Achieving effective selective coordination design


Selectively coordinating power transfer switches

Power transfer switches are essential in emergency, legally required standby and optional standby power systems. Consequently, they are essential to selective coordination systems as well. Optimized selective coordination systems that incorporate power transfer switches achieve fast fault-clearing times and coordination of overcurrent protection at reasonable cost. Transfer switch design affects cost, reliability, maintenance, and personnel safety throughout the system’s lifecycle.

Transfer switches must withstand and close on fault currents until the downstream protective device clears the fault. Its ability to accomplish this is typically rated in terms of the device’s withstand and close-on rating (WCR). The WCRis the highest level of current that can be carried by a given transfer switch for a specific amount of time. This time must be long enough for the upstream overcurrent protective device to clear the fault. Transfer switches with integrated overcurrent protection typically selectively coordinate with other devices.

Whether selected to satisfy requirements of NEC Articles 517, 700, 701, or 708, transfer switch ratings for a specific 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 WCR can be based on either a specific device rating, or an optional “any-circuit-breaker” rating. A switch may have both types of ratings, which provides greater flexibility in the selection of the overcurrent protective devices. 

A transfer switch also may have an optional short-time rating. This rating is intended for use with an upstream circuit breaker having a short time rating of “X” cycles (sec). The overcurrent device’s clearing time is typically provided by trip curves indicated in sec, but is frequently translated to ac cycles in a 60-Hz system. In other words, a transfer switch should be able to withstand a short-circuit current of 65 kA for 0.5 sec or 30 cycles, for example.

Typically, power transfer switches facilitate the selection of fuse-clearing times or breaker settings using increments between 0.5 to 30 cycles, with multiples of two or three cycles being popular. There are no ideal time-delay settings for selective coordination in design schemes. Also, UL doesn’t require a specific time or specific number of cycles to qualify for short time ratings, although it does provide standard recommended values.

The transfer switch location is significant in terms of effective selective coordination. Transfer switches located closer to their loads translate into:

  • Higher reliability
  • Smaller circuit breaker or fuse sizes for feeding the transfer switch
  • Fewer levels of distribution
  • Lower fault currents at the transfer switch terminals
  • Faster fault-clearing times
  • Improved load protection.  

Locating a switch closer to the source (as opposed to the load) can lower system or facility reliability, may cause downstream breakers to trip frequently, and may not isolate—or start—an alternate power source. Larger circuit breaker or fuse sizes may be needed. Higher fault currents may be experienced and short time protection may be required. Bottom line: locating a switch closer to the power source typically means relatively poor load protection. 

Specifying cycle times

Consulting engineers should recognize that specifying transfer-switch cycle times is necessarily project-specific. Most transfer switches specified by the co-author’s company are 3-cycle switches. The firm has specified 30-cycle switches where at least one of the following conditions exists:

  • Larger projects with high emergency-system fault current
  • Where the instantaneous trip setting is defeated to achieve selective coordination (note that this may change because of the acceptance of the latest NFPA 99)
  • Where transfer switches are served by ANSI switchgear, which also has 30-cycle withstand ratings.  

Every project should be considered to be custom. For example, even if two chain restaurants are built in two different locations using the same drawings, the coordination would be different. Effective coordination depends on the electrical utility and the available fault current. Inevitably, electrical characteristics and situations vary from place to place, building to building, design to design, and utility company to utility company.

It might seem easy to specify 30-cycle transfer switches as a cookie-cutter approach for both the ceiling and floor of selective coordination timing. However, that decision introduces safety, cost, and reliability issues. In some installations, personnel safety and equipment integrity may be compromised by letting energy levels flow for 30 cycles within those electrical systems. 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 hazards.

Specifying 30-cycle-rated transfer switches for every application can increase equipment and spare parts costs—sometimes as much as 15% to 30%. These switches may also require rear entry, a larger footprint, and more expensive maintenance.

Specifying 30-cycle-rated transfer switches may also necessitate installing larger feeder cables than normally required in order to safely carry the available short-circuit current for 30 cycles. This can cause a significant increase in the cost of the feeder cabling. 

Of the engineers who participated in a survey following a webcast on selective coordination, 88% agreed selective coordination may not be optimal if 30-cycle transfer switches are used for an entire facility.

Figure 4: The value of short-circuit current at any point in a circuit is a function of the conductor sizes, the distance from the electrical source to the short circuit, and the current available from the source. When the service is a dedicated transformConsidering the custom nature of selective coordination, the better decision is specifying equipment, components, and overcurrent protective devices that precisely satisfy the requirements of that unique design.

Achieving selective coordination can often be accomplished with transfer switches between 3 and 18 cycles. Matching time-based ratings to timing requirements of a given selective coordination design often better serves the design and the facility owner. If the design settings are at 3, 6, 9, 12, or 18 cycles, there is no reason to specify 30-cycle rated switches universally.

Developing a transfer switch schedule that includes fault-current levels and time requirements helps optimize equipment performance and cost.

Selective coordination issues

Properly coordinating ac electrical power distribution systems with overcurrent protective devices can be complex and difficult. When selectively coordinating electrical systems, many issues confront engineers. These issues include:

  • Selective coordination may not be fully achievable for every system.
  • Selective coordination requires a high level of analysis and engineering judgment.
  • Typically, best-fit solutions are sought.
  • Cost plays a major role due to potentially increased design time, space requirements, and equipment.
  • Rote insistence on full selective coordination may impede the ability to deploy desirable alternate power protection.
  • Selective coordination is often impossible to achieve on conventional designs without major reconfiguration; it cannot succeed with device selections alone.

Source: Consulting-Specifying Engineer webcast, Feb. 2008

General selective coordination guidelines

Although there is no effective cookie-cutter approach to engineering selectively coordinated systems, the following list provides general guidelines that can help ensure balance among potentially competing interests:

  • Obtain information from the utility (main facility power source) that identifies the maximum available fault current and transformer size.
  • Conduct overcurrent-coordination and short-circuit studies to lay the foundation for an effective design—this work should also include equipment full-load rating verification.
  • Reduce the number of levels of protective devices—the fewer the levels, the easier the task of selectively coordinating overcurrent protective devices.
  • Reduce the available fault current by increasing system impedance or by using step-down or isolation transformers (see Figure 4).
  • Select long time settings to protect equipment from overload; select instantaneous timing and short time settings to selectively coordinate; or use tables for overload protection that coordinate up to the available fault-current level.
  • Select current-limiting type molded-case circuit breakers where possible for branch devices; they respond very quickly, significantly limiting let-through current so they coordinate better and even reduce the required upstream device trip-timer settings.
  • Consider changing a molded-case circuit breaker to either an insulated-case breaker or a low-voltage power breaker, either of which can increase the level of selective coordination with a downstream device.  


Mandated selective coordination has changed the way design engineers think about designing electrical distribution systems. Engineers responsible for developing and vetting selective coordination systems that meet NEC requirements often face difficult challenges. Balancing the need for business continuity, equipment protection, personnel safety, and managing costs can be fraught with pitfalls. Knowing NEC requirements, observing design optimization methods, coordinating transfer switch ratings with circuit breakers and fuses, and satisfying special application needs are essential for effective selective coordination.

Dan Caron is principal and the head of the electrical department at Bard, Rao + Athanas Consulting Engineers in Boston. He is also a principal member of NEC Code Making Panel 13. Ron Schroeder is director of applications engineering and product management for ASCO Power Technologies in Florham Park, N.J.

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