[Diabolical Artificer]

Thanks Dave. I found an interesting protection circuit whilst reading an old book, it used a "voltage dependant relay" which was in a BJT PSU, from memory it was a resistor on the base of the series pass device, the went to one end of the relay coil, the other end being fed by Vcc+. The SW shorted the rails, blowing a fuse, I guess a VDR is a normal relay + resistor.

Andy.