Overcurrent Protection in AC Power Systems

Get an intro to overcurrent protection and OCPDs (overcurrent protection devices).

In this article, we will cover types of overcurrent, what overcurrent protection devices are, and their place in an electrical circuit.

Types of Overcurrent

The three major categories or types of overcurrent are overload, short-circuit, and ground-fault.

 

Overload Overcurrent

Overload overcurrent is self-defining: Any current in excess of rated-load current is, in effect, an overload. An overload occurs when an electrical circuit, whether by the original design of a new circuit or by modification of an existing circuit, is required to convey load current in excess of the rated-load ampacity of the circuit conductors.

For example, a 20-amp branch circuit is modified with an additional lamp, which increases the load current to 22 amps: this would be a circuit overload.

Overload conditions can occur at the service, feeder, or branch-circuit level of a building's electrical-power distribution system.

An electrical overload overcurrent also occurs when a motor is mechanically overloaded. This may be caused by excess friction within its internal bearing surfaces, excess heat (due to high ambient temperature or another failure), or by the binding or some other mechanical overload in the utilization equipment it drives. Overload is a controlled overcurrent situation, normally of low magnitude.

 

Short-Circuit Overcurrent

Short-circuit currents (as well as ground-fault currents, which we'll touch on next) are high-magnitude fault overcurrents that, in effect, place a low resistance in parallel with the impedance of the connected load(s). Short-circuit overcurrent normally involves an accidental cross-connection of at least two circuit conductors (supply and return). This places a short-circuit across the supply-transformer winding.

Figures 1 and 2 represent the more common transformer supplies to a structure. 

Figure 1 is the drawing of a single-phase AC, 3-wire, 120/240-volt supply to a building such as a home or small industrial facility). A single primary winding in the transformer supplies (by induction) two 120-volt windings wired in series in the secondary. A utilization-equipment load will operate at 240 volts when connected between the two ends of the two series-connected 120-volt windings. A utilization-equipment load will operate at 120 volts when connected between either end of the two series-connected 120-volt windings and the third wire shared by the two windings (see Figure 1).

Figure 1. Voltage relationships of the three supply lines from the secondary of a single-phase AC residential service power transformer

 

A three-phase AC electrical power distribution system, as shown in Figure 2, will normally have a higher value of short-circuit overcurrent because the short will normally involve more than one single-phase AC transformer winding.

 

Figure 2. Voltage relationships of the four supply lines from the secondary of a three-phase AC commercial or industrial service power transformer

 

Ground-Fault Overcurrent

Ground-fault overcurrent is also a short-circuit condition that normally affects only one of the circuit conductors and the grounded metal raceway or electrical distribution or utilization equipment enclosure.

Ground-fault overcurrent can occur only if the electrical power distribution system of the building or structure is referenced to earth ground. “Reference grounding” requires the common connection of one end of one or more of the single-phase AC transformer windings (wye transformer configuration) to a grounding-electrode system, creating both grounded and ungrounded circuit/supply conductors.

The magnitude of ground-fault overcurrent is normally less than the magnitude of short-circuit overcurrent available from the same transformer. The short circuit can be across two or more transformer single-phase AC windings. The ground-fault overcurrent normally affects only one single-phase AC winding in the transformer supplying power to the faulted condition.

Both short-circuit and ground-fault currents are high-magnitude overcurrents caused by an accidental low-resistance parallel connection to the connected load resistance. Without some form of overcurrent protection device installed in series with the circuit conductors, the only limit of the fault overcurrent is the conductor resistance and the amount of power available from the transformer.

Overcurrent Protection

As shown in Figure 3, full overcurrent protection for the conductors and the connected load can only be provided by a fuse or circuit breaker installed at the point where the circuit originates (or where it receives its supply).

If an OCPD is located downstream from the supply, the overcurrent protection is technically subdivided with short-circuit, ground-fault protection located upstream, as well as separate overload protection located downstream. The fuses or circuit breakers located downstream provide full overcurrent protection for any circuits or equipment located on their load side while providing only overload protection for their line- or supply-side circuit.

 

Figure 3. Split overcurrent protection for a transformer circuit

The Form and Function of Overcurrent Protection Devices

There are three principal components of an electrical circuit: a power source, a load, and a connection between the two. 

These three principal components are supplemented with a means of ON/OFF control and a means of limit control. Both types of control restrict the amount of current that can flow in the circuit. The means of ON/OFF control is normally in the form of a switch (either manual, automatic, electronic, or electromechanical). The means of limit control is normally an overcurrent protective device, which at the electrical-power distribution level is a fuse or circuit breaker (as seen in Figure 4).

 

Figure 4. Overcurrent protection devices

 

As shown in Figure 5, the electrical power distribution system within a building or other structure has three major classifications: the service, the feeder circuits, and the branch circuits.

In general, the conductors of all of these circuits must be provided with a means of overcurrent protection at the point where they receive their electrical supply. The OCPD must be installed in accordance with the requirements of the National Electric Code. Both the conductors and the connected load they supply must be protected at the correct amperage.

 

Figure 5. The electrical power distribution system within a building

 

The rated ampacity of the conductors, the full-load current rating of the connected load, and the size or load-rating of the OCPD are interrelated. The full-load current rating of the connected load dictates the size (by rated ampacity) of the supply conductors and the rating or setting of the OCPD.

By the same token, the rating or setting of the OCPD and the rated ampacity of the circuit conductors dictate the maximum full-load current that can be supplied from the service, feeder, or branch circuit. Any current magnitude that is greater than the rated ampacity of the conveying wires or the rated-load current of the electrical utilization equipment—such as light fixtures, motors, or transformers—is described as an overcurrent.

The primary purpose of a circuit overcurrent protection device (a fuse, a circuit breaker, or some other type of current-limiting device) is to limit the temperature of the circuit conductors to a value that will not damage the conductors or their insulation. This is achieved by limiting the amount (value) of current the conductors are required to convey. Protecting the circuit conductors against overheating by limiting the amount of current the conductors are required to convey inherently protects the supplied electrical distribution and utilization equipment (the connected load) from the effects of overcurrent.

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I hope that this article has helped you achieve a better understanding of overcurrent and overcurrent protection devices. If you'd like to know more about a specific topic relating to overcurrent, please share your thoughts in the comments section below.