MCC Bucket Changeout Question 2

[richanton]

I have to replace a size 1 with a size 2 starter. It's the same 12" space requirement. I've done an Arc Flash calc showing it's just Category 0. Are there any other issues with removing and replacing the bucket without shutting the rest of the MCC down.

 

[oldfieldguy]

1. Wear your PPE. Make sure any helpers wear their PPE.
2. Make sure that you control the conductors you may have to disconnect to remove the bucket.
3. Make sure you know the locations of ALL the screws and locking devices that hold the bucket in place.
4. Have a competent standby person(s)outside the blast area. This person should know the location of the best communications device AND the switch/breaker that turns off the MCC you're working on.
5. Before work commences, have a meeting to discuss safety, the process of the task, and emergency measures.
6. When you start the process, if at any time things don't seem to go right, i.e., too much force is required, things stick, etc. STOP and investigate.

old field guy

 

[richanton]

Thanks, oldfieldguy.

 

[itsmoked]

That's why he's old.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[rbulsara]

Is the bucket designed by mfr. to be replaced while the MCC bus is hot?

 

[dpc]

I'd be willing to make a small wager that if you read the operating instructions from the MCC manufacturer they will state that buckets should not be withdrawn or inserted while the bus is energized. This is, of course, one of the most widely ignored CYA statements ever written. But it's worth keeping in mind these days.

Star for Keith - ain't it the truth. I was once told 'we have old electricians and we have bold electricians - we don't have any old bold electricians.'

"An 'expert' is someone who has made every possible mistake in a very narrow field of study." -- Edward Teller

 

[jraef]

Actually, it's always been my understanding that MCCs are SPECIFICALLY designed to be able to remove buckets without shutting off power to the bus. The original design came out of the Automotive industry where a bucket change would not mean shutting down the entire line and having to do a re-start, which might take all day. That's why the buckets are "buckets"; they provide shielding from the bus bars and any flashes that might occur until the unit is all the way out, plus there are mechanical interlocks that ensure you cannot even begin to withdraw it until the disconnect is open.

Looking at a Siemens MCC instruction manual I have, it makes no mention of having to kill power to the bus first.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[ScottyUK]

Bear in mind when choosing where to stand when withdrawing it that a starter bucket becomes a projectile if an arcing fault occurs as it comes off the bars. Keep out of the immediate trajectory.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[dpc]

Jeff,

I'm glad I said a SMALL wager I do recall a discussion with some Cutler-Hammer engineers at an IAS safety conference on this subject a few years ago. They indicated that their instructions recommend de-energizing the bus to insert/remove buckets, while at the same time acknowledging that the removeable bucket has always been a major selling point for MCCs.

But I just checked a GE instruction book and don't find anything specific in there either. So I stand corrected.

Cheers,

Dave

"An 'expert' is someone who has made every possible mistake in a very narrow field of study." -- Edward Teller

 

[oldfieldguy]

jraef--

You're absolutley correct, but that was back in a time where rules were somewhat more relaxed. I swapped out dozens in my time, from live MCC's.

There were a lot of tasks we USED to do differently than today: manually racking out circuit breakers, rolling out PT drawers, etc. but in today's safety environment it's a different story.

old field guy

 

[jraef]

Actually, I don't disagree that there are undoubtedly new safety related issues now with 70E and PPE etc. I was not implying that proper safety procedures should not be adhered to. My point was just that from a design standpoint MCCs were, at least originally, designed to allow buckets to be removed under power. I'm going to check with my people now to see if there has been an update on bucket removal procedures in light of this. It will be good to know.

Regarding the "projectile" comment from ScottyUK: Keep in mind that NEMA MCC design requirements are likely different from IEC MCC requirements. I was involved in a redesign of an MCC structure years ago at a better-left-unmentioned manufacturer because the base design came from Europe and we were to "NEMA-nize" it. A big issue was that NEMA required that there is a detente or stop point at which the bucket cannot be "jerked" off of the bus without taking another active effort to continue after it has come free. Some mfrs do it with a simple catch point and tab in the frame, requiring that the bucket be lifted off of the tab and designed so that this can only take place after the bucket has traveled far enough to clear the bus. Others do it by having a second mechanical device such as a quarter turn screw that must be turned. We had to create one from whole cloth because the IEC design had no provisions as such.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[resqcapt19]

richanton,
Did you do your arc-flash calculations based on the available current on the MCC bus? Unless this MCC has a very weak supply, Category 0 doesn't sound right to me.

 

[dpc]

Category 0 is possible if there is a main CB at the MCC and credit is taken for that.

"The perfect is the enemy of the good." -- Voltaire

 

[richanton]

As far as the ArcFlash calc, I checked my numbers and they seem correct. I'm using PTW software. I think the reason the PPE is Category 0 is that the MCC is fed from a relatively small 480 volt switchgear(1000kva) and the available SC at the swgr is only around 22k. Then I run parallel 500MCM conductors about 200 feet to the MCC. But I'm going to check my numbers and the options also one more time just to make sure.

Even though it is Category 0, we are still going to have the electricians wear gloves and face shields.

 

[Zogzog]

Category 0 is possible if there is a main CB at the MCC and credit is taken for that.

A main with an INST function.