A1 A2 DC Relays?

Where did the assignation come from originally?
I'm currently playing with a system that has a 24V DC control panel and every relay has A1 connected to 0V.
Not really a problem currently, but a few years back I took out a channel of a dedicated control unit when thoughtlessly adding a diode quenched relay to a digital output through connecting A2 as per drawing. i.e to the digital output of the controller.
The control panel wiring in that was 24V too, although the panel builders had obviously not wired to the "as builts", which showed every A1 grounded.
The damaged controller was got around with the aid of a software Engineer back home who managed to nominate a spare i/o, but since then I have been very wary of the consequences.
Does A1/2 misidentification cause problems generally? is it common to ignore polarity in systems with "unquenched" relay's?
AB
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On Sun, 07 Jul 2019 08:01:24 +0100, Archibald Tarquin Blenkinsopp
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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On Mon, 08 Jul 2019 09:57:15 +0100, The Other Mike

No idea if 0V was ground.
AC wouldn,t matter
AB AB
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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.
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On Tue, 09 Jul 2019 14:11:38 +0100, "Dave Plowman (News)"

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.
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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.

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