earth leakage unit failure

[murdock]

been having some challenging issues with earth leakage units lately.

i have 2 problems at present which are still not sorted out.

maybe i am missing something...

1 st one...

get a call out because an earth leakage unit has tripped...

step 1...isolate the entire installation and remove all the sockets and switch off the isolators...
step 2...disconnect all the neutrals
step 3...do an insulation resitance test on all the circuits...found 2 circuits with low readings...identified the circuits and checked the installation...found an unused cable which was grounded...disconnected and isolated from the installation...2nd circuit was a cable in the garden which had been hit by a spade and the red wire was exposed but only grounding when it got wet from rain...retest...everything clear...reconnected and switch on...everything working.
2 days later called back earth leakage tripped again...disconnected and removed the earth leakage and swopped with the outbuilding...house has not tripped since...however the out building has started tripping now.

so i asume it is the earth leakage unit is faulty and replace the unit with a brand new one...2 days later i am called back...earth leakage tripping again ...so i do the same thing to the outbuilding...disconnect and do an insulation resistance test...find a plug in the garden full of ants and moisture...disconnect and isolate...retested...all clear except the light circuit...a low reading of just below 1 meg...normal on fluorescent fittings with electronic ballasts...so i leave...2 days later earth leakage unit tripping again...and so it has been for the past week every 2-4 days it randomly trips...reset and works for another couple days.

the 2nd...

do all the test like above...find no low reading...plug everything back in...4 days later it trips again...go to site and reset doesnt trip for a couple days again...

so i do some other tests
mA readings on the earth wires
circuit
1. 0.4 mA
2. 0.0 mA
3. 2.6 mA
4. 7.8 mA
6. 0.2 mA
7. 24.3 mA
main incoming earth 123 mA (this i dont understand)

the earth leakage unit is warm...but the load is only 37 amps...unit is rated at 60 amps

the installation has been working for at least 5 years without a problem the way it is wired...with everything on earth leakage.

the only thing that has changed...is earth leakage tripped a couple of weeks ago for the first time and ever since then this random tripping occurs.

after dicussing this issue with another person...something which i need to look into are the surge arrestors on all the plugs for the computors...that they have not been damaged...which i would asume should be causing the earth leakage to trip all the time not just randomly.

there are a few thing i have thought about...
replacing the earth leakage...which i am not convinced is the problem.
seperate all the circuits and remove the lights and a/c units from the earth leakage which has been working for more than 5 years without an issue...why now suddenly is there a problem.
take all the computers...servers...battery charges...ups etc off the earth leakage and fit red dedicated sockets...but again if it worked for 5 years the way it is why suddenly now would it need to be changed.

i do realise because the current adds up to a point...they could eventually get higher than the rated 25-30 mA and will start tripping...

any ideas?