any know what error 79 is on a megger 1552, set up i was testing is proteus 8way main cu with just mcb's no rcd, just fitted a bg 40A 30mA 2 way rcd unit with 32A and 6A mcb's as a submain which is fed from the proteus board off a spare 40A mcb fed in 6mm clipped direct which is about 10inch in length positioned above the first c/u, every time i came to do a zs at the new bg board kept getting trp at first so did it on the no trip with the 3 lead method and just kept getting err 79, eventually after keep trying it give me a reading which is what i expected it to be, but tried it a few times again after getting a reading and same again hit and miss sometimes err79 and then some times ok, tested zs at new ring main and lighting circuit off the new bg board and got readings everytime just kept getting the error at the incomer on the bg board cheers