Help please

[STI]

I have just read the topic about a tripping RCD and my neighbour has just called me yesterday with a similar problem can you help.

In this case she has a old type board with glass fuses in the old brown rewirable type of holders. 3036's ?. Between this and the main incoming supply is a RCD switch.

The main RCD switch keeps tripping. Having been told she thinks its the fridge freezer in the outhouse which is on a fused spur supplied from the downstairs ring i spend much time checking sockets etc. Worthwhile as a few loose earths where found. Reset the RCD Everthying ok for a couple of mins then the RCD trips.

So i start to isolate all other circuits by removing the breakers switching on waiting to see if we get a trip. Finally this confirms it is the circuit that the outhouse is supplied from. This time it tripped after a few seconds.

Anyway cutting a long story down she suddenly remembers that the sceptic tank pump is also run off this circuit. So i remove this element from the circuit switch everything back on at the main and in 15 mins no trip.

Reconnect the pump and within a seconds it trips again.

I check all the connections to the pump and appart from a little rust on a choc block connector in the chamber all seems ok. We flushed the pump. Switched it back on and after a good 10 minites nothing tripped. I thought great cured.

You guessed it, this morning i get a call, its tripped again.

The bit i cant get my head round is why are the trips occuring over different time periods and not instantaneously the pump is switched on and active (its one of those automatic ones with a float switch).

The other difference to the other post about the RCD tripping is that in this case everything else in the house is switched off, so i dont think its a gradual build up of leakage.

The neighbour is elderly and not rich so i dont want hr to go out and buy a new pump at £150+ to find the problem is not the pump.

Any suggestions what else i can do to find the problem or do you think it is just the pump.

Sorry for the long post.

 

[chippy5]

bad earth in the pump?

 

[1john]

I think it is just the pump, but you could get an insulation test done on the circuit wiring or the pump pat tested to confirm, it is hard to tell without taking readings but my money (all three quid of it) would be on the pump being at fault.

 

[STI]

Thanks both for the replies. Re the bad earth at the pump, thats a possible as it was that terminal on the choc bloc that was corroded up. Unfortunately there is so little cable going to it i didnt want to cut and refit unless essential, it looks like it is going to be.

Re the pat test yes i thought about that and i am going to do it, i was trying to avoid it as it means getting a rather messy pump out and wiring it up to my nice clean PAT tester.

A'h well what else could i be doing on a hot sunny sunday!!

 

[GaryMo]

Did you disconnect this part of the circuit at the pump or as it leaves the house, therfore still cabling connected to the pump?
If it's the latter then you need to rule out the external cabling along with any potential joints by carrying out an Insulation Resistance test.
If that's fine then it could just be the pump coming to the end of its life or a little water ingress.

Regarding the fuses, are these what you're looking at?

If so, they're BS1361 and are found in these carriers:

 

[STI]

Thanks Garymo, yes they're the carriers and fuse, i never know whether they are 1361's as they dont have a switch but now i do. cheers.

I did disconnect the cabling at the house, so i will do the IR test now with the pump disconnected, i will also run an IR test on the wiring after the supply to the pump with the PAT tester.

Thanks all, it does seem to be all ponting to the pump at this stage..

 

[GaryMo]

Thanks Garymo, yes they're the carriers and fuse, i never know whether they are 1361's as they dont have a switch but now i do. cheers.

BS 3036 are semi enclosed (rewirable) and have fuse wire.
If you see a device with a switch (MCB) it'll either be BS 3871 or now more commonly BS EN 60898. RCBO's are BS EN 61009.

 

[TicklyT]

Have you considered Neutral > Earth faults?

The RCD may be responding to a N > E fault anywhere on the installation, with the N > E current through the fault dependant on the total load on the system, not just the load on the faulty circuit / appliance.

Just pulling fuses / switching off breakers will not affect the fault, as the neutrals are still all connected, but it may mask the fault by reducing the total load on the installation, so the fault current stays below the level required to trip the RCD.

 

[STI]

Hi TicklyT,

The straight answer is not directly, but as the fault only occurs when the pump circuit forms part of the total load, ie everything on, does this not eliminate this possibility. I dont particulary want to start testing all of the circuits although i suppose a mass IR test would show this up or am i looking at the wrong thing.

However the reason for the delayed response is that i have had my head down the old tank and i wired the pump up to a plug and stuck the PAT tester on it, the IR test came back with a result of < 0.10Mohm. So looks like the pump to me.
I also ran a IR test on the wiring to the pump from the house, with the pump disconnected that came back at >299Mohm.

So i think everyones first thoughts that its the pump are correct. I will have the joy of fitting it tomorrow.

One thing i still cant get my head around though is why the delay in the trip, why is it not instantaneous well milli seconds. Is it just that the fault in the pump takes a bit of time to warm up the wires before the short occurs.

 

[bongos]

One thing i still cant get my head around though is why the delay in the trip, why is it not instantaneous well milli seconds. Is it just that the fault in the pump takes a bit of time to warm up the wires before the short occurs.

At a guess, I wonder if it is some sort of intermittent fault on the pump. At some point there is enough earth leakage to cause an instantaneous trip, at others maybe something dries out or moves in the tank and so there isn't a problem until things move around again and settle.

Either way, these sort of trips are usually a nightmare to locate. You may find that if left, over time it would start tripping more often/quicker during use.

Possibly a high resistance L>E fault that changes depending on atmospheric conditions around it/in the tank?

 

[STI]

I've just got everything crossed that changing the pump fixes the problem. Otherwise having tested/eliminated everything i can think of its panic stations