The 6A Ravistat one looks like its decent quality. See
the manufacturer's specs. For UK bench use, I'd mod it to add a 6A panel mount circuit breaker (or 5A if you cant find a 6A one) on its output if it doesn't already have one, and add a IEC C13 socket for its output so you can easily hook it up to various adapters e.g. a
Quicktest QT1 or a trailing BS1363 13A socket, or your step-down transformer for linear PSU design testing. To minimise the metalwork you may choose to use a trailing lead mounted IEC C13 socket. I wouldn't advise using its binding posts for anything other than monitoring its output voltage on a DMM - keep them screwed down so they aren't an immediate touch hazard.
Limiting the current from any AC PSU except a fully electronically controlled four quadrant bench PSU is *HARD*. Its easy to trip a breaker on over-current but to limit the load current without distorting it and maintain voltage regulation up to the current limit requires fast acting feedback to the output voltage control, which cant be achieved with a mechanical control like a Variac. Even if you motorise it, mechanical inertia will limit its response speed to a significant fraction of a second.
A poor-man's alternative for AC current limiting is a
dim bulb tester which uses a series incandescent bulb to limit the current utilising the x10 resistance increase of a tungsten filament between cold and its normal operating temperature. Either use a low voltage one on the output of your step-down transformer, possibly utilising 24V truck bulbs, or a mains voltage one on its input. Only use genuine incandescent filament bulbs - no LED or CFL bulbs! Its advantageous to have a couple of bulb sockets wired in parallel for more choice of limiting current by fitting different combos of bulbs. You can test the effective limit by putting a heavy resistive load on your Variac (a 500W to 1.5KW oil filled heater or bar fire or radiant heater or water heater would be appropriate, but a fan heater would not as the fan wouldn't run well at reduced voltage and the element might overheat) with the dim bulb tester in the feed to it, and cranking up the output voltage while monitoring the input voltage and current. If you are careful not to exceed the 6A output current rating of the Variac, an electric kettle filled with cold water will do for brief testing, though for a 3KW kettle, you wont be able to draw more than about 4A into the Variac before you reach its max output current rating.
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