This article provides recommendations on fused switches in a MCC for short circuit protection of motor and feeder branch circuits.
The Electrical Engineer selecting the appropriate device should understand what the potential available fault current is at the MCC incoming and select devices whose rating is greater than the available fault current. It is also important to understand what the ratings mean and under what conditions they are tested so the protection devices used can perform properly and not create additional hazards in themselves.
Fusible disconnects offer a number of important safety features such as the availability of visible blades and the ability to withstand and limit the maximum fault current available downstream if properly rated. Conversely, it is these two requirements that have been the biggest barrier to widespread implementation of molded case circuit breakers (MCCBs). Improvements in MCCBs, however, have made a re-examination and comparison of their features advisable in light of the present business needs. Current limiting MCCBs are approaching the capabilities of fuses in current limitation and interrupting capabilities. In addition to degree of protection, other factors deserving evaluation are operational, maintenance, and safety considerations, and cost, both initial and replacement.
The decision process on whether to specify MCCBs or fusible disconnects in MCCs can be very straight forward based upon technical requirements or very complex due to local requirements based upon operations and safety procedures.
Downstream Equipment Protection
In order to properly apply protective devices, Electrical Engineer should first understand the function of the fuse or circuit breaker in typical MCC usage. The function of these protective devices is somewhat dependent upon whether they are used in a branch feeder or a motor circuit. In a branch feeder circuit, the protective device provides both short circuit (fault) protection and overload protection for the cable and, in some cases, for the load if it is not protected by another device. In a starter or other MCC mounted motor controller the protective device protects the components within the "bucket," the cable and the load for short circuit only since the thermal overload device is the primary protection for motor overload.
The degree of protection offered by fuses or circuit breakers is dependent on their interrupting capabilities and their ability to limit energy (electromagnetic and thermal). A device that is incapable of passing or interrupting the currents and energies available in the circuit can fail in two unsafe modes:
- It can blow apart causing molten metal and hot gases to be violently propelled from the device.
- One or more of the contacts could weld closed making it impossible to open the contact(s) of the disconnect.
In the case of a fused device, the switch is not intended to interrupt the fault, but it must withstand the short circuit let-through of the fuse. The fuse is the interrupting device and, as such, must be capable of interrupting the maximum fault current available. In the case of circuit breaker protected motor and feeder branch circuits, the circuit breaker is the interrupting device and it therefore must be capable of interrupting the maximum fault current available.
There are important differences in how interrupting ratings for fuses and circuit breakers are determined under UL test procedures. In UL198C (fuse Classes G, CC, J and L) and UL198E (Class R) the test for interrupting rating ensures the fuse carries the full current values for the interrupting value it is being tested, i.e., 100,000 Amps, 200,000 Amps, etc. However, a circuit breaker's interrupting rating test parameters are described in UL489.
This interrupting test provides for 10 inches of "rated wire" on the line side and 4 feet of "rated wire" on the load side of the breaker. This means that, even though the test circuit has been calibrated to generate the rated interrupting current, the additional impedance of these lengths of rated wire reduces the actual current interrupted by the circuit breaker under test. To further confuse matters, when used in MCCs, an MCC starter can be rated under UL845 and will be labeled a "Unit short-circuit-current rating." The test for this rating allows for 4 feet of wire on the load side of the unit before the fault initiation; therefore, not only does it introduce the impedance of the wire but also the additional impedance of the internal starter wiring, the contactor and the overloads. This information is significant when considering circuit breakers for MCC applications especially on applications with relatively high available fault.
Table below shows the calculated currents interrupted by circuit breakers under test conditions required in UL489 and UL845. Note that in the UL845 column that the additional impedance shown is only for the overload heaters that should be the most significant resistance contributor. The 200 amp and 400 amp units typically use CT (current transformer) fed overload heaters and are therefore not calculated in this table (shown with"*").
Circuit Breaker UL Test Comparisons