High-reliability environments need selective coordination to avoid downtime caused by main breaker nuisance tripping
You may have experienced a short circuit on a 15A to 100A branch circuit that also tripped the main breaker in that same panelboard, resulting in widespread downtime. This short circuit may have affected an IT cabinet, trader's desk, glass-making process, operating room, or other critical loads. This common occurrence causes millions of dollars in business losses every year.
Data processing centers are affected as well. They typically have numerous lines of businesses fed from each computer room panelboard that would experience downtime if such an event occurred.
With this problem in mind, a top 10 U.S. bank formed a design team with EYP Mission Critical Facilities, Inc. and a major circuit breaker manufacturer and set out to overcome this downtime vulnerability. The bank was early into the design of its new corporate data centers.
The conflict. At locations where distribution is typically carried out through multiple 42-circuit panelboards (referred to in computer room vernacular as “RPPs,” an acronym for remote power panels), a short on a branch circuit that also trips the upstream panel main breaker would result in the consequential loss of 84 or more processes or end-users. As a facilities or data center manager, trying to explain to management that little can be done to overcome this situation with present circuit breaker technology is a very difficult task.
Today's panelboard circuit breakers are molded case UL 489-Listed products that respond to peak currents during a short circuit event. Fig. 1 (click here) shows how these circuit breakers respond to high-current conditions. There is a distinct “area of conflict” where the trip curves overlap. This is the Achilles' heel of today's thermal magnetic circuit breaker technology, and it's the reason why these products won't selectively trip. Consequently, uptime is threatened in computer rooms, broadcast studios, telecommunications switching facilities, and other critical environments with high availability expectations.
The area of conflict shown in Fig. 1 is the “magnetic trip” region, which is where circuit breakers respond to high peak currents, such as those of a short-circuit condition, and trip within three cycles. For a 2,500A rms short circuit on a 20A branch circuit, there's a distinct overlap of the 20A and 225A trip curves. Therefore, it's likely that the 225A RPP main breaker will “nuisance” trip.
It should be noted that the 2005 NEC now mandates selective coordination for Art. 700, “Emergency Power Supply Systems” and Art. 701, “Optional Standby Power Supply Systems.”
Understanding the problem. A major circuit breaker manufacturer performed an array of short-circuit tests on these circuit breakers to establish empirical data on peak let-through current values that would hold and trip these main breakers.
Present day molded case circuit breakers and molded case switches used in power distribution units (PDUs) and RPPs trip at five to 10 times the breaker or switch-handle rating. Peak let-through currents of downstream branch breakers are high enough to engage the magnetic trip levels of the main breaker. But how much current would it take to trip these main breakers?
Fig. 2 shows the simplified test circuit that was used for these tests. It's a typical 120/208V series circuit where a short is applied on a branch circuit. Upstream of the branch circuit is a 250A panelboard main and a 400A distribution breaker. Fig. 3 shows the results. As you can see, a 3,000A rms fault on a 20A or larger branch-circuit breaker results in a trip of the upstream 250A circuit breaker. When the manufacturer ran these tests for faults up to 18,000A rms on branch-circuit breakers ranging from 20A to 100A, the conventional 250A circuit breaker located upstream would nuisance trip in most cases. They concluded that a high percentage of panelboard mains would trip on branch short-circuit faults.
Fig. 4 better illustrates the coordination conflict between a conventional 225A circuit breaker and branch-circuit breakers ranging from 15A to 100A. The vertical axis represents the available fault current, and the horizontal axis represents the branch breaker rating. Short-circuit events shown in the red zone will nuisance trip the upstream 225A main breaker. Here again, you can see that conventional main breakers fail to provide much selectivity or discrimination for faults on the branch circuits.
The solution. The bank's design team first considered replacing the RPP's main circuit breaker with non-automatic switches. However, switches will trip automatically for a high-current condition because of fixed magnetic trip elements intended to protect these products from high-fault currents. Actually, switches offer little improvement over a standard circuit breaker because both trip at approximately five times to 10 times its handle rating.
The design team then evaluated fused branch circuits, which were also ruled out because fused panelboards are about twice the size of circuit breaker panelboards. The team also learned that, in most cases, today's branch breakers that are of a blow-open design clear a fault within 8 milliseconds, which makes them as fast as fuses.
The design team found that technology isn't capable of solving this common occurrence. The team then went to a major circuit breaker manufacturer to request a breaker that would eliminate this selectivity and coordination problem.
As a result, the team developed a circuit breaker with extremely high magnetic trip settings. It also has new thermal components to provide a certified withstand rating up to 20 times its handle rating, providing thermal integrity at those higher magnetic settings. Fig. 5 shows a standard breaker 10X curve and the new high magnetic breaker 20X curve.
The high magnetic trip setting on these breakers will allow, for example, a 225A main panel circuit breaker to hold or stay closed while a fault downstream of a branch is cleared by the branch breaker. The upstream 225A breaker won't trip. In Fig. 3 the high magnetic 250A breaker (labeled “250A HM breaker”) is represented with a straight red line, and all of the data points below that line are main breaker hold values. Fig. 6 illustrates the difference more clearly. The green boxes show a main breaker hold, and the red boxes show a main breaker trip for the new 250A high magnetic breaker. Short circuit events shown in the green zone won't nuisance trip the upstream 250A main breaker.
Welte, P.E., is a principal at EYP Mission Critical Facilities, Inc. in New York City, and Winter is a senior staff engineer with the Square D Co., in Palatine, Ill.