New Jersey -- Selectively coordinating power distribution and electrical systems in critical facilities remains a hot topic among professional engineers and their facility-owner clients.
Properly coordinating AC electrical power distribution systems with over-current protective devices can be complex and difficult. It requires integrating different components, technologies, manufacturers and standards. There is no standard, cookie-cutter approach that can be applied effectively across system designs.
Selective coordination requirements for emergency and legally required standby systems are challenging electrical engineers to consider how systems respond to short circuits. In fact, full selective coordination has changed the way engineers must think when designing electrical distribution systems.
When selecting circuit breakers as over-current protective devices, manufacturers' tables can help determine proper upstream and downstream circuit breakers. Such tables and time-current curves ought to be used in tandem to meet selective coordination requirements. Each manufacturer's table provides data only for its own over-current protective devices (Fig. 1).
Selective coordination, unquestionably, involves trade-offs of personnel safety and the threat of arc flash (Fig. 2), and of maintaining power to critical systems operating while preventing damage to electrical wiring and equipment. It's mandatory for emergency electrical systems for healthcare facilities, emergency systems, legally required standby systems and critical operations power systems.
Optimizing the Design
While the design engineer may select an over-current protective device that may seem well suited for satisfying the requirements of the short circuit study, it may not be the best choice for selective coordination. If the system has been expanded or upgraded over time, it may include both circuit breakers and fuses, and those products may even be from different manufacturers. In these cases, ensuring selective coordination becomes more problematic.
Effective selective coordination during a system’s lifecycle could require using the same type of over-current protective devices from the same manufacturer over time.
Although there is no effective cookie-cutter approach to engineering selective coordination systems, the following seven general guidelines can help ensure balance among sometimes competing interests.
Seven General Guidelines
1. Identify the maximum available fault current from the utility that provides normal source power and related information such as maximum transformer size.
2. Conduct over-current coordination and short-circuit studies to lay the foundation for an effective design. This work also ought to include verifying equipment full load ratings.
3. Reduce the number of levels of protective devices. The fewer the levels, the easier the task of selectively coordinating over-current protective devices.
4. Reduce the available fault current by increasing the impedance of the system (Fig. 3), or by utilizing step-down or isolation transformers.
5. Select long-time settings to protect equipment against overload. Select instantaneous timing and short-time settings to selectively coordinate. Or, use tables for overload protection that coordinate up to the available level.
6. Select current-limiting type molded case circuit breakers where possible for branch devices. These breakers respond very quickly and significantly limit let-through current so they coordinate better and even reduce the required trip timer settings of upstream devices.
7. Or, consider changing a molded case circuit breaker to either an insulated case breaker or a low-voltage power breaker, which can increase the level of selective coordination with a downstream device.
Power Transfer Switches and Selective Coordination
Optimized selective coordination systems that incorporate power transfer switches achieve fast fault clearing times and coordination of over-current protection at reasonable cost. Transfer switch design, including specific features, affects cost, reliability, maintenance and personnel safety throughout the system’s life cycle.
Transfer switches must withstand and close on fault currents until the downstream protective device clears the fault. Its ability to accomplish this typically is rated in terms of its “withstand” current. The withstand rating is the highest level of current that can be carried by a given transfer switch for a specific amount of time. It needs to be long enough for the upstream over-current protective device to clear the fault. Transfer switches with integrated over-current protection will need to selectively coordinate with other devices.
Whether selected to satisfy requirements of NEC Articles 517, 700, 701 or 708, transfer switch ratings for a fault current must be greater than, or equal to, the available fault current and system voltage as determined at the power source terminals of the switch. The withstand-close-on ratings can be based on a specific device rating or on an “any-circuit-breaker” rating.
Effective August 1, 2014 the 7th Edition of UL1008 deleted the terminology, “Any Breaker,” rating and replaced it with time-based ratings. Specifically, these times are 0.025 seconds for switches rated 30-400A and 0.05 seconds for 600-4000A. Most transfer switches today have both time-based and specific breaker WCR ratings listed on their label.
A transfer switch may also have an optional short time rating, which must be longer than 6 cycles (0.1 seconds) according to UL1008.
The ratings must be evaluated based on the pairing of the transfer switch and the immediate upstream over-current protective devices (Fig. 4).
The over-current device’s clearing time typically is provided by trip curves in seconds, but is frequently translated to AC cycles in a 60Hz system in transfer switch provider’s marketing documentation and communications. In other words, a 1600 amp transfer switch would need to be able to withstand a short circuit current of 65,000 amps for 0.3 seconds and 18 cycles.
There are no “ideal” time delay settings for selective coordination in design schemes. And, Underwriters Laboratories doesn’t require a specific time to qualify for “short time” ratings, though it does provide standard recommended values.
What matters is the calculated available fault current at that point in the system, and the ability to clear the fault as fast as possible while maintaining coordination.
Specifying Cycle Times
As discussed earlier, effective selective coordination design does not follow cookie-cutter approaches. So it is, too, with specifying transfer switches that will be integrated into such designs.
It might seem easy to specify 30-cycle (0.5 seconds for 60-cycle systems) transfer switches as a cookie-cutter approach for both the ceiling and floor of selective coordination timing. What accompanies that decision, though, are safety, cost and other issues. As discussed earlier, in some installations, personnel safety and equipment integrity may be compromised by letting energy levels flow for 30 cycles within the electrical system. Using short-time-rated trip units in low-voltage circuit breaker settings may allow fault currents to flow for 30 cycles, perhaps negating equipment protection and increasing arc-flash hazard.
Specifying 30-cycle rating switches for all applications carries premiums in terms of equipment and spare parts costs—sometimes as much as 15 to 30 percent or more. They also may require rear entry, a larger footprint and more expensive maintenance.
Considering the custom nature of selective coordination, specifying only what’s necessary, what more precisely satisfies requirements, is the better decision.
Specifying 30 cycle ratings also may necessitate the installation of larger feeder cables than normally required to safely carry the available short circuit current for 30 cycles. This can significantly increase in the cost of the feeder cabling.
Engineers who attended a webcast on selective coordination and responded to a survey overwhelmingly agreed. Eighty-eight percent* agreed that selective coordination may not be optimal if 30-cycle transfer switches are used for an entire facility.
Engineers responsible for developing and vetting selective coordination systems that meet code requirements face an often difficult challenge.
Knowing code requirements, of course, methods to optimize design, matching transfer switches with circuit breakers and fuses, and satisfying special application requirements is essential for effective selective coordination.
About The Authors
Dan Caron is Principal of Bard, Rao + Athanas Consulting Engineers, a national consulting engineering firm headquartered in Boston. Ron Schroeder is Director of Applications Engineering and Product Management of ASCO Power Technologies, Florham Park, N.J.