Siingle phase MCBs for a three phase socket

[ohmer]

Can anyone find a problem in using single phase MCBs for a three phase socket. I would consider it bad practice and prefer a 3 phase MCB.
Doing a PIR and cant find anything in the regs to fail it.

 

[DurhamSparky]

hmm thats a good one!! effectively all that joins them is the switch lever!! but should one phase trip all poles will trip where as your single verison wont take all 3 out..!! cant see much wrong off top of head but there will bound to be some body who can shed light..!

 

[Sparky Ninja]

If your mcb's are in line, you may find yourself able to put a rod through all three,Then if one isolates, they all do.

This is the only way I would consider this acceptable.


Have done it myself twice, had an old crabtree polestar with a load of single mcb's but no three phase that would accommodate the load.

 

[ohmer]

If your mcb's are in line, you may find yourself able to put a rod through all three,Then if one isolates, they all do.

This is the only way I would consider this acceptable.


Have done it myself twice, had an old crabtree polestar with a load of single mcb's but no three phase that would accommodate the load.

No thats the problem the board is 3 rows L1 top L2 middle L3 bottom.

The socket is protected by a integeral RCD so the only isolation is the 1 pole mcbs. I am concerned that if I mcb trips equiptment could be damaged. I know this is the same as fuse's. But common practice seems to be 3 phase mcbs.

 

[Sparky Ninja]

No thats the problem the board is 3 rows L1 top L2 middle L3 bottom.

The socket is protected by a integeral RCD so the only isolation is the 1 pole mcbs. I am concerned that if I mcb trips equiptment could be damaged. I know this is the same as fuse's. But common practice seems to be 3 phase mcbs.

Dunno then....




You'll also likely have high neutral currents if one of the phases goes down..

 

[davelerave]

there's a risk of single phasing obviously-bad practise anyhow for 3-phase final circuits

afaik fuses are more common on the distribution side of things these days

 

[scotsparky]

use the 3 single mcbs and install into a small module with a triple pole mcb. That way any fault takes everything out and you have put it in as a sub main.

 

[Sparky Ninja]

Is it at all possible to install a little TP&N unit off of the distribution side? Or is this not accessible or not in the budget?

 

[ohmer]

My problem is finding a reg, that says this is wrong. any ideas

 

[J@mes]

Not sure if there is a reg that covers it as such, but in the information that comes with the DB/ MCB's there may be something about either them being able to be used to protect 3P or not, which would lead to the reg of "complying with manufacturers instructions".

Is there anything about a faulty piece of equipment (the bit being plugged in) having to be totally isolated when under a fault condition, even though it may only be on 1P of the 3 available?

It's not the kind of thing I would do I don't think.

 

[Andyfm]

This used to be fairly common with the older rewirable fused TPN boards, so it must have been ok at one time .

 

[caddy]

An interesting question and i have come across a similar thing in the past where 3 single mcb's have been used where a 3phase MCB could have been installed and on the face of it looks completly wrong. However how is it any different then the old MEM / BILL 3 phase rewirable boards, they have 3 phases across 3 rows and either use fuse wire or fuses. Even 3 phase switch fuses have seperate fuses.

So MCB's Fuses or Fusewire if its ok for one why cant it be ok for all ?

 

[davelerave]

it's preferable to break all phases/lines simultaneously on 3-phase final-circuits-in the event of a fault or if the circuit needs to be isolated

 

[mechelec]

I have learn somthing from this as I always thought that a 3 phase circuit had to have all poles of the MCB trip at the same time. I was wrong, check out 431.1.1 only the phase with the overcurrent needs to disconnect unless it will cause danger, so your installation is OK.

 

[Risteard]

It's not really ideal when a triple pole circuit breaker could be used but it's no different than using say BS88 fuses. That's not going to break each phase.

 

[davelerave]

another iisue could be mechanical damage etc. to a nym or swa on a 3-phase circuit


with the SP mcbs you could in theory have 3 sequential short-circuits wheres with the TP you have only 1 max

 

[Sintra]

Have a read at 431.1

 

[mechelec]

Have a read at 431.1

I could have sworn I said that earlier

 

[Sintra]

I could have sworn I said that earlier

So you did mate sorry.