In the past, electrical engineers had two options when specifying power circuit breakers: air or oil. Circuit breakers are often an afterthought today, as established technologies exist for interrupting and insulating methods. The 2kV to 38kV AC air circuit breakers have virtually disappeared, but they still play a major role in equipment of the 250V to 2000V range, especially in AC and DC power systems.
Remember that when standards for AC and DC circuit breakers were developed, your application may not have been considered. This could lead to unforeseen problems, such as equipment malfunction.
AC Circuit Breaker Operation
AC breakers can be problematic when fault currents are high and X/R ratios are high (i.e., AC systems can experience high fault currents with high X/R ratios). Low fault currents in an AC system do not cause concern, since the AC breaker benefits from a zero-current crossing during the AC cycle. The arc energy dissipates sufficiently during these zero crossings, and the breaker is able to establish a successful circuit at the next voltage spike.
DC Circuit Breaker Operation
Different DC electrical systems deal with fault interruptions. A DC breaker’s maximum fault current is just as important as an AC breaker. For DC breakers, however, the L/R (i.e., the rate of time constant rise) is also important. DC circuit breakers perform well with high fault currents and low L/R values. This is due to the powerful forces that push an electric arc into the magnetic blow-off quenching zone. A DC breaker is unable to interrupt higher L/R and low fault currents. The lack of zero-current crossings and the weak magnetic forces, which would otherwise push the persistent arc into the quenching area, are credited with this aspect. The DC breaker can fail to trip in extreme cases where the fault current is low.
For industrial power systems, such as rail electrification and traction power, it is important to have precise short circuit study models. You should use either reliable data or experienced engineers to obtain accurate X/R and L/R parameters. Conduct an accurate short-circuit analysis at each bus faulted to identify AC breakers that have been exposed to X/R and fault currents beyond their published test ranges. Also, perform an accurate DC short circuit analysis to ensure that the rise times are equivalent for each bus.
For nearly half a century, Swartz Engineering has been at the forefront of industry safety. They are a family-owned company specializing in power distribution for the electrical industry. They are the leading manufacturer of DC 82. DC 82 relay is designed to be used on light and heavy rail systems.
