Running an arc welder off a 16amp MCB

[creekybones]

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.

Creeky

[chriscba]

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

They can withstand higher serge currants.

[chriscba]

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...

[ericmark]

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.

[creekybones]

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.

[ericmark]

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!

[sparx]

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.

[creekybones]

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

Creeky

[moggy1968]

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

[creekybones]

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?

Creeky