Identify the ‘right’ limit on selective coordination
In 2005, the National Electric Code was changed to require selective coordination of circuit protection for emergency circuits and legally-required standby systems. Its goal was to ensure that if one circuit overloaded or faulted, it would not cause a blackout or other circuits to lose power in places like hospitals and elevators, where losing power could be hazardous.
Common sense says that a circuit breaker with a lower current rating than the upstream circuit protection device by a ratio of 2:1 will always open first. Actually, if plant managers and electricians fully understood how selective coordination works, they would see why this isn’t always the case, and why the standards were written. What’s more, they would see the value of upgrading to current-limiting devices to meet the selective coordination standards and reduce electrical hazards.
According to the code
The NEC defines selective coordination as “localization of an overcurrent condition to restrict outages to the circuit or equipment affected, accomplished by the choice of overcurrent protective devices and their ratings or settings.” At first the NEC selective coordination requirements covered only elevator feeders (Article 620.62) and healthcare facilities (517.26), but in 2005 the NEC authors added emergency systems (700.27) and legally-required standby systems (701.18). Emergency systems supply power to systems such as emergency lighting and fire pumps. Legally-required standby systems supply power to equipment that aids in firefighting, rescue and control of health hazards. Many state and local building codes require compliance with the NEC for selective coordination, at least for certain types of facilities.
The point of selective coordination is to isolate the faulted portion of the circuit while maintaining power to the rest of the electrical system. It’s not hard to see how this is handy %%MDASSML%% nobody wants a plant’s emergency exit signs to go dark when one load is faulted. Selectivity is accomplished using coordinated circuit protection.
Coordinating circuit protection
The data sheet for an overcurrent protective device will show a time-current curve that describes the time the device will take to clear a fault at a given value of fault current. Electrical system designers use these specifications to put the fastest-clearing devices closest to the load (Fig. 1).
Circuit breakers can be used in selectively coordinated electrical systems, but specifiers must overlay the time-current curves of all upstream (line side) and downstream (load side) breakers to ensure that the downstream circuit breaker will open under a short-circuit condition before the upstream circuit breaker operates. To ensure that a circuit-breaker-protected system is selectively coordinated, the time-current curves must not overlap at any possible fault current.
With fuses, selective coordination is achieved as long as specifiers maintain manufacturer-recommended ratios. For example, an upstream fuse rated 200 A and a downstream fuse rated 100 A provide a 2:1 ratio. Plant managers can ask their supplier for a fuse coordination table that lists the recommended ratios for fuses by model number, type and rating. When using a UL Class J fuse (30-600 A) line side, and a UL Class RK-1 fuse load side, the recommended ratio is 2:1. For a Class J fuse line side and a UL Class RK-5 fuse load side, an 8:1 ratio is recommended because an RK-5 fuse has a longer time delay at high fault currents.
During moderate overload conditions, circuit breakers and fuses work just fine, operating selectively. However, during very high overcurrent conditions (such as a fault), circuit breakers do not always operate selectively. Because most circuit breakers have a range of trip times that overlap each other at high potential fault currents, system designers cannot be assured that the lower current rated circuit breaker will open before the higher rated circuit breaker begins to open (Fig. 2).
If fuses are used on the line side as well as on the load side, then a similar situation can occur if the ratings ratio is not sufficient. This points to the importance of understanding the electrical system and knowing what kind of circuit protection is in place at every piece of equipment.
Plant managers may be intimidated by the idea of a selective coordination study of their entire plant. Fortunately, there is an easier way to achieve guaranteed selective coordination: current limiting devices.
UL-Listed current-limiting fuses or current-limiting circuit breakers must open within the firstger than a typical current-limiting fuse or current-limiting circuit breaker.
A properly sized current-limiting fuse near the load will open before a circuit breaker located upstream. However, there are differences in the degree of current-limitation. For example, UL Class RK-1 fuses are more current-limiting than UL Class RK-5 fuses. And UL Class J, T and CC fuses are more current-limiting than Class RK-5 or Class RK-1 fuses.
Fuses are not without their drawbacks. In higher current ratings, fuses can be physically larger than molded case circuit breakers of equivalent rating. Generally, current-limiting fuses cannot be used line-side if there is a non-current-limiting circuit breaker load-side because the fuse would operate faster than the circuit breaker at high fault currents.
Using a fuse load-side and a circuit breaker line-side may not provide selectivity if there is a rare phenomenon called dynamic impedance. Under high faults, if the line side circuit breaker’s contacts begin to open momentarily before the arc is extinguished by the fuse, the decrease in current due to the air gap will slow down the response of the fuse so that the breaker may also open. Specifiers must compare the minimum possible response time of the circuit breaker and maximum possible clearing time of the fuse to determine if dynamic impedance is a factor.
Current-limiting overcurrent protective devices have other advantages for the plant, starting with safety. Current-limiting devices dramatically reduce the destructive energy of a short circuit or arc flash. The instantaneous peak current during the first half cycle after a fault can reach as high as 2.3 times the available RMS bolted-fault-current available at the equipment. For example, if current-limiting overcurrent protective devices are not used and the available fault current at an industrial control panel is 100,000 A, the maximum possible instantaneous peak current reached could be as high as 233,000 A. If a 200-A rated current-limiting fuse were used, the maximum instantaneous peak let-through current during the first cycle would be only 20,000 A. If a 30-A rated current-limiting fuse were used, the instantaneous peak let-through current would be only 6,000 A.
In addition to limiting the instantaneous peak let-through current, current-limiting devices reduce arc flash incident energy. It is estimated that there are 10 arc flash incidents everyday in the United States, and the resulting burns are sometimes fatal. Current-limiting fuses and breakers shorten the duration of an arc flash event. A
Also, current-limiting fuses can increase the Short Circuit Current Rating of industrial control panels, which according to the 2005 edition of NEC, Article 409, must be clearly marked on the panels. Most states and authorities having jurisdiction have adopted the 2005 edition of the NEC as their standard for safe electrical installations.
The motivation for selective coordination %%MDASSML%% from the NEC perspective %%MDASSML%% is safety, by preserving power to critical systems. However, selective coordination in a plant can also reduce unplanned work stoppages and help speed troubleshooting. Although not required by NEC for all systems, selective coordination is best practice, and it can be easily accomplished by replacing traditional fuses and circuit breakers with current limiting fuses.
|Kenneth Cybart is a senior technical sales engineer at Littelfuse, Des Plaines, IL.|