Overcurrent Protection – Part One

1- Introduction to Overcurrent Protection:

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:


  • Fuse
  • Electronically Actuated Fuse
  • Controlled Vented Power Fuse
  • Non-vented Power Fuse
  • Vented Power Fuse
  • Expulsion Fuse Unit (Expulsion Fuse)
  • Power Fuse Unit
  • Multiple Fuse
  • Thermal Protector (as applied to motors)  
  • Thermally protected (as applied to motors)  
  • Switching Device
  • Cutout
  • Disconnecting (or Isolating) Switch (Disconnector, Isolator)
  • Interrupter Switch
  • Oil Cutout (Oil-Filled Cutout)
  • Oil Switch
  • Regulator Bypass Switch
  • Switch, General-Use
  • Switch, General-Use Snap
  • Switch, Isolating
  • Switch, Motor-Circuit
  • Switch, Transfer
  • Switch, Bypass Isolation
  • Circuit Breaker
  • Adjustable (as applied to circuit breakers
  • Nonadjustable (as applied to circuit breakers
  • Interchangeable (as applied to circuit breakers)
  • Instantaneous Trip (as applied to circuit breakers)
  • Inverse Time (as applied to circuit breakers)
  • Setting (of circuit breakers)
  • Device
  • Disconnecting Means

You can find the definitions and explanation for above terms in the following articles:
  1. Circuit Breaker Definitions
  2. Electric Switch Definitions
  3. Fuse and Switching Devices Definitions

I recommend a good review for the above articles before going on with below paragraphs.

2- Additional Important Terms:

The following terms are also very important for better understanding of the Overcurrent protection issues.


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.


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.


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:
  • One type of overload is when there are too many devices on a circuit. You might notice that the lights dim when your air conditioner comes on, or that a cable or equipment is hotter than normal.
  • An overload may also occur when a single high amperage device is added to a circuit that is already marginally sized for the demand.

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,
  2. Arcing faults
  3. Ground faults

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.

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.

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.

Interrupting Rating: (see below image)

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.

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. branch-circuit overcurrent protection
  2. supplementary overcurrent protection.

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:
  • First, the use of a supplementary device is specifically limited to a few applications.
  • Second, where it is used, the supplementary device must be in addition to and be protected by the more robust branch-circuit overcurrent protective device.

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.

  • Overcurrent protective devices, such as fuses and circuit breakers, have time/current characteristics  (TCC) that determine the time it takes to clear the fault for a given value of fault current. Selectivity occurs when the device closest to the fault opens before the next device upstream operates.
  • For example, any fault on a branch circuit should open the branch-circuit breaker rather than the feeder overcurrent protection. All faults on a feeder should open the feeder overcurrent protection rather than the service overcurrent protection.
  • When selectivity occurs, the electrical system is considered to be coordinated. With coordinated overcurrent protection, the faulted or overloaded circuit is isolated by the selective operation of only the overcurrent protective device closest to the overcurrent condition.
  • The main goal of selective coordination is to isolate the faulted portion of the electrical circuit quickly while at the same time maintaining power to the remainder of the electrical system.
  • The electrical system overcurrent protection must guard against short circuits and ground faults to ensure that the resulting damage is minimized while other parts of the system not directly involved with the fault are kept operational until other protective devices clear the fault.

For more information about Coordination (Selectivity), Please review the following Articles:

After reviewing above terms definitions, now we come to answer the following questions:

  • 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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