Low Voltage Power Systems Utilizing Fuses and Industrial Circuit Breakers

Many plant managers in both industrial and commercial industries use both fuses and industrial circuit breakers in concert in low voltage power systems. However, Traditional Time Current (TCC) analysis is known not to completely and fully identify which is used when. Therefore, manufacturers of fuses and industrial circuit breakers are careful to provide specific guidelines for product selection.

Traditional Time Current Analysis and Industrial circuit breakers

When incomplete or incorrect TCC data are used in analysis, incorrect decisions can be made regarding what type of fuses or industrial circuit breakers should be used within a system. Traditional selectivity assessments are based on the use of TCC overlays. These have been proven over the long-term as a useful way to evaluate the long-term operation ranges for different overcurrent devices, such as fuses and industrial circuit breakers.

When industrial Circuit Breakers are examined, the curves are used to document the operation of overcurrent devices as well. Therefore, if the overcurrent devices operate faster than expected, the TCC is a limited assessment for accurately predicting device behavior.

Molded-Case Industrial Circuit Breakers

When molded-case industrial circuit breakers are used, they usually show instantaneous clearing times of 1.5 cycles or less. Additionally, the industrial circuit breakers instantaneous trip system may be responsive to peak amperes. This implies that the faults of equal root mean square (RMS) value power factors will be sensed by the industrial circuit breakers trip system differently. Since fuses are energy-based, they may also be affected by fault current unevenness.

Industrial Circuit Breakers

There are two different types of physical mechanisms that may result in current-limiting behavior in industrial circuit breakers. Magnetically opposite forces are created at the point where contracts touch due to current construction. This leads to conduction only at a few spots on the contact’s surface. As the current moves towards those contact surfaces, opposite forces are created across the contacts. When this occurs, mechanism spring forces should keep the contacts closed. When currents move above the industrial circuit break’s maximum simultaneous pickup, those opposite forces may start to overcome the spring forces, resulting in the contacts to temporarily part, resulting in an arc voltage known as “popping.”

A second current limiting industrial circuit breakers design is the reverse-current loop. In this design, the current is routed through parallel contact arms so opposing magnetic forces are formed.