Hi all,
We are being presented a marshalling board with many (50-odd) volt free contacts to be picked up to a SCADA. The supplier has put one side of the volt-frees onto a single common, wired up to a single terminal. So if there are N signals, there are N+1 terminals.
At the other end are multiple input devices, but the inputs take a common voltage source and form a single circuit per BS7671 definitions. The volt-free circuits are 24Vdc operating solid-state inputs imparting a negligible load.
Being so many signals, we have to use multiple cables. One option is to fit a few additional terminals on spare rail and disconnect the common bus wire in a few places to make groups which match the cables. The alternative proposed is just to use multiple cables using just one core for the common; due to 521.8 I would counter setting aside a core for the common in each cable with them all coming together (via some suitably neat method!) into the designated common terminal.
The latter is obviously simpler in that it avoids modifying an arrangement by others. My concern is that this still doesn't meet the intent of 521.8. It also doesn't seem to meet the principle of least surprise for maintenance teams.
(Yes, I know it would be easier to fit the SCADA DI unit in the marshalling box, but there are reasons)
... Which one (or something else) would you do?
Ta
Jam
PS - Enjoy the sky-show everyone! (c:
