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Interposing Relays

01/10/2018 1:43 PM

I want to get ON/OFF/TRIP status from my switchgear to the PLC. The PLC also issues ON/OFF commands to the switchgear.
The issue here is the status signals are of 24V DC and the command signals are of 120V AC and I would like to have all the signals in the same cable.
To overcome this, I plan to deploy a Interposing relay in the switchgear that will get energised by 24V DC. My question is if there are any problem by deploying Interposing relay in the switchgear for this purpose.

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#1

Re: Interposing Relays

01/10/2018 2:10 PM

<...any problem by deploying Interposing relay in the switchgear for this purpose...> That would depend upon the applicable local standards, which cannot be read from here. However, what is proposed in the original post is commonplace, as it is best practice to segregate the two styles of signal.

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#2

Re: Interposing Relays

01/10/2018 6:39 PM

An interposing relay is normally used to allow a command signal to operate a device which requires a higher current or higher/lower voltage.

I'm not sure of the problem you are facing, whether you need the relay for that purpose, or it is the inclusion of differing voltages in the same cable that is concerning you?.

In most jurisdictions, wiring carrying different voltages, in your case ELV with LV may be enclosed in the same cable or trunking provided the lower voltage wires are insulated to the voltage level of the highest voltages present in the cable or trunk

If your concern is that interference from one set of wires may inadvertently trigger a false signal in the other set, then the relay may not overcome that, and separate cables may be the only solution.

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#3

Re: Interposing Relays

01/10/2018 10:54 PM

Selk,

If your switchgear has a section or compartment for this purpose then I don't see any problem with the question of making modifications that are not approved by the manufacturer. Whatever you end up doing, please do it safely! In the USA the most recent publication of the NFPA 70E has a much more explicit requirement to de-energize unless there are some very clear exceptions, and then the documenting of the safety plans is clearly required with approval from many levels.

--JMM

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#4

Re: Interposing Relays

01/11/2018 12:15 AM

I assume your switchgear is reporting status to the PLC via auxiliary contacts (vs a data communication network). In that case, you may introduce a separate interposing relay panel and just run wires from the switchgear to the new panel. That way, modification of the switchgear is minimal.

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#5

Re: Interposing Relays

01/11/2018 9:41 AM

<...would like to have all the signals in the same cable...> An alternative would be to use some sort of multiplexer or bus system, which would satisfy the <...same cable...> criterion.

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#6

Re: Interposing Relays

01/11/2018 9:51 AM

I do not see any reason why not, as long as this interposing relay has the required isolation value to it.

24VDC status/switching signals are not all that unusual, and neither is 120 VAC command signal. Make sure you follow the NFPA 70E guidelines (as pointed out by others), and follow your local codes.

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#7

Re: Interposing Relays

01/12/2018 1:47 PM

The answer to your question begs for more specifications such as cable length, level of total harmonic distortion in the power line, average distance between conductors, etc.

In general, most PLC's will have some level of over voltage protection on the input cards else they would be a lot less popular.

In terms of mixing AC and DC signal lines in a single cable, the PLC world has it best. Because of the 24 Volt signal level used in a PLC, the signal to noise ratio is very high. In lower voltage circuits (TTL and below) the signal to noise signal is higher because the signals are much lower to start with.

I'm curious how you are detecting a "TRIP" but some equipment has a spare contact for that purpose. Otherwise, as others have said, be extremely careful around live or energized equipment. Better safe than dead.

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