Switchgear versus Switchboards 1

[cslater]

Conventional wisdom has been that switchgear is more robust and more reliable than switchboards - and that it should be specified for critical and industrial applications.

However, I know that there have been a lot of changes in molded case equipment over the last few years and I'm wondering - is it still the case that switchgear should be specified if uptime and durability are critical?

Any opinions and/or pointers to reading material would be very much appreciated.

 

[dpc]

I'd suggest you start by searching this forum - this has been discussed a few times. Here's a start:

thread237-155576

A few quick points - assuming you are talking about an installation in the US with equipment built to US and UL standards:

Switchgear implies use of low voltage power circuit breakers (LVPCB).
Switchboard implies use of molded case circuit breakers(MCCB).

LVPCB:

Maintainable
Through-fault withstand rating (instantaneous trip not required)
Designed for frequent operation
Electric operation basic to the design

MCCB:

Non-maintainable
Require Instantaneous trip (no withstand rating)
Intended for relatively infrequent fault interruption
Electric operation is a jury-rigged affair, in many cases





David Castor

http://www.cvoes.com

 

[cslater]

Great - looking at all of that now.

When you say that electric operation is integral - are you talking about operation by PLC's?

 

[dpc]

I was referring to electrical trip and close of the breakers via contact inputs. Standard breakers are manually operated, but electrically-operated LVPCBs are common and have an integral operating mechanism. This is not always the case for MCCBs.

The ability to remotely close large 480 V circuit breakers is more desirable now than in the past due to arc-flash concerns.

 

[jraef]

It also has a lot to do with fault tolerance. In a coordinated system, you want the breakers farthest upstream to be the LAST to trip. That means that the highest devices, i.e. the breakers in the primary switchgear, have to hold in against the fault long enough for the down stream devices to try to clear it. In that case, they are subject to the physical forces created by the fault current magnetics more than anything else in the system. The test procedures for LVPB devices is different than for MCCBs, different UL files, different ANSI classes, everything. That reflects in the price as well.

Power Breakers: UL1066, ANSI C37.13, .16, .17
Used in:
Switchgear: UL1558, ANSI C37.20.1
Switchboards: UL891*

MCCBs: UL489
Used in:
Switchboards: UL891
Or panelboards: UL67

The big difference as far as the fault tolerance is that UL891 Switchboards and the MCCBs (or fused disconnects) used in them will RELY on the Instantaneous Trips (clearing) of the devices to allow the structure to withstand the fault. In UL1558 gear, the UL1066 breaker can allow you to turn OFF the instantaneous trips so that the down stream devices clear the fault. In that case, the gear has to withstand those forces while waiting for the other trips to take effect.

* You can use some UL1066 breakers in UL891 Switchboards if you use a trip unit that disallows the disabling of the instantaneous trips. In other words because the equipment design RELIES on it, you MUST have it.

Another key factor is maintenance. MCCBs are not really designed to be maintained (although people do it), whereas LVPBs are specifically designed to be maintained, parts can be replaced in the field, etc. This is necessary in continuous process environments with regular scheduled maintenance programs. The cells of the switchgear are also designed to isolate a catastrophic failure without damaging nearby breakers, a Swbd is not; any catastrophic failure that occurs will take out other gear and require a complete shutdown to take care of it.

Using UL891 Swbds means you are willing to take a chance and suffer a major shutdown when something goes wrong in order to save initial capital costs. If down time is unimportant to you, go that way.

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