mcbs for isolation

M cebee

are mcb's suitable for use as isolators on sub-boards

i see this done quite a bit

i know they can be used as switches if suitable

http://www.farnell.com/datasheets/99157.pdf

there's a link to MK brand which suggest's they're suitable for this purpose

2011

M cebee wrote: »

are mcb's suitable for use as isolators on sub-boards

i see this done quite a bit

For the final circuits fed from the sub-boards? Yes.

Personally I alway like to have a clearly labeled isolator that is very local to the apparatus (when possible).

Or supplying the sub-boards? Yes also. Just do your checks for discrimination.

M cebee

i mean as an incomer on dom sub-boards

i don't do it -but i see it done

Bruthal

I never did it either but often thought about it. As long as it has a fuse beside it then it would seem little different from the red switches used in the swith fuses now, except the switches dont trip.

So maybe they be suitable as a main on off switch on any domestic board.

The thing about the red switches is that its easy to tell someone how to switch off their main switch if required for some reason, which is the red one.

2011

M cebee wrote: »

i mean as an incomer on dom sub-boards

i don't do it -but i see it done

I don't see an issue. We are doing this on a large industrial job.

M cebee

what mcb exactly

M cebee

robbie7730 wrote: »

I never did it either but often thought about it. As long as it has a fuse beside it then it would seem little different from the red switches used in the swith fuses now, except the switches dont trip.

So maybe they be suitable as a main on off switch on any domestic board.

The thing about the red switches is that its easy to tell someone how to switch off their main switch if required for some reason, which is the red one.

mcb -no fuse on sub-boards

i don't do it

Bruthal

M cebee wrote: »

mcb -no fuse on sub-boards

i don't do it

Woulnt see much problem using an MCB as main switch on sub boards without a fuse. Fuses are used for the high breaking capacity, so the one in the main board would be enough for that.

If the normal switch is used, it stays on during a high fault current, and other devices operate, where as an MCB would trip, so its hard to see many problems using the MCB as main switch, except maybe they might trip along with a 20 amp socket MCB etc, and so the whole installation would be off, where as a switch fuse will more likely hold while the socket MCB trips. Not so bad on a sub board but on the house main DB maybe it could be a slight extra problem.

A 63A or 40A MCB on its own would seem a decent main switch on a sub board such as in a shed.

2011

I have seen it in many places and would see nothing wrong with it. The important thing is to clearly mark up the schedule.

On the job I am working on at present we have lots of sub-distribution boards that are fed from 20A type C MCBs. The final circuits in these boards are fed from 2A type C MCBs.

M cebee

are you talking about the protection on the distribution cct

or the incomer on a sub-board

2011

20A MCB in a main board provides a supply to a sub-distribution board.

M cebee

yes no problem there


im referring to people's habit of using an mcb as an incomer on a sub-board

c32 on distribution circuit and a b25 incomer in the final board-the b25 serves as the isolating switch


that sort of thing-never bothered with it myself

2011

im referring to people's habit of using an mcb as an incomer on a sub-board

c32 on distribution circuit and a b25 incomer in the final board-the b25 serves as the isolating switch

I see what you mean, sorry I misread.

It is a little odd, but not unsafe.

I would prefer to have an isolation switch on the sub-board, but an MCB is acceptable.

M cebee

i wasn't sure

you think mcb is acceptable for isolation on final DB

Bruthal

Cant see much difference though, as using a 63 amp MCB as the sub board incomer will trip at whatever its instant trip level is, where as the proper switch will never trip, so whats the difference, even if the sub board is fed with a 20 amp MCB.

There are contact gap distances and all that, other than that, cant see much difference.

M cebee

i was doing a bit of research on this

breakers marked bs en 60898 are suitable for use as isolators and switches anyhow

Bruthal

Do they have a sort of switch symbol on them?

I wonder would a double pole main switch be of benefit in domestic DB`s. I seen one or two cases where it definitely would of been anyway.

M cebee

not sure on the symbol

i checked a few breakers earlier

some have the bs60898 some don't

all the dom boards have DP isolation in uk-but they have TT as well as TN


prob a good idea and rcbo all final circuits even


give it time:D

Stoner

don't ABB have an MCB with the correct operation curve to take the place of the fuse now. I know it's not the original question, but last time i was talking to someone from ABB about some domestic boards they had them coming through the approval process, much better situation IMO do away with the fuse and free up a mod on the board.

Bruthal

The high current interrupt capacity of the fuse compared to MCB`s i think was the reason fuses are still used i think. Did ABB say anything about that?

Stoner

yes they did also I think it was due to the was the short circuit current response and also i think a different response time on the fuse coupled with the response of the MCBs gave a better spread of coverage, these new MCBs seemed to satisfy the requirements and match the various characteristics of a fuse to the required level.

Like all new products they were not cheap and were being marketed as a maintenance saving device (I guess they would be, I'd rather one on my house) and not a cost saving device.

I'll get the name of the unit and pop it up.

Bruthal

Would be handy not needing the fuse in the DB alright, they are often a source of problems as you know, and a couple of less main connections in the DB as well as the extra space.