Michael Chare formulated the question :
On 07/02/2019 23:22, Paddy Dzell wrote:
I have a hybrid car and the 12V battery is under the boot floor, which is
awkward to get to if I ever need to jump start the car so I've installed a
socket in a more convenient and accessible place and I've taken two wires
directly from the battery to that socket.

The socket is keyed so there's no chance of any reverse polarity accident
occuring if I ever need to use my jump start battery pack but I need to
protect that cable run from any sort of accident resulting in a
short-circuit, so I bought a 12V circuit breaker rated at 30A (the
computers draw less than 20A to get the hybrid system to the 'READY'
state).

My question is - the circuit breaker has one connection labelled as 'Line'
and the other one as 'Load', so is there anything in there that means it
will only operate with current flowing in that direction?

The wires from the battery to the socket will have a permanent 12V supply
on them. If I were to plug anything into that socket then current would
flow from battery to socket. However, it's purpose is to do the reverse and
provide 12V from an external source to the flat battery so current flow
would be from socket to battery, so will the breaker work 'going one way'
as an overcurrent device in case of a fault but also allow current flow in
the opposite direction when/if needed?

Or should I just put a 30A in-line fuse in instead of the breaker?

Google for 'starter motor current'. If a car wont start, I either put it on
charge or just go an buy a new battery using a different vehicle.

A normal starter motor draws upwards of 300A when cranking the engine
but there's no starter motor in a hybrid and all the 12V battery does
is power the entry/exit system and boots the computers to get the
hybrid system to the READY state, a draw of less than 20A.