Dan,
In a previous incarnation (before I sold my soul to EMC) I did product
design (redesign after the compliance department tore it apart) for industrial
control systems. I recall being shown a clause in 60204-1, Electrical Safety
for Machines, that prohibited switching the grounded side of a control system
(there were a number of other clauses about the hoops you need to jump through
if you don't ground one side of your control system). I am not currently
involved with safety requirements so I can't quote directly from the standard
(but there must one or two or our faithful contributors who can help me out);
but I believe it went something like 'when switching control devices one side
of the device shall be connected to the protective bonding circuit'.
I believe the reason for this (and for the other requirements if you have a
floating control system) is to prevent an inadvertent ground in the control
wiring from initiating a control action. I have seen 'negative logic' allowed
as a deviation when the logic and control device are all on a single circuit
board (there are no wired connections to get grounded, and an inadvertent short
to +V is as likely as an inadvertent short to ground), and also for cases where
you would rather have the control function occur than have it disabled by a
fault.
I hope this helps, I hope some of our dedicated safety members will jump in and
shed some light.
Paul McCoy
"Dan Kinney (A)" wrote:
> All,
>
> I have been told negative logic Input/Output devices are not allowed in
> Europe. I have found the following in EN61131-2:1994 (IEC 1131-2) Page 28,
> Para 3.3, note 1
>
> "Current souring inputs and current sinking outputs which may be required
> for certain applications are not covered in this standard. Special care
> should be exercised in their uses."
>
> This says current sinking outputs are permitted. Can anyone point to
> anything that prohibits them?
>
> Dan Kinney
> Horner APG
>
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