[AMPS] Solid state relays and HV fusing

[MM@plextek.co.uk (Mark Marsden)]

A disadavantage of SSRs in primary supply / soft-start
switches of a HV transformer is that Triacs continue to conduct
(in a flashover fault situation) until the next mains zero-crossing.
It could take up to 10ms at 50Hz to interrupt the fault.

The time constant of a 40uFcapacitor bank with 
a 50 Ohm fault-current limiting resistor is 2ms.

If a HV fuse in the anode supply interrupts a fault discharge it's
energy from the capacitors (in a capacitor filter supply) that 
blows the fuse. 

Then the loss of anode voltage (through interlocking) can
switch off the PTT and mains supply (but not the blower).

Then the primary-side SSRs don't supply the fault current as
they wait it out till the next zero-crossing


Mark G4AXX & M4R,  Granta Contest Group
                                                                        
                                                                        
                                                                        
                                                                        
                                                                        
                                                                        
                                                                        
                                                                        
                                                                        
                                                                        
                                                                        
                                                                        
                                                                        
                                                                        
                                                                        
                                                                        
                                                                        
                                                                        
                                                           

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