Hi Fi MCB on Consumer Board Tripping

I wonder if any of the more electrically savvy Members can give some advice on an issue I am having with my Mains Supply.

I have a dedicated circuit for my main system with 8 switched sockets on which are 3x250DR, 552PS, Supercap DR and 2x555DR together with a Roksan Xerxes PS.

The system is on 24/7 and normally there is no issue. Problems appear to come when there is a power cut which, it goes without saying, is obvious but more regularly when there appears to be a momentary ‘blip’ in the power supply and the system needs to ‘partially’ power up. This causes the MCB to trip.

It appears that the effect of everything trying to power up together is overloading the system and this is understandable I guess. I can power on again individually in the prescribed order but since my electrician replaced the MCB (see below) I have to power up the 250DRs in a specific order for some reason.

My electrician has been and checked all the sockets which are fine and replaced the 16A MCB with a 20A one but this has had little effect and it appears if anything to have made the problem, as mentioned above, slightly worse.

Has anybody had a similar issue or able to suggest a solution?

It’s transformer inrush current, something all transformers do when initially powering up as the magnetic field builds.
Having all the boxes power up at the same time is the problem, they should be powered up one at a time, starting with source end first, then pre, & last is power.

Thanks Mike, that is what I thought was the case. Normally I would not power up everything at once, beyond anything else it would be physically impossible. It’s what appears to be very short power drop outs that are causing the problem.

If i remember correctly the massive transformers on the Statement amps do a gentle start up, just for that reason to avoid an overload on the circuit breakers.

Hi Gazza, yes they have ‘soft start’ in the Statement.
Soft start in one form or other is common in industrial applications.

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Here is what you need to do. Change your mcb to a C type rather than a b type.

Hopefully this shows you.


From the Internet…
Type C Circuit Breakers

This type of circuit breakers is used for
loads with a moderate inductive
component, typically electrical equipment
which uses low-HP motors or specific
types of lighting: Air conditioners.
Residential / Commercial Pumps.

The key bit being inductive loads. I.e. in this case transformers.


Thanks. The MCB I have is a B Class. I will look into changing as suggested.

It should take an electrician about ten minutes and the circuit breaker itself will only be a few £s, so this should be a very cheap fix for your problem!



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Yes, it’s Type C you need. We have a 50amp breaker on our spur and it’s never tripped. Funny that.

Crikey, my electrician is nervous about putting on a 25!

But don’t forget, after CB gets changed, its still best for to power up one at a time, starting with source end first, then pre, & last is power.
(and power down in the reverse order)

It’s important that the breaker is compatible with the cable, as I understand it. Mine was done by a fully qualified electrician and everything was checked for safety by the distribution people when they installed the smart meter recently. It’s absolutely critical to get it right and to follow the regulations to the letter.

Naim’s recommendation to me was to use a 32 Amp MCB, and that’s with a smaller system than yours with fewer big PSUs. So I would think that a 50 Amp wouldn’t be excessive for you if the circuit permits. (Mine is 10mm T&E cable, also as per Naim’s recommendation.)
It’s the Type C breaker that will prevent tripping, though.


Type B - trips between 3 and 5 time full load current

Type C - trips between 5 and 10 times full load current.

But the other thing that has to be considered is the maximum earth loop impedance of the circuit.
For a 16 Amp B type mcb the Max earthloop is 2.73 ohms so that the mcb will disconnect the socket circuit within 0.4 sec should a short circuit occur.
For a 16 Amp C type mcb it is 1.37 ohms.

If the earthloop of your circuit is above either of the above, depending on type of mcb, then a 16 Amp RCBO of either type would be installed. The Max earthloop for a 30mA 16 Amp RCBO is 200 ohms.
Your electrician will have to take the earthloop readings and then determine which type of protective device can be fitted taking into account the type of cable, the cable size and other installation factors.

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I had 3 radials put in this year following the instructions in another thread on this forum, and by a very understanding electrician. He used 32 amp type B breakers, and so far, with many power ups of the system, no tripping. Main fuse to house is 80 amp.

However, if I do get any problems with tripping, I know what to do.

(No acid was harmed in this post :crazy_face:).

Similar here, just had three new radial circuits installed to a new consumer unit. Went for 6mm2 cable and 32A RCBO Type C.

However, the Electrician initially installed Type B as the Stockist provided these in error. These tripped immediately upon switch on.

He has been back and changed them to Type C and now no tripping when switching on.

FYI, I do keep it switched on 24/7.

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