Let say we have a relay rated for 12 Vdc - 80A

https://www.amazon.com/uxcell-12V-SPDT-JD1914-Relay/dp/B00LGKRSMG

If this relay connect to load 4 ohm and power supply ( charged capacitor ) 300 Vdc for a few milliseconds The current in this case is 75A

My question when the voltage of circuit exceed the voltage rating of the relay but no for the current will cause damage to relay?

 

You could see insulation break down when the voltage is excessive, also, breaking a voltage of that magnitude, may cause a sustained Plasma arc across the contacts, when they open.

 

You could see insulation break down when the voltage is excessive, also, breaking a voltage of that magnitude, may cause a sustained Plasma arc across the contacts, when they open.

Can this happen even in a few Milliseconds?

 

The voltage rating is more important for when the contacts open (closed to open), then when they close (open to closed).
The contacts can likely tolerate the 300V when open but not for closed to open.

What will be the voltage when the contacts open?

 

My question when the voltage of circuit exceed the voltage rating of the relay but no for the current will cause damage to relay?

Exceeding the nominal working voltage by 25X is almost certain to cause damage.

 

If the relay is being used to discharge a capacitor (charged to 300V) into a 4Ω load, then it is likely that the relay contacts will survive such switching – whether the capacitor can withstand such a rapid change in voltage/stored energy is another thing.

 

Can this happen even in a few Milliseconds?

Yes,
Typically relays that switch DC of this magnitude use a arc blow out magnet close to the contacts, effective when they open.

 

The voltage rating is more important for when the contacts open (closed to open), then when they close (open to closed).
The contacts can likely tolerate the 300V when open but not for closed to open.

What will be the voltage when the contacts open?

The relay will be a switch to coil 4 ohm, to supress the back emf, a diode will be used, so the voltage when the contacts open 300v

 

If the relay is being used to discharge a capacitor (charged to 300V) into a 4Ω load, then it is likely that the relay contacts will survive such switching – whether the capacitor can withstand such a rapid change in voltage/stored energy is another thing.

Yes the relay will be switch for capacitor to discharge
Yes capacitor can discharge rapid

 

Yes,
Typically relays that switch DC of this magnitude use a arc blow out magnet close to the contacts, effective when they open.

That because the back emf of the relay coil itself?

 

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