Breaker Discrimination in the Computer Room

A dynamic technological breakthrough was needed to solve the inevitable problem of circuit breaker discrimination in panel boards. This is especially a concern in mission-critical operations. It was certainly the case for Bank One, when it teamed with New York City-based EYP Mission Critical Facilities, Inc., to overcome this downtime vulnerability in Bank One's new corporate data centers, where there are numerous business lines fed from each computer room panel board.

At locations where distribution is typically through 42-circuit panel boards—what is referred to in computer room vernacular as a remote power panel (RPP)—a short on a branch circuit that also trips the upstream panel main breaker would result in the loss of 42 or more processes or end users. Today's panel board circuit breakers are molded case UL 489 products that respond to peak currents during a short-circuit event.

The Bank One design team considered replacing the RPP's main circuit breakers with "non-automatic switches." However, switches would trip automatically for a high current condition because of fixed magnetic trip elements intended to self protect these products from a thermal runaway.

Actually, switches offer little improvement over a standard circuit breaker; both will trip at approximately 10X. Fused branch circuits were evaluated but also ruled out, based on the fact that the size of fused panel boards is about 2:1 in comparison to circuit breaker panel boards.

Also, "blow open" branch breakers, which clear a fault within eight milliseconds, were researched and found to be as fast as fuses.

Finally, the design team learned about a recently developed mission-critical breaker that is a hybrid of the company's type-LA molded case circuit breaker, a technology that has been a tried and proven workhorse of many critical sites for years. When used as a panel main, it delivers selective tripping for short-circuit conditions. This applies to panel boards incorporating 20- to 100-amp single- and three-phase branch circuits and the breaker as the main.

The high magnetic trip setting on these breakers will allow, for example, a 250-amp main panel circuit breaker to hold or "stay closed" while a fault downstream of a 30-amp branch is cleared by the branch breaker. The upstream 250-amp breaker will not trip.