Backup power promotes operational continuity by mitigating the risks of power issues such as utility outages or faults. One approach to protecting commercial buildings from power interruptions is practicing selective coordination to localize overcurrent conditions. Specifying transfer switches properly rated to withstand fault and short circuit currents helps make selective coordination possible. This article lists considerations when selecting transfer switches for selectively coordinated systems.

Background Information
Selective Coordination is the practice of setting overcurrent protection device trip times to minimize the amount of equipment deenergized when faults are cleared from electrical circuits. Where the same trip times are applied to multiple circuit levels, the opening of a single breaker to clear a fault current could depower an unnecessarily large amount of load equipment, reducing the impact to facility operations. Additional information is available in the ASCO Power Technologies Technical Brief entitled Selective Coordination Basics.

Figure 1 compares systems without and with a selective coordination strategy. The diagram at left shows an uncoordinated system where breakers at each level use the trip time. The diagram at right shows a selectively coordinated system that tiers trip times, causing only the breaker closest to a fault to open. This clears the fault while depowering the least amount of load equipment.

Selective coordination strategies help commercial facilities avoid outage-related losses in several ways. Because selective coordination ensures that electrical problems depower the least amount of equipment, it minimizes the scope of any power interruptions to avoid wider electrical impacts that might lead to further loss of productivity and impact safety systems.

Localizing a power interruption leaves remaining systems available for use, such as the information and data systems that drive business activity and even lighting needed for occupants to remain safe and productive. For commercial warehouses, this can include electrically powered equipment such as forklifts and conveyors that move goods, refrigeration that preserves perishable inventory, and all of the systems and functions that enable facilities to ship and receive goods.

The 2020 National Electrical Code® (NEC®) requires hospitals and other public facilities to practice selective coordination in power distribution circuits serving life-safety, legally required, and Critical Operations Powers System (COPS) loads. Nevertheless, the practice can limit outages and increase resilience in any facility.

For more info on NEC codes for backup power systems, review the ASCO Power Technologies document entitled, Standards for Backup Power.

A primary function of transfer switches is to withstand fault currents until they are cleared by over-current protection devices. Consequently, each ATS must be appropriately rated to withstand fault and short circuit currents until overcurrent protection devices, usually circuit breakers, clear over-currents that occur. Primary factors that affect selection of appropriate ATS are discussed as follows.

Figure 2 below shows two common approaches to selective coordination. At left is a “slow trip” arrangement with trip times ranging from 0.5 seconds at the primary breaker to 0.05 seconds on the tertiary breaker. At right is a “quick trip” scheme with trip times ranging from 0.2 to 0.05 seconds. Both will comply with code, but what are the ramifications of choosing one scheme over the other?

Fault Currents
The amount of energy transmitted during a fault or short circuit event increases with time. As a result, longer time ratings require larger transfer switches (and potentially larger distribution devices and conductors) that can handle the larger currents until they are cleared by a breaker or fuse. For a specific ATS, the amount of fault current it can carry decreases with longer times. Whereas an ATS may be able to withstand a certain fault current for 0.2 seconds, its rating at 0.5 secs, if available, will be reduced. As a result, a larger or different, and likely more costly, ATS may be required to achieve the short circuit ratings needed for that location. Figure 3 illustrates the relationship by showing the larger and smaller ATS needed for each approach.

Importantly, ATS are rated for nominal capacity as well as the amount of fault current they can carry. The amount of fault current required rises with the ampacity of the load. Consequently, the position of the ATS in a power distribution system has a significant effect on the ratings required to provide adequate short circuit performance.

ASCO Power Technologies typically suggests locating transfer switches close to the loads they serve. One reason is that the location of an ATS in a power distribution system affects the ratings required to serve downstream loads. Transfer switches located close to loads can use switches with lower ampacity and short current ratings.

Figure 4 shows two configurations. The configuration at left offers the advantage of simple installation and operation of a single switch. However, an excess capacity rating is needed to achieve an adequate short circuit rating. At right, transfer switches are located closer to the loads they serve. To handle the amount of load and the amount of fault current on each tertiary circuit, 600 Amp switches may provide adequate nominal capacity and short circuit rating.

Longer trip times increase the amount of energy available within equipment. Any work inside equipment must comply with requirements of NFPA 70E - Standard for Electrical Safety in the Workplace. Incident energy values in a slow trip approach could increase required levels of protection, or in worst cases, preclude servicing without deenergizing the switch. Consequently, using a selective coordination scheme with shorter trip times can reduce electrical safety risks to workers and promote serviceability.

See the ASCO Power Technologies white paper entitled Arc Flash Studies and Transfer Switch Serviceability for more information.

To implement selective coordination, designers must determine the instantaneous clearing times of breakers from their published trip curves to confirm that these times are less than or equal to the time-based ratings of transfer switches. If the instantaneous trip time of a breaker exceeds 0.05 seconds or if the breaker has a “short time response”, a short-time rating of the transfer switch must be applied, which is typically a lower current level. If the transfer switch has no short-time rating, the Short Time response cannot be enabled on the Circuit Breaker ahead of the switch, which can hinder the ability to selectively coordinate the system.

The challenge for specifiers is to produce a specification that will require fewest changes. As a rule of thumb, an approach can be selected based on the type of breaker specified ahead of the ATS. Molded case breakers generally offer relatively short time ratings and limited adjustability. Conversely, power breakers typically offer longer trip times and larger ranges of adjustment.

Because of these generalities, providing 0.05-second rated ATS for locations served by molded case breakers and 0.3 second switches for locations served by power breakers can often provide workable solutions. This approach makes changes to specification submittals less likely and can reduce change orders and conflicts between vendors, specifiers, and contractors.

Closing
Because each power distribution system is unique, the adequacy of selectively coordinated systems and their ATS must be verified by a qualified expert following the performance of a selective coordination study. The guidance herein is provided for conceptual purposes only. A qualified engineer, an overcurrent protection device provider, or a transfer switch manufacturer can provide additional guidance for specific applications.

Summary
Choosing a slow trip arrangement can offer simpler transfer switching. This scheme can also require a more costly ATS with higher short circuit ratings. Conversely, quick trip arrangements locate smaller ATS units closer to loads. Both arrangements affect factors such as fault current, ATS location, and arc flash safety characteristics.
Specifiers can select transfer switches based on the identified breakers before the ATS to minimize changes to submittals. Workable solutions are achievable by either providing 0.05-second rated ATS for locations served by molded case breakers and 0.3-second switches for locations served by power breakers. This also reduces change orders and conflict between vendors, specifiers, and contractors.