Can Consumer Unit Be Modified?

My house must have been rewired about 30 years ago, I've had it checked out previously and all is safe and above board.

Recently I have had a problem with random tripping of the power. It's always been the case that if a bulb fails, this can trip out the a lighting circuit, sometimes the main switch. I think I understand why this is and can live with it.

Now I have a problem that SOMETIMES, a combination of the boiler and dishwasher being on will trip the power. Each works fine on its own. And very occasionally another combination will trip it. I've had a guy look at it once (a pretty sensible and well-experienced electrician who has done work for me before and whom I trust). We could not duplicate the fault whilst he was here. He disconnected various conductors and took various readings and muttered something about leakage currents adding up to the trip threshhold.

So I'm wondering, can the individual circuit breakers in the consumer unit and/or the main circuit breaker be replaced with one with a higher (but safe) trip threshhold? I really don't want to purchase a whole new consumer unit and have it wired in, particularly when it's not clear what the fault is. These components look modular - are they a standard fit? Or do you have to find parts to fit a particular make and model of consumer unit?

Plus, as a matter of curiosity, what does the electrician do when working on/changing a consumer unit? Have the supplier switch off the power for a day? Because as far as I'm aware, there's no switch beyond the consumer unit (NB NO WAY am I going to touch this myself!).

Take a photo of your CU and post it, identify in your post which breakers are tripping.

I assume you mean that the RCD trips knocking out all the circuits than run via the RCD.

On an aged installation this is likely to be everything.

Did your spark consider the ageing of the RCD and ramp test it? Reason I ask is that your RCD when if left the factory would have required in excess of 30mA leakage to trip. As time goes by that figure could have dropped down and maybe a natural leakage that was was tolerated by the RCD is no long okay given the RCD ageing.

On modern boards either two RCDs are used or better still RCBOs, the advantage being that the 30mA whole system trip point you currently have is shared between two RCDs, in the example of RCBOs each individual circuit has its own 30mA trip threshold.

The surge from a lamp which fails will often trip an MCB. Possible fixes include:

• Replacing with a Type C breaker //www.diynot.com/wiki/electrics:mcb2
• Replacing with a fuse carrier
• Replacing with a higher rated MCB supplying a fused connection unit outside the CU
• Replacing incandescent lamps with fluorescent and/or LED

Should not trip the main RCD though.

What your electrician muttered is common - individual leakage currents adding up to get near the trip threshold of the RCD, and it doesn't take much to push it over. Can be a right b***er to find. The price of RCBOs has fallen a lot over the last few years, so if they are available for your CU it could be cheaper to just replace the main RCD with a switch and swap MCBs for RCBOs than pay for a lot of fault-finding time.

As for your last question, the correct approach is for the electrician to ask the supplier to come and remove, then later replace, the service fuse, but a great many just cut the seals and pull it themselves, which is possibly dangerous and arguably illegal.

Here are some pictures:

//www.diynot.com/network/andrewjameshoward/albums/14947.

The unit is in a corridor - I cannot get far enough away to get proper focus. The unit is labelled Merlin Gerin AMG15. I believe Schneider took this company over. As far as I can see, everything routes through the RCD (9multi9 RCCB residual current CB 100A 240V 0.03A EMG 100 030 2) and every circuit has an MCB.

The lighting circuits have 5A MCBs (C45 240/415V M3 8017). A 40W light bulb blowing on the 2nd floor will trip this MCB. I think from the above numbering that the MCB is a type C already. I'd rather keep this as an MCB and I'd rather not change the light fittings. So maybe I could have the MCB replaced with a higher rated one and a fused connection unit added. But this issue is just a niggle and I could just as easily leave it alone.

Tripping of the RCD (and hence all the power in the house) is occasionally caused by the downstairs lighting circuit - when a 70W halogen blows. This may have something to do with it being on a dimmer switch. I can live with this also - it's only when the bulb goes and that's not very often.

The problem circuit seems to be one of the 32A downstairs ring circuit MCBs (C45 240/415V M3 32A 8135) (although if its a question of residual currents adding up to a trip threshold, maybe other circuits are also to blame). The dishwasher doesn't like to be on at the same time as the boiler (and the dishwasher only needs to be on at the power point, not actually switched on and working for this to happen). This trips the RCD every other day if we let it.

If my system was being installed today, I suppose at the very least there would be 2 RCD's and even better an RCBO on every circuit. To convert the existing system to split RCD though, someone's got to fiddle around with the busbars and do some re-wiring inside the unit. To convert to RCBO's, there are 12 MCB's to replace, so what with the labour and paying the supplier's disconnection and reconnection charges, this could become an expensive way of fixing a minor problem.

So I am hoping (a) that Chri5's theory about the RCD is correct and that (b) this component can be replaced (are you allowed to fit a higher leakage current than 30mA?). If that doesn't work, splitting across 2 RCDs might be the next thing to try.

Now that I've posted the pics and the specs, can anyone tell me if new RCDs/MCBs/RCBOs are available to fit this unit?

andrewjameshoward said:

the dishwasher only needs to be on at the power point, not actually switched on and working for this to happen

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Could be the heating element on the way out.

are you allowed to fit a higher leakage current than 30mA

Click to expand...

No.

It's a safety device telling you something is wrong.

A 30mA RCD as the main switch is not a good design, especially with that many circuits.

It could be a faulty appliance, or wiring. It could be a faulty RCD, but unlikely.

You need an electrician to carry out some insulation resistance testing. Get him to test the dish washer, both when cold and after it has been running for a short period of time.

So if the dishwasher is unplugged, the fault does not occur regardless of what combination of other appliances/equipment and lighting is on load?
Does the trip occur if the dishwasher is plugged in to another socket outlet (via extension lead)
Does the trip occur if another appliance is plugged into the outlet used by the dishwasher?
A few things that I would consider, depending on the result of above:
*Ramp test on the RCCB and IΔN test
*inspection for damage to flex/plug of dishwasher and socket outlet/accessories connected.
*Inspection for water ingress/excessive moisture around the dishwashers electrical components and socket outlet/accessories.
*Continuity and IR test on heating element of dishwsher

If the RCD trips when the dishwasher (DW) wall socket switch is ON and the DW power is off, the problem is a small residual current between the neutral and earth conductors anywhere between the wall and everything in the DW itself or the live conductor and earth in the flex to the DW on/off switch. It isn't significant in itself until another device with similar leakage is connected to the circuit and takes the main RCD over the threshold.

Pull the DW away from the wall and check the flex for dampness. Open the DW (plug removed, obviously) and check around for dampness.

I think the big clue is that the RCD trips occasionally when you use the appliances that also use water. Damp is the cause, not electrical power control.

Although all the mcbs are modular and changeable, you should trust that they are all properly functional and keeping you safe. Rather than employ a lecky to "upgrade" your mcbs, etc, you are better advised to buy a new DW.....

First to

Can Consumer Unit Be Modified?

Click to expand...

answer is no they are type tested and not permitted to be modified. However what is being talked about is not modifying the consumer unit it's only changing its configuration.

As already said there are many ways to get around the problem. But before any electrician starts on the job he will need to take some reading and decide what options are available. So only one option open to you is call an electrician. As to power off while working officially yes should call DNO and have power turned off. But in real live this changes area to area and in some areas the DNO turn a blind eye to breaking seals and removing fuse.
Again leave it to your electrician he will know what he can get away with in your area.

The C45 range is defunct, the ones on ebay are most likely copies or secondhand

The C60 range visually is virtually the same and can be used to replace the C45 range and new ones are now branded as schneider

Rcbos are one module and only single pole, the same width as the Mcbs but are not cheap

Thanks - you've given me some ideas for one or 2 more bits of testing/diagnosis. Can't do these properly until the fault recurs, so I'm going to start leaving the dishwasher switched on at the mains all day again, wait for the fault to recur and then try a few tests.

Having read around now on this subject, it seems to me that these 2 appliances are only the last straw, and we must have several leakages of 1mA or so from all over, eg due to RFI suppressors. As I have a lot of time on my hands, I am tempted to hire an RCD tester, do several ramp tests with only one mcb on and find out where they all are.

Watch this space.