BG RCBO ramp test on one of them shows trip at 15ma every other one is 24 ma

[The Ghost]

So just did an EICR few problems mostly rectified. Customer wanted new CU so put in BG with RCBO just five circuits. Tests on EICR were fine except for high ohms on Zs for lighting first floor so tracking that down. All RCBO are fine except this one 32a ring final circuit. RFC tests all fine IR around 400 + Mohms. 10ma leakage with clamp on tails. So it was the end of a long day. Said I will replace the 32 rfc RCBO.
What really made me wonder is when the downstairs ring is used i.e. kettle on, the upstairs ring trips. I just can't right now formulate an approach to it. And I do seem to remember BG RCBO being quite often defective, is that true in your experience. My first thoughts were that there is some cross connection between the RFC, not that I picked that up on testing. Now I can assure you that I have not mixed up the RFC neutrals I triple wipple checked on that.

 

[telectrix]

try kettle in upstairs RFC see if it trips the downstairs RCBO. i'd suspect a N-E fault that your MFT is not detecting.

 

[The Ghost]

A little more info. The upstairs RCBO tripped when certain items were plugged in. With the ramp test showing the RCBO for up trips at 15ma which is very low, then plugging a newish tv in or lamp trips the RCBO. If I take them downstairs the RCBO for down does not trip. So I take that as the RCBO for up is defective. So replace it... But as I say what throws me is why would a separate circuit trip when an appliance is plugged into another circuit. Now there is one socket that is downstairs at the back of the house on an extension that comes from the upstairs RFC. I am thinking considering the lighting wiring that I had to re-do in the shower room (really bad wiring)

 

[Midwest]

Know we've had reported instances of faults on one RCD, causing other RCD to trip in dual CU. Perhaps this can happen with RCBO's?

 

[westward10]

Have you IR tested between the two circuits as there maybe some interconnection. A continuity test between the two circuit conductors may also show a connection.

 

[Richard Burns]

Tel has covered it really, if there is already a NE fault on the upstairs ring the RCBO is tripping when the leakage current and test current reach (say) 24mA but the leakage of (say) 9mA causes the test current to only read 15mA.
Previously posted diagram of NE fault causing RCD to trip, the fault is on the circuit that trips even if the load is on another circuit.

 

[The Ghost]

Have you IR tested between the two circuits

No I have not as yet. Having done this loadsa times never occurred to me to IR between separate circuits although that is a good idea and will have to in this instance.

 

[westward10]

The ramp test at which most are tripping at, 24ma is low. Manufacturers do not design a device to trip at 30ma and allow some leeway normally around 27ma.

 

[Octopus]

At 15 ma I would change the rcbo ..

 

[telectrix]

is it tripping @ 15mA with circuits connected or dissed?

 

[Wilko]

Agree there's likely a cabling fault. Also, if rcbo is isolated from the circuit it should not trip at 50% of rated current, just saying

 

[Midwest]

Wylex told me once, a 30mA RCD tripping at 21mA, was within design tolerances (circuits disconnected).

 

[Risteard]

Wylex told me once, a 30mA RCD tripping at 21mA, was within design tolerances (circuits disconnected).

In fairness you don't need Wylex to tell you that the standard permits anything above half the rated residual operating current and not greater than the rated residual operating current. So I wouldn't have questioned it tripping at 21mA. In fact I never ramp test RCDs as the standard test sequence tells me what I need to know. (Other than perhaps to ascertain what leakage is being caused by connected appliances or something like that.)

 

[Midwest]

In fairness you don't need Wylex to tell you that the standard permits anything above half the rated residual operating current and not greater than the rated residual operating current. So I wouldn't have questioned it tripping at 21mA. In fact I never ramp test RCDs as the standard test sequence tells me what I need to know. (Other than perhaps to ascertain what leakage is being caused by connected appliances or something like that.)

Twas contradicting a previous post

 

[The Ghost]

15mA with circuits connected or dissed

Hi just got back from work. Errm... circuit disconnected, I am not sure I understand what you are asking? Anyway nipped in to change the RCBO no change to the situation. So looks like get in and break the circuit down bit by bit. Needle in a haystack it seems like at the moment but anyway it's the weekend so maybe come Monday I'll have a vastly good idea. Sometimes works like that for me. Still stewing over @Richard Burns' post. I meant to phone kewtech to ask how the RCD function works exactly. i.e. does it discount existing leakage then give you an answer and so on. It may be the leakage of 10ma on the tails is all on the upstairs sockets. Will know more Monday anyway. Now for a rest.

 

[Wilko]

To ensure there is no circuit leakage adding to the result, test the rcbo on its own

 

[The Ghost]

In fact I never ramp test RCDs as the standard test sequence tells me what I need to know

Well it wouldn't in this case as you could not keep it on long enough to standard test it.

 

[The Ghost]

circuits connected

Anyway the circuit was connected and tripping at 15ma.

 

[telectrix]

so, if the circuit itself has a leakage of, say, 10mA, you tester pushes in 15mA @ x 1/2, the the RCD sees 25mA and trips. FAIL.

 

[westward10]

This is where you need an old style rcd tester so you skip the 1/2 × test, can't remember the last time I did this test

 

[Dave OCD]

I think if there was any sort of interconnection between the circuits both RCBOs would be tripping constantly when current was drawn on either circuit.

 

[The Ghost]

both RCBOs would be tripping

That is an interesting and potentially valid view. Certainly food for thought.

 

[The Ghost]

The BG RCBO is the new compact type and only has a neutral fly lead by the way.

 

[The Ghost]

This is where you need an old style rcd tester

 

[westward10]

I think if there was any sort of interconnection between the circuits both RCBOs would be tripping constantly when current was drawn on either circuit.

I agree but never say never.

 

[westward10]

View attachment 43390

Nice meter Vorts, not sure that is safe to use on the fast trip anymore as it could create dangerous voltage across exposed metalwork.

 

[The Ghost]

dangerous voltage across exposed metalwork

I know it sounds crazy but many years ago when we did this test we had to make sure everyone in the house kept away from metal work radiators and x bonded pipes and had to almost shepherd them to the middle of the room while doing it.

 

[marconi]

Just wondering - what is the earthing system, what is EP bonded to the MET, any supplementary bonding still in place, and have you measured the N-E voltage at the CU for no load (Main Switch off), and before and after an RCBO trip for the four combinations of the two RCBO closed/open states? Are any equipments or socket strips with EMI filters present?

 

[marconi]

No mention by you yet but I assume you checked the ground floor socket at back of house and run to it off the up FRC (#3) and inter FRC IR and found nothing untoward.

Could you also test for PEFC and PSSC:

a. Supply
b. Sockets on Up FRC
c. Sockets on Down FRC

to see if you have a higher resistance neutral path for 'return' currents (cf RB's #6).

You mention the Tv and lamp(incandescent, CFL or LED?) when switched on (no cpc) cause the up FRC's RCBO t0 trip. Makes me wonder if you have a high resistance Neutral path or connection which is shown up when a brief high current (ie: the surge when TV powered up) flows - I'd try kettle on up FRC and measure L-N and L-E voltage at the socket it is plugged in to and also observe up L-N and N-E at CU as kettle turned on. I am wondering if the brief drop in L-N or rise in N-E pd is being detected by RCBO as a loss of neutral (but then one might expect the down FRC RCBO to trip too albeit it is not biased by the 8mA earth leakage).

No doubt you have thought of doing FRC standard tests.

Happy hunting. Better wear your deer-stalker hat!

 

[The Ghost]

These are the figures from the EICR RFC IR order LN-LE-NE r1-rn-r2 (first three figures). Thanks @marconi for your input. Ze is 0.16 PEFC 1.4 kohm. The earth DNO supplied was TN-S converted to TN-C-S with braid from outer sheath in there with the neutral. I suspect the utility room socket and the way it has been supplied as this is part of an extension and looking at the lighting in shower room, very poor wiring (see pic). Going back Monday now to get to the bottom of this problem. As you can see tests have been done as I did an EICR before fitting a new box. The client is about to go to Australia for a year and is renting the premises and wants a satisfactory cert. for the estate agent. So I have time to sort this and it is a semi empty house at the moment. But it is going to be a bit of a shock on the pursestrings methinks. Just musing... I wonder what the position would be if I changed all RCBO to RCD and did not supply RCD at all? After all I am required to leave the property no less safe then when I started. And in fact it would be much safer as the fuse carrier for an ex shower had gone into melt down due to replacing fuse on a short under load, i.e. not switching off the main switch. MCB would obviate this problem.

 

[The Ghost]

OK went back and sorted it. Managed to get it down to middle room spurred socket opened it up and neutral and earth connected incorrectly. Owner had installed a couple of sockets. Didn't take long at all. Thought I would be under the floorboards all day. Phew!

 

[marconi]

So Richard Burns was spot on - an N-E fault as a result of N and E reversal at a spurred socket, which was switched on (dp switch?) or switched on or off(sp switch)on the up FRC with an appliance plugged in to it using L,N and cpc?

 

[The Ghost]

In fairness to @telectrix as @Richard Burns acknowledged Tel got it. And Richard illustrated it. But otherwise that is about the size of it @marconi

 

[telectrix]

the important thing is always the result. ( as Croatia said after the semi-final).