Working in a live Motor Control Center 480V 200 Amp

mjc1060

Member
Hi,
I have a 40 year old Square D MCC where the control transformer failed. The 480V fuse block is in front of the transformer. My question is when I shut off the disconnect switch I remove the power to the enclosure but once the fuse block is removed I don't know if the bus is exposed. Also the rest of the MCC is still energized and this switch is on top. Is this a violaton of 70E. I am not comfotable doing this and I am not going to but my supervisor is pushing me to do it. Can anyone shed some light on how 70E covers this type of a situation.
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
I know you didn't ask this, but it sounds like you are in an MCC bucket. Can you pull the bucket?

ice
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
Hi,
I have a 40 year old Square D MCC where the control transformer failed. The 480V fuse block is in front of the transformer. My question is when I shut off the disconnect switch I remove the power to the enclosure but once the fuse block is removed I don't know if the bus is exposed. Also the rest of the MCC is still energized and this switch is on top. Is this a violaton of 70E. I am not comfotable doing this and I am not going to but my supervisor is pushing me to do it. Can anyone shed some light on how 70E covers this type of a situation.
Removing the fuse block does not cause additional exposure to bussing.
There is no way to remove the voltage from the line side of the bucket disconnect switch.
The bucket does not provide true compartmentalization, therefore the incident energy on the MCC bussing would be applicable to the starter bucket. There are gaps around the starter bucket that would allow screws and such to 'fall into' the MCC and contact live bus.

The MCC should be powered down, and the bucket removed from the structure.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
Thank you for the response I was intrested in what NFPA 70E requirements would be in this situation.
I know it is a common misconception, but 70E is not a step-by-step or how-to-guide for safety.

For all intents:
OSHA says you must protect your employees from known dangers in the work place.
It is a company's responsibility to create an Electrical Safe Work Practices (ESWP) program/policy.
NFPA70E is an industry accepted standard for creating an ESWP (e.g. assessing dangers in the workplace and determining appropriate PPE).

It is your company's requirement to justify, maybe in a court of law, the safety of their ESWP should an incident occur.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Assuming your company IS using NFPA 70 E as the basis for your program:

When you open the door of an MCC bucket, you are at the maximum Hazard Risk Category as it exists in that MCC. So if you have done your survey and identified it as HRC 3, then you will require PPE as appropriate for HRC 3, plus you need an Energized Work Permit.

By the way, at a recent talk given by OSHA, they mentioned that they are going after prosecuting managers and supervisors who knowingly demand that employees violate their own program, or FAIL TO IMPLEMENT ONE AT ALL, as a criminal offense, i.e. jail time.
 

meternerd

Senior Member
Location
Athol, ID
Occupation
retired water & electric utility electrician, meter/relay tech
I can't really picture what you have. If the disconnect is ahead of the fuse block, then the only energized portion should be the top side of the disconnect. If there are no exposed energized conductors, then is it a hazard? Could the wires in the top of the disconnect decide to fault while you have the door open? Maybe...but not likely. If shutting down the entire MCC is impractical, then appropriate PPE seems suffucient. Just the way I would do it, though. All companies are different. Bottom line is, if you aren't comfortable doing it, don't. Just because the "boss" said it was OK, and it's his rear end if something happens, it's you who gets hurt.
 

fmtjfw

Senior Member
Is the danger caused by deenergizing the MCC greater than the hazard of an accident that might occur while you are doing repair work on energized equipment?

Note that proper PPE is designed to limit the damage to your body to 2nd degree burns.

Note that if an accident occurs, the time to order a new MCC, remove the burnt up one and install the replacement may cost a whole bunch more that just turning the thing off for the 1/2 hour it would take to replace the part.

I would have a manager and your safety director sign off on the hot work.
 

kwired

Electron manager
Location
NE Nebraska
Is the danger caused by deenergizing the MCC greater than the hazard of an accident that might occur while you are doing repair work on energized equipment?

Note that proper PPE is designed to limit the damage to your body to 2nd degree burns.

Note that if an accident occurs, the time to order a new MCC, remove the burnt up one and install the replacement may cost a whole bunch more that just turning the thing off for the 1/2 hour it would take to replace the part.

I would have a manager and your safety director sign off on the hot work.
Hopefully the employer is also concerned about the life and health of the employee also and not just the cost of the lost MCC, or cost of downtime:happyyes:
 

big john

Senior Member
Location
Portland, ME
If you are afraid, maybe you should become a plumber.
I wouldn't want a guy on my crew who wasn't afraid.

I've spent a lot of troubleshooting time with my face in front of MCC buckets that been de-energized at the internal breaker, so were not completely isolated.

I've also seen the results of a completely spontaneous line-side failure on a breaker in one of those buckets. It blew the door off. I'd have needed a new face if I'd been in front of that one at the time.
 

kwired

Electron manager
Location
NE Nebraska
If you have no fear at all, it may be because you don't understand what could potentially happen, and that could get you into trouble someday.

There is a difference between being afraid, and respecting it because of some fear.
 

meternerd

Senior Member
Location
Athol, ID
Occupation
retired water & electric utility electrician, meter/relay tech
I wouldn't want a guy on my crew who wasn't afraid.

I've spent a lot of troubleshooting time with my face in front of MCC buckets that been de-energized at the internal breaker, so were not completely isolated.

I've also seen the results of a completely spontaneous line-side failure on a breaker in one of those buckets. It blew the door off. I'd have needed a new face if I'd been in front of that one at the time.
I too have seen line side of breakers fail. Yes, it is ugly. Could it happen while you are working in an MCC bucket that has the main turned off and has no load on the circuit. Possibly. All of the failures I have seen are due to loose connections that overheat under load until the insulation fails. I'm NOT saying it will never happen when the breaker is off. All I'm saying is that PERSONALLY....meaning "JUST ME", I would likely work on the bucket components without the required PPE for energized work. Not all MCC buckets are removable. Troubleshooting with power off is not possible. In a pump room with poor lighting and difficult access, it just makes it literally impossible to troubleshoot wiring and components in a flash suit, rubber gloves with protectors, etc. If something bad were to happen, would I be saying the same thing? Who knows. I think there's some "wiggle wording" that says that if the safety equipment increases that hazard, it can be modified. I'm no expert, though.

Like I always say, "It's just my opinion...If your is different, I'll patiently explain why you are wrong.":D
 

kwired

Electron manager
Location
NE Nebraska
I too have seen line side of breakers fail. Yes, it is ugly. Could it happen while you are working in an MCC bucket that has the main turned off and has no load on the circuit. Possibly. All of the failures I have seen are due to loose connections that overheat under load until the insulation fails. I'm NOT saying it will never happen when the breaker is off. All I'm saying is that PERSONALLY....meaning "JUST ME", I would likely work on the bucket components without the required PPE for energized work. Not all MCC buckets are removable. Troubleshooting with power off is not possible. In a pump room with poor lighting and difficult access, it just makes it literally impossible to troubleshoot wiring and components in a flash suit, rubber gloves with protectors, etc. If something bad were to happen, would I be saying the same thing? Who knows. I think there's some "wiggle wording" that says that if the safety equipment increases that hazard, it can be modified. I'm no expert, though.

Like I always say, "It's just my opinion...If your is different, I'll patiently explain why you are wrong.":D
All MCC buckets are removable, some are easier than others though.:p

Employer is required by OSHA to provide safe working conditions. If lighting is poor, or there is not enough room for usual tasks, then compromising by working live with no proper protection is not the solution, more light is needed, more clearance is needed...
 

meternerd

Senior Member
Location
Athol, ID
Occupation
retired water & electric utility electrician, meter/relay tech
All MCC buckets are removable, some are easier than others though.:p

Employer is required by OSHA to provide safe working conditions. If lighting is poor, or there is not enough room for usual tasks, then compromising by working live with no proper protection is not the solution, more light is needed, more clearance is needed...
Ok....maybe I have had experience with the ONLY non-removable buckets in existence. Several of our older pump stations have the main breaker hard-wired to the vertical bus behind the bucket. No way to remove it except de-energizing the MCC. Many are not in buckets at all, but are control cabinets with a fused disconnect in the same enclosure as the controls. Built in the 50's and 60's. As I stated before, troubleshooting a control circuit is often impossible unless the control is energized. Most are located in the same bucket as the 480 starters. Working in the now required PPE while doing so is EXTREMELY difficult, especially when using small tools or test leads. We do the best we can, but sometimes reality gets in the way. No more arguments from me, though. We all should know the rules and follow them. Nuff said.
 
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