MCC Bucket disconnect

[davidb123]

Gentlemen,

Let's say I've a MCC bucket 480V, FVNR with a AB E300 O/L. When I disconnect this bucket, what if there is still 120V on the E300 module. Is there any NEC Violation in there ?

Thanks

 

[Jraef]

Gentlemen,

Let's say I've a MCC bucket 480V, FVNR with a AB E300 O/L. When I disconnect this bucket, what if there is still 120V on the E300 module. Is there any NEC Violation in there ?

Thanks

It's not an NEC violation in and of itself, but may represent a safety issue. Where does the 120V come from? Generally if you order an MCC bucket and say that the 129V control will come from a "separate source", the MCC design software will force the engineer to add a door interlocked aux contact attached to the disconnect handle operator. For you to have ended up without that, someone had to work at it a deliberately defeat the safety checks. But if it was overridden, the quote system will specify to the buyer that they will be responsible for applying warning labels stating that there is a foreign source if control power on each bucket door.

 

[petersonra]

we had a customer that did that on hundreds maybe thousands of MCC buckets we provided them.

we used to buy std mccs with xfmrs from AB and take the xfmrs out of the buckets and add in the aux contact for the disconnect because it cost less and was much faster to get.

by the way - the code requires a controller disconnect as well as a motor disconnect.

 

[Cow]

It's not an NEC violation in and of itself, but may represent a safety issue. Where does the 120V come from? Generally if you order an MCC bucket and say that the 129V control will come from a "separate source", the MCC design software will force the engineer to add a door interlocked aux contact attached to the disconnect handle operator. For you to have ended up without that, someone had to work at it a deliberately defeat the safety checks. But if it was overridden, the quote system will specify to the buyer that they will be responsible for applying warning labels stating that there is a foreign source if control power on each bucket door.

Are you talking about the breaker aux contact or a separate disconnect handle or door contact?

I've ordered a lot of AB MCC where the "120v is from a separate source" and I've never seen more than a breaker aux contact?? If there is something above and beyond that to kill control power when the door is opened, it would seem my local AB rep has been nix'ing that option without telling me.

 

[wireman]

Often times we are req'd to wire the disconnect's AUX contact back to a PLC and the power on the contact comes from the PLC.

We put a label on the bucket saying comething like "Foreign Power May Stiil Be Present Even Though Disconnect is Off". Orange wire is used between the MCC and the PLC.

 

[Jraef]

Are you talking about the breaker aux contact or a separate disconnect handle or door contact?

I've ordered a lot of AB MCC where the "120v is from a separate source" and I've never seen more than a breaker aux contact?? If there is something above and beyond that to kill control power when the door is opened, it would seem my local AB rep has been nix'ing that option without telling me.

You can do it either way and the cost is the same either way. In the olden days when breaker aux contacts had to be ordered as part of the breaker, it was often easier to just use the handle aux contacts, which are what you use if it's a fused disconnect anyway. Now with the simpler field retrofit breaker aux contacts, it doesn't matter. The thing is, the breaker aux contact will change state if the breaker trips, the handle aux contact will not, which is a slight difference from a safety standpoint. If you open the door, the handle aux contacts are always going to be open and if you close the breaker while the door is open, the handle aux stays open and the control circuit stays off. The breaker aux will change state with the breaker main contacts, regardless of why so if you close the breaker while the door is open, the control circuit gets energized too. Like I said, slight difference. But more dangerous (in theory) is that if the breaker main contacts WELD, the breaker aux contact will not open when the handle is moved, even though it should.

 

[templdl]

All good stuff. This is why it is imperative to have a qualified person accessing the bucket. But on additional thing. The MCCs that I as m familiar with also have pull sppart terminal block on the side of the bucket which are accessable by opening the wire way door.

 

[davidb123]

Guys,

Thanks for you answers. However I was refering to article 430.75, which looks like I'm a violating. Maybe exception 2 could be a way to escape since loss of communication could results in potentially unsafe conditions.

Any thoughts ?

 

[Jraef]

Guys,

Thanks for you answers. However I was refering to article 430.75, which looks like I'm a violating. Maybe exception 2 could be a way to escape since loss of communication could results in potentially unsafe conditions.

Any thoughts ?

Hard to say, what is "unsafe" about losing communications?

Even so, you STILL must comply with conditions a and b of exception 1, which is the signage and the access limited to "qualified persons".

 

[davidb123]

Well,

I guess my questions is more about 430.75 itself. Am I violate it or it doesn't apply to a MCC Bucket ?

Thanks guys!

 

[Jraef]

Well,

I guess my questions is more about 430.75 itself. Am I violate it or it doesn't apply to a MCC Bucket ?

Thanks guys!

Yes, it applies to an MCC bucket.

So if you somehow managed to thwart the system that was designed to NOT allow you to set up a situation were a foreign voltage source is not disconnected by the MCC bucket disconnect, then you will have to have an external means somewhere (usually where that source originates) and signs on each bucket door.