This is the last in a series of posts discussing the National Electrical Code® (NEC®) requirements for selective coordination. In this post I will address some frequently asked questions and misconceptions regarding means for achieving high levels of selective coordination with molded case circuit breakers (MCCBs).
Q. Is there a rule of thumb regarding the ratio of the size of upstream and downstream circuit breakers required to achieve selective coordination?
A. There is no hard and fast rule other than that the upstream circuit breaker should be at least one frame size larger than the downstream circuit breaker. What the ratio actually turns out to be in a specific application is dependent on the electrodynamic withstand capability of the upstream circuit breaker, the peak current (Ip) let through by the downstream circuit breaker and the available short circuit current where the downstream circuit breaker is located. However, Schneider Electric does offer circuit breakers specifically designed for selective coordination applications where a 2:1 ratio does apply.
Q. Do I need to know the available short circuit current throughout the system?
A. Yes! And if selective coordination is required up to both the normal and alternate sources you need to know the available short circuit current from the potentially highest-producing sources, all the way down to the last bus in the system. For the lesser source, you need to know the available short circuit current down to the first bus downstream from the transfer switch.
Q. Do I need to be concerned about the available ground fault current?
A. In most systems the three-phase short circuit (aka bolted fault) current is higher than the single-phase-to-ground current. However, in systems with very large generation capacity, the single-phase-to-ground current close to the generators can actually exceed the three-phase short circuit current.
Q. Can I use the bus bracing or short circuit current rating of the equipment?
A. Engineers often specify the short circuit current rating of panelboards and the like in their specs. But usually this specified level far exceeds the actual available short circuit current. Using the actual available short circuit current will result in a more practical and economical design.
Q. If I want a system to be selectively coordinated can I just specify that the system be coordinated to 0.01 sec?
A. No! Doing so implies relying on the use of time-current curves (TCCs), but the information presented on TCCs alone may be insufficient to determine selective coordination (see the post “Selective Coordination with Molded Case Circuit Breakers”). To achieve selective coordination with fuses, the fuse ratio tables must be employed. For circuit breakers, if there is an overlap of the TCCs below the available short circuit current at the downstream bus, then the circuit breaker selective coordination tables must be employed. If the system you are designing requires selective coordination, then that is what specifications should state, not “coordinated to 0.01 sec”.
A. The GFPE pickups should be set so that the GFPE devices are not only coordinated with one another, but also with the downstream phase overcurrent devices. If this is not done properly, a low level ground fault in a branch circuit may cause a GFPE device on a feeder to unnecessarily open.
Q. Does the NEC permit the use of circuit breaker selective coordination tables and fuse ratio tables?
A. Yes! The NEC simply states that certain systems must be selectively coordinated but does not specify what resources may or may not be used to achieve the requirement. Manufacturers publish TCCs and tables for just that purpose. The use of TCCs alone, depending on the available short circuit current, may be insufficient and may result in a design that is not selectively coordinated (see the post “Selective Coordination with Molded Case Circuit Breakers”).
Q. Where should selective coordination studies begin, at the normal source or at the alternate source?
A. Studies should always start at the bottom of the system, i.e. at the bus farthest from the source, using the highest available short circuit current, which will most often be from the normal source, then work your way up to that source. Then for the source that is capable of producing less short circuit current, start at the first bus downstream from the transfer switch and work your way up to that source.
Q. If TCCs overlap in the instantaneous region of their TCCs (see Fig. 1 in “Selective Coordination with Molded-Case Circuit Breakers), does that mean the circuit breakers are not selectively coordinated?
A. Not necessarily. What will cause the upstream circuit breaker to open is the peak let-through current (Ip) of the downstream circuit breaker. But TCCs don’t tell you anything about let-through current, so in such an instance the coordination tables provided by the circuit breaker manufacturers must be consulted. Such tables take let-through current into account.
Q. Is it necessary to use electronic trip circuit breakers to achieve selective coordination with MCCBs?
A. Electronic trip units in MCCBs offer the advantage of being able to shape the TCC in order to achieve better coordination or protection. But when two circuit breakers are not coordinated in the instantaneous region of their TCCs (see Fig. 1 in “Selective Coordination with Molded Case Circuit Breakers”), what may be needed in the upstream circuit breaker is higher electrodynamic withstand capability. That is a function of the design of the current path in the circuit breaker, not what kind of trip system it has. The type of sensor feeding the electronic trip unit is also an important factor, with air core current transformers (Rogowski coils) being better than iron core CTs. And has previously been mentioned, the selective coordination tables provided by the circuit breaker manufacturer may need to be employed.
Q. Why is coordination to only 0.1 sec required in health care facilities by the NEC and NFPA 99?
A. In my opinion, the health care engineering community has recognized that high level faults rarely occur, hence requiring selective coordination to the available short circuit (bolted fault) current is not necessary. After all, when can a bolted fault occur? Certainly not in an operating electrical system! So one is most likely to occur at initial installation or after major service work or modifications to the system have taken place. In such instances, testing can determine whether a wiring error has been made or a grounding jumper has not been removed before energizing the system.
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