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Multicore signalling cables

Jam

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:

  • 0

    60 core cable ? https://www.farnell.com/datasheets/427000.pdf 

    one per relay contact and then the rest can be common return.

    You may need to find someone with good colour sense to wire both ends, as 'tan' 'orange' and 'brown' are perhaps easily confused. If you struggle with white, snow and pale cream at the paint shop, this cable is not for you.

    If its not going very far, ribbon cable may be easier. 


    Mike

  • 0

    If you have a cable with just returns/feeds, rather than common (for each of the returns/feeds), you could well run into problems with EMC, and not following RTU/outstation and/or field equipment manufacturer's instructions for EMC.

    The options are:

    • Second marshalling cubicle arrangement so you can match the terminal arrangement to the planned cables (but this takes up more space)
    • Get the marshalling panel manufacturer's permission to add more 'common' terminals.

    Assuming this is a wider programme of work, who is responsible for coordination of the designs? Sounds like project change procedure might be required?

    The worst thing I've had to address with design coordination and SCADA, is a designer of monitored field equipment assuming LV switched signals, but the outstation being 24 V DC PELV, and despite both design teams being in possession of the Client's specification for SCADA stating 24 V DC and interposing relays, where needed, having to be in field equipment ! Luckily no-one got hurt!

  • 0 in reply to gkenyon

    Thanks both. Good to hear I’m not overthinking it.

      have been trying to stick with easy to obtain BS6724 or YY flex as it’s not a particularly long run so MOQs are a consideration.

    Will look to have the marshalling board modified. Shouldn’t be that difficult just needs coordinating as you say.

  • 0 in reply to Jam

    it’s not a particularly long run

    How about single "hookup" wires in some kind of containment? (flexible conduit or trunking or possibly even heat-shrink) - 0.25mm² typically have an outside diameter of only about 1.3mm - so if neatly arranged 51 cores should fit into a bundle hardly half an inch in diameter. You'd have to mark up the ends of each core for identification of course, but that's more tedious that difficult.

      - Andy.

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