RCBO achieving 40ms disconnect only when phase reversed

[bearlyable]

Howdi
I have a 32a ring that when I test the RCBO it won't achieve 40ms when tested at 5x in 0 degrees but when switched to 180 degrees it trips well within 40ms The Zs measured is 0.49 ohms the circuit trips at 1x okay so Im a little confused and am looking for help to point me in the right direction so as to avoid pulling everything apart so any help is most welcome. thank in advance ps I have swapped the RCBO

 

[chris gray]

im sure that the 40ms applies to the 1x and the 5x is 200ms.

 

[ian.settle1]

im sure that the 40ms applies to the 1x and the 5x is 200ms.

Wrong it is 200ms for x1 and 40ms for x5. For the new BSEN type it is 300ms

If it fails on any test 0 and 180 at x1 or x5 it has failed as far as I am concerned --- needs changing

 

[Deleted member 9648]

Make sure you have no load connected when you test,this will slow the trip time.If necessary disconnect the ring and test at the rcbo terminals,only if it fails that test condemn it.

 

[bearlyable]

Condemn what though? I have swapped the breaker and re the diconnection time by example it is disconnecting at 27ms on 1x my megger simply states >40ms on the 5x on 0 degrees however it disconnects in 17ms on x5 when phase reversed??? Thank you so far for replies

 

[stumas]

Condemn what though? I have swapped the breaker and re the diconnection time by example it is disconnecting at 27ms on 1x my megger simply states >40ms on the 5x on 0 degrees however it disconnects in 17ms on x5 when phase reversed??? Thank you so far for replies

does the crcuit need to be on a rcd if not and the zs readings are within the regs swap for a normal mcb

 

[bearlyable]

The circuit is the main ring through the downstairs of what is classed as a new build

 

[stumas]

is there other rcds in the installation working ok ? is it domestic industrial or a commercial premeses ? is the ring main a long one?

 

[bearlyable]

Its a TNS system domestic and yes it is quite a long ring but no longer than one with an identical RCBO which is working ok This is a MK sentry box filled to the brim with Niglot RCBO's

 

[stumas]

Its a TNS system domestic and yes it is quite a long ring but no longer than one with an identical RCBO which is working ok This is a MK sentry box filled to the brim with Niglot RCBO's

only thing i can sugest is swap over the rcbos and see if the readings swap over with rcbos if so its rcbo if not do r1 and r2 test and insulation resistance test to see if they show anything up and test every outlet on rcd test to see if its only at furtherst point of the ring if this is the case it could be to long hope this helps oh can you test it on another meter which tells you how many ms over the 40ms you are, so if its only 1 or 2 ms i wouldnt worry

 

[Des 56]

I suppose a lot depends on how far over the 40m sec the reading was

Our testers are hopefully calibrated to a standard that does this test fairly accurately
However,they are not always completely accurate,cable temperature,circuit loads and whatever else may affect them

The manufacturers sometimes make their devices with trip times that are very near the stated limits
If the meter is known to be accurate and the circuit tested is not loaded then a call to the manufacturer may give information on the product that may account for the high reading
They set the margins for their own particular devices and they may give reassurance for its compliance

 

[Deleted member 9648]

Condemn what though? I have swapped the breaker and re the diconnection time by example it is disconnecting at 27ms on 1x my megger simply states >40ms on the 5x on 0 degrees however it disconnects in 17ms on x5 when phase reversed??? Thank you so far for replies

If you test the RCBO with the circuit cables disconnected to ensure there are no hidden loads and it still fails the RCBO must be faulty,assuming it is correctly connected to the busbar/neutral.If it passes then those are the results you will use

 

[Deleted member 9648]

only thing i can sugest is swap over the rcbos and see if the readings swap over with rcbos if so its rcbo if not do r1 and r2 test and insulation resistance test to see if they show anything up and test every outlet on rcd test to see if its only at furtherst point of the ring if this is the case it could be to long hope this helps oh can you test it on another meter which tells you how many ms over the 40ms you are, so if its only 1 or 2 ms i wouldnt worry

I cant see how the legnth of the circuit will affect the rcd trip times...please explain.

Re the question of meter accuracy,I believe bearlyable stated that another rcbo in the same db tested ok which would suggest the meter is not the problem.

 

[stumas]

I cant see how the legnth of the circuit will affect the rcd trip times...please explain.

i was doing some testing once and tested a curcuit with a extensoin lead in it and tested it there and it failed i rmoved the lead and pluged it in the socket it passed so persumed it must be thje lenght?

 

[Deleted member 9648]

i was doing some testing once and tested a curcuit with a extensoin lead in it and tested it there and it failed i rmoved the lead and pluged it in the socket it passed so persumed it must be thje lenght?

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.

 

[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