RCBO achieving 40ms disconnect only when phase reversed

[stumas]

The maximum allowable Zs for a 30ma rcd to achieve a 0.4s disconnection time is 1667ohms,if you had a Ze of say 0.5 ohms....in theory an R1R2 of 1666.5 ohms would still allow the rcd to operate.......That would be one hell of a long ring circuit.

so why did it fail then? explain in detail please, i do know the above maimiums ive qouted these in prevois post

 

[Deleted member 9648]

Dont know mate, unless the fault current the tester applies is affected by a long lead....but it would have to be a hell of a long circuit I would have thought for a significant resistance to have an effect. Cant say I've come across it before but maybe you have something there.

 

[pennywise]

Where are you testing the rcbo at the board?

 

[andekoch]

I suggest you try swapping the rcbo with another as the first test, which is what a gather you did from what you said and the 'new' rcbo also failed but passed when in its original location.

I have heard of cases where a rcbo can fail a trip time test if not connected to any circuit at all since some designs apparantly need the impedance of a circuit to work correctly. Not seen this myself though.
As said in another post check the insulation resistance of the circuit to make sure it is ok first and make sure all equipment is unplugged (not just switched off).

 

[bearlyable]

i'll give that a tickle this weeek cheers

 

[bearlyable]

I am testing the circuit on a socket outlets using meggers 3 pin plug
Zs was 0.49 r1+r2=0.48
Ze was 0.03
end to end line .69
n .74
cpc 1.13

megger tests all >299 M ohm

 

[pennywise]

I would test as others have said at the RCBO with no load if it fails there, then swap it out.

 

[bearlyable]

Having visited the site again I can proudly confirm... I am an idiot and would like to thank all those concerned with advise such as 'Check there are no loads attached when testing!' This discovery of moronic misshap is highlighted by the fact that I was testing on saturday which was a warm day and I needed to open the windows which are opened by 230v motors supplied from the ring by a switch spur that I myself fitted but without the info I probably would have been confused somewhat longer so thanks to all again.

 

[robsparky1975]

good to hear,noticed that you put ze at 0.03 ohms so what was your pfc,is it under 6ka?

 

[bearlyable]

No it was 6.8Ka but the supply has a 1361 type II b fuse

 

[pennywise]

The Reading for your Rn is slightly different form you R1 this indicates a slight load as both reading on a new ring would be closer in my experience. Sometimes a good way to find out and see If the circuit is clear of loads is to put the tester onto buzz and test between live and neutral, if you get a buzz it's quite a good indication you have a load obviously disconnecting it form the breaker etc .

 

[bearlyable]

Iv had to work with an impossible premadonna of a builder who kept throwing his toys out of the pram so I wasn't surprised by the difference in R1 and Rn as he kept screwing my cable coils up when he wanted to work within the area.