Circuit breaker question

Hi Darren. I take it the RCD covers numerous circuits? You could have an electrician change the RCD for a switch and then fit RCBOs on each individual circuit. This would also help identify the faulty circuit. 

Hi Finkfan,

Thanks, the RCD covers 2 "sockets" circuits, presumably upstairs & downstairs plus a cooker circuit so that could narrow things down as you say.

As it's random times day & night this occurs I'm thinking items such as fridge / freezer / heating pump are most likely culprits.

I would say out of those 3, the heating pump could be the culprit. 

When I installed a dedicated mains circuit for my HiFi, Naim suggested that I used type C MCBs instead of the type B more commonly used in domestic consumer units. Before I did this, turning on the power amp would, more often than not, cause the MCB to trip. Since the change, it hasn't happened once.

Type C MCBs are more commonly used in commercial premises, especially on circuits that run fluorescent lighting, refrigeration compressors, and other appliances with a high inrush current. It may be worth talking to your electrician about reconfiguring your consumer unit to reduce the chances of tripping. 

Thanks Chris, I was wondering if the "sensitivity" to the culprit could be adjusted and that could be the ticket.  I assume if there's something actually wrong the thing will still trip and I can have it addressed.

Don’t forget that an RCD and an MCB do different jobs. The RCD will detect an imbalance in the supply when there is an earth fault say, but an MCB is an over current device like a fuse. If the RCD is tripping, changing the type of MCB won’t help. 

If it is the RCD then the problem is an intermittent residual current that shouldn’t occur, such as an earth leakage - i.e a fault. That is what needs tracing and eliminating. Alternatively, though maybe less likely, the RCD is the incorrect type or oversensitive (i.e. faulty)

I have come across situations where switching something on or off has occasionally, but ot always, tripped the RCD. As it happens when no-one is home, something that are stays running but which switches on and off is a likely contender: e.g central heating boiler or pump, fridge, freezer, external proximity sensing security lights etc. Of these I’d start by considering the last as they are particilarly vulnerable to water ingress. If something records the time when the power has failed enabling you to know the time of tripping, is there any common theme? (e.g only hours of darkness if security lights are inactive during the day, or a switching time of central heating controller)

One thing you can do is manually force possible suspects to turn on and off a few times in succession, in case one triggers the random event. With security lights include wet weather (or simulate with a long prior drench with a hose).

Of course, an uninterruptible power supply is still worth considering for critical things.

The UK RCD requirements for 13A socket outlets ( i.e normal power circuits), and for all circuits including lighting in bathrooms etc is that they must be protected by a 30mA RCD. So if that is what you have it must NOT be changed for one of a higher rating to reduce tripping.

One thing I did in my own house is use a split CU, having one half with common RCB covering circuits unlikely to have problems (e.g lighting), and the other half with no common RCD but useing RCBOs (effectively MCB combined with RCD) on the individual power circuits and the outside lighting circuit, apart from one power circuit on that half of the CU that has just an MCB (i.e no RCD), feeding the small number of essential permanently-on things - but that circuit is strictly limited, with sockets labelled and positioned differently from any normal circuit in the area. (N.B. Changes to the UK wiring Regs in 2015 means that if the latter was done now there would have to be a written risk assessment.)

Innocent Bystander posted:

If it is the RCD then the problem is an intermittent residual current that shouldn’t occur, such as an earth leakage - i.e a fault. That is what needs tracing and eliminating. Alternatively, though maybe less likely, the RCD is the incorrect type or oversensitive (i.e. faulty)

I have come across situations where switching something on or off has occasionally, but ot always, tripped the RCD. As it happens when no-one is home, something that are stays running but which switches on and off is a likely contender: e.g central heating boiler or pump, fridge, freezer, external proximity sensing security lights etc. Of these I’d start by considering the last as they are particilarly vulnerable to water ingress. If something records the time when the power has failed enabling you to know the time of tripping, is there any common theme? (e.g only hours of darkness if security lights are inactive during the day, or a switching time of central heating controller)

One thing you can do is manually force possible suspects to turn on and off a few times in succession, in case one triggers the random event. With security lights include wet weather (or simulate with a long prior drench with a hose).

Of course, an uninterruptible power supply is still worth considering for critical things.

Thanks.  I hadn't considered security lighting and agree it would be high on the list of culprits; I recently replaced one for that reason and the new one seems to have water / condensation inside the IR unit.

I wonder if I could obtain RCD safety adaptors with a more sensitive /faster trip specification than that in the CU?

I don’t know if an RCBO in place of the MCB on the errant circuit would trip before the main RCD, or not or both - don’t know off hand if they are available with faster trip speeds, but maybe that is worth a try if your make/age of CU accepts them. 

I’m assuming everything in your CU goes through the one RCD.

of course, even better would be to also replace the lights if it is them.

Im not sure if the RCD protects all the circuits or just those to the left of it.

For the faster/more sensitive device I was thinking on the lines of a plug adapter which could catch the problem before it gets to the CU?

Yes RCD just covers those circuits to the left. Main switch will isolate the whole board 

Finkfan posted:

Yes RCD just covers those circuits to the left. Main switch will isolate the whole board 

 

Agreed.

2 options present themselves: put circuit you don’t want cut off on the right, or put the errant circuit on the right. But if you use an electrician (and given that that is beyond UK permissible DIY that is the only legal way), you may find he/she is bound to upgrade all on the CU to current standard, eithe fitting RCBOs to right bank, or change whole CU to one with 2 RCDs. (And the upgrade would be wise anyway. ) The RCBO approach giving  individual circuits protection is more flexible in being able to not trip other things if one circuit has something thta habitually trips it, and keeping one circuit for the things that you really don’t want being cut off.

Thanks guys, I appreciate your advice.