MCB question : B20 or C20 ?

bertie 56

Hi everybody,

Simple question for the brightest people here:

Is a C20 MCB acceptable in Ireland , instead of a B20 ? ( same question with C10 or C16... ).
I know that a C will trip with a bit more of current and will take a bit more time as well to trip, than a B, hence my question.

Any information about this in the Regulations ?

Thanks.

Dardania

it's based on the load being supported.

The ETCI regs suggest B as far as I know generally, but then C for other large inrush loads like motors.

What's your load?

2011

bertie 56 wrote: »

Is a C20 MCB acceptable in Ireland , instead of a B20 ?

Yes it is acceptable once the maximum disconnection times are not exceeded. This may mean that a larger CPC is required.

In general B type MCBs are only used in domestic installations.

Irish Steve

From experience, a 2300 W 9" angle grinder will take out a 20A B rated breaker on some startups, but since it was swapped for a C rated breaker, there's not been any issues. Had no problems getting the C rated breaker from the local wholesaler, though it was a while back now.

bertie 56

Hi everybody,
Thanks for your answers.
It's for a domestic installation. I've ask my electrician if he can fit some double MCBs , instead of the single pole ones.
I have difficulties to find some double pole B20 / 16, etc.... The only ones I find are two single ones that are connected together, but it takes too much space in the fuse board.
I know where to find some double pole ones that are similar in size than the single poles ones.
But they are in C, not in B !
Hence my question...

2011

Double pole MCBs in B or C characteristic should be easy to find (Edit: As you said 1 mod wide is difficult).
Try ABB or Hager.

If they won't fit in the board you could install a larger board.
The price differential between one enclosures size and another is not that large.
Why do you want double pole?

bertie 56

Thanks, 2011 !

Yes, it's a size of board problem , and price too, but not only.

To be precise, the 2 module ones are bipolar, they protects live and neutral separately.

The 1 module ones are unipolar + neutral , they protect only the live, but cut live and neutral in the same time.

So, I called them " double pole" because they have two poles , but they are unipolar + neutral, in reality.

And the reason why I would prefer to have this in my house, is that I'll be more confident knowing that both live and neutral are isolated from the circuit if the MCB " jumps ". I always found strange to have power in the neutral even when the MCB has jumped...

And the other reason is a " cultural " thing too, these single pole MCBs as we find everywhere in Ireland are banned for ( I think), around 20 years in my country...
Now, maybe I worry too much !

2011

bertie 56 wrote: »

To be precise, the 2 module ones are bipolar, they protects live and neutral separately.

The 1 module ones are unipolar + neutral , they protect only the live, but cut live and neutral in the same time.

In a single phase electrical circuit the current in the phase conductor is equal to the current flowing in the neutral (assuming that it is wired correctly).
So if the current flowing in the phase is not of sufficient magnitude to cause the MCB to operate the current in the neutral will not be either.

This will not provide any additional protection to the neutral.
Let's not forget that in a neutralised single phase installation the neutral is connected to earth. Once the phase is switched is switched off the neutral potential is at 0V with respect to earth.

And the reason why I would prefer to have this in my house, is that I'll be more confident knowing that both live and neutral are isolated from the circuit if the MCB " jumps ".

There is a reason that this is not a requirement in the National Rules for Electrical Installations.....

I always found strange to have power in the neutral even when the MCB has jumped...

Please explain, I do not know aha you mean.

And the other reason is a " cultural " thing too, these single pole MCBs as we find everywhere in Ireland are banned for ( I think), around 20 years in my country...

What country and what type of supply is used in your country?

Double pole MCB's are used in 2 phase installations.
For example on building sites with the 110V supply is centre tapped. This means that the 110V supply is 2 phase with each phase at 55V with respect to earth.

Now, maybe I worry too much !

If I wanted to spend extra on my distribution board I would rather install an RCBO for every socket circuit. I see no benefit to instilling double pole MCBs in a domestic installation.

bertie 56

Reading what you write, I think my confusion comes from the fact that , yes, the type of supply is different ( France, TT system ).

Wouldn't be that the reason why a double pole MCBs in a domestic installation is useless ???

Risteard81

bertie 56 wrote: »

Reading what you write, I think my confusion comes from the fact that , yes, the type of supply is different ( France, TT system ).

Wouldn't be that the reason why a double pole MCBs in a domestic installation is useless ???

Not useless, but as the Earth in a TT system cannot usually be guaranteed to be of a sufficiently low impedance a potential difference between Neutral and Earth could develop and therefore a single pole device would not be suitable for isolation. This isn't to suggest that an MCB necessarily need be suitable for isolation, however. But if it isn't then omnipolar isolation would be required elsewhere. (I am not terribly au fait with French wiring practices though.)

Bruthal

Isolation is just that, disconnection of all poles so any event on one side of the isolation point won't affect the other side.

At least on the current carrying conductors anyway.

2011

Bruthal wrote: »

Isolation is just that, disconnection of all poles so any event on one side of the isolation point won't affect the other side.

At least on the current carrying conductors anyway.

Yup.

230V equipment installed in ATEX zoned areas require double pole MCBs to ensure disconnection of the neutral conductor (complete electrical isolation). However this is for very different reasons.

Risteard81

Bruthal wrote: »

Isolation is just that, disconnection of all poles so any event on one side of the isolation point won't affect the other side.

At least on the current carrying conductors anyway.

Indeed, which is why I stated that the MCB needn't necessarily be selected for use as an isolator.

bertie 56

Ok everybody. I noted the advice of a RCBO for every socket circuit.Thanks 2011 for this.
Still doubting on the utility or not for the double poles ones ( unipolar + neutral ) though...
( and I don't have your technical level to see what would be best ! )