Thank you for explaining these parameters, I knew some of them. Sometimes a data sheet does get a bit overwhelming, maths isn't my strongest subject, I do struggle a bit with it. This afternoon I've had a play with a thyristor zener diode and 470 ohm half watt resistor, oh and a 10 Amp 20mm fuse. The load was a single 50 watt halogen lamp. It works, but there where a few bumps along the way. First I tried a TYN1225 SCR and a zener diode 1N5351B and a 470 ohm resistor also 0.1 uf capacitor. The SCR just died if I raised the voltage at the point the zener reverse voltage started conducting. The zener survived, the fuse didn't blow. I changed the SCR to a 2N6509G, this combination worked, I tested it over and over blowing 10 Amp fuses just to see if the components where survivable. They where, one thing I noted is the 50 watt lamp load had the briefest of glows, but didn't distroy the lamp filermant. Almost like the very shortest of delays, but after about 6 blown fuses the lamp survived each over voltage scenario. I powered it back up through the crowbar circuit with 12 Volts, and it was fine. I don't know the issue with the TYN1225 that died twice, but the 2N6509G SCR and the zener diode 1N5351B worked ok I think. I'm unsure of the very tiny delay in allowing the lamp the briefest of power just a tiny glimmer before the 10 fuse blew. Maybe the fuse should have been a bit lower rating, I'm not sure.