On Sun, 07 Jul 2019 08:01:24 +0100, Archibald Tarquin Blenkinsopp
wrote:

Where did the assignation come from originally?

EN50005

BS5472


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?

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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