A1 A2 DC Relays?

EN50005

BS5472

Switching a 'live' is generally preferable to switching 'earths'

But it often depends on the earthing scheme, for instance you could have a 48v positive earth with a switched negative and a 110v negative earth with a switched positive

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No idea if 0V was ground.

AC wouldn,t matter

AB AB

Saol fada agus breac-shláinte chugat.

Unless of course you are a Brexit voting moron or child molester

Really? On car EFI systems, most is ground switched. Gets rid of the need for a high(er) current positive rail on the PCB - the ground layer normally being pretty massive.

I agree the likes of injectors (and ignition coils?) are low side driven, maybe an architecture that evolved from the early days when N channel mosfets were (and are?) always more rugged?

Switches such as oil pressure warning also switch to earth, but the likes of lamps on modern vehicles still seem to be live switched from a body control ecu or local canbus node with an earth to the bodyshell at or close to the lamp

Back to the original 'industrial' scenario switching 'live' reduces somewhat the possibility of a stray earth causing an unwanted operation / indication, said stray earth possibly introduced by four or eight legged wildlife exploring a control panel.

For controlling things switching both +ve and -ve together with circuit integrity monitoring is the norm when you need assurance it's going to work when required and avoid false operation.

Yes.

Quite - where you can happily share a ground, the car body make sense. Reduces the wiring needed for the power side.

Never thought of that.