[AMPS] Solid state relays and HV fusing

[Phil T. (VE3OZZ)]

>
>
>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.
>

Yea I agree. But I don't think mechanical relays on the mains are any
better at this either.  I haven't looked at make/release times for the
usual clunky mechanical units but I don't think they're noted for
lightning-quick response.

>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
>
>

Maybe this is why many of these SSRs are available either with zero-cross
or with random operation.  I wondered why they would offer the option when
I was looking at the data sheets.

Good points Mark.

Phil

>Mark G4AXX


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