RCD constantly Tripping on long cable run

[Herisson]

I'd be very grateful if someone can help with this.....

I've added a diagram to help explain. This is single cabling not a ring (I'm in France). All the cabling is underground which is split into 3 at a junction box by MCB's. There are 5 spurs above ground on the cable in question and each has it's own RCBO.

I have an RCD (Type A) on the distribution board (RCD 1) which intermitently trips. Sometimes it takes an hour sometimes 24 hours, rarely shorter or longer. I was actually standing at the board by chance on one occasion when it tripped and it refused to go back on again despite several attempts, after a few miutes it went back on ok and stayed on for a few hours.

Using the MCB's I isolated the problem to the cable in the diagram. Having switched off all the RCBO's at each spur the tripping continued.

To try and isolate the problem further I disconnected the cable at each spur working backwards from spur 5 until the tripping stopped. This occurred after diconnecting the cable running to spurs 4 & 5 at spur 3.

To allow the continued use of the system up to and including spur 3 (and the other 2 MCB circuits) I installed an RCD (let's call this RCD 2) on the cable from spur 3. With this switched off there were no trips. Once switched on RCD 1 started tripping again.

Other than one RCD being more sensitive than another the reason for this is beyong my knowledge, so I interchanged RCD 1 and RCD 2. The trips were then on RCD 2 at the distribution board.

The installation has been untouched for 2 years and the connections at the spurs are secure. No power is being consumed during all this and no appliances are connected.

So my questions are:

1. Does this mean there is a problem with the underground cabling between spurs 3 and 5 and if so, how can it take so long to trip?
   
   
2. When I interchange RCD's 1 and 2 and the problem is beyond spur 3 why does only the RCD on the distribution board trip and not the one nearer the problem?

Many thanks for any help.

 

[Richard Burns]

I would suggest that insulation resistance testing of the cables may provide the answer.
It does sound as if the fault is on the cable leading to spurs 4 and 5 from spur 3.
However there may be a significant leakage on the earlier cable but not at a level that causes the RCD to trip, adding the leakage from the last section of cable may then be enough to cause the origin RCD to trip.
The delay in tripping seems to indicate that the fault is just at the limit of where the RCD will trip, this is indicative of a progressive fault that has got worse and worse over time until the RCD trips and investiagtion starts.

 

[mache]

Have you done an IR on the cables / circuit in question, is this a damp property ??
My first guess would be a moisture problem developing allowing some tracking at the junctions. Your RCD downstream of spur 3 is only looking at two spur junctions. RCD 1 upfront is looking at the sum of all 5, except when you isolate RCD2.

 

[Herisson]

Many thanks.
Sounds like I need an Insulation Tester. Can someone suggest a good quality tester at a reasonable price?

 

[Wilko]

Hi - at this stage you need to ask what will you do if you find a fault? Or even what does an insulation fault look like? We can always provide incremental advice, but it perhaps would be best if you called in your local Spark to test the RCDs and the cable, hopefully find the exact fault for just a call out fee? (I would )

 

[Octopus]

IR tester ... Speak too sponsor test meter...

You will also need a device to test the rcbo's and possibly and earth leakage tester too

Plus the knowledge to use them and interpret the results....

 

[Pat H]

Where in France are you? I have a tester, Knowledge and could do with a few days away

 

[dmxtothemax]

1. Does this mean there is a problem with the underground cabling between spurs 3 and 5 and if so, how can it take so long to trip?
   

When I interchange RCD's 1 and 2 and the problem is beyond spur 3 why does only the RCD on the distribution board trip and not the one nearer the problem

1. Answer - 1, Yes. The leakage is not constant, so the trip point is not always reached
2. Answer - 2, Variances between the two units, no two are the same.