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interlocks between panels

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dwanderson

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We are adding motor control panels to an existing system of motor control panels. The 480vac power source for our new motor control panels is fed from a different power source used to feed the existing motor control panels.
We have to run interlock wires between the old and new motor control panels and see a 220volt difference of potential between the hot wires from each system. We didn't realize this until we blew up a PLC input card in one of the existing system's motor control panels.

What, if any, NEC codes address this situation?

I think we will have to install a junctiom box with relays to keep the wires of different potentiol out of the motor control panels, but we just move the problem to the junction box. Is this allowable per the NEC?
 
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don_resqcapt19

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Why are you using the voltage from the motor control circuit for the inputs to the PLC? The power source for the inputs should originate in the PLC.
Don
 
DGrant041

DGrant041

Senior Member
Location
Peoria, Illinois
Can I ask what brand of PLC and what model? Some things I notice people are cryptic on when it comes to brand names but I think it's important in situations like this.

There are many different way to accomplish what you want to do. The separate relay idea will isolate the two systems from each other but still let you see interlocks.

You could also use an unused aux contact on the motor starter and run your PLC power through that. (Of course you'll need to train your people, label for multi-power sources, etc.)

One company I worked for pulled 120V from the control transformer via the aux contact to drive the PLC input. These were usually PLC 5's but they also used other A-B PLC's. I'll try to scan a quick sketch. It's pretty slick!

Edit for wording. . .
 
D

dwanderson

Member
If you take a hot wire from an existing system MCP and a hot wire from my new system MCP and measure between them you get 220 volts. When we connected our PLC output to their PLC input it blew the input cards circuit.
 
D

dwanderson

Member
DGrant041 said:
Can I ask what brand of PLC and what model? Some things I notice people are cryptic on when it comes to brand names but I think it's important in situations like this.

There are many different way to accomplish what you want to do. The separate relay idea will isolate the two systems from each other but still let you see interlocks.

You could also use an unused aux contact on the motor starter and run your PLC power through that. (Of course you'll need to train your people, label for multi-power sources, etc.)

One company I worked for pulled 120V from the control transformer via the aux contact to drive the PLC input. These were usually PLC 5's but they also used other A-B PLC's. I'll try to scan a quick sketch. It's pretty slick!

Edit for wording. . .
Click to expand...
AB 1756 Control Logix and I/O
 
D

dwanderson

Member
The MCPs are powered with 480vac 3ph for the motors. A 480 to 120 control transformer provides 120vac power for the PLC and I/O circuits. The motor starters have 120volt coils connected to PLC outputs.
 
don_resqcapt19

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
A 480 to 120 control transformer provides 120vac power for the PLC and I/O circuits. The motor starters have 120volt coils connected to PLC outputs.
Click to expand...
The power from the CPT at the starter is for the starter coil and should be controlled by a relay or contact output at the PLC. The power for the inputs to the PLC should originate at the PLC, not the CPT at the starter.
Don
 
hockeyoligist2

hockeyoligist2

Senior Member
Location
close to greenville sc
dwanderson said:
The MCPs are powered with 480vac 3ph for the motors. A 480 to 120 control transformer provides 120vac power for the PLC and I/O circuits. The motor starters have 120volt coils connected to PLC outputs.
Click to expand...

I must be missing something here....... You said an input card blowed up. The inputs are powering thru a transformer which, even with a variation of the input power to the transformer should produce close enough voltage for the card to handle it. And you are powering the starter coils from it also. Are you sure it wasn't an output card that blew? Are the starters bigger? Could you be overloading the outputs with a starter coil that draws too many amps? You also said there was a 220volt difference of potential between the hot wires from each system. Can you explain that a little better?


I want to help and I'm sure someone here knows the answer but we need some more details I believe. :)
 
petersonra

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
This may be a case where it would make sense to get an engineer experienced in such things involved to make sure the interfacing is done properly.

The wiring between the two system should be down in yellow to indicate that there exists a potential for voltage to be present even with the disconnect shut off.

For LOTO purposes, you will need to be able to isolate the voltages that might be present on these wires in a reliable way. keep in mind that under normal conditions these wires may or may not have power on them, so merely checking for voltage on them is not an adequate safety measure.

If it is convenient to sense foreign power, many PLC manufacturers have isolated input modules that can have a separate source on each input. Isolated outputs are also available.
 
DGrant041

DGrant041

Senior Member
Location
Peoria, Illinois
don_resqcapt19 said:
The power from the CPT at the starter is for the starter coil and should be controlled by a relay or contact output at the PLC. The power for the inputs to the PLC should originate at the PLC, not the CPT at the starter.
Don
Click to expand...

Don-

I've tried a couple of different ways to drag over a picture of what I was trying to say--no luck.

To summarize: We used a dry contact pilot relay to control the starter but used 120V from the aux contact to provide relay status input to the PLC. The pilot relay was 24 Vdc coil from PLC, input cards were 120 Vac direct from the aux contact. Make sense? In my two years there we didn't have any card or power supply issues.

One of the advantages of this set-up is that if you open the disconnect to the starter can, you open all power to the control circuits. (I remember reading that?s a requirement but I don't remember the specific reference.)

One of the disadvantages whenever you start adding pilot relays (for the output AND input) you add a new reliability factor (new place for failure). The relays were easy to swap out and were fairly reliable. The bases were much more difficult to swap because they used the bus-style jumpers.

OK that's my 2.75 cents worth. . .

If someone could PM me how to copy and paste a picture, that will at least clear up what I'm trying to say.
 
D

dwanderson

Member
Forget about transformers and input and output cards. that is not my issue.

I am going to have a junction box with relays in it to isolate the wiring between the two motor control panels. The junction box will contain hot wires [120vac] from each motor control panel.

The junction box will have a hot wire, [connected to a PLC output], and a neutral wire from one motor control panel that will control the relay. There will also be a hot wire [120vac] and a signal wire, [to a PLC input] from the other motor control panel that will be wired to the relay contacts. When the relay is energized by the first MCP the PLC input in the other MCP will turn on. Basic interlock circuit.

The issue I have is that the two hot wires, [one hot wire at 120 vac when the relay is energized from one MCP and the other hot wire from the other MCP wired to the relay contacts], have a difference of potential of 220 volts because the 2 MCPs are powered from seperate power sources coming into the building. We tried connecting the transformer to different phases of the 480vac [L1-L2, L1-L3, L2-L3] with no luck. we always saw the 220volt difference of potential.

I guess my question is: Is it allowable by the NEC to have an enclosure, MCP or Junction Box, that contains 120volt circuits powered from seperate sources that have a difference of potentiol of 220volts?

Mayby all I have to do is put a sticker on the junction box saying it contains voltage from seperate sources. I may also need a warning sticker saying it contains 220vac. mayby all the wires need to be yellow, I dont know. I'm looking for some help.
 
D

dwanderson

Member
hockeyoligist2 said:
We tried connecting the transformer to different phases of the 480vac [L1-L2, L1-L3, L2-L3] with no luck. we always saw the 220volt difference of potential.
Where are you getting this difference of potential? To ground? Are the transformers bonded?
Click to expand...


Between the two hot wires. Normally you would expect to see a 0 to a couple volts difference but we are measuring 220. Transformer neutrals are grounded.
Click to expand...
Click to expand...
 
hockeyoligist2

hockeyoligist2

Senior Member
Location
close to greenville sc
dwanderson said:
hockeyoligist2 said:
We tried connecting the transformer to different phases of the 480vac [L1-L2, L1-L3, L2-L3] with no luck. we always saw the 220volt difference of potential.


Between the two hot wires. Normally you would expect to see a 0 to a couple volts difference but we are measuring 220. Transformer neutrals are grounded.
Click to expand...


Between the two hot wires? Nowwwww it is starting to make sense..... it is a step down to a 120/240 transformer. One leg (hot wire) to the common or grounded conductor will give you 120. Two hot wires will be 240 (or in your case 220) if you are running two "hots" then you are feeding to much voltage! I'm sure that would blow a lot of 120 V stuff! Do you have two "hots" running to the PLC? If so remove one and run a wire to the "neutral" or grounded conductor!
Click to expand...
Click to expand...
 
wirenut1980

wirenut1980

Senior Member
Location
Plainfield, IN
The fact that you are measuring 220 V between two different sources must be because the two sources are fed from different phases somewhere upstream. Or maybe from different transformers, one wye-wye and another delta-wye, or something like that.
 
hockeyoligist2

hockeyoligist2

Senior Member
Location
close to greenville sc
wirenut1980 said:
The fact that you are measuring 220 V between two different sources must be because the two sources are fed from different phases somewhere upstream. Or maybe from different transformers, one wye-wye and another delta-wye, or something like that.
Click to expand...

But he said "between two hots" ? wouldn't you think it's from the same transformer?
But of course that would be to easy (not toward you wirenut) I'm beginning to think someone didn't follow directions ! :)
 
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