You need some chunky switches when you’re connecting and disconnecting 300V. Hence, contactors. These units I bought from one of the members of the OpenInverter forum. They are American made Gigavac GV240 units (full datasheet here)rated for 800V and 400A. I expect to be drawing a maximum of about 330V and 200A so these are well within spec.

The last four characters in the product’s name tell you a little bit more about its specifications:

• ‘2’ means that it is side-mounted with a single M8 bolt
• ‘M’ means that it has a built-in PWM economiser for the 12/24V coil that drives the switch. What this means is that the contactor can draw a chunky amount of current when it needs it do to the initial switching, but then drop that current right back to hold the switch closed. This extends the life of the coil and reduces power consumption.
• ‘A’ means that the driving coil is terminated in flying leads, which should make hooking them up nice and easy.
• ‘B’ means that is a single pole, single throw (SPST) relay that is normally open (NO). In other words, there are only two switch terminals to connect and the switch remains open until you apply some current, which is what we want.

Each contactor has two M8 threaded terminals for the high voltage contacts and four wires for the low voltage conections. These are:

• Red: LV coil positive – apply 12 or 24V here to switch the contactor on
• Black: LV coil ground – the return for the switching circuit
• Blue: Auxiliary contact 1 – if I have understood correctly, this allows you to sense the contactor’s state using a mechanical connection to the main switch
• White: Auxiliary contact 2 – the other connector for sensing the contactor’s state