Hi all
Had a strange thing happen yesterday.
RCD on split DB tripping on a ring circuit fault. Tested all three conductors to each other at 0.02 meg.
Separated circuit and found redundant part of leg going though kitchen had fault. Kitchen had new ring and redundant part was put in strip connectors and plastered in wall. Disconnected kitchen part and made ring into 2 16A radials. (DB had spare 16A MCBs).
Anyway, the strange part.
Before splitting circuit I tested Zs on 3 wire low setting. Tripped. This was not expected as at 0.02 meg the leakage should only be approx 0.01mA.
The other circuits only had approx 2mA leakage current which was found out when I ramp tested the RCD at the DB with all circuits off, then with all circuits but the faulty one on. All off tripped at 23mA. All on (except faulty one) 21mA leaving 2 mA leakage on circuits. Low current Zs at 15mA plus leakage from other circuits = 17.01mA. Shouldn't have tripped, but did.
Second:
This is the really confusing one. On testing the RCD from a socket on the faulty circuit it would not trip, even at 500mA.
However, when testing at the DB with the faulty circuit's MCB off it tripped fine on x1 on both sides of the cycle.
After rectifying the fault the Zs and RCD tests done from the sockets were okay.
Strange behavior on live tests from an IR fault.
I cannot work out why:
1/ An insulation resistance fault with not enough earth leakage would cause the RCD to trip on a low current Zs test.
2/ An insulation resistance fault would prevent an RCD from tripping even when using 500mA. (Even with part of the test current flowing back though neutral through the fault 500mA should have still tripped it).
Any thoughts about this?
