MOLDED CASE BREAKER OR POWER CIRCUIT BREAKERS? 3

[ZOBLITZ]

Hi everyone!

Our client is going to install a CO2 new plant; therefore they requested us install a main power distribution switchboard with service mains. Immediately I started to design the panel and I chose a 1250_A, 480_V, 45kA low voltage power circuit breaker as the main disconnect device. When I had advanced making the drawings for the panel someone asked me “Why not choose a motorized molded case breaker is cheaper than your option?”

I think 1250_A is on the border between a molded case breaker and a power circuit breaker so Right now I'm not so sure if there are others technical decisive aspects (excluding economic matters) for stay with a molded case instead of a power circuit breaker.I appreciate any comment regarding the subject.


I appreciate any comment regarding the subject.


Thanks.

 

[dpc]

Not sure what border you are referring to - you can purchase 5000 A molded case circuit breakers and they will be cheaper than a LVPCB. There are a lot of advantages to LVPCBs over molded case, the main one being that they will provide much better coordination since they can be purchase without an instantaneous trip, whereas an MCCB (at least in the US) cannot.

Molded case CBs require basically no maintenance, other than periodic testing, but cannot be maintained. LVPCBs can be maintained, but must be periodically maintained.

The list goes on. Space and access requirements for LVPCBs will be greater than for MCCBs.

Bottom line: If you want it cheap - go with MCCBs. If you want it to coordinate and have a long life (and you have the space) go with drawout LVPCBs. If you want it REALLY cheap, go with fused switches.

 

[DRWeig]

Last building I worked in did it on the cheap. Pair of 3000A molded-case breakers with a PLC in lieu of a transfer switch for the standby generator.

After 13 years, all is working just fine. The generator breaker stuck in the open position once and we were nice and black during the outage in which we needed it. I got the manufacturer's tech support on the phone and was advised to pick up something heavy and smack the breaker in the center of its front. Did so, breaker closed, no problems since. They do now have a spare sitting on a pallet nearby.

Best to you,

Goober Dave

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[Zogzog]

Go with a reconditioned, modern, LVPCB and get the best of both.

 

[jraef]

Since you are putting it into a Switchboard, you have a choice. But generally, the main advantage of an LVPCB is the ability to "hold" in on a fault for up to 3 seconds until a lower level protective device clears it. This capability is part of the UL1066 listing of LVPCBs. So you want to have LVPCBs in your primary Switchgear, which would be listed for the above purpose under UL1558. You can think of it as a last line of defense.

Putting an LVPCB into a switchboard, which most likely is listed under UL891 just like any other panelboard, kind of wastes that capability because nothing else in the Swbd will be rated for that anyway. So as a gross generality, people tend to only use LVPCBs in Swbds when they exceed the ratings of the MCCBs available from that mfr. most of the majors go up to 2000AF in their MCCBs, a few go higher.

A couple of other important issues though, LVPCBs are MADE to be rebuilt and serviced, MCCBs are not really. People do it, but it is much more involved. So your long term cost of ownership may be lower for an LVPCB if the user has a regular scheduled maintenance program (and they should). But if the user is more of a "set it and forget it" company anyway, you may as well go with the MCCB. In addition to that, LVPCBs often come with much more sophisticated electronic trip and monitoring systems now. It's not that you can't get that in an MCCB, but it's pretty much inherent in an LVPCB. So if you plan on doing some energy monitoring and want to use the Main device as a data gathering point, the LVPCB may offer better options for you. But also check your Swbd mfr to see if they offer a UL489 listed version of the LVPCB. Since the Swbd can't handle the UL1558 requirements anyway, the UL489 listed version is good enough and will cost less, but still offer the other advantages of LVPCB designs.




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