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| Protective Device |
In order for an overcurrent protective device to operate
properly, the overcurrent protective device ratings must be properly selected.
These ratings include voltage, ampere and interrupting rating. Of the three of
the ratings, perhaps the most important and most often overlooked is the
interrupting rating.
If the interrupting rating is not properly selected, a
serious hazard for equipment and personnel will exist. Current limiting can be
considered as another overcurrent protective device rating, although not all
overcurrent protective devices are required to have this characteristic.
Voltage Rating
The voltage rating of the overcurrent protective device must
be at least equal to or greater than the circuit voltage. The overcurrent
protective device rating can be higher than the system voltage but never lower.
For instance, a 600V fuse or circuit breaker can be used in
a 208V circuit. One aspect of the voltage rating of an overcurrent protective
device is a function of its capability to open a circuit under an overcurrent
condition. Specifically, the voltage rating determines the ability of the
overcurrent protective device to suppress and extinguish the internal arcing
that occurs during the opening of an overcurrent condition.
If an overcurrent protective device is used with a voltage
rating lower than the circuit voltage, arc suppression and the ability to
extinguish the arc will be impaired and, under some overcurrent conditions, the
overcurrent protective device may not clear the overcurrent safely. The voltage
rating is required to be marked on all overcurrent protective device labels.
Ampere Rating
Every overcurrent protective device has a specific ampere
rating. In selecting the ampere rating of the overcurrent protective device,
consideration must be given to the type of load and code requirements. The
ampere rating of a fuse or circuit breaker normally should not exceed the
current carrying capacity of the conductors.
For instance, if a conductor is rated to carry 20A, a 20A
fuse is the largest that should be used. As a general rule, the ampere rating
of a fuse or a circuit breaker is selected at 125% of the continuous load
current. Since the conductors are generally selected at 125% of the continuous
load current, the ampacity of the conductors is typically not exceeded.
However, there are some specific circumstances in which the
ampere rating is permitted to be greater than the current carrying capacity of
the conductors. A typical example is the motor circuit; dual-element fuses
generally are permitted to be sized up to 175% and an inverse time circuit
breaker up to 250% of the motor full-load amperes.
Interrupting Rating
NEC® Article 100 defines interrupting rating as: The highest
current at rated voltage that a device is intended to interrupt under standard
test conditions. An overcurrent protective device must be able to withstand the
destructive energy of short-circuit currents. If a fault current exceeds the
interrupting rating of the overcurrent protective device, the device may
actually rupture, causing additional damage.
NEC® 110.9, requires equipment intended to interrupt current
at fault levels to have an interrupting rating sufficient for the current that
must be interrupted. Protective devices such as fuses and circuit breakers are
designed to clear fault currents and, therefore, must have short circuit
interrupting ratings sufficient for all available fault levels. Equipment such
as contactors and switches have interrupting ratings for currents at other than
fault levels, such as normal current overloads and locked rotor currents.
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