300ma RCD Tripping

[toasted]

Hi,

New to the forum and newly qualified. Please go easy on me.

Went to a customers site today to look at why "the power keeps tripping out"

The board is a three phase Merlin Gerin with a 100A 300ma SRCD Main switch.

I've checked all of the circuits CPC's for current using an earth leakage clampmeter and all seem ok.

I've also I.R. tested all circuits which came up as >999

I've PAT tested all of the appliances that they are owning up to and found no faults.

The lighting is a mixture of tubes and CFL's which at first I thought were a likely candidate but as I said i'm not picking up any current on the CPC's.

Does anyone have any suggestions? I didn't have an RCD tester avaliable today but will have tomorrow.

 

[Spark123]

Were all the switches on the light circuits closed when you did the IR test (L&N connected together and tested to earth?)
Any sign of damage to any cables?
Any signs of water ingress in any fittings outdoors?

 

[toasted]

Switches were closed, yes.

I did rush the IR tests slightly today because I only had a limited amount of time in which I could keep the supply isolated because of a demanding customer.

As far as outdoor fittings go, there were some but on opening the dist board I found them disconnected and labeled as disconnected from '06

This is a portacabin by the way, apparently rats have chewed alarm and data cables in the past, but would have thought such damage would show on the I.R. test?

 

[Spark123]

Would depend on wether or not the fault was there at that actual moment i.e. it may be tripping as a rodent crosses the wires, then the rodent runs away.
Could be when someone walks on a floorboard, two wires touch.
Could be the wind.
I take it the sockets are on their own 30mA devices?

 

[RF Lighting]

There is a thread on here about a job I did a little while ago.

New circuit wired in PVC/PVC cables. RCBO intermittently tripping. Once every few days.

I.R. readings all >200Megohms.

That was rodent damage. I can only assume the rodent to earth fault was tripping the RCD. The cable itself, although well ruined, did not have a short on it.

Untitled

• RF Lighting
• 19 Jul 2008
• 5

 

[toasted]

Thanks for that both of you.

If it does turn out to be rodent damage, its going to be extremely unfunny.

Apparently the cabin was moved to its current location around 3 years ago.

There is now precisely ZERO access to underneath because it has been bricked all the way round up to about two feet and had a disabled access ramp built.

Not looking forward to work tomorrow! Already feel like i'm being chucked in at the deep end.

 

[JohnD]

the 300mA 3-ph RCD is presumably intended as a precaution against faults causing a fire. It does not give much personal shock protection.

is there any scope to provide closer protection per phase or circuit as well? If the portacabin is on a submain, could you fit an RCBO (or to the other most suspect circuits)?

BTW if the fault has only recently started occurring, I wonder if it is cold weather related (heater, or condensation pooling somewhere)?

 

[toasted]

the 300mA 3-ph RCD is presumably intended as a precaution against faults causing a fire. It does not give much personal shock protection.

is there any scope to provide closer protection per phase or circuit as well? If the portacabin is on a submain, could you fit an RCBO (or to the other most suspect circuits)?

BTW if the fault has only recently started occurring, I wonder if it is cold weather related (heater, or condensation pooling somewhere)?

Hi JohnD,

If this mornings testing is fruitless my next line will be to aquire MG RCBO's to do as you say.