Good evening all,

I have been asked to install a 400v single phase welder. The welder is to be supplied from an existing three phase board. A similar welder that was installed some time ago is connected into two separate MCBs in the distribution board - is this correct? Or is it possible to get two pole MCBs for 400v single phase equipment?




Thanks

I'd be looking for a double pole MCB if it was my job.

They are available to fit most boards.

Stu

Thanks for the quick reply

So it's not single phase welder then

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Nope, I suspect it wants 400 volts and the only way that it's going to get it is across two phases

Stu

Couldn`t you use a three phase MCB on two phases.

The equipment would see 400 (415) as a single phase anyway if it was derived from two actual phases of a three phase supply or if it was two phases 180 degree out of phase. It don`t care.

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Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik

Single phase, connected across two lines. 17th Ed terminology is a slight improvement in that respect.
- Andy.

Agreed.

In a previous life I was involved in the sales/installtion/servicing of a range of industrial welding equipment. Most of the larger oil cooled arc welders & AC/DC TIG's were like this. If they are connected by a BS4343 socket just connect across two phases & the earth as normal. Same if wired directly into a 3ph isolator. Some machines also have an input connector which you have to select the correct input voltage for the supply.

I was for ever getting called out to supposedly faulty equipment which turned out to have been wired across a phase & neutral rather than across 2 phases!

One of the dangers of letting hairy a***d welders loose on electrics!

Make very certain that the welder ONLY requires two phases, it may need a neutral as well.

Conventional arc welders are in effect a large step down transformer.
Some designs connect the primary between phase and neutral for the lower output settings, and between two phases for full output.

I have seen one such connected to 2 phases, without a neutral.
It worked fine, but only on full or higher output settings.
On being switched to the reduced or lower output settings, nothing would happen.

I would use a 2 pole MCB if available, if not use a 3 pole one with one pole unused.

Wouldn't the phase angle have to be zero, Andy

It's two phase 3 wire without the N being distributed (Fig 3.3) - ?

Regards

OMS

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Failure is always an option

The supply might be, yes, but I'd still argue the load is single phase... (sorry I should have quoted the context).
- Andy.

OK - I can follow that Andy

regards

OMS

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Failure is always an option

I have this at quarries where the quarry regulations allow portable equipment (welder) provided protected by RCD. Attempting to comply (bluff) you could find on inspection two phases connected through a single phase double pole RCD, don't work on button (wonder why) or on trial (just to confirm once again RCD live probe to neutral earth combined don't work. However hopefully now superseded, bought one myself £50 at B & Q reduced because the mask was missing from the box. Carry it like a handbag with welding power more powerful and better than the big old-fashioned oil cooled stick welder. (The small none transformer inverter stick welder is a great invention).

The welder does not require a neutral - and I will try and get a double pole mcb, otherwise I will use 2 poles of a 3 phase mcb.

Thanks for the responses.

I put my MIG welder away last week until I find some PPE.......

I didn't properly assess the risk of getting under my 6 x 4 x 1/2 steel bench top to weld it to one of the steel staunchions. I'd got the safety goggles, welding mask, gloves, trousers over the rigger boots and thought that was enough. *
I hadn't counted on some weld splatter hitting the underside of the top and landing in my ear........ Don't laugh

Regards

BOD

* In addition to my usual work clothes before anyone suggests otherwise

Ouch - still it clears out those stray ear hairs that men of a certain age get BOD - so it's an ill wind I guess

and that's one hell of a bench I might add !!

regards

OMS

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Failure is always an option

It was a road plate that I "picked up"

Yes, I had to visit the Drs but when she looked in, she didn't find anything there

Regards

BOD

Not on your own you didn't - or if you did, I've a wide belt you can borrow

That's just what you want to hear from the MedicO - all clear

ear defenders or a balaclava helmet or both - but wait till it's cooler eh -

Best regards

OMS

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Failure is always an option