3 Pole MCB queery

[Ziggy Sparks]

Chaps,

New to the forum with an opening queery as follows:

I am required to install a 3phase PSU. PSU is essentially a bunch of rectifiers to power DC coms equipment. Clients design is for a 40A 1pole type C MCB x3. I suggested that a Type D 3pole is more suitable and that as best I know it 3x 1 pole MCB's would be contray to BS7671. Client asked why and my reply was that in the occurence of a fault on 1 phase all 3 should be disconnected for safety reasons. NICEIC were inclined to disagree and said that it was acceptable as it would cause safety only if a motor was liable to stop a piece of equipment and that for the equipment in question it would be ok. Pondering the matter further I thought, how can it be safe if someone who assumes the kit is dead, as they only test on the de-energized phase during safe isolation, plays around with it while 2 phases are still live?

 

[Silly Sausage]

I'd go for 3 pole as well, for the same reasons as yours and especially if the NICEIC disagree!


oh yes...and Hi & Welcome!

 

[trev]

I understand the point you're making Ziggy but the safe isolation procedure checks all conductors, anyone not carrying out the proper procedure is in line for a Darwin award anyway. In your position I'd go for the 3 pole.

 

[Adam]

Although a TP mcb would be more suitable for isolation, and disconnecting all phases under fault conditions as you mentioned, it is not necessary.

Does BS7671 specifically state that all phases should be disconnected if a fault occurs on one?

The reason I ask, is that many TP circuits are protected via BS 3036 or BS 88 fuses. If one phase has a fault, the other two are going to remain live.

Food for thought?

 

[snowhead]

You'll need a 3pole (if not 4pole) Isolator adjacent to the equipment, so safe isolation is not a problem.

If it's 3 stages, they probably don't want total loss caused by partial failure.

If it's 3 ph rectifier it should all go off together.

If you're using the MCB as the only means of isolation it must be 3pole

 

[Darkwood]

You need to understand the nature of what your supplying i.e. would the tripping of a single mcb allow a feedback voltage down the dead phase from the other phases if so the 3 phase isolation needed, like a motor or transformer the phases are linked together through the load and dropping one phase out leaves a floating voltage on the phase thats down creating a potential hazard or even damage to equipment

 

[Des 56]

They want single pole,you want tripple pole

In an attempt to reconcile the warring parties and provide arbitration to such a conflict :33:

I suggest getting single pole and link them physically with a rigid pin through the levers
:8::yes:

 

[Darkwood]

With regards to the NICEIC's reply they are a bunch of numpties, its alright qouting regs and reading it word for word but in the real world the regs are applied to circumstances where you may need to use your own judgement on risk assessment, check my post 6 and see if said situe could occur.

What NIC should have done is made you aware of relevent reg's that address this area then said however if the regulation they want to apply to allow individual mcb's could create a dangerous situation then disconnection of all phases should be implemented in fault conditions, this can be achieved either with a TP mcb or a phase monitoring device.

It might be the case that NICEIC are fully aware of the PSU you intend to connect (unlikely!) and know this not to be a situation that would occur but they should have been more informative with their reasoning. They wonder why they constantly get flack when they always seem to dodge the more technical issues by simply quoting Regs.

Regs are what we should follow but you must remember they are a guide and dont fit every situe... i remember when it was brought up many yrs ago that putting overload protection on a electromagnet could create a larger danger in an overload condition than the letting the overload occur..... clearly a situation the Regs of the time hadn't thought of but guess what; it was in the next edition.

 

[Risteard]

As said fuses would not disconnect each line conductor of a three-phase circuit, although I would question the logic of installing three single-pole circuit breakers instead of a triple-pole linked one.

 

[snowhead]

They want single pole,you want tripple pole

In an attempt to reconcile the warring parties and provide arbitration to such a conflict :33:

I suggest getting single pole and link them physically with a rigid pin through the levers
:8::yes:

There's really only one way to solve this issue, 3pole or 3 x 1pole

FIGHT.

:30: