120VAC Solenoid Control Panel

[Jack Reyes]

In an application where a PLC uses a 16 point relay output module to switch 120VAC solenoids with each relay module having a 10A total output rating, is there a requirement to fuse each individual solenoid output or is it sufficient to fuse only the output module? The solenoid cables would be 18AWG/3C so I belive that fusing just the output module would mean that I could not go any higher than a 7A fuse, is this correct. I am confused on how this circuit is viewed by the NEC. Is it considered a power circuit, control circuit, class 1, ect.

If you have a remote panel, say within sight of the main panel disconnect, with multiple output modules in it would I bring multiple 120VAC circuit for each output module or just one large circuit and divide it up at the remote panel? An electricain I am speaking with says in that case he would prefer the small multiple circuits (All derived from the same single phase control transformer). I am trying to figure out the best way to handle this.

 

[jim dungar]

Totally a design issue.
Personally, the more isolated circuits the better.

 

[masterinbama]

Totally a design issue.
Personally, the more isolated circuits the better.

Unless a single output failure will create a hazard I prefer to protect them individually.

 

[GeorgeB]

PLCs before the market became so heavily cost driven typically had individual fuses on each output. While your module has a 10A limit, I doubt an individual relay is rated for full module current, either.

Designs I do use the fused disconnecting terminal strips for the outputs if there is substantial current involved

More commonly, I use plug in 24V coil external relays which can be used with the 32 point modules. The external relays eliminate the PLC contact damage and need to replace a module for one burned relay.

 

[pfalcon]

In an application where a PLC uses a 16 point relay output module to switch 120VAC solenoids with each relay module having a 10A total output rating, is there a requirement to fuse each individual solenoid output or is it sufficient to fuse only the output module? The solenoid cables would be 18AWG/3C so I belive that fusing just the output module would mean that I could not go any higher than a 7A fuse, is this correct. I am confused on how this circuit is viewed by the NEC. Is it considered a power circuit, control circuit, class 1, ect.

Control circuit. Design issue. NFPA79 has more to say about this than the NEC.

Individual fuses are preferred (not required) for troubleshooting purposes. If you must drive high volt, high amp (relative to a PLC) then use a 24VDC control circuit and drive DC coil relays with rated contacts. 120VAC with loads measured in Amps are something to keep away from most controllers. "ON" time of the relays may overheat the unit.

If you have a remote panel, say within sight of the main panel disconnect, with multiple output modules in it would I bring multiple 120VAC circuit for each output module or just one large circuit and divide it up at the remote panel? An electricain I am speaking with says in that case he would prefer the small multiple circuits (All derived from the same single phase control transformer). I am trying to figure out the best way to handle this.

Multiple output modules do not require separate conductors. Separate conductors only make sense where you need to isolate power. In the case of a machine tool that might be the case if each run was fused (probably for troubleshooting). That would isolate which card had the fault. Normal practice is to bring two 120VAC conductors:
1) 120VAC for pilot lights, annuciators, inputs, and 24VDC power supplies for controls.
2) 120VAC that passes through the Estop circuit for motion outputs.

Normal practice (preferred not required) is also to individually fuse motion outputs that drive solenoids (and sometimes coils but often not).

Downtime on our machines is far more costly than adding a dozen fuses to a panel. Pays the first time service looks for what died.

 

[petersonra]

Putting a fuse on each output does nothing but add cost in most cases. It will not protect anything, nor is it generally possible to coordinate an individual output fuse so it won't blow the branch OCPD.

It may have some value in isolating a circuit, but I don't believe that 70E would let you take advantage of that.

 

[jim dungar]

It may have some value in isolating a circuit, but I don't believe that 70E would let you take advantage of that.

The entire advantage is in being able to isolated the circuit components.

Why do you think NFPA70E would have a problem with isolating something?

 

[tom baker]

The NEC considers PLC input or outputs to be either Art 725 Class 1 power limited or non power limited circuits, or Class 2. The difference between the two the amount of energy available, as determined by the power supply . A class 1 circuit is wired with a chapter 3 wiring method while Class 2 is what you would call a low voltage wiring method, although the NEC does not use that language.
Read the scope of Art 725 and the first sections on Class 1 and Class 2 wiring.

 

[petersonra]

The entire advantage is in being able to isolated the circuit components.

Why do you think NFPA70E would have a problem with isolating something?

I did not say they would.

What I was trying to suggest is that the ability to isolate the circuit may not be especially useful. At one time it was common to do so on the theory that it would make it safer to work on things live that way. Since working on them live has become an issue, isolation may not be as useful as it may seem at first glance. As a practical matter I think it has merit.