Protective Devices Maintenance As It Applies To The Arc/Flash Hazard
By Dennis K. Neitzel, CPE
Director, AVO Training Institute
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One of the key components of the Flash Hazard Analysis, which is required by NFPA 70E-2000, is the clearing time of the protective devices, primarily circuit breakers and protective relays. Fuses, although they are protective devices, do not have operating mechanisms that would require periodic maintenance; therefore, this article will not address them. The primary focus of this article will be the maintenance issues for circuit breakers and protective relays.
Molded-case and low-voltage power circuit breakers (600-volts or less) will generally clear a fault condition in 3 to 8 cycles. To be conservative a clearing time of 8 cycles should be used. The majority of older medium-voltage circuit breakers (2300-volts or greater) will clear a fault in around 8 cycles with the newer ones clearing in 3 to 5 cycles. Protective relays will generally add approximately 3 to 4 cycles to the clearing time of the medium circuit breaker. Where proper maintenance and testing are not performed, extended clearing times could occur creating an unintentional time delay that will effect the results of flash hazard analysis.
This article will address some of the issues concerning proper maintenance and testing of these protective devices, according to the manufacturer's instructions. It will also address how protective device maintenance relates to the electrical arc/flash hazard.
Molded-Case Circuit Breakers
Generally, maintenance on molded-case circuit breakers is limited to proper mechanical
mounting, electrical connections, and periodic manual operation. Most lighting, appliance,
and power panel circuit breakers have riveted frames and are not designed to be opened for
internal inspection or maintenance. All other molded-case circuit breakers that are UL
approved are factory-sealed to prevent access to the calibrated elements. An unbroken seal
indicates that the mechanism has not been tampered with and that it should function as
specified by UL. A broken seal voids the UL listing and the manufacturers' warranty of the
device. In this case, the integrity of the device would be questionable. The only exception to
this would be a seal being broken by a manufacturer's authorized facility.
Molded-case circuit breakers receive extensive testing and calibration at the manufacturers' plants. These tests are performed in accordance with UL 489, Standard for Safety, Molded- Case Circuit Breakers, Molded-Case Switches and Circuit Breaker Enclosures. Molded-case circuit breakers, other than the riveted frame types, are permitted to be reconditioned and returned to the manufacturer's original condition. In order to conform to the manufacturer's original design, circuit breakers must be reconditioned according to recognized standards. The Professional Electrical Apparatus Recyclers League (PEARL) companies follow rigid standards to recondition low-voltage industrial and commercial molded-case circuit breakers.
Click here to download the full article in pdf format.