Transformer secondaries in an industrial environment?

[rhec boss 1]

URGENT! I have been asked to comment about a proposed industrial installation in NC. There are upwards to 30 individual combination motor starter enclosures currently installed in this industrial environment. It has been proposed by the customer to bring each neutral from the individual combination motor starter enclosures to a central location including the overload trip contact, the run contact and coil wire to control it by a PLC. The customer then wants to tie all secondary grounded conductors (neutral) from each transformer to the new PLC control transformer grounded conductor. I have read myself in and out of it and need confirmation from someone. The NFPA-79 Electrical Standards for Industrial Machinery does not appear to answer the question. Reading in the NEC NFPA 70 Article 450.6 makes it sound like the customer can do this. I have never seen this done in the 27 years I've worked as an industrial electrician or as a project engineer. I have always removed the transformer at the individual combination motor starter enclosure and sent the new transformer 120 VAC source to the field enclosure with control wires to monitor, control and acknowledge operation from the new PLC control panel. Can anyone supply a reason citing NEC code or NFPA-79 article numbers for me to review?

[ July 22, 2005, 08:04 PM: Message edited by: rhec boss 1 ]

 

[augie47]

Re: Transformer secondaries in an industrial environment?

Probably be best that I not comment, as I don't fully understand the question, however, in an attempt to be helpful to you I will mention a concern

and coil wire to control it by a PLC.

It would seem to me that this application will be a violation of NEC Sec 430-73.

(also, I may be misreading the post, but it does not appear it would work, electricaly speaking--two separate voltage sources?? )

{For those posters who like to "kill the messenger", please refrain--I'm only trying to be helpful by making a suggestion here}

 

[rhec boss 1]

Re: Transformer secondaries in an industrial environment?

My response to the reference of my "and coil wire to control it by a PLC."

Normally I would not have this discussion with a customer as they usually go with my suggestions.

But in this case I would unwire and eliminate the 50VA transformer and related start/stop circuits utilized by the individual combination motor starters. I would then send a 120 VAC control circuit source and I/O to each individual combination motor starter as such: ten wires minimum plus 2 spares.

120 VAC source [red]
120 VAC Grounded conductor (neutral) [white]
Equipment grounding conductor [green]

120 VAC Output from the PLC to one side of the motor starter coil
120 VAC Output from the PLC to the combination motor starter for a ?Run? light [green PL]
120 VAC Output from the PLC to the combination motor starter for a ?Fault? light [red PL]
120 VAC from the run aux on the motor starter as an input to the PLC
120 VAC from the O/L aux on the motor starter as an input to the PLC
120 VAC from one side of the ?Start? PB as an input to the PLC
120 VAC from one side of the ?Stop? PB as an input to the PLC

Once at the individual combination motor starters:

?Terminate the 120 VAC source [red] to the Run aux, to one side of the O/L aux, to one side of the ?Start? PB and to one side of the ?Stop? PB.
?Terminate the 120 VAC grounded conductor (neutral) to one side of the motor starter coil via an O/L unit aux contact
?Terminate the 120 VAC grounded conductor (neutral) to one side of the combination motor starter ?Run? light
?Terminate the 120 VAC grounded conductor (neutral) to one side of the combination motor starter ?Fault? light
?Terminate the 120 VAC Output from the PLC to one side of the motor starter coil
?Terminate the 120 VAC Output from the PLC to the combination motor starter ?Run? light [green PL]
?Terminate the 120 VAC Output from the PLC to the combination motor starter ?Fault? light [red PL]
?Terminate a wire from the run aux on the motor starter as an ?Run? input to the PLC
?Terminate a wire from the O/L aux on the motor starter as an ?O/L Tripped? input to the PLC
?Terminate a wire from one side of the ?Start? PB as an input to the PLC
?Terminate a wire from one side of the ?Stop? PB as an input to the PLC

Does this help explain? I just haven't never heard of tieing all of the individual neutrals from so many different control transformers back to a separate control transformer for PLC control...

 

[thinfool]

Re: Transformer secondaries in an industrial environment?

Code issues aside, I would think a phone call to a PLC Mfr describing the issue would shed some light on this.
From a safety aspect, if you take the neutral beyond the control enclosure to use in the control scheme, any accidental grounding will complete the circuit and activate the device (motor, solenoid, etc), allowing it to operate. There can be a serious potential for injury (and liability, $$$$$$) in this case.

 

[templdl]

Re: Transformer secondaries in an industrial environment?

It has been proposed by the customer to bring each neutral from the individual combination motor starter enclosures to a central location

Why do the starters have to have a neutral? Is there a L-N load? Has the customer misunderstood the difference of grounded and grounding conductor? If there is no need for a grounded neutral conductor do they understand that it may be a non problem?