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Two Power Sources in a Multipair Cable?

Two Power Sources in a Multipair Cable?

Two Power Sources in a Multipair Cable?

(OP)
I encountered checking a design that calls for 24VDC power supply for a field pressure transmitter. The power supply was taken from a field I/O building with a 24VDC panelboard fed from a DC UPS. The designer proposed to route power conductors from the panelboard to a spare terminal strip at existing analog marshalling cabinet located on the same building. From this terminal strip, he utilized one spare, #18AWG shielded conductor out from a 12PR-#18 analog cable pre-terminated and routed from the marshalling cabinet to a field analog terminal box near the field instrument. From that existing I assume this was done to reduce material and labor cost since he is intending to utilize a pre-routed and pre-terminated homerun instrument cable. On the other hand, a phase 2 of this project will tend to have the instrument power from the DCS through a voltage FTA. This sounds a good idea since come on phase 2, they will simply disconnect the 24VDC panelboard fed power supply and connect one spare of the existing analog homerun cable already routed and terminated to the DCS PLC.

However, the existing homerun instrument multipair cable used between the analog marshalling cabinet at the I/O bldg and the field analog terminal box will be have 10 pairs powered from the DCS' 24VDC and a pair powered from a separate 24VDC panelboard. Its like have two separate 24VDC sources in one multipair cable...is this acceptable???

Is there a code violation or a standard practice violation for this....I find it technically unacceptable and I do hope somebody will agree and provide a code reference section being violated. I am dealing with Instrument practioners here.

RE: Two Power Sources in a Multipair Cable?

It's not an NEC violation.  

What is your reasoning for finding it technically unacceptable?  
 

RE: Two Power Sources in a Multipair Cable?

Sorry but I have no problem with it.

However, I would label that odd-man-out 24V line everywhere it is accessible in some manner that alerts future modifiers.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

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