TECHNICAL BRIEF: REPLACING OLDER MEDIUM-VOLTAGE CIRCUIT BREAKERS WITH VACUUM CIRCUIT BREAKERS  

By Baldwin Bridger

Many older installations of metal-clad switchgear are candidates for modernizing or upgrading. The interrupting duty may have grown past the interrupting capacity of the existing circuit breakers, maintenance costs may be getting out of hand, or circuit breaker parts may no longer be available.

Modernization may be accomplished by either of two methods, conversion of the existing equipment or replacement of the circuit breaker with a new, modern vacuum breaker. In some cases, modernization may involve conversion of the switchgear equipment and replacement of the circuit breaker. Modernization may also involve an increase in one or more ratings of the switchgear and/or circuit breaker.

In any case, users should insist that modernization be done in accordance with applicable industry standards to insure that the modernized switchgear meets the required ratings. For replacement circuit breakers, this is fairly simple. The breakers should be tested in accordance with ANSI/IEEE C37.09 and ANSI/IEEE C37.20.2, just like any other new circuit breaker that is used in metal-clad switchgear. The only caution is that certain tests, including continuous current, momentary current, and BIL should be performed in a switchgear cell of the basic design in which the breaker will be used. All of these tests involve interaction between the cell and the circuit breaker. For instance, we have performed full wave impulse (BIL) tests where both the breaker alone and the cell alone passed the test, but the combination would not pass, requiring additional work to the breaker to achieve the desired result.

ANSI standard, ANSI/IEEE C37.59, IEEE Standard Requirements for Conversion of Power Switchgear Assemblies, covers conversion of both switchgear equipment and power circuit breakers. This standard specifies the design and testing requirements for conversion, including uprating, of existing equipment. Requirements for new documentation, including nameplates and instructions, are included. Any user considering converting low-voltage or medium-voltage switchgear equipment should review this standard and insist that vendors conform to it.

Baldwin Bridger is retired Technical Director of Powell Electrical Manufacturing Co., Houston, Texas. He has worked as an engineer and engineering manager in the design of low- and medium-voltage switchgear since 1950, first at GE and since 1973 at Powell. He is a Fellow of IEEE and a past president of the IEEE Industry Applications Society .

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