A normal ring main is designed to have a maximum of 20 amp draw at the mid point and 12 amp evenly distributed around the main. If the ring main was to get over 20 amp draw at a point close to the consumer unit then in theory an overload could occur and if the maximum of 32 amp is drawn at mid point then the volt drop could be exceeded in practice this is unlikely.
However heavy current using equipment like washing machines, tumble dryers etc are often feed with a radial or dedicated ring main. And any built-in appliance like ovens and immersion heaters will also have their own supply.
Although 3kw or 13 amp should be the limit with a BS1363 plug as 13A is the largest BS1362 fuse made but the fuse will allow 20 amp for some time and welders with intermittent use can draw high currents for a short time. Normally this is not a problem.
Where it does become a problem is where there is a second fault. Mistakes like broken ring mains or spurs off spurs even lose connections when combined with the use of devices like welders drawing over current can cause problems.
Where there have been no DIY work undertaken and the house has been checked every 10 years as required by BS7671 then no problems but where the DIY man has fitted 2 x 32A MCB’s to a single ring main which I have found in the past or cavity wall insulation has been added to walls with lose cables in then there can be some real problems.
So if you are using the welder for a very short time once in a Blue Moon then OK. But if used on a regular basis then I would run in a dedicated supply for it.
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