Strange Fault on a Farm!

[GJH]

Hi Guys

I am new to the Forum so here goes......

We have been called to a Farm which is 40+ years old, it is a TT 3 phase supply and has a 100mA main RCD switch at the incoming supply.

There is a main DB in the barn and out of that is an SWA sub main which feeds a stable with only one lighting circuit on it. And one 400w SON and weatherproof switch.

We were called out as everytime the light is operated the RCD will trip.After some investigation and testing one of the cables had been chewed which feeds the light as the previous spark had wired it in T&E!

We have replaced the wiring to the switch and light and the light itself is in good working order.

The problem we have is that everytime we operate the light, the incoming SWA gland of the sub main cable will spark and go bang and trip the RCD.
No fuses have blown and the Lines to Neutral and Earth are all clear in the IR test.
I have got 230v from each Line to Neutral and Earth so all seems good.
Not sure what else it can be? - there is no load on the board only this SON fitting and there are no Eddy Current issues.
Does anyone have an idea what it could be?
I can distantly remember a similiar fault when i was an apprentice and im sure it was a loose/partially damaged neutral along the cable?

Thanks in advance for any help on this

 

[littlespark]

Bust swa cable somewhere. Is it buried anywhere?
Sounds like the neutral and the armour earth are just about touching.... no fault showing up, but when a big enough load is put on, the return path is trying to use the armour.

40 year old electrics on a farm is unusual. Normally 60 - 70 round these parts.

 

[chris gray]

IR neutral to earth reading?

 

[davesparks]

It could be that there is a fault to earth and the SWA gland is poorly fitted and/or has a poor connection to earth so fault current flowing through it is arcing.

 

[ruston]

Is the gland really secure , no loose rusty strands allowed to float and short similar to @littlespark has suggested

 

[Deleted member 26818]

Two faults.
Chewed cable and deteriorated SWA cable.

 

[buzzlightyear]

The problem we have is that everytime we operate the light, the incoming SWA gland of the sub main cable will spark and go bang and trip the RCD.
No fuses have blown and the Lines to Neutral and Earth are all clear in the IR test.

I me thinks damages to swa .

 

[ipf]

It could be that there is a fault to earth and the SWA gland is poorly fitted and/or has a poor connection to earth so fault current flowing through it is arcing.

deleted.

 

[PEG]

Hi,how is the earthing arrangements of the SWA,incorporated in to the earthing system,at the incomer?