RC circuit

SCRs & Diodes are prone to damage their pn junctions with dv/dt very high. For inductive loads, as all know well, voltage spikes induces across the terminals & so, across the SCRs / diodes.

tO ELIMINATE THE DV / DT I.E. FAST RATE OF RISE OF VOLTAGES w.r.t. time, We use RC snubber circuits , with proper time contant. This reduces the peak voltage spikes also , & keep the operations within the ratings of SCRs.

Yes, they are most likely there to stop false turn on.

My former employer made a TV camera crane that used a bridge of SCRs to control the motor driving the jib up and down. They worked perfectly on our normal 240V but when we tested the 110V export versions in the factory they kept blowing their (higher current rated) fuses and stranding the tester in the elevated position. It turned out that the 110V transformer supply had a higher impedance than true mains, making every spike and surge generated by the system that much worse. As it neared the top of the movement the geometry created maximum load, creating big spikes every mains cycle as the pair of SCRs that were supposed to be driving switched on. This coupled through the stray capacitance inside the SCRs and in the PCB tracks to turn on the opposite pair at the same time. And bang goes your fuse. It needed an R-C network soldered directly across the SCR terminals to solve the problem. I believe the C can be a problem to source, due to high spike voltages and the low inductance requirement - a wound capacitor construction can act as an inductor and limit its high frequency usefulness.

The snubber is also wired across the start contacts of things where a current surge, as may be caused by a motor starting, might affect a high impedance circuit.

• In student broadcasting several millenia ago, the snubber as described in the original post was wired across the start contact for the Garrard SP25 record deck motor, so that the 'pop' didn't appear on the pickup signal going into the mixer, and didn't come out on the listener's AM radio. It's probably all changed by now....