Will a zero-crossing SSR activate with shorted load contacts?

Question

If I have a zero-crossing SSR switching 240VAC (e.g. this LS240D12), will the zero-crossing detection function if the load side is shorted?

I know that zero-crossing SSR's can't be used with DC loads because the load never crosses zero. What I'm wondering is if with an AC load the SSR only cares that the voltage is zero, or if it actually needs the voltage to cross zero.

E.g. in the following schematic:

If the switch is already closed and the SSR receives an activation signal on the control side will it actually activate, or will it only activate once the switch is opened?

I'm asking because I'm hoping to use a combination of SSR and mechanical relay in parallel (per advice received elsewhere). The idea is to prevent arcing in the mechanical relay, and to prevent heat buildup in the SSR.

To turn on: SSR activates at zero crossing, small delay, mechanical relay activates, small delay, SSR deactivates.

To turn off: SSR activates, small delay, mechanical relay deactivates, small delay, SSR deactivates at zero crossing.

My concern is that the "turn off" procedure described above may not in fact reduce arcing in the mechanical relay if the SSR fails to truly activate without alternating voltage on its outputs.

Answer 1

From the internal block diagram, we can see how it works:

The power to trigger the thyristor gates comes from current flowing through the load near the zero crossing.

So, if you apply the appropriate control voltage to the SSR then open the contact then the voltage could rise very quickly and not allow a long enough pulse to actually trigger the thyristors. I suspect it will be erratic and the erratic behavior will depend on the load, any snubber, and the exact time during the AC cycle that the contact happens to open.

Confining our thoughts to the linked datasheet, the random trigger type of SSR would likely be better for this application (there would be more EMI, but less than just a relay contact, and the thyristor power dissipation and relay contact life issues would be mitigated).

Answer 2

That data sheets says it has an SCR output stage, an SCR cannot be started with 0V across it.

As for whether shorted output will trigger the zero crossing detector it's impossible to tell. maybe it will maybe it will not. it's probably easier to use the immediate trigger version.

So the question now becomes will the SCR restart fast enough or stop slow enough to mitigate relay contacts bouncing. some of that depends on the trigger circuit. if the datasheet does not suggest using this SSR in parallel with a mechanical relay it may be unsuited for that task.