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Welder Inrush Current

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peak412/01/2020 19:21:48
1322 forum posts
147 photos

A general query about welders/transformers and MCBs in the workshop.
I have a stick welder which trips the MCB when powered up, I think due to the quick saturation/inrush current.
The Backstory
When we moved over to Buxton, I had a new garage/workshop built, with single phase supply fed off the house dis board via a suitable armoured cable. (Can't remember now, I think 10mm².) My builder is also a qualified sparky, and all wiring is correctly tested, recorded, and registered.

The house end of the cable is terminated in a C40 breaker in a spare non-ELCB position on the normal domestic board.
It terminates at the garage end on a new 12 way dis board with ELCB protection supplied at this end, so I don't trip anything in the house.

Amongst the facilities in there, I have a dedicated 16A socket to run a 110v transformer. Originally I bought an ex -WD 1.5 KW torroidal one, but the fast saturation was enough to trip a D16 breaker, I didn't want to go to anything higher as I don't want to trip the breaker in the house board.
I duly replaced it with a 2KW conventional transformer running off a C16 and all now OK with that side of things. The chap I bought the torroidal one off demonstrated it at his house; it ran fine off his D16, but it was much further away and a far older install. My cable runs are quite short and have good low impedance(s) all round.

I later picked up an older oil cooled stick welder, with the intention of running that off another dedicated 16Amp socket. The (different) seller ran it off an MCB, rather than a fuse, again I believe a D16, but said that he just needed to power it up initially at one end of the current adjustment, and then after the transformer had saturated, he could weld at all of the available ranges without a problem. He ran it in his other workshop via a fused outlet.
Unfortunately, at my workshop it trips a D16 breaker on power up. I'm again wary of going too high, as I don't want to risk tripping the C40 at the house end.

The 16Amp sockets are each fed with 2.5mm² twin and earth, radially to dedicated MCBs, so the cable should be good for 20A
I guess I could try a C20; I'm the only user working in there at the time, so wouldn't actually be welding simultaneously whilst running a 110v grinder etc.
I'm aware I'd have to consider the 3HP compressor kicking in whilst I'm welding.

I'm confident that the welder isn't actually faulty, as I've had a play with a megger, so I think it's purely a speed of saturation problem, with a large inrush current causing the tripping; I suspect it would be fine running off proper old fashioned 16A fuse wire.

I have a smaller 140A SIP stick welder, and a 140A Cebora MIG which work fine in the same workshop off a conventional 13A ring main fed from a B20

Does anyone have any useful suggestions please?
I know we have folks on here who actually do/did workshop/industrial electrics for a living, so experience is better than me studying the trip curves of various MCBs.
I also appreciate that it's not good for the MCBs to keep tripping them, so I've limited my experiments thus far.

I know you can get a slow starter for electric motors, but for welders???

Cheers Bill

JohnF12/01/2020 19:56:32
1034 forum posts
148 photos

Hi Bill, Electricity is not my forte other than simple ring main and lighting wiring etc, however I had the same problem with my welders. The MIG ran just fine but my TIG which also has a stick welder outlet was a different story, for many years I had only a fused supply but we had the house upgraded to a modern RCD / MCB unit and I also did the workshop with a dedicated circuit for welders only as well as the ring main and lights.

From that point the TIG welder would start OK but after a singe run it would trip the MCB thereafter when trying to start a weld so contacted our spark who came and changed the MCB to a different rating - problem cured.

Trust this may be helpful but I am not competent to offer definitive advice but I can if you wish look at the MCB rating he fitted please let me know.


Ian McVickers12/01/2020 19:59:47
195 forum posts
91 photos

Try a D20. This will supply a higher inrush current.

peak412/01/2020 20:20:23
1322 forum posts
147 photos
Posted by Ian McVickers on 12/01/2020 19:59:47:

Try a D20. This will supply a higher inrush current.

Unfortunately, I don't think it's quite that simple.
A "C" curve nominally trips @ 5-10 times rated inrush current, whereas a "D" trips @ 10-20
Not happy about the idea of upping the house end of the garage supply to a higher rating, as there's then the danger of tripping the house board completely.

Hence my problem, that a D20 at the workshop end will trip at about the same current as the C40 at the house end.
I need the MCB equivalent of a slow blow fuse at the garage end's 16A supply.


Dave Halford12/01/2020 20:26:29
1142 forum posts
11 photos


It's possible your 16A D type is a bit quick off the mark if it's a budget brand. Get a Schneider

A 20A type D will give you the discrimination you need with the house, I wouldn't drop to a C you still need the high inrush protection the C can't supply. Don't forget a D is just slower to operate so the inrush is in effect ignored the figures you are worried about are instantaneous values

Edited By Dave Halford on 12/01/2020 20:31:44

Edited By Dave Halford on 12/01/2020 20:43:44

Neil Wyatt12/01/2020 20:29:56
18409 forum posts
718 photos
78 articles

Best suggestion I was given is - use an extension lead.


peak412/01/2020 21:31:03
1322 forum posts
147 photos

Dave, I'll double check, but I think the D16 is a Schneider

Neil, I did think about that whilst I was typing up my original query. Why I never considered it before, I'll never know.
I do have a couple of long 2.5mm extensions, so I might re-purpose my 110v one of those to 16A connectors and see what that does. It's quite long so it might do; just a shame it would overheat if I left it on the reel.

Anything will have to wait a day or two though as I'm otherwise engaged tomorrow.


Emgee12/01/2020 21:35:27
1825 forum posts
240 photos

Hi Bill

I would advise to remove the existing garage supply from the consumer unit and fit a SP&N Switch fuse with a 63A BS88 fuse for protection supplied direct from the incoming supply tails.
This will provide means to isolate the garage supply and lock it off for maintenance if required and overcome any problem associated with tripping the house consumer unit.

Then as suggested uprate the mcb supply to the welder to a D type.


Steviegtr12/01/2020 21:58:43
1778 forum posts
235 photos

We always used D type circuit breakers for welders in industry. Or the earlier ones were HRC fuses. Now out of date really. Cannot remember because of my old age, but the trip capacity of a D type breaker was very high. Always used C type for Ring mains & A or B for lighting. On control panels we used a device called a MMS or manual motor starter. These had auxiliary contacts for other uses.

Mark Rand12/01/2020 22:40:30
946 forum posts
8 photos

If it's an Oxford/Pickhil etc. Bantam or equivalent welder, it really needs a D32 to supply it. Cost me a few breakers to work that out. If you don't want to automate anything, wire up a box with either a 15Ohm 100W resistor or a kettle element and a switch to short it out, in series with the mains lead plug the welder in with the switch open, then short the resistor out before starting to weld. That'll limit the inrush current to no more than 16Amps.

I recently sorted the same problem on my electronic multi process welder that has replaced the oil cooled one. That had the problem that the inrush current to its power supply regularly welded its switch contacts closed angry. Said resistor, coupled up with a time delay relay has tamed the problem.

PS, anyone need an oil cooled welder?

Steviegtr12/01/2020 22:53:39
1778 forum posts
235 photos

Don't forget with the bantam & any oil cooled welder to at some point change the oil. It's dialectric strength breaks down & they end up taking more current than needed.

Steviegtr12/01/2020 22:54:42
1778 forum posts
235 photos

To add you can usually tell when the oil needs changing by the smell of the oil. Hard to explain.

larry phelan 113/01/2020 14:19:58
903 forum posts
17 photos

Not sure if this will help, but when I installed my Transwave rotary converter, I was advised to make sure I fitted trips suitable for motor running [I think they are type "D" ] It seems the D type can cope better with the surge when the motor starts. Anyone out there know more about this ? I bet there is !!!

Howard Lewis13/01/2020 16:18:33
4136 forum posts
3 photos

Possibly, you need a MCB with a longer reaction time, equivalent to a slow blow as opposed to a quick blow one,

Not being an electrician, this is probably the difference between the A, B, C and D types.


Steviegtr13/01/2020 16:30:42
1778 forum posts
235 photos

Yes it is. Simply starting with the ( A ) type mcb for light standard type resistive none surge loads. Lighting etc. If a lot of fluorescent lighting then up to ( B) . (C) Nearly always for Sockets & ring mains etc. The (D) type for motor circuits or anything with a large surge. Sorry cannot remember the KVA ratings of them any more. Hope this helps. Retired Electrical contractor.


Nicholas Farr13/01/2020 16:59:43
2549 forum posts
1210 photos

Hi bill, is there a ratings plate on your welder? It is quite possible that you will need a 32amp supply for an oil cooled welder, even if you can get it powered up. It won't mean that it'll draw 32amps all the time that you are using it, but it may need the extra power for striking the arc up.

Regards Nick.

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