Problem Measuring Thyristor Gate Signals

Was there an isolating transformer between the oscilloscope and the local single phase supply powering it? It sounds like your technician accidentally grounded the thyristor output using the ground-referenced ground probe lead causing a low impedance (high current) phase to ground fault, resulting in the explosion and blowing the fuses.

This may be the case as poster is writing, he measured between anode and gate. Imagine whats going to happen if un-isolted ground wire is on anode?

Brings back memories for me. If the internal oscilloscope circuitry was definitely isolated from the test circuit, I would be wary of probes slipping off of components. I had to place heat shrink tubing over my probe tip except for the very, very end to prevent it causing shorts in the circuit under test.

What is the specified min firing current of Thyristor? In my opinion you should have measured between ground and gate. and also thyristor should have been unloaded before any measurements.

if anode was at 400 volts,20 MΩ can pass 0.4mA through gate,not sure if it can fire thyristor.more info about circuit may help.

Dear Rakesk Semwal,

Thanks for advise , Igt is 300mA AND IL is 800mA.

May i know why if i measure ground and gate may cause explode ?

Regards