Under-protected circuits leave equipment vulnerable to damaging electrical
surges, and over-protected circuits add cost and can lead to nuisance tripping.
are the most common mistakes appliance engineers make in specifying circuit
protection and some tips that can prevent future mistakes, save costs, and
issue: March 2003 APPLIANCE Magazine
Specifying Circuit Protection
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While a circuit breaker may seem like a simplistic component, there is enough complexity and confusion when it comes to specifying circuit protection that many engineers are designing devices with too little or too much protection.
Specifying too high a rating to avoid nuisance tripping caused by
in-rush or transient currents. Most engineers are concerned about
nuisance tripping, as
they should be, but they often specify a circuit breaker rated much higher
than they should. Part of the reason is confusion between fuses and circuit
Engineers are used to oversizing fuses as a way to prevent nuisance
However, there is no need to oversize a circuit breaker.
Unlike a fuse rating,
a circuit breaker rating tells you the maximum current that the circuit breaker
will consistently maintain in ambient room temperature.
Thus, a 10-A circuit breaker will maintain a 10-A current without nuisance
tripping. In fact, a typical 4-A circuit breaker with a slow trip profile
a temporary 10-A current surge without nuisance tripping.
Often times, nuisance
tripping is caused by in-rush currents associated with certain electrical components
- primarily motors, transformers, solenoids,
and big capacitors. In such cases, the designer needs to specify a circuit
that has a delay. Thermal circuit breakers have a natural delay, and magnetic
circuit breakers can have added hydraulic delays. Match the delay to the
duration of the expected in-rush currents.
Over specifying or ambiguously
specifying the degree of protection. Terms such as drip-proof, ignition
protection, water-splash protection, and dustproof
commonly used, but may be misleading unless standard definitions are applied.
When specifying, use the established standards as a measure, such as EN
60529/IEC 529, which defines Degree of Protection of Electrical Equipment.
standards, decide which protection is correct for the application.
a combination switch-breaker installed in medical equipment might need
a water-splash protection rating, but it probably does not need
for continuous immersion in water. Truly watertight and dust-tight circuit
breakers are available, but they are expensive and usually unnecessary.
a fuse when a circuit breaker would be better. Although fuses
provide inexpensive circuit protection, the cost savings should be weighed
low total cost of ownership of circuit breakers. Foremost, circuit breakers
can be quickly reset, enabling the circuit to be restored with a minimum of
downtime. In addition, there is no assurance that a replacement fuse will be
of the proper
rating. If a fuse is replaced by a higher rated fuse, overheating and catastrophic
equipment failure may occur. Circuit breaker performance is relatively stable
over time, but as fuses age, their trip characteristics change. This may
lead to nuisance tripping and increased downtime.
Circuit breakers offer designers
more options than fuses do. An auxiliary contact may be added that can communicate
an alarm condition to an LED indicator or software.
In addition, a circuit breaker can be combined with a switch, saving space
and adding overload protection. Remote trip is another option available with
breakers but not with fuses. Furthermore, unlike fuses, circuit breakers
have a variety of types and trip profiles, and therefore, can be more precisely
matched to loads and environment.
Finally, fuses cannot be tested without destroying them. How can you be sure
the fuse you specify will open if there is an overload?
This information is provided
Circuit Breakers, Mt. Prospect, IL, U.S.