Molded Case Breakers for a Highly Variable Load

[cslater]

I am in the planning stages of a project to replace the 480V switchgear for a process that can't tolerate much down time.

One approach we're considering is to set a temporary line-up of switchgear nearby, and move the loads to the temporary line-up while we remove and replace what's there.

The existing line-up uses draw-out gear, as will the new, permanent line-up. However, since the temporary gear will not be needed after the upgrade, our plan is to use molded case breakers.

What concerns me is that one of the process loads we're supporting has extreme fluctuations. On an analog current meter, it appears to swing between 0.5 MW and 1.5 MW every second or two. I suspect it may be fluctuating much faster than that and the analog meter is just not fast enough to show it.

Can molded case breakers support a load like this (assuming they are of the proper amperage) or is there a risk that they will trip?

 

[jraef]

Breakers trip on current over time (i²t) or on instantaneous magnetic trips (inrush) during a fault. Assuming you had ACBs or VCBs in the draw-out gear, the tripping characteristics would not be that much different from that standpoint. But there is a MUCH bigger difference in their fault tolerance and interrupt capability. Have you looked into that aspect? Also, most likely your draw-out gear was coordinated to hold in during some faults to allow downstream gear to interrupt at the lowest possible level. Putting MCCBs in it's place could put you at risk for more widespread interruptions because the trip characteristics may be too similar to the downstream gear.


"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

 

[RajeevCell]

1.5 MW load at 480 volt? Usually ACBs are used in that range, not MCCBs.

 

[cslater]

Just to be clear - I will be putting in draw-out breakers for the final replacement. I just want to use MCCB's for the temporary distribution panel to "keep the lights on" while we demo the old gear and put in the new.

It sounds like it will probably work as long as I get MCCB's with adequate settings.

As far as upstream issues, this lineup is the heart of the facility, and it has a dedicated substation, so I should be protected I think.

 

[jraef]

...so I should be protected I think

Famous last words...

The fact that this has a dedicated substation should make you MORE nervous about it, not less. I highly suggest you contract the services of a qualified EE for this, even though it's temporary. You can't be temporarily burned or dead...


"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

 

[cslater]

We will be working with an EE to do the coordination study for the new equipment. Whatever settings are determined for the new gear will be used on the temporary gear as well.

Thanks for the warning