120v inputs / dual phase

Okay guys...... I was working on a PLC today with five 120vac input cards, 16 inputs each for a total of 80 possible. 75 are actually in use, all with the same color wire. Of the 75, 74 use a 120vac source from phase A of the plant's three phase electrical system, but one of the inputs is sourced by phase B.

This results in any ONE input having 120vac to neutral, but a possible 240vac from any of the A-phased inputs as measured to the B-phased input.

Now this system has been in operation for 4 or 5 years, and as far as I know, has not had any undue frequency of failure on the input card with the B-phase input, it just struck me as being an odd way to design something.

The input in question is used to MONITOR the B phase, and was wired directly to it. I promptly (almost as a reflex action) rewired the installation with an ice-cube in between so that the source for all 75 inputs is now the same phase.

My question is this: Is this a common occurrence? Have others run across similar situations? Is there a code, or spec that allows or disallows doing this? I searched the archives but may not have worded my search perfectly.

As always, thanks in advance for your input.

Stationmaster

Stationmaster, I do not believe you actually have a three phase 120VAC system in your plant. You may have a three phase 208/120 Y system and that may account for part of why your system was working. The neutral required on the 120VAC input cards has to come from the same source as the hot side does, and this certainly can happen with a 208/120 Y. So, did you have 240V between inputs? No! Possibly 208V but then again, maybe not. The isolation relay is a good idea and that eliminates the possibility of problems so I agree with that choice.

240v systems are generally single-phase, so I'm not clear on your use of the term dual-phase.

Most of the AC input cards I've used have been wired to use a common Neutral. So long as your Phase A and B share the same Neutral there should be no problem.

A little clarification. The plant system is 3phase 240vac. Anywhere in the plant the potential between phase A and phase B is aprox 240vac. The PLC system's function is to start up the plant in groups, or drop preceding groups up-line if a subsequent group drops out from say an overload trip.

My term "dual-phase" was misleading, I simply meant "two phases".

The function of the PLC was not my concern as I stated that it has worked fine for several years.

My concern was more along the line that I haven't seen this arrangement before, that is: two different phases on inputs of the same PLC /card. The B-phase lead is the only terminal in the entire 3' by 4' PLC panel where one can measure 240vac potential. With all of the wires the same color, there is nothing to warn a technician that there is a different phase, or the possibility of 240v on that single wire.

The phases DO share a common neutral.

The entire panel IS controlled by a single disconnect.

My question remains: is this common practice? Two different phases on two inputs of the same PLC?

Stationmaster

I would say that this is not common practice since most plc i/o is usually fed from either a 24 volt power supply or a 120 volt single phase control transformer. I agree with you that it could be dangerous since all the wiring is the same colour, therefore it would be hard to identify a 240 volt potential. The relay is a good idea, but you should identify the panel as having a 240 volt potential.



Derek

since the panel is contained under one disconnect, then the relay really isn't needed. You stated that the one input was to monitor the phase (for power loss, I'm guessing) and since it is the one input, and not several...just document it. If the wiring is confusing, then change that wire from it's source to the input, then document it.

Unfortunately, there is no standard for common practice. What is acceptable in certain regions is considered illegal in others, and this pertains to electrical installations as well.

Stationmaster said:

The entire panel IS controlled by a single disconnect.
Stationmaster

Click to expand...

My guess is the voltage for the input in question is not controlled by the single disconnect. If this is true, then in my quite often flawed oppinion this is the real hazard.

It is also my guess that you have one of the two 3 phase systems below in this pic.




.
Note that the voltage on the wild leg to neutral is 208 volts!

Hey, Stationmaster,

Yes, I have run into that before. I had this concern with designing an Army project out in Nevada. This hot gas decontamination burner had three 120 volt power circuits to feed five 120 VAC input cards and three relay output cards. I was at first concerned about unbalancing the 3-phase 208/120 volt circuit breaker panel if I put all 3 circuits on the same phase.

Although it would have worked just dandy if the 3 circuits had been on different phases, it is possible that 2 circuits could be next to each other in a conduit or box, and somebody would check the voltage and wonder what the heck was going on!

To prevent any troubleshooting difficulties and calls from frantic electricians at 3 AM, I re-arranged the 3 circuits so that they were all coming from Phase A of the panelboard. Thus no higher voltage than 120 volts is ever measured inside the control cabinets, regardless of which terminals are checked.

I do not think it is a code violation to use two or more phases in a 120 volt control scheme, but it may certainly lead to troubleshooting problems. I think it is best avoided where possible. It would take a very large system to make it impossible to put all controls on the same phase.

Finally!

I should have known you would be dead on Lancie1.......How have you been?

While I'm thankful for EVERYONE'S input, your perception of my concern was the most helpful. (except I wasn't really "frantic", lol).

: )

Thanks again,

Stationmaster

Steve,

I did not think that YOU would be frantic over this, but it is a valid concern. I can visualize a situation where a PLC has two 120 circuits connected to 2 different input cards. The circuits come from diffent phases of a 3-phase circuit breaker panel.

One of the PLC inputs goes bad (no longer works) and that wire gets moved to a spare on the other card. (After all, they are both 120 volts, aren't they?) Now you have the two phases connected together (short circuit) and that will cause a lot more problems.

Overall, using two or more phases for the control power in a control panel is not wise.