If you're saying the RCD in the left hand position fails (regardless of which RCD it is) - my first guess would be a N-PE fault on that side (the fault then allows a proportion of the PE test current to return to the load N conductor on the load side of the RCD - so the RCD doesn't see the imbalance). Theoretically that should have shown up on the insulation test before energisation - but in practice such faults can occur in appliances or extension leads that were unplugged or switched off (via DP switching) during the test.

      - Andy.

Thank you Andy I tested the rcd with the circuit breakers off then put the live proton the bus bar yhen earth clipped on cpc bar and neutral on its bar 

Hmm, if the circuit breakers are single pole, then the load-side neutral and CPC are still connected during that test, and it is possible that is confusing matters.

Against an electrode of 200 ohms, you could have even a 10 ohm NE fault  perhaps in say an immersion heater or something, and that might not trip the RCD in normal operation, (depends on N-E offset voltage) but would also mean that during the test, 200/210 of your test current would pop down the CPC of the circuit with the fault and come back up the neutral and home via the RCD so it would not register as out of balance.... so No Trip.

Try un-shipping the neutrals as well as the lives (at the breakers), retesting, and then, assuming that then trips OK, then reconnect neutral for each circuit covered by that RCD in turn and retest. The result may be revealing, let us know what you find.

Mike.

Hmm, if the circuit breakers are single pole, then the load-side neutral and CPC are still connected during that test, and it is possible that is confusing matters.

Against an electrode of 200 ohms, you could have even a 10 ohm NE fault  perhaps in say an immersion heater or something, and that might not trip the RCD in normal operation, (depends on N-E offset voltage) but would also mean that during the test, 200/210 of your test current would pop down the CPC of the circuit with the fault and come back up the neutral and home via the RCD so it would not register as out of balance.... so No Trip.

Try un-shipping the neutrals as well as the lives (at the breakers), retesting, and then, assuming that then trips OK, then reconnect neutral for each circuit covered by that RCD in turn and retest. The result may be revealing, let us know what you find.

Mike.