Fuse vs. Circuit Breaker?

Protect Me, CB!
Question posed in the Dec. 9 issue
(Exam level: CBRE)

When is a fuse more desirable than a circuit breaker as an overcurrent protection device?

a. A fuse is never more desirable in a circuit
b. A fuse is always more desirable in a circuit
c. A fuse is more desirable when the current is over 100 amps for economic reasons
d. A fuse is always more desirable because it means you get a service call
e. A fuse is more desirable when instantaneous interruption is needed

Society of Broadcast Engineers certification is the emblem of professionalism in broadcast engineering. To help you get in the certification exam taking frame of mind, Radio World Engineering Extra poses a typical question in each column. Although similar in style and content to the exam questions, these are not from past exams nor will they be on future exams in this exact form.

A fuse for every muse. The lineup starts with the ubiquitous 3-AG format and a typical holder on the left, to a 480 volt motor protection one-time use (as opposed to a cartridge with replacement fuse strips) on the right. Two Society of Auto Engineering (SAE) standard automobile fuses are on the bottom. Color coding indicates ampere rating.
The correct answer, or more aptly the most correct answer, to the question at right is the final one.

A circuit breaker (CB) is an electromechanical device. Due to the intrinsic mechanical transit time needed for any action to take place, a fast-blow fuse can provide faster circuit interruption than a CB.

A circuit breaker senses over-current in two ways: First, over the long haul, by thermally heating a sensor; second, in the relative short term, by magnetic trip, where the excess current flow builds a field in a magnet until the magnet pulls in a trip actuator opening the circuit.

A fuse (“fusus” is Latin for melted) can be constructed of various conductive materials in many different enclosure formats. In any given fuse, the conductive material is selected in type and sizing such that when a predetermined, rated maximum level of current is exceeded for a forecast amount of time, the conductive material fails by melting thus opening the circuit. Ultra-fast fuses can “blow” in under 0.1 second.

Two pedestrian circuit breakers: a bolt-in type with lugs as often found in transmitters, and a panel ‘snap-in’ version.
To review the other answer choices:

Answer (a) is wrong on its face; as noted, the faster-acting fuse over the slower CB can be more desirable.

Answer (b) is wrong as both of these protection devices have many unique desirable qualities. For instance, a CB can be reused by simply removing the overload and resetting the CB. In contrast, more choices of incremental current values and response curves are available in fuses to protect delicate or expensive components closer to their operating values.

Answer (c) is wrong as both CBs and fuses have ratings to 1000 amp plus and the economies involved are not that far apart. The decision for which device to use is really a judgment based on device design and application.

Answer (d) is pure humor and if you don’t think it is, you’re in this profession for the wrong reasons.


Actually, humor aside, an important point is reflected in answer (d): the one-time-use feature of a fuse.

When a fuse is used in the devices found in broadcast plants, a blown fuse signals a failure circumstance. Replacing the fuse and restarting the device before an evaluation can be made may enlarge the damage or reactivate a dangerous condition.

Exposed view of inner mechanism of a circuit breaker. Courtesy Stefan Fassbinder/Leonardo Energy
The designers of many devices put the fuses inside for this very reason, and your designs and construction may want to emulate this arrangement so that your overly ambitious staff members don’t keep replacing the fuses until they have fried everything back to the input line cord.

Since we’re mentioning fuses in equipment you build, let me note some configuration conventions.

For chassis-mounted fuse holders, the supply is always on the back pin and not the ring connection point. This places the supply potential furthest from your possible contact. When you make your fuse and holder decisions, size them for the expected maximum current and voltage potential present.

In this age of shared stations, where someone as unfamiliar with your plant as a competitor’s engineer may be covering for you in an exigent circumstance, always mark the type and current rating for any critical fuse next to the holder. Also helpful is to note the normal current that would be flowing through that fuse to help in troubleshooting.

A closing comment related to me by an instructor at a Motorola two-way radio school: Be wary of people who try to sell you “100 percent pre-tested fuses.”

The deadline for signing up for the next cycle of SBE certification exams is March 26, 2010 for exams given at the NAB Show on April 13, 2010.

Missed some SBE Certification Corners or want to review them for your next exam? See the “Certification” tab under Columns at radioworld.com.

Charles “Buc” Fitch, P.E., CPBE, AMD, is a frequent contributor to
Radio World.

Watch That Drop
Question for next time
(Exam level: CPBE)

You are preparing an executive summary for management outlining the projected cost of various aspects of new facilities at your station. You need to calculate the wire size for the main power feed to the remote truck.

This truck will be used as the ultimate studio backup in the event of a long-term utility power outage. The HVAC and terminal equipment in the truck make this a notable load, so the wire is a large part of the cost. An NEC requirement is to limit voltage drops at the load to under 5 percent of the nominal supply voltage.

The worst-case load, with both HVAC units running on a 95 F day and all lighting and gear running, is 62 amps at 240 volts. The wire distance to the trucks reserve parking space is 250 feet.

Ignoring power factor (PF) and incidental issues, from the panel CB to the local disconnect supplying the truck, which size wire will limit the voltage drop to under 5 percent?

a. #18 copper TW
b. #16 copper TW
c. #12 copper THHN
d. #8 aluminum USE
e. #4 copper THHN

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