Disconnet located inside enclosure

[gmozur]

We have machine tools, grinders, broaches, etc. that have main 480 vac electrical enclosures with the main disconnet means. It has been the practice to have a 480v to 24v single phase power supply (Electromatic brand) with disconnect mounted within the main enclosure. The power supply is fed off the line side of the main disconnect. Is there a code issue in any of this design? The main encloser disconnet is E1 and the power supply is E2.

 

[james_mcquade]

NFPA 79- Electrical Standard for Industrial machinery 1997 edition.
page 79-13 chapter 7 Supply circuit disconnecting means.
7.1 general requirements - a disconnecting menas shall be provided for each incomming supply circuit.

my concerns

1. With the power supply on the line side of the disconnect, how will you turn off the power to the power supply?
you have bypassed the main machine disconnect and put it on the line side of the disconnect. the plant bussbar / disconnect is
25 ft in the air and not readily accessable. Put yourself in this situation, YOU are the one who forgets or is unaware the power supply
is not turned off with the machine disconnect, YOU touch a live wire and also connectyed to ground, and YOU are now the one being electrocuted. How will your coworkers get YOU
free? I forgot one time that a machine had 2 sources of power during the july 4th shutdown at a plant, soaking wet with sweat. Had my elbows not been the source
of contact from 110 to ground, i would not be here (the electrical shock caused my elbows and muscles to contract and knocked me out of the panel).

Explain that to OSHA and and corporate management as your family files a mega lawsuit against the company.

2. machines with more than one source of power must have a legend stating this. typically this is interface wiring to another
machine or controller with yellor or orange wires.

to me, its a very bad idea. i was hoping that someone else would reply with a better answer.

regards,
james

 

[james_mcquade]

NFPA 79 - 2002 edition.
chapter 5. 5.1.1
"where practicable, the electrical equipment of a machine shall be connected to a single power supply circuit. Where it is necessary
to use another polwer supply circuit for certain parts of the equipment (e.g., electronic circuits, electromagnetic clutches), that
supply circuit shall as far as practicable, be derived from devices (e.g., transformers, converters) forming part of the electrical equipment of the machine.

i was unable to edit my last post.

regards,
james

 

[petersonra]

We have machine tools, grinders, broaches, etc. that have main 480 vac electrical enclosures with the main disconnet means. It has been the practice to have a 480v to 24v single phase power supply (Electromatic brand) with disconnect mounted within the main enclosure. The power supply is fed off the line side of the main disconnect. Is there a code issue in any of this design? The main encloser disconnet is E1 and the power supply is E2.

It is not a good design choice IMO.

However, I am not convinced that it creates an especially dangerous circumstance.

 

[cadpoint]

To the average electrician, they should keep in mind that the Code states all wiring be of the same insulation with in the same enclosure. Most 24V wire is of 300V value.

Separation by a physical barrier not just space is required

All power source(s) like this case all need to be labled correctly.

 

[jim dungar]

Definitely put a warning label on the enclosure door, stating that there are two sources of 480V in the panel. Internal signage and barriers is also helpful. colored wire insulation is part of NFPA79, however that only applies to the wiring internal to the panel, and it becomes meaningless if panel was wired 'following the NEC' with something like a BOY color code for phasing (NFPA79 says all normal source 480V wiring should be Black with phase identification done by 'marking').

Having voltage in a panel with the main disconnect off is not uncommon, I would not be surprised to find it in >10% of equipment.