Service or Machinery???? 1

[StanMan122]

Working on a 575VAC machine line. A couple of the machines have several 230VAC loads. Using step down transformers and am uncertain of the implications, as I usually just build the machines and don't have to worry about electrical service. Current plan - 1 Customer supplies disconnect. 2 - Run service feeder from customer supplied disconnect to a distribution block in the machine's electrical enclosure (Enclosure contains several motor starters and controls, a couple of VFD's, etc). 3 - Run wires that will feed 63KVA transformer from distribution block to a Fuseblock. 3 - Run wires from fuseblock to the transformer, located outside of enclosure. 4 - Run wires from secondary of transformer back into enclosure and to a second distribution block.
Any implications on panel SCCR? Any disconnect requirements? ("Machine within sight of disconnect" or does it require a service disconnect at the machine/ transformer?) Would it actually be considered "Service electrical" and, if so, is it allowable to have service electrical enclosed in the machine's electrical enclosure? Code seems somewhat vague.
Management says, basically, "If it isn't explicitly forbidden by NEC/NFPA 79, then it is implicitly allowed". Not sure I agree.

 

[jraef]

Assuming that because you mention the SCCR, you are using the 2005 NEC. I'm not sure what the CEC has done in that regard, but not all states here in the US have adopted the 2005 NEC, most of use still use the 2002, which didn't have the SCCR requirement.

There are different rules for self contained machine controls, specifically Articles 409 and 670. If your transformer is attached to the electrical enclosure, even on the outside, it would fall under those rules. I'm not sure about it if you separate the transformer and mount it somewhere else though.

Personally, I would include the disconnect as a safety feature, mechanically interlocked with the enclosure door. This ensures that all power to the machine is off when the door is opened (unless the interlock defeated by a qualified person). Having the customer supply the disconnect and you just providing a distribution block may save a few dollars up front, but puts your machine at risk of a bad installation or future problems if someone gets hurt. Also, without your own main breaker or fuses, your SCCR will revert to the LOWEST rated device in the enclosure.

Since a transformer is not a switching device, it shouldn't have any effect on the SCCR rating of the system by the way.

JRaef.com
Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[mc5w]

The secondary of your step down transformer is a separately derived system if there is no connection between primary and secondary other than equipment ground. This is a different section of NEC article 250 for separately derived systems. The difference between a service and a separately derived system is that for a service supplied system the transformer and some of the wiring is owned by the electrical utility. Grounding electrode and equipment grounding wires on the secondary of a separately derived system are bonded to the neutral either inside the transformer, in the secondary disconnect, or rarely in a box or wireway in between.

New 2005 U.S. rule is that the neutral cannot be smaller than the equipment ground so that a phase to neutral fault will not toast the neutral.

Now if you are using an autotransformer that is 347Y600 primary 120Y208 volt secondary ( and there is a way to build such a sucker using standard U.S. components ) then the secondary is NOT a separately derived system. However, the primary equipment grounding conductor and neutral minimum size need to be sized for the secondary ampacity, not the primary ampacity. In this case the secondary neutral is not bonded to the grounding conductor. In this case, any secondary fault current in a hot lead splits at the transformer about 1/3 into the primary hot lead and 2/3 into the primary neutral depending on how the primary taps are set. The primary equipment grounding conductor carries the same amount of current as the secondary equipment ground and flows all the way back to the source of primary power.

Mike Cole

 

[mc5w]

I forgot to mention that if a separately derived system, such as control power, is internal to the machine you still theoretically need a grounding electrode conductor and grounding electrode.

However, for 1 KVA and less control transformers most electrical inspectors allow the equipment ground of the machine main power to serve as the grounding electrode.

Over at International Exposition Center in Cleveland, Ohio we have a number of portable separtely derived systems that step down from 480 volts to 129Y208 volts. For temporary power the equipment ground of the primary is allowed to be used as the grounding electrode of the secondary but this is an NEC 90.4 exception and not an explicit exception. Putting in dedicated grounding electrode conductor running to the nearest building columun would in some senses be less safe not to mention inpractical.