the other thing im not understanding is i set my metrel to 30mA type A rcd and it goes through the test's it comes back o.oo on all 4 test's but if i change it to type Ac i get trip times . this is the for the right hand rcd 

Thank you for all the advice from you all 

Does the RCD physically trip?

Yes ,I also changed the rcd to one i had in the van that did the same ,im going back tomorrow and go over the installation again .

aligarjon said:

I'll add that i always presumed that it possibly being unstable meant if the ground were to get very dry as it is round here at the moment. Am i wrong about that?

Freezing etc also. It can also be an indicator that the electrode has corroded.

Paul Berryman said:

Yes ,I also changed the rcd to one i had in the van that did the same ,im going back tomorrow and go over the installation again .

Again, is this test method 1, or 2, or both?

split board ...  Its a tt system earth

Going off on a tangent - metal split CUs are not often used for TT these days - the problem being the single insulated wiring upstream of the RCDs. How are you providing shock protection for faults before the two RCDs? Is there another upstream RCD (S-type) or is the wiring double/reinforced insulated?

   - Andy.

Nothing but metal boards used in new dwellings here even when tt. No upstream RCD. Usually a chunky plastic trs gland or special kit used with the tails of double/reinforced insulation.

Agree with your NE fault. Had almost exactly the same issue recently.

Hello went back to job today and checked insulation res test had a N/E fault on the lighting cct . Then tested rcd at the board and it tripped with the cct disconnected. stripped down the cct and found nothing wrong. the odd thing was ,i took out all the neutral's and had a dead short between neutral and earth i then switch of the main isolator and the dead short went its on the incoming main when i spoke with the supplier i was told it could be that the joint in the road is n/e combined and the strip a earth out of the joint and bury it in the ground ,he thinks i was picking it up via the earth rod .

strangely when i put the lighting cct back i rechecked the rcd tripping  time the left rcd now trip's could it bee then led lights that is causing it not to trip its a a A type rcd   

Afraid that is making no sense at all. If the supply is "n/e combined" it is TN-C-S (PME). So my first question is what is the earthing arrangement?

Is there PME next door ? Are the services metal . Could be connected through the bonding. The rcd's need testing before anything else is connected to them to eliminate faulty rcds then go from there.

Gary