Choosing the Best: Series Rated vs Fully Rated Protection

In power systems and electrical engineering, the choice of protective schemes is significant. Two key strategies, Series Rated and Fully Rated systems, play a key role in safeguarding power distribution networks. Understanding these methodologies is essential for engineers and professionals entrusted with designing and maintaining robust electrical systems. This blog offers a comprehensive exploration of Series Rated and Fully Rated protection schemes, diving into their applications, benefits, and considerations. Join us in uncovering the details of these schemes, shedding light on their vital contributions to the reliability and safety of modern electrical infrastructure.

Fully Rated System:

In a fully rated system, the short-circuit ratings of all protective devices, including components like circuit breakers and fuses, are either equal to or exceed the available short-circuit current of the system. Furthermore, in instances where breakers are housed within equipment, the bus's short-circuit withstand rating and equipment short-circuit rating must also meet or surpass the available current. This alignment of ratings guarantees that all elements within the system can adeptly handle potential short-circuit currents they may encounter.

Figure 1. Fully Rated System (circuit breaker)

Figure 2 Fully Rated system (fuses)

Series Rated System:

NEMA® defines a series rating as follows:

“Series Rating–A short-circuit interrupting rating assigned to a combination of two or more overcurrent protective devices which are connected in a series and in which the rating of the downstream device(s) in the combination is less than the series rating.”

Figure 3. Series Rated System (circuit breaker & fuse)

Figure 4. series rated system (circuit breakers)

A Series Rated System is a specific configuration in a power distribution network where multiple overcurrent protective devices are connected in series. In this arrangement, the upstream device possesses a short-circuit interrupting rating equal to or greater than the available fault current. However, the downstream devices have lower interruption ratings compared to the available fault current at their respective locations. Series Rated Protection refers to the precise short-circuit interrupting rating allocated to a combination of two or more overcurrent protective devices connected in series.

Underwriters Laboratories (UL) permits the assignment of a short-circuit rating to a combination of protective devices, which may include molded-case circuit breakers and/or fuses, that are connected in series. This assigned rating can surpass the rating of the lowest protective device in the combination.

Article 240.86 in the National Electrical Code (NEC) and NFPA 70E pertains to the series ratings of circuit breakers. It provides guidelines on how circuit breakers can be connected in series to coordinate their operation and ensure effective protection against overcurrent . This article outlines the conditions and requirements for safely implementing series ratings in electrical systems, aiming to enhance the reliability and safety of power distribution networks. Engineers and electricians refer to this article to ensure compliance with industry standards and best practices when designing and installing electrical systems.

Two Tier Series Rating & Three Tier Series Rating:

Following is a comparative description of Two tier & three tier series rating:

Table 1 Table showing a difference between Two/Three Tier series rated system.

Figure 5 (Top) Two Tier system & (bottom) Three Tier system.

Comparing Fully Rated & Series Rated Systems:

A Fully Rated system is known for its high reliability and flexibility, accommodating a wide range of devices, but it tends to be more expensive due to the need for higher-rated circuit protection. It also requires complex coordination for selective operation. In contrast, a Series Rated system, while very cost-effective, offers simplified coordination but lacks selective coordination capability. It is particularly suitable for applications where space is a constraint. However, it's important to note that it may have limited applications, especially in critical systems like healthcare or elevators due to specific requirements. Additionally, it's crucial to ensure that manufacturers' recommendations and UL listed protective devices are adhered to.

Table 2: Table showing comparison between series & fully rated system

How to apply Series Rating:

Let's delve into practical scenarios to better understand the application of series rating in power systems.

Example 1:

Three-Tier Series Rating

In this example, we'll explore a three-tier series rated configuration involving three subsequent circuit breakers. These breakers have been carefully selected based on their interrupting capacities to ensure effective fault protection. The main circuit breaker, with a higher AIC (Amperes Interrupting Capacity), acts as the upstream protective device. It is followed by a feeder breaker with a medium AIC and a branch breaker with a lower AIC. This arrangement allows for a hierarchical response to faults, ensuring that the device closest to the fault initiates the trip sequence. The combination of these breakers must be UL listed/Manufacturer recommended to guarantee seamless operation.

Figure 6: Picture showing series rated system.

Figure 7 Manufacturer Recommended List for Three tier series rating

Example 2:

Enhancing WCR Rating of ATS

Another practical application of series rating lies in enhancing the Withstand Current Rating (WCR) of Automatic Transfer Switches (ATS) . Manufacturers often recommend specific breakers to be used in conjunction with their ATS units to achieve optimal performance and safety. By adhering to the manufacturer's recommendations, you can increase the WCR of the ATS, providing a higher level of fault protection. Deviating from these recommendations by using non-manufacturer approved breakers may result in a de-rated WCR and compromise the overall reliability of the system. It's imperative to follow the manufacturer's guidelines to ensure the ATS operates within its specified parameters.

Figure 8 ATS WCR Rating (left) non-manufacturer recommended CB. (right) Manufacturer recommended CB.

Figure 9 ATS WCR for non-manufacturer recommended breakers (time base rating)

Figure 10 ATS WCR for manufacturer Specified breakers.

Factors Considered in the Selection Process of Series Rating

When choosing a series rating for your system, consider factors like equipment compatibility, understanding short circuit currents, coordinating protective devices, selecting the right type, matching device characteristics and following manufacturer standards. Don’t forget that the most important part is to ensure that the breakers are UL listed, ensuring they work well together in series. The reliability and safety of protective devices within the electrical infrastructure are greatly enhanced by these considerations, which contribute to better overall system performance and safety.

Importance of series Rating

Series ratings are significant in enhancing system reliability by allowing deployment of various combinations (UL) of protective devices. In the event of fault, the current-limiting device operates quickly to limit the magnitude of the fault current. In power distribution systems, where high fault currents are common, series rating provides a cost-effective and space-saving solution. Many regulatory standards and codes require proper overcurrent protection in electrical systems. Series rating provides a method of achieving compliance with these standards while also optimizing costs and space utilization.

Another huge advantage of series rating lies in its ability to enhance the Withstand or closed-on ratings (WCR) of the ATS. It ensures that protective devices such as fuses and circuit breakers work together seamlessly during power source transitions, minimizing disruptions and ensuring safe operation of ATS. Series rating can be particularly valuable when upgrading or retrofitting existing electrical systems. It allows for the integration of new equipment with the existing protection scheme, potentially avoiding the need for extensive modifications.

Selecting the right series rating for a power system requires solid understanding of electrical gear, engineering design, coordination studies, simulation software, and adjusting trip settings to ensure protective devices work together effectively. AllumiaX specializes in series rating services, offers tailored solutions for optimal coordination, design and safety in your electrical network.

Our expert engineers use advanced techniques to ensure your protective devices are perfectly aligned. With a commitment to industry standards, AllumiaX enhances the efficiency and safety of your electrical network. Partner with us to elevate your system's resilience and minimize downtime and risks.

AllumiaX is a licensed Engineering company headquartered in Seattle, Washington USA. We work with sub-contractors to deliver various Power System Engineering studies including DC arc flash, DC short circuit, and more. Get in touch with us or Request a Quote.