LIFE EXTENSION STRATEGIES FOR MODERN POWER CIRCUIT BREAKERS

Satin American has manufactured retrofit systems for 600V class power circuit breakers since 1988. Back then, most upgrades were performed because aging electromechanical trip devices were no longer serviceable. Since that time, OEMs have standardized on more reliable electronic protection. Now, after decades of dependable performance, these modern systems are beginning to approach end of life.

Circuit breakers make up the backbone of power distribution infrastructure. They are robustly constructed and expected to have a service life spanning several decades.   Industry depends on this equipment to operate safely and reliably long after OEMs have introduced new models.
Modern breakers are controlled by an electronic control unit (ECU) that interfaces with current sensors and a flux trip device.  The ECU determines the magnitude of current flowing in each phase conductor by measuring the output of its corresponding current sensor. It then compares this current to preprogrammed thresholds. If a threshold is exceeded, the ECU executes a predetermined time delay, verifying that current remains above pickup and then sends a pulse to fire the flux device and open the breaker. Drift or failure of any component in this system can result in compromised coordination, nuisance tripping or complete loss of protection.  While the protective system is arguably the most critical subsystem on the breaker, experience has shown that it is also most susceptible to developing performance issues over time.

Like any electronic device that has seen years of service, ECU’s are vulnerable to numerous modes of failure. These include degradation of components (especially capacitors), contact and circuit board failures due to electrical, mechanical or thermal stress, as well as corrosion and development of tin whiskers. Figure 1 depicts a “bathtub” curve that is widely used in reliability engineering to model the lifecycle of a population of products. The left side of the curve shows a high but rapidly decreasing failure rate that occurs when the product is new. These failures are referred to as “burn-in” or “infant mortality” and are frequently caused by improper handling or installation and are usually identified during the commissioning process. After infant mortality issues are eliminated, the population enters its “useful service life” which is a long period of high reliability and low failure. After time, “end of life” failures begin to manifest and the mortality rate across the population increases rapidly.

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