intermittent house electrical fault

[keithfixit]

Intermittent fault on house
A new boiler was fitted and ever after the house electrics RCD (Residual Circuit Device) goes off at random. I have tried isolating every circuit using the MCB's (Miniature Circuit Breakers) but nothing immediately showed-up (*but see later), I also switched off individual appliances on each circuit but no conclusive results. I had no electrical problems before the new boiler was fitted.
The CU (Consumer Unit) is a MK 5780s with a single RCD and 10 MCB's - the lighting circuits have C Type MCB's fitted about 7 years ago because light bulbs going kept putting out the RCD.
Before switching the RCD back on, all the MCB's were put to, 'off' and then each MCB switched on in turn with 10 seconds between each MCB going on to see if a fault was on that circuit.
*Occasionally, when switching the garage MCD 'on' the RCD immediately tripped, 'off', but this was not consistent, perhaps 1 in 3\4 times.
Questions:
- would replacing our CU with a twin RCD unit and having the MCB to the boiler and the MCB to the garage on the separate MCD's be a cure or only a 'patch' that does not find the fault?
- would fitting RCBos (Residual Circuit Breakers, overload) in place of the MCB's isolate where the fault is?
- if RCBos can be fitted into the existing CU are they, plug out, plug in? and is a reasonably competent DIY person allowed to do this?
- what kind of RCBos?, I have heard of 'single' and 'two module' types and 'short and tall'.
If what needs to be done is other than, 'plug and play' then I will use an electrician.
Any help much appreciated.

 

[Taylortwocities]

None of what you describe is in any way "plug and play"

You have a fault, somewhere, and putting in a dual RCD, or multiple RCBO consumer unit won';t make the fault go away.

Also, please understand that an RCD trips if there is an imbalance (eg fault) on either the line or the neutral legs of a circuit.
The MCBs that you have been turning off ony isolate the LINE conductor. So the neutral leg is still connected to the RCD and it will/may still trip.

To return to your oringinal post. Everything was OK until the new boiler was fitted. I would be first looking at that. There are pumps and valves as part of the boiler system that can cause a problem.

You need a boiler friendly electrician to do some specialist testing.
Personally, I'd turn the heating system off at its isolator switch (it should be double pole) and use the immersion heater for hot water for a while. If the RCD doesnt trip then the blame lies with the boiler or associated stuff.

 

[keithfixit]

Hi TTC,
Many thanks for reply.
Understood, not DIY, not 'plug and pray'.
Thanks for tips on how the RCD works.
It does seem like the only isolation is the double pole isolator to the boiler with everything else, including appliances, on.
If this problem is solved I'll post it.
Thanks again.

 

[keithfixit]

"You have a fault, somewhere, and putting in a dual RCD, or multiple RCBO consumer unit won';t make the fault go away"
Understood. The objective of the thought of RCBos was to find the circuit with a fault. I know that some mods have to be done on the CU wiring to fit RCBos (from my electrician) but the question is, if there is a fault on a circuit will the fault only trip-out a single RCBos so I can identify the circuit? Many thanks.

 

[PrenticeBoyofDerry]

It could be purely coincidental that your system trips after new boiler installation, but worth looking at that first.
The nature of RCD tripping has already be mentioned, but what you must do if investigating the nature of the trip, is not just switch off the appliance, but remove both live and neutral from the load. So just switching single pole devices is not removing the appliance from the circuit, with regards to investigating the trip. Items should be unplugged. Also if it is a circuit fault rather than an appliance fault, this could mean that the neutral at the busbar be disconnected, to investigate the fault further.
The most common appliance to cause trips are those with heating elements (cookers, irons, toasters, laundry machines etc...)
Cooker generally I find to be a favourite.

 

[JohnW2]

The objective of the thought of RCBos was to find the circuit with a fault. I know that some mods have to be done on the CU wiring to fit RCBos (from my electrician) but the question is, if there is a fault on a circuit will the fault only trip-out a single RCBos so I can identify the circuit? Many thanks.

If you had separate RCBOs for each circuit then, yes, that would obviously enable you to determine which circuit was causing the trip (and prevent the problem it affecting other circuits). However, to replace your ten MCBs with RCBOs (even if they would fit in your CU) would probably cost you about £250, just for the RCBOs - which is quite an expensive route to fault-finding your current problem.

Without RCBOs, all you can do is try to eliminate each possible cause by 'trial and error', by unplugging (or switching off with a double-pole switch/isolator) each appliance etc. in turn and seeing if that cures the problem. As has been said, switching off a circuit with a single-pole device (like an MCB) will not stop the tripping if it is due to a neutral fault.

Kind Regards, John