RCBO Tripping

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Mr_Bean

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Hello all,

Please go easy, I have never used a forum before but this scenario has me stumped, having searched online for potential answers for some weeks and my electrician seemingly equally confused.

Some time ago I got an electrician to replace the consumer unit, from a rewireable fuse type. The layout of the new board (all BG products) is each of the 6 circuits connected through a 1 pole RCBO with a main isolating switch covering all circuits. Everything seemed to be ok when it was fitted with the electrician completing all his checks without there being any issues.

However, the garage socket (a single socket on a circuit all of its own, total length of cable 1m from the consumer unit with 16A RCBO) would regularly trip, even with double insulated items with no earth connection in the plug. Every one of the items which caused the garage circuit to trip would always work without issue when powered from any of the sockets in the house on a separate circuit (through exactly the same type of RCBO). After a short period of time the RCBO would no longer reset at all, even with nothing plugged into the circuit.

The electrician returned and fitted a new replacement RCBO and this also would regularly trip. So, we decided to replace the garage socket and short length of cable to eliminate any issues with the circuit, but it would still trip (new circuit is 1m of 2.5mm cable and 2 x double sockets). The electrician then fitted a slightly older RCBO (same make and specification but just a slightly older model) and it seemed to work. But every so often it would still trip, particularly if using a power tool and not allowing it to fully come to a stop before restarting, and now again the RCBO will not reset at all.

I am generally a competent DIYer and a professional engineer who can work through most problems logically, but my electrical understanding is at a very basic level. At present with the still broken RCBO fitted and remaining in the open position I have measured that there is a 50 Ohm resistance between the neutral and live on the garage circuit, but if I remove one of the wires from the RCBO there is an 'infinite' resistance (the same as all the other circuits). This would suggest that the broken RCBO is somehow connecting the L and N. The RCBO will not stay set even with the wires removed from its L and N out terminals so this suggests to me that the RCBO is broken and in need of replacement.

A further observation is that some fairly non-scientific experiments conducted before the RCBO stopped resetting showed that if I connected a radio to the circuit and put it on quite loud then the RCBO was less likely to trip.

Rather than having to replace the RCBO every few months and have to regularly reset it I would like to try and resolve the underlying cause of the frequent tripping.

My questions are:

Does anyone have any idea what might be causing the RCBO to continually trip when a load, of over circa 400W, is applied?

Has anyone else had problems with RCBO's when supplying only a single relatively large load?

Are there any other basic checks or tests that I can conduct to try and get to the bottom of the problem?

Many thanks for taking the time to read my essay, and I would be pleased to hear your views. Let me know if any further information would help in offering suggested answers.

 
Your terminology is confusing.  Can I ask you to not the following, then edit your post to make it correct and make sense.

A 2 module wide, usually red, switch, is a main switch.

A 2 module wide switch that has a test button and will say "RCD" is a 2 pole RCD. there are usually 2 of these in a normal split load board.

A 1 module switch that has no test button and usually says "MCB" somewhere is a single pole MCB

It would be VERY unusual to have rcbo's and RCD's in the same split load consumer unit.

When you have revised your description I will be able to understand exactly what is tripping.

If you can take a picture and put it on an image hosing website you will be able to post a picture to show us the consumer unit and which one is tripping.

 
 The electrician then fitted a slightly older RCBO (same make and specification but just a slightly older model) and it seemed to work. But every so often it would still trip, particularly if using a power tool and not allowing it to fully come to a stop before restarting, and now again the RCBO will not reset at all.




What sort of "power tools" are we using?

A 16A Type B  RCBO could be tripping due to start-up current of the "Tools" you are using....

i.e.   Tools ok off a 32A ring..

but trips on a 16A radial!

Why did the electrician only rate this circuit at 16A?  

Was it just what came in a pre-equipped CU, or is there some obscure design reason?

I am guessing either a 16A type C...

or a 20A may stand a change of working better...

Guinness  

 
The switch on surge of the universal motors in many power tools is large. Such a short circuit length offers very little circuit impedance to help limit that surge.

I have a 16amp RCBO feeding an outdoor socket (normally used for lights and a pond pump), and I can't use my chainsaw on it for this reason.

To prove a point, if you have a reasonable length extension lead, try connecting your tools via the extension, in the garage socket. 

 
Thanks for such prompt reply's.

ProDave - apologies for any confusion, I have failed to find how I can edit my original post.

The consumer unit consists of 1 Main Switch and 6 RCBO's, as per the image here:

https://ibb.co/4FF8y5H

3rd from the left is the problematic RCBO.

Special Location - You may well have spotted the problem! How had I not noticed that the house sockets were on a 32A and Garage sockets on 16A?....

I do not know why a 16A was fitted, when the electrician came to replace the RCBO last time I asked whether a different 'Type' of RCBO would be worth trying but he seemed reluctant.

The 'power tools' that have caused the problems include:

240W Orbital Sander

450W Hedge trimmer

700W Drill

1100W Vacuum cleaner

1500W Router

Could the start up current on each of these exceed 16A?

Geoff1946 - Thanks for the suggestion, but the problem seemed to happen just a frequently when using a 20m extension lead.

 
Thanks for the clarification, that is not a common configuration (thought it is good) and sorry for doubting your descriptions.

Is it the Garage rcbo that is tripping (I note the water mcb is also off) 

For anyone else, here is your picture

Image.jpg.07efb4329aa5fc28681211578243ee5b.jpg


I suspect he used a 16A rcbo as there is only a single 2.5mm cable.  A better solution would be pull another 2.5 cable through and make the two double sockets in the garage a 32A rcbo as a ring final.

The problem at the moment is you don't know if the rcbo is tripping due to over current or earth leakage.  That wiring alteration and going up to a 32A rcbo should eliminate inrush current from most power tools.

And I would ask the electrician to PAT test all your power tools.

 
Wonder why the water heater is on a MCB?

Never trust IR readings "across" a RCD or RCBO - they normally give very low readings.

Did said sparky give you any certificates when he changed the fuseboard - be interesting to know what the IR readings were

 
Whilst it is labelled Water Heater, as that was what the previous unit had it labelled as, the power doesn't reach the immersion heater so the circuit is left off. I haven't yet bothered scrabbling around to find where the wire actually goes.

I can't recall why the Water Heater is on an MCB but I have a feeling it might be that when he came to change the RCBO on the garage circuit he used the one that had been fitted to the Water Heater to put onto the garage circuit but then that tripped even more than the original so he wanted to see if it was a dodgy RCBO by taking to test on another job and the most suitable thing he had in the van for the heater circuit was the MCB, not having another RCBO.

Yes, I have a certificate for the work and the IR readings are 200 MOhms.

Thanks ProDave for the suggested way forward, it will be easy enough to run another wire so will propose this to the Electrician.

Thank-you for all the response, I feel pleased that I at least have a way forward to try and get this resolved 🙂

 
Did this "electrician" swap the dodgy RCBO for an MCB??

as you appear to be missing an RCBO?

Basic starting point for investigations if a "Double insulated" appliance....

i.e.  2 core flex..  NO EARTH..  it tripping an RCBO..

Then its more likely the Overload protection aspect that is operating rather than the Earth leakage protection..

as you have No earth path.... 

UNLESS you have some major appliance fault as well  including a suitable path back to electrical earth..

Your electrician sounds a bit incompetent IMHO?

As well as the current ratings.. 16A, 20A, 32A etc..

MCBs / RCBOs also have operating characteristics described as "Type"

Yours are Type B typically used in domestic installation..  

other Types are less prone to operation due to start-up currents..

( But do have other limitations on the circuits length / cable sizes etc..)

e.g. https://www.bgelectrical.uk/public/downloads/data-sheets/devices/circuit_protection_MCBs.pdf

I have know a fair few  "builders power tools" trip a 20 type B MCB / RCBO intermittently  on start up..

so a 16A wouldn't stand much chance

especially tools that have a 13A fuse in their plug..

(i.e. NOT 5a rated appliances)

:C  

 
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