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100% Rated Square D 2000 Amp Circuit Breaker

December 15th, 2010

MIDWEST lost a battle with the maintenance dollar.  A manufacturing plant had a 2000 amp Square D circuit breaker that was loaded to over 1800 amps, sometimes hitting 1900.  We detected the heavily loaded breaker during an Infrared Scan of the facility.  This was an old Square D circuit breaker mounted in a tap box feeding a 2000 amp bus duct. The local contractor recommended they just replace the breaker with a 100% rated breaker. He said they could use Square D, Cutler Hammer, or General Electric, whoever was cheapest and would fit. Because of the bus configuration, it only made sense to use the same Square D circuit breaker. The customer bit on the idea that just replacing the old circuit breaker would solve their problem and save them a lot of money. We strongly disagreed. Replacing the circuit breaker would make zero difference, in this case. The old Square D molded case circuit breaker was an 80% rated breaker.  More recent replacement Square D circuit breakers, same frame, model number, current rating, were 100% rated, ie 2000 amps. The 100% rated replacement circuit breaker was specified 100% if it was in an enclosure with a much larger volume than the old breaker. In other words, to achieve the 100% rating, the breaker must be in a much larger enclosure so as to properly dissipate the heat generated from 2000 amps. This all makes sense. But to just replace one breaker, whether GE General Electric, Cutler Hammer, Siemens, or Square D, with a 100% rated breaker and not address the installation requirements to achieve the higher rating can be a waste of dollars.  A quick check of the old and new circuit breaker specifications, will tell what the installation requirements are. This is the technical added value that is often ignored, or just not known, in too many facilities, as experienced personnel disappear.

            This facility decided to replace the circuit breaker, but later transferred load off the bus duct.

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