Article 240 provides the requirements for selecting and installing overcurrent protection devices (OCPDs). Depending on your application, other Articles may apply (see Other Articles in table 240.3 in below).
Our objective here is to cover the basics in order to provide the information needed for best understanding of the Overcurrent protection. So, we need to start with the basic definitions for used terms along article 240 for overcurrent protection as follows.
Actually, I explained a lot of terms definitions in previous articles in our course " Understanding the NFPA 70 (NEC code) ", the definitions that are related to our subject; overcurrent protection are as follows:
Definitions:
You can find the definitions and
explanation for above terms in the following articles:
I recommend a good review for the above
articles before going on with below paragraphs.
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2- Additional Important Terms:
The following terms are also very important for better understanding of the Overcurrent protection issues.
Ampacity:
It is the maximum current, in amperes, that a
conductor can carry continuously under the conditions of use without
exceeding its temperature rating.
The Ampacity of a conductor varies with the
conditions of use as well as with the temperature rating of the conductor
insulation.
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Overcurrent:
It is any current in excess of the rated current
of equipment or the ampacity of a conductor. It may result from overload,
short circuit, or ground fault. They can occur as a result of normal conditions such as motor starting, or abnormal
conditions such as a fault.
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Overload:
It is the Operation of equipment in excess of normal,
full-load rating, or of a conductor in excess of rated ampacity that, when it
persists for a sufficient length of time, would cause damage or dangerous
overheating. A fault, such as a short circuit or ground fault, is not an
overload.
Examples for overloads:
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Short Circuits:
Short circuits usually occur when
abnormally high currents flow due to the failure of the insulation of the
conductors. When the insulation between phases or between phases and ground
breaks down, short circuit currents can be expected to flow into the fault. A
short circuit is limited only by the capabilities of the distribution system.
Large currents are associated with
short circuits which can cause heating, magnetic stress and arcing. The
amount of current that is available in a short circuit is determined by the
capacity of the system voltage sources and the impedances of the system,
including the fault.
There are several types of short
circuits:
1- Bolted Fault:
A bolted fault is a short circuit of
very high magnitude. The magnitude of a bolted fault is greater than that of
an arcing fault and is the value used for most fault calculations. The
principle effects of a high value short circuit are heating and magnetic
stresses that vary as the square of the current.
Excessive values of magnetic stress
can result in damage to insulation, conductors and components involved in the
fault. It can also be extreme enough to burn through raceways and equipment
enclosures.
Insulation damage in electrical
conductors is usually the result of overload conditions. When an overload
condition exists, the temperature builds up between the conductor and the
insulation, which can reduce the life of the conductor and result in a short
circuit as the insulation fails.
2- Arcing Fault:
An arcing fault results from a gap
between two electrodes (such as a loose wire on a terminal block).
3- Ground Fault:
Ground faults normally occur either
by accidental contact of an energized conductor with normally grounded metal,
or as a result of an insulation failure of an energized conductor. Normal
phase overcurrent protective devices provide no protection against low level
ground faults.
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Ground-Fault Protection of Equipment:
It is a system intended to provide
protection of equipment from damaging line-to-ground fault currents by
operating to cause a disconnecting means to open all ungrounded conductors of
the faulted circuit. This protection is provided at current levels less than
those required to protect conductors from damage through the operation of a
supply circuit overcurrent device.
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Short-Circuit Current Rating:(see below image)
it is the prospective symmetrical fault current
at a nominal voltage to which an apparatus or system is able to be connected
without sustaining damage exceeding defined acceptance criteria.
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Interrupting Rating:
It is The highest current at rated voltage that a
device is identified to interrupt under standard test conditions.
Equipment intended to interrupt current at
other than fault levels may have its interrupting rating implied in other
ratings, such as horsepower or locked rotor current.
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Current-Limiting Overcurrent Protective
Device: (see below image)
It is A device that, when interrupting currents
in its current-limiting range, reduces the current flowing in the faulted
circuit to a magnitude substantially less than that obtainable in the same
circuit if the device were replaced with a solid conductor having comparable
impedance.
There are two levels of overcurrent
protection within branch circuits:
1- Overcurrent Protective Device,
Branch-Circuit:
It is a device capable of providing protection
for service, feeder, and branch circuits and equipment over the full range of
overcurrents between its rated current and its interrupting rating.
Branch-circuit overcurrent protective
devices are provided with interrupting ratings appropriate for the intended
use but no less than 5000 amperes.
The protection provided may be overload,
short-circuit, or ground-fault or a combination, depending on the
application.
2- Overcurrent Protective Device,
Supplementary:
It is a device intended to provide limited
overcurrent protection for specific applications and utilization equipment
such as luminaires and appliances. This limited protection is in addition to
the protection provided in the required branch circuit by the branch-circuit
overcurrent protective device.
The definition of supplementary overcurrent
protection device makes two important distinctions between overcurrent protective devices:
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Coordination (Selectivity): (see below image)
It is 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.
For more information about Coordination
(Selectivity), Please review the following Articles:
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After reviewing above terms definitions, now we come to answer the following questions:
In the next article, I will answer the above questions regarding the overcurrent protection devices (OCPDs). Please, keep following.
- How to size the overcurrent protection devices (OCPDs)?
- How to select the proper overcurrent protection devices (OCPDs) for certain application?
- Where to locate the overcurrent protection devices (OCPDs)?
In the next article, I will answer the above questions regarding the overcurrent protection devices (OCPDs). Please, keep following.
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