Running an arc welder off a 16amp MCB

Postby creekybones » Mon Nov 02, 2009 9:37 pm

Hi all,
I am enquiring about what should be done about my garage circuit. I have a new split load consumer unit for my home circuit, this feeds the garage via a 16 amp mcb and armoured cable. the garage is detached from the house about 15ft away. The feed in the garage rises through the floor to a fused spur.
I have several sockets in the garage and some strip lights. When I switch the welder on the initial load trips the mcb (most of the time). If I leave the welder on then reset the mcb it holds ok, until I need to cycle the power on the welder again.

My question(s) is what could be done to prevent the mcb from tripping? Can I install a 240v 16amp socket in the garage and plug the welder into it,would it prevent the tripping? Or would I need to install a 16 amp mcb in the garage to save me going back in the house everytime I turn it back on?

I am not interested in up grading the cable, or running another supply to the garage as I cannot hide the cable.

Is there any other options I have?

Lots of questions, Thanks in advance for your replies.

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Postby chriscba » Tue Nov 03, 2009 7:59 pm

Try a industrial rated mcb, I think they are type C, instead of domestic type B.

They can withstand higher serge currants.
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Postby chriscba » Tue Nov 03, 2009 9:04 pm

I didnt realise your welder was fed from a fused spur !!!

The garage should have its own CU, fed from the house via 32a mcb,(if ring circuit in garage, if radial 20a) then in garage 16a mcb type C for welder, 32a for ring ect...
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Postby ericmark » Tue Nov 03, 2009 9:09 pm

The 16A MCB has three types B, C, and D which have two tripping devices the thermal is set to 16A but the magnet is for a B type 5 x 16A or 80A and for C its 10 times for 160A and for D its 20 times or 320A.
To ensure these will trip in time should there be a fault going to earth with a D type using ohms law 230/320 = 0.72 ohms against the 2.87 ohms for B type so not simply a matter of changing first one has to know the earth loop impedance (ELI) and this should be shown on the installation certificate or periodic inspection report but in practice this is rare. Only since Part P in 2004 have these readings been provided by registered electricians but since it would likely need Part P registration to change the MCB then likely you will need an electrician to do the work anyway.
So method one is call an electrician who may be able to help.

Second wedding sets are in essence big transformers and these tend to have a large inrush and also poor power factor correction however there are now welding inverters which draw far less current on switch on and they normally have a better figure for power factor nearer to unity. They are also lighter and give a high frequency output when on AC giving the feel of using a DC set so likely changing to one of these more modern sets could also help.

Sorry no magic wand solution. If you only weld once in blue moon then I would call an electrician to test insulation and see if the MCB can be changed.

If you do a lot of welding then better to change to using an inverter instead of transformer.
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Postby creekybones » Wed Nov 04, 2009 8:19 am

Thanks for your replies. Some good advice there.
Ericmark: You seem to know alot about welder's!
I will give the sparky that fitted the house consumer unit 2 years agoa call, If he is still around.
Is there a rough figure of fitting a consumer unit in the garage, will he have to test the whole house again or just the garage circuit? Trying to guage cost as if its as much as the house CU I'll have to wait till after crimbo!

Also you mention feeding the garage with a 32amp MCB, would that not overate the garage supply cable?

Thanks for the replies.
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Postby ericmark » Wed Nov 04, 2009 1:06 pm

Over the last two years there have been changes in the rules which could impact the work involved.
1) May now need RCD fitting.
2) May need to be 110 volt old demarcation was 3Kw now 32A depends on how garage is classed I would not think this applies to you.

I did a google and you can get a 120A inverter for around £180 and welding voltage is 25 volt plus 1 volt per 25 amp so at 120A approx 30 volt so 30 x 120 = 3600W which at 230 volt = 15.65A so is about the max on a 16A supply working on these figures likely cheaper to change welder than to modify wiring plus new type welders are so much better anyway.

For old type welders because not as efficient 90A was about limit on 16A socket.

Both will run on 13A plugs because the 13A fuse will allow quite an overload before blowing but try a google for welding inverter and see what you think!
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Postby sparx » Wed Nov 04, 2009 6:47 pm

Hi, not to detract from any of the comments so far, it would seem your welder is not blowing the 13A fuse in spur so clearly it's only inrush current at switch on that is causing the problem.
As said there are different types of mcb, it may be a change to a type C or even D may solve problem BUT they each have a reduced total loop impedance limit.
You [or probably your leckie] would need to check the earth loop impedance reading in garage, this will tell the type of mcb you can use, as ERIC suggests.
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Postby creekybones » Thu Nov 05, 2009 9:13 am

Thanks for those replies.
I have been looking at the inverter welders, the small portable ones look quite good compared to my old arc. So will look into. I think I'll get the sparky round as well to look at the ELI as it sound like its worth sorting.

Thanks for your help guys

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Postby moggy1968 » Sat Nov 07, 2009 7:29 pm

Usually the earthing is good enough to be able to meet the disconection time even with a Ctype, if it isn't you could change to a C type RCBO which gets around the problem of tripping in the required time as it is rated at 30ma (ion the RCD side and whatever you need on the overload side).
Thats how a house (or detached outbuilding) with TT type earthing (i.e. through a rod or other similar means rather than the suppliers earth) gets around the problem. TT systems always have pants EFLI and without this get out of jail card would never be able to pass a test!
bewareof uprating the circuit until you know the capacity of the cable
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Postby creekybones » Wed Nov 11, 2009 11:43 pm

Thanks Moggy,
Just out of interest and feedback from you guys the cable feeding the garage is 2 core amoured and using the sheaf as the earth going back to the earth terminal at the main board.

Whats your thinking on this?

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Postby ZanderJay » Fri Oct 22, 2010 3:03 am

To ensure the which type of tripping device, it might be better to use the 16A mcb C. It could make a good changing matter for the installation process. I think that the the part of having that comparative of welder.
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Postby ericmark » Fri Oct 22, 2010 3:52 pm

The problem only flex which under regulations should not be covered has a fixed amp ratting according to cable size. Once one starts with fixed cabling the routes make a huge difference. For example a 2.5mm cable with range from 13.5A to 33A according to how it is run and the temperature rating of the cable.

I am sure we all break the rules from time to time and don't check the earth loop impedance especially where we know there is an RCD. However we do in the main test and as a result have a good idea what the results will likely be. I could normally give a fairly close result from an estimation if caught out and asked. Or at least arrive a few hours latter if I forgot the meter.

But with the price to hire tester or to get an electrician to test you could put it instead toward an inverter. Because even the AC output type use a high frequency they weld very like a DC set. Of course still 50:50 on heat not the 75:25 of DC set but the feel is like a DC. If you have ever used a mighty midget welder you will know the feel. AC but not as we know it.

The humble 13A fuse will allow something like 25A to flow for short time without blowing and I always check welder plugs to see if there is a fuse the number I have found with two neutral pins I have lost count of. Then people wonder why the socket burns?

Earthing is a big problem with welders. Years ago with oil filled welders it was common to find the output earthed as when they are dropped the bare copper wires inside can bend and connect the incoming supply to the output. However this produced other dangers as the welding current could easy end up going down the mains earthing wires and burn them out. After a few nasty accidents the HSE put out a warning and where I worked the old oil filed welders were only used in the workshop where all earth cables were 50mm so bad earths on welders could not burn out the mains earth wires.

So in the main all welders have to have isolation transformers and they have a little shield with two circles to show they are isolation type transformers to EU standards. This means oil filled are no longer used nearly all welders are now air cooled. Even so many old welders no longer comply with HSE directives.

Welders are a big problem and I have seen old non double insulated drills with completely burn out cables where they were left lying on a metal table that some one else welded on. I have found so many unearthed sockets due to welders burning out earth wires and skipped so many extension leads.

If you are a welder you will know the problems and will be super careful connecting the welder earth. I hate magnet earth clamps with a vengeance as they so often make bad connections nothing like a good brass clamp.

Do say how you get on.
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