For 3 years I used a 3600 watt inline RCD and welded off it, it also had the workshop lights, 800watt angle grinder and a 750watt cold saw running off the same 4 socket extension lead............(not all at the same time), but often the transformer welder on, 6 x 5ft fluorescent tubes, 800watt angle grinder and DAB radio. The largest welder I used on this set up was my Sealey 210 amp mig adjusted to the top of its range. stitch welding 40 X 10mm to 40 X 8 and 30 X 10 MSF. It sometimes tripped out on cold mornings when going straight into welding, but if I switched everything on and let the welder transformer warm up for about 10 minutes it was no problem. In the warmer months I often exceeded the welders duty cycle and its auto cut out tripped but I don't ever recall this tripping the inline RCD.
It did save me from shocks on a few occasions; mainly me putting hot work down on the (live) grinder cable, once cutting the cable with a spinning down angle grinder, once from dropping a sharp steel edge on the cable and once from a friend's stupid dog who chewed the cable.......... who never even noticed what his stupid mutt was doing while I welded a hinge on his wrought iron gate...... dog didn't even get shocked!