I wonder what its life would be if it was seldom switched under
load.
You might ask, what good is a 300A relay if you never switch a
load with it...
In a previous life I designed motor control electronics at a
forklift company. The forklift had a 600 amp main power
contactor that switched power to the motor controllers. But
under normal operation the main power contactor was never opened
or closed with the motors powered. In the software, the motor
controllers were all shut off first, then the contactor was
opened. So the contactor only opened in an error situation
where the software determined there was something wrong, such as
a failed motor controller.
I did something like that. It's a power amp for MRI imaging. There's a
BIG 3-phase transformer with multiple secondary windings and
rectifiers to give, basicly, floating bipolar power supplies of 100,
50, and 25 volts. A bunch of smallish relays switch power supply
sections in and out, so we wind up with a dual 3-bit DAC, 25 volts per
LSB, 100 amps max. The relays are selected to set the power amp
voltage rails, depending on the expected load. To keep the relays from
exploding, the firmware makes sure they're switched at zero current.
There is an aux supply to precharge the filter caps so the relays
don't see the surge. It's the world's dumbest switcher: transformer,
rectifier, mosfet, and a big power resistor, with the uP turning the
fet on/off as needed.
This all works great, and when, rarely, something goes wrong, we get
smoke and stuff.
John