An important part of the safety system is the contactor: this connects the high voltage of the battery to the motor controller. Technically, this could be done manually: just bolt the wire onto the right part of the controller. However, it's a bit more convenient to use a remote one: essentially a high-current, high-voltage relay that connects the circuit when the key is turned. Obviously, it's important to make sure it's rated for the right voltage/current so using a standard relay isn't possible as it can't stand up to the sort of current that we need. Finally, a pre-charge resistor is needed to make sure no "arcing" happens within the controller. This is essentially a resistor that allows a little bit of current to flow through to avoid a huge inrush when the contactor is closed.
