Interesting problem

[sponge23]

I was called out to look at an RCBO tripping that powers a ring final circuit in a youth cafe.
Checked the IR values at the DB - and all >299. I removed the conductors from the RCBO and checked just that circuit.
Now weirdly, when I go to one of the sockets and test either way, the value for L-E is 0.12 when the conductors are back in at the DB end. To prove a point I then removed the conductors at the DB and stuck in some wago's, i.e. L-L, N-N and E-E. When testing at the socket again, >299.

Also checked the RCBO on its own, that seemed to function just fine.

Anybody got any ideas?

 

[markc123]

You're reading through the RCBO.

Disconnect the flying earth and neutral.

 

[Octopus]

A rcd on a circuit will always give you a very low IR reading!

 

[TJ Anderson]

An RCBO on a circuit will always give you a very low IR reading L/N-E!

Corrected that for you.

 

[conrad]

Dont know why you would use an IR tester to find this fault?

 

[gnselectricals]

Dont know why you would use an IR tester to find this fault?

It's a pretty good start ? Rcd tests inc ramp as well would be good

 

[conrad]

It's a pretty good start ? Rcd tests inc ramp as well would be good

Suspect faulty appliance or something being plugged in. Generally the case. Id do Continuity tests. Difficult Without more detail i.e intermittent or permenant fault, when is it tripping etc..

 

[davesparks]

Dont know why you would use an IR tester to find this fault?

What else would you use for fault finding? An analogue megger has been the tool of choice for fault finding since it was invented.

 

[gnselectricals]

Suspect faulty appliance or something being plugged in. Generally the case. Id do Continuity tests. Difficult Without more detail i.e intermittent or permenant fault, when is it tripping etc..

Well I'd be sending the client round unplugging whilst I had a look round as well then unpack my megger and start with IR and cont, then Rcd testing first offload then onload as well.
That's after a chat with client to diagnose problems/symptoms which is usually the best course of action. I was told in college with fault finding the best way is to grill the client regarding the fault and what's been going on in the property drilling walls, new appliances etc otherwise you could be going round in circles. Found a faulty kettle once on the phone form 200miles away in 5minutes lol

 

[sponge23]

Nothing plugged in, that was my suggestion on the phone when the client first rang me. Oh and yes neutral fly-lead + earth lead removed whilst testing.
With either side of the ring connected to the RCBO (in turn) will operate fine. When both all are connected, RCBO trips.
I had all the sockets off and removed the conductors from terminals and IR tested all conductors individually, all >299.

 

[marconi]

I was called out to look at an RCBO tripping that powers a ring final circuit in a youth cafe.
Checked the IR values at the DB - and all >299. I removed the conductors from the RCBO and checked just that circuit.
Now weirdly, when I go to one of the sockets and test either way, the value for L-E is 0.12 when the conductors are back in at the DB end. To prove a point I then removed the conductors at the DB and stuck in some wago's, i.e. L-L, N-N and E-E. When testing at the socket again, >299.

Also checked the RCBO on its own, that seemed to function just fine.

Anybody got any ideas?

Could you tell us:

Is the RCBO tripping upon being closed or randomly some time afterwards? Has the fault just started happening after a prolonged period of satisfactory state?

What types of equipment are connected to this RFC?

Could you clarify this figure by including the units please? eg: 0.12MOhms, or 0.12 Ohms or what?

What is the type of supply: TT, TNCS or TNS?

What is the make of RCBO. Do you know if it a basic type or electronic type?

 

[Hellmooth]

Well I'd be sending the client round unplugging whilst I had a look round as well then unpack my megger and start with IR and cont, then Rcd testing first offload then onload as well.
That's after a chat with client to diagnose problems/symptoms which is usually the best course of action. I was told in college with fault finding the best way is to grill the client regarding the fault and what's been going on in the property drilling walls, new appliances etc otherwise you could be going round in circles. Found a faulty kettle once on the phone form 200miles away in 5minutes lol

Inspector Gadget?

 

[sponge23]

RCBO trips immediately after connecting both legs of the ring. Figures are 0.12MOhm. On a TNCS 3P Supply.
Oh and MEMshield 2 C Type RCBO.

 

[Octopus]

What are the end to end results for the ring?

 

[marconi]

RCBO trips immediately after connecting both legs of the ring. Figures are 0.12MOhm. On a TNCS 3P Supply.
Oh and MEMshield 2 C Type RCBO.

Is the RCBO single pole(SP) OR single pole with switched neutral(SPSN)? To find out out which, isolate the RCBO and measure continuity between Nin and Nout when RCBO open and then closed. (There is a circuit capacitance effect which can be troublesome to residual current detectors when a switched neutral conductor is first energised (at a similar(but not absolutely identical) moment to the switched line conductor becoming energised).

Have you done standard FRC checks ? What where the result ? Of particular interest are the figure of 8 readings at each socket taken in step from the first socket on the 'outward leg' and ending on the first on the 'home leg'. As well as the L-N and L-E figure of 8 also do an N-E figure of 8 set of tests. (This is to explore whether the ring is symmetrical to currents flowing clockwise and anti-clockwise or if there is in fact a direction around the ring with a lower conductance path for the line currents which is opposite to the direction of lower conductance path for the neutral currents). Or in other words, the currents at the beginning of a leg should equal eg: 10A/10A and 20A/20A not 10A/15A and 20A/15A.