Which is better protection: Fuses...or breakers?

Either can be effective; here are some points of comparison

WHAT DO WE MEAN BY protection? We mean prevention of damage. We do not mean fault prevention. No protective device can function until a fault has occurred, triggering device operation. And a fault, by definition, represents some sort of damage.

In a motor circuit, one type of protection is furnished by the starter overloads. They don't prevent overloading, blocked ventilation, or other causes of winding overheating or overcurrent. What they do is sense excessive current and act to take the motor off the line before damage results.

The second type of protection, with which we're concerned here, comes into play when overheating, a voltage surge, or some other condition causes a fault within the motor winding. Without this protection, the resulting "fault current" would quickly escalate the damage. That could occur either within the winding itself (possibly complete burnout with core damage), in the motor starter, or in the circuit wiring. Attention in recent years has centered particularly on preventing that starter damage, by providing what the International Electrotechnical Commission has called Type 2 circuit protection.

Type 2 circuit protection involves what the National Electrical Code (in Section 430.1) defines as "Motor branch-circuit short-circuit and ground fault protection." Immediately upstream from that protective function, the Code also mandates a "Motor disconnecting means." These two functions are normally combined in one of two packages: either a fused disconnect switch, or a circuit breaker (see diagram, next page).

Again-this is not "motor protection." Some types of fuses will indeed respond to fault current within a winding. But no such current can exist until a damaging fault has already occurred; all the fuse can do is prevent additional damage beyond that. Hence, either the fuses or the breaker are customarily considered "circuit protection."

Which method is better? Properly designed, either is effective, making the choice usually a judgment call. In some industrial installations (especially utility generating plants), breakers are favored because operators cannot readily determine fuse integrity. When fused equipment is supplied, some

plant personnel have been known to replace fuses with solid copper rods

(and add upstream circuit breakers) so no question of fuse condition will arise.

For decision-making on more objective grounds, here are some points of comparison:

1. Favoring fuses:

a. Lower cost.

b. No moving parts; contact welding not a problem.

c. No need for periodic "exercising" to verify capability.

d. Current-limiting capability to 300,000 amperes.

On the other hand: repeated motor starts can fatigue fuses.

2. Favoring breakers:

a. Common trip bar eliminates single-phasing possibility; all phases trip together.

b. More compact.

c. Many can be fitted with electric shunt-trip attachments for GFI protective schemes.

d. Can be reset following a fault; no need to replace.

e. Not subject to accidental operation through fatigue.

f. May operate faster than fuses at low levels of fault current.

On the other hand, re-setting a tripped breaker can be dangerous because it may be reclosing on a fault.

Other concerns affecting either alternative: Individual fuses may be replaced with an incorrect item; the same is true of the smaller circuit breakers. Also, fuses are subject to high resistance connections, resulting in overheating and unbalanced downstream voltage, at their mounting clips. Circuit breakers, using screw lug connections instead, have been subject to the same problem (particularly where aluminum/copper interfaces exist). A sound maintenance program will include periodic infrared surveys of all such connections. Knowing when the device has operated once required testing of fuses, whereas the breaker's status could be seen from its operating handle position. However, "blown fuse indicators" are now widely available.

At one time, the fast-acting (generally in less than a quarter cycle) current-limiting capability of some fuses offered a clear advantage. Current-- limiting breakers have now been developed, however, with ratings up to 200,000 amperes. In most circuits, then, either fuses or breakers can limit let-through current and energy equally well.

Don't let such numbers fool you, however. A 300,000 ampere interrupting rating for a fuse simply means that the device will quickly interrupt the short-circuit current that can be drawn from a system capable of supplying 300,000 amperes. Actual current flow through the fuse never approaches that value. If it did, the fuse would be vaporized. The same applies to a circuit breaker.

With either type of protection, proper operation depends upon selective coordination between branch-circuit protection and what exists upstream. In addition, device function needs to take into account the protection requirements of the circuit conductors-- not just the motor rating.

By Richard L. Nailen, P.E., EA Engineering Editor

Copyright Barks Publications Mar 2003
Provided by ProQuest Information and Learning Company. All rights Reserved