MCC disconnects don't match up to circuit breaker, install shunt trip

[bdn2004]

We are replacing some power feeders to some new custom equipment. The design calls for changing out the existing 2P 480v circuit breakers with 100a 3 pole breakers at the MCC. The selected circuit breakers do not match up with the disconnect mechanism on the door. And they are a different brand. The existing breakers range from 100a to 350a and each one located in a full section of MCC. The existing breakers are different physical sizes.

This would work ok if we rendered that disconnect on the MCC inoperable and made the operator open the MCC door to turn off the power. The power would rarely ever be cut off and the receiving end of the feeders have disconnects. Or we could try to fabricate a new piece to get the mechanism to work.

I thought also about installing a shunt trip in the new breakers and leaving the old ones in place and running that through a control circuit. Turn off the power at the disconnect switch it turns off the existing circuit breaker that circuit breaker shunt trips the new breaker.

Is this way too much rigmarole or a reasonable design?

 

[LionelHutz]

Why don't you use the same brand of circuit breaker?

 

[jghrist]

Requiring the operator to open the door to turn off the breaker potentially exposes him to different arc flash hazards. If the current PPE requirements for operating the breaker with the door closed are lower than that required for the full incident arc flash energy, a risk assessment should be done to determine new PPE requirements.

Using the existing breaker as a control switch for a shunt trip would not be acceptable for use in Lockout/Tagout procedures. Control switches are not acceptable disconnecting means. See OSHA Std 1910.147.

 

[bdn2004]

I am looking into using the same brand. They are ABB. The problem is the existing breakers are 250Amp frame, up to 400A frame. The new ones are 100A. The existing mechanism looks like it is specifically made for the frame size. So even the smaller frame size won't fit the 100A.

I'm not sure how small a trip unit you can put in the frame sizes. I'm looking into that too.

It also looks like the existing 2P breakers are the same physical size as their 3P relative. Is there actually a set of power contacts in a two pole breaker and just not the terminals on the top and bottom? Can you retrofit a 2P breaker and make it into a 3P?

jghrist - The only thing in the entire full MCC section will be this breaker, everything else is being gutted. There is no exposed bus. I also considered just getting a standard enclosed breaker with a side disconnect and mounting it to the back panel. Would that be acceptable? Just a thought on the shunt trip...you torpedoed that one pretty good.

 

[jraef]

If the old MCC cabinet is just being used in lieu of an ordinary box and the bus bars have been removed, then you are completely on your own for adaptations anyway. If it were me, I would not mess with modifications and use one of the existing larger breakers to feed a panelboard somewhere else that houses the new feeders. Safer, legal, etc.

A better alternative to your complex shunt trip rigamarole would be to finish gutting that box and put in a full height back panel in each unit, then mount the breakers and use a rotary through-door mechanism for them that interlocks with the door. You will likely need to fill the openings left by the old operating mechanisms, but that's why they make steel plates and rivets...

As to the 2P vs 3P, most NEMA design breakers do not know or care if one pole is used or not, but some IEC breakers might give you nuisance trips when near full load if one pole is left with no power flow. You will have to ask the specific breaker mfr or look in the instructions. If it is sensitive to phase loss, you will see the instructions tell you to loop one pole back to the 3rd so that all 3 have power flow through them.

"Will work for (the memory of) salami"

 

[bdn2004]

I think we're going with one of these. See attached. This is the same as the existing except except a different breaker.
They don't make the breaker that's there anymore. So we are going to have to adapt it to the existing holes.

I like the rotary through the door switch alternative.

 

[ScottyUK]

You might want to review

http://www.eng-tips.com/viewthread.cfm?qid=309140

if you are using a shunt trip: you're using the same (or very similar) range of breakers albeit with a US part numbering scheme. The source of the original problem has never been satisfactorily answered by ABB although the factory did produce a replacement shunt trip coil for us and replace those affected.

 

[LionelHutz]

In NEMA type equipment, the cutout for the flange handles from the different manufacturers in the 15A - 600A breaker range is a standard size. Also, the different manufacturers tend to have a single flange handle that works for all their breakers. We mostly use Schneider and Siemens breakers and it requires us to stock 2 different flange handles. So, it should be easy to re-use the handle and just change the mechanism it drives, or change it all to a different manufacturer. Cable operators do make it easier to place the breaker.

Otherwise, if all else is being removed and replaced then use a rotary handle with whatever breaker you like as jraef has suggested.

 

[bdn2004]

That flange assembly size on all NEMA equipment is the same size? If that is the case why would we even have to change it out? They all look like they work through a cable sort of like what's on a lawnmower. Too bad the breakers aren't a standard size too.