RCD tripping

[Spudlov]

Hi guys,
I'm having trouble with a 63A 300mA RCD in my meter cupboard that is randomly tripping. I'm suspicious of the smartish meter that was installed not so long ago. The RCD covers my tails into my consumer unit which is around ten metres away. The tails are in plastic conduit under the floor. In my consumer unit there is a 63A 100mA RCD covering eight circuits switched with MCB's from 6 Amps to 32 Amps. The 100mA RCD doesn't trip. I've tested the tails and they are clear. It's a TT system with a couple of rods knocked down in the garden.
Any ideas anyone? I was thinking maybe I could replace the RCD in the metre cupboard with a double pole MCB but I don't know whether I would need 1P+N or a standard double pole breaker would do the trick.
I've just started taking note of when the trips are occurring. Three days ago it tripped at 10 am and it tripped today at 10.10am. I don't have any timers in the house and all breakers are live continually.
Cheers

 

[ericmark]

If the 100 mA is type S and the 300 mA is 40 mS then very likely a fault will take out the 300 mA before the 100 mA. Back in about 1992 I was working on the building of Sizewell 'B' first time I had seen multi RCD protection, and it was common for a fault to take out the larger RCD, at that time I did not understand type AC, A, F, and B but it is likely DC was stopping the lower rated RCD's from tripping before the higher ones, as the higher rated ones tended to be better quality.

Any markings like type S or

etc on the RCD's?

I would start with an insulation tester (500 volt mega) as it could actually be a fault, but the 300 mA is faster acting or the 100 mA frozen due to DC.

 

[AndyPRK]

post a picture of both RCD's.
Does the internal one have a marking S
Press the Test button on the internal one, and ensure it trips

 

[Spudlov]

Thanks for your response ericmark. The 300mA rcd is a Legrand 086 48. The rcd in the dist board is a Doelec AUR3 ( I think it must be Greek as I'm in Cyprus). Both units show 230v and an ac sinewave. I don't have a megger with me, just a crumby multimeter

 

[Spudlov]

 

[Spudlov]

 

[Spudlov]

The trip works on the internal RCD