Rating of relay is 40A or 30A?

[treez]

Hello,
Do you know why the following relay is said to be rated for “30A” at the top of the page, but lower down it says “40A”.
We need this relay for an offline PSU which is 7kw (230VAC).

T92S7D12-12 relay
https://www.farnell.com/datasheets/1729804.pdf

 

[mtwieg]

It's rated 30A, even though it can tolerate up to 40A. Just like how an IC may tolerate a 6V supply voltage in its absolute maximum specs, but is rated at its nominal supply voltage of 5V.

 

[FlapJack]

30 A continuous.
40 A intermittent.

 

[treez]

thanks, the boss wants us to use this relay for a relay that will carry 32 Amps rms continuously into an smps.

 

[Warpspeed]

Current carrying capacity is a different thing to current making and breaking capacity.

There are a number of issues, but I suppose the big one would be how frequently this relay has to make and break under full load.

Sitting there with 32 amps flowing through it should not present a problem.
Neither should very infrequent operation of something like a power up soft start circuit.

If it is to cycle constantly, for instance in a thermostatic temperature control, I would worry a lot more about the marginal rating.

 

[mtwieg]

Yeah I definitely would keep the switching current within the rated spec, except in rare and exceptional circumstances. Make sure inrush current due to capacitor banks is limited as well, otherwise your contacts will fuse quickly.

 

[Warpspeed]

Its often possible with a capacitor charging soft start, to keep the relay closing current fairly low.
Then gradually ramp up the action in the switching part of the circuit so that input current only starts to really climb after the soft start relay has fully closed.

In that case the relay only has to carry 32 amps, not switch 32 amps, which should be a walk in the park.

 

[mtwieg]

I meant the actual input capacitance of the SMPS, which causes inrush regardless of whether the SMPS is actually active or not. Especially with boost converters where you also see the output capacitance through the input.

 

[Warpspeed]

It depends on the value of inrush limiting resistor.

Given sufficient time, all the capacitance can fully charge before the relay closes across that start up resistor, and before PWM switching commences.

 

[treez]

thanks all, this is indeed a relay that switches out an inrush resistor at the input to an offline smps with boost pfc. We only switch it when it has zero current in it.