I need to take the overcurrents off a motor starter in an MCC. The problem I have is that the motor starter is racked into a live 600 Volt bus. I want to know if I can rack out the motor starter without isolating the whole MCC (there is only 1 bus in the MCC). Is this usual practice. Any bad experiences when racking the starter back onto a live bus.
Any rules in any standard on how to do this.
Any web site on how to do this.
Any help would be appreciated.
Thanks, Blair
All the mcc's I've racked out have interlocks etc. to prevent the unit being racked out whilst under load. There is usually no problems with racking out a properly designed mcc with the buss live, thats what they're made for :-)
Just be sure the thing isn't under load. The higher voltages I've worked on like 4160 have interlocks for that. I saw an interlock fail once though, so be careful, and take it step by step. Most MCCs are designed to be able to rack out while the main buss is energized.
- There is an arc flash hazard, and you need to use appropriate safe work practices and/or the proper personal protective equipment. Only someone who is properly trained should do this, and from your questions you obviously do not have the knowledge. Asking out of curiosity is fine, but I highly recommend that you not perform this task.
Ben Miller
- Benjamin D. Miller, PE B. MILLER ENGINEERING
I'm not doing it but I will be getting guys to do it. I have racked out many breakers but they were on a dead bus. The guys are highly trained and have done it before. I wanted to know if it was regular industry practice. It appears they will be racking it out for maintenance. Proper PPE will be worn I assure you.
Probably be best to ask the guys doing it, no doubt they will require certain conditions to carry out the work.
As others have said, MCCs are specifically s=designed for this purpose, otherwise you could just put a bunch of starters in a box.
In order to open the door, you will need to turn off the disconnect or breaker in that bucket. Once off (open) the load is disconnected and the bucket can be pulled out. most good MCC designes have a vertical ground bus and each bucket stabs on to that bus. When pulling the bucket out, the ground stab will be the last one to disengage, meaning the bucket is still grounded until after the main bus has been cleared.
Still, not something to be undertaken by novices.
I know an electrician who sports some nasty scars on his arm from racking out a 480V MCC breaker. He was the safety man, and was about 20 feet away. The two doing the work were in full PPE. One stabs broke and twisted sideways at disconnect, causing a line to ground fault and then a full three phase fault. The MCC main did trip very fast, but there was quite a bit of damage done in the few cycles of the fault. The two men doing the work suffered no injury. But the guy I know got hit with some of the flying molten copper and steel. Nothing life threatening and most fortunately none in the face and especially eyes, but painful nonetheless.
If you're watching, give them a lost of room. A long sleeve cotton shirt and blue jeans would be a good idea if you're not wearing full fire retardant clothing.
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