the end and remake is this likely to help the IR readings?

 

[Pete999]

I may be being a numpty but here goes, I had three hours today to sort this out and had to leave it disconnected in the end as had other urgent committments.

Client has an outside light which is tripping RCD, this is fed via a fused switch from a rfc this circuit also feeds an adjacent fuse switch which supplies a number of pond pumps, all connections are made in a metal junction box from which 5 x swa cables are connected, two of which feed switch and provide 3 x returns to then go onto feed 3 x seperate 3 core swa cables which in turn then are connected externally to 3 x 2 core MI cables to feed 3 x external lights.

With me so far.............?

The insulation resistance of external light feeds are low, 1.10Mohm 1.12 Mohm and the third shows a fault bertween connected live and neutral and earth as I cacall about 0.5ohm I think a downpipe has leaked near the end of the MI cable and possibly soaked the end.

I disconnected all lights with a view to return later in the week to rectify and then the pond circuit started to trip the RCD which before had been running fine, the pond circuit shares an earth with the lighting circuits via a metal junction box but all the neutrals were all disconnected, I cannot understand why the RCD is now tripping a different circuit which was previously fine uless the RCD is on it's way out? The circuit is fine until the pond pump and UV filters are energised.

Any ideas before I go back especially with regards to the MI cable, if I strip back the end and remake is this likely to help the IR readings?

If it was my job I would isolate supply, disconnect all the cables and test them IO wise individually. Before you go stripping down the MI end , make sure you have the correct replacement gland, pot etc,if it's an old install the MI will probably be Imperial so tracing a new gland, pot etc may prove difficult.

 

[rolyberkin]

If it was my job I would isolate supply, disconnect all the cables and test them IO wise individually. Before you go stripping down the MI end , make sure you have the correct replacement gland, pot etc,if it's an old install the MI will probably be Imperial so tracing a new gland, pot etc may prove difficult.

I agree, have all the tools but last made off an MI cable in 1986 as I recall! I wondered whether to just run a new tuff cable next to it, quite an old building so they are keen to retain the MI for aesthetics.

 

[Bobster]

If the pot has been made correctly on the MI it should be water tight.

However if you cut the end to re make (only after making sure it is the cable at fault.

You can usually dry the ends out by running a blow torch over the cable in the direction that's been cut.