It is currently Thu Feb 22, 2018 3:55 am



Post new topic Reply to topic
Author Message
ekstra   ara
 Post subject: Machine with 2 Sided Cabinet
PostPosted: Tue Dec 24, 2013 5:17 am 
Offline
Plasma Level
User avatar

Joined: Wed May 07, 2008 5:00 pm
Posts: 711
Location: Rutland, VT
Hello,

I have my ideas on this but thought it might be worthy of discussion, so here is the scenario.

The piece of equipment has 240V coming into it fed from a panelboard breaker. The incoming is connected to a circuit breaker and then the load side feeds controls and such in this side of control cabinet. The load side of the 240V breaker also feeds a set of fuses which then feed a 66.5 kVA transformer located next to the control cabinet. This transformer steps the 240V to 460V delta and the 460V comes into the other side of the 240V cabinet. It comes into a breaker and then on the load side of that breaker is a drive unit for a motor.

Therefore, one has one shock and arc flash hazard on the 240V side and a different shock and arc flash hazard on the 460V side.

So, the choices are:

1. Label each side with the respective shock and arc flash hazard
2. Label each side with the respective shock hazard and with the worst case arc flash hazard.
3. Other??

_________________
Barry Donovan, P.E.
www.workplacesafetysolutions.com


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Tue Dec 24, 2013 9:37 am 
Offline
Arc Level

Joined: Mon Jan 18, 2010 11:35 am
Posts: 445
Location: Wisconsin
This is very common in data center Power Distribution Units (PDUs) and some transformers.
In my experience the industry practice is to follow your #2 - worst case regardless of source.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Tue Dec 24, 2013 10:52 am 
Offline
Plasma Level
User avatar

Joined: Wed May 07, 2008 5:00 pm
Posts: 711
Location: Rutland, VT
I guess the next phase of this discussion could be what PPE to wear when troubleshooting as the 240V side contains circuit boards, contactors and other misc items. The 460V side would contain the drive for a motor. So, how do you wear HRC 3 PPE and connect probes to small test points on a circuit board or connect up an oscilloscope.
This could be where one is not interacting in a manner to cause an arc flash but let's hear the discussion on this also.

_________________
Barry Donovan, P.E.
www.workplacesafetysolutions.com


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Fri Dec 27, 2013 8:02 am 
Offline
Plasma Level
User avatar

Joined: Tue Oct 26, 2010 9:08 am
Posts: 1878
Location: North Carolina
This is a very common problem in industrial control panels for the same reason. These are panels such as used in assembly lines, machine tools, and so forth that hold multiple open face contactors mounted either on DIN rail or screwed to a back panel. Panel fuses or breakers, jf present, are mounted the same way. There are often relatively small instrument transformers stepping down to lighting voltages, which then run controls. If you are building these, I highly recommend compartmentalizing into 3 compartments, and sheet metal is cheap. Compartment 1 contains an incoming protective device. Compartment 2 contains the motor contactors, drives, and voltage conversion. Compartment 3 houses controls. Many companies (GM and Ford I know for sure) have tried to go to under 24 VDC only in the control compartment. The problem with this is that it is difficult to operate actuators for hydraulics and pneumatics (solenoids typically exceeding 1-2 A), and contactors usually end up with a very nonstandard voltage like 48 VAC. Or a layer of isolation relays gets added which reduces reliability and consequently increases exposures to the 'high voltage' compartment. I recommend instead using 24 V where practical and lighting voltages (120/240 in US) where needed in isolated terminal strips. Cut and strip everything to proper length, use fusing or circuit breakers in the noncontrol cabinet, and use hardware that is 'touch safe' which means screws, terminals, etc. are recessed. That is how most gear is built today. Even if you have an older panel, it is usually relatively easy to achieve touch safe status. Open frame transformers are usually the biggest problem and they can be insulated or caged relatively easily. Once this is done then look at how an arc flash can be created. Depending on the environment, opening the doors may or may not be prone to causing an arc flash such as if there is conductive dust or liquids around that could start something. Once the doors are open then visual inspection does not require crossing the restricted approach boundary. If the tools are insulated to prevent both phase to ground and phase to phase contacts (using insulated screwdriver, insulated meter probes with the tip covers on) then at that point even with 480 Volts, initiating an arc is very remote. I dealt with this exact problem in a foundry that had a lot of very old equipment and a lot of industrial control panels. The trick as was already mentioned is attacking the source of the problem (initiating an arcing fault). Throwing PPE at it is not really the recommended approach. Eliminating sources of arcing faults engineers the hazard out of the system rather than simply accepting it and doing what we can to protect personnel. Predicting arcing faults is NOT an exact science. IEEE 1584 shows that if proper PPE is worn, there is a 95% chance it will work. The other 5% is due to the variable nature of arcs. If we can engineer out the things that electricians can do to accidentally cause an arc, the remaining equipment failure related cases run at about 0.0001% to 0.00001% or lower (about 1 in a million), based on statistics on older designs, compared to human error rates that are around 0.1 to 40% depending on a large number of largely uncontrolled variables as verified by various military studies. Even with the PPE, that still puts arc flash injury likelihood in the absolute best of circumstances well above an engineered approach. H/RC 3 is only going to lower visibility, dexterity, comfort, ergonomics, heat stress, and otherwise contribute to increasing odds of an arc flash. It is better to take human error out of the equation as much as possible. And if the remaining arcing fault likelihood is tolerable, then adding PPE would no longer be necessary, and may even be counterproductive to increasing safety because it increases other risks such as dropped tools, stabbing injuries, heat stress, slips/trips, and misidentification due to poor visibility.


Top
 Profile Send private message  
Reply with quote  
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 4 posts ] 

All times are UTC - 7 hours


Who is online

Users browsing this forum: No registered users and 1 guest


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Jump to:  
© 2017 Arcflash Forum / Brainfiller, Inc. | P.O. Box 12024 | Scottsdale, AZ 85267 USA | 800-874-8883