Thyristor protection circuit, Gate terminal, over current protection,

 BTA16 Series 600 Vdrm 16 A Flange Mount Snubberless Triac Thyristor - TO-220AB

https://www.futureelectronics.com/p/electromechanical--circuit-protection--power-thyristors/bta16-600cwrg-stmicroelectronics-9285586