That could just be the problem, winding the tensioner down that hard puts strain on the motor that it simply isn't designed to handle.
Tripping electrics might imply a fault. A common one on the SIPs is the power wire that goes to the torch can fray where it joins to the wire feeder, and can touch the casing. Zippy clipping the outer sleeve to the end of the wire feed mechanism can help take tension from that area, and a steel liner should help too.
Otherwise might be worth a peep inside to see if there are any wires touching anything they shouldn't.
I would have expected the trip to go when the arc is started rather than when the trigger is pressed without an arc. I'm no expert in these things though - someone else might have a better idea.
Normally a 13 amp fuse would be OK on the high setting of one of those, but trip switches are more sensitive to initial surge current (transformers filling up, starting the arc). If the trip switch is a 13 amp B type then you might well be restricted to the lower settings. What spec of trip switch is it?