A circuit breaker is an overcurrent protection device (OCPD) designed to protect electrical devices and individuals from overcurrent conditions. Unlike most fuses, circuit breakers can be reset, which makes them a popular choice for overcurrent protection. Circuit breakers use an electromagnet and/or a bimetallic switch to detect an overcurrent condition.
Circuit Breaker Types and Characteristics
A circuit breaker may be reset by moving the trip lever handle to the full OFF position and then returning the handle to the ON position. Individuals must ensure the source of an overload is cleared before attempting to reset a breaker. There are three types of circuit breakers differentiated by their internal mechanisms for tripping:
Regardless of which internal mechanism a circuit breaker uses, most circuit breakers look the same externally, with the exception of the circuit breaker fuse. A circuit breaker fuse is a screw-in OCPD that has the operating characteristics of a circuit breaker.
The advantage of a circuit breaker fuse is that the fuse can be reset after an overload. Circuit breakers are available in a variety of amperages, but the voltage is typically rated as 110 V for single-pole residential breakers or 220 V for double-pole residential breakers.
Figure 1. Circuit breakers are available in a number of configurations, including single-pole and double-pole breakers.
To gain access to the circuit breaker connections in a service panel, the cover of the panel must be removed.
A magnetic circuit breaker is an OCPD that operates by using miniature electromagnets to open and close contacts. The basic idea is shown below.
Figure 2. Electromagnetic solenoids are an example of using electromagnetism to do work.
As you can see, an iron plunger is surrounded by an encased coil of wire and a set of contacts are attached to the iron plunger. With an electric current passed through the coil, the contacts attached to the iron core are pulled toward the coil. In this way, we can open or close the solenoid contacts. Note the figure shows both normally-open and normally-closed contacts.
As illustrated in Figure 3, the produced magnetic field can be strengthened by increasing the applied current and the number of turns per unit length as well as inserting an iron core through the coil.
Figure 3. An electromagnet can be strengthened by increasing the amount of current, increasing the number of turns in the coil, and inserting an iron core through the coil.
A solenoid in a magnetic circuit breaker opens the circuit based on the current limit of the breaker.
When the current through the coil exceeds the rated value of the breaker, the magnetic attraction becomes strong enough to activate the trip lever handle and open the circuit. See Figure 4.
Figure 4. In a magnetic circuit breaker, passing an electric current through the coil causes the contacts attached to the iron core to be pulled toward the coil. The solenoid in a magnetic circuit breaker opens and closes the contacts based on the current level.
Once the overload is removed, the trip lever handle can be reset to the original position, reactivating the circuit.
Thermal circuit breakers use a bimetallic strip attached to a latch mechanism. The bimetallic strip is made of two dissimilar metals that expand at different rates when heated. The bimetallic strip bends when heated and opens the contacts. See Figure 5. The bimetallic strip may be heated directly by circuit current or indirectly by the rise in temperature caused by an increase in the circuit current.
Figure 5. Thermal circuit breakers use a bimetallic strip attached to a latch mechanism to open the circuit when a short circuit or overload occurs.
Thermal circuit breakers are designed so that the bimetallic strip bends to release the contact under spring tension based on the amount of continuous current flowing through it. The bimetallic strip must cool and return to its normal condition (size) at room temperature before the circuit breaker can be reset.
Thermal protection of a circuit is not instantaneous. It requires time to heat the strip and for the strip to bend far enough to cause the contacts to snap open. A magnetic circuit breaker is used in applications where this delay can cause damage to a circuit. Thermal circuit breakers can be reset by pressing the pushbutton only after the bimetallic strip has cooled.
Thermal-magnetic circuit breakers include both a magnetic-tripping function for short-circuit protection and a thermal-tripping function for overload protection, as illustrated in Figure 6.
Figure 6. Thermal-magnetic circuit breaker.
Thermal-magnetic circuit breakers are also called inverse-time circuit breakers. As the alternative name inverse-time indicates, the higher the overload, the shorter the time it takes the circuit breaker to open.
When an overload condition occurs, the excess current generates heat, which is sensed by the bimetallic heat-sensing element. After a short period, depending on the breaker’s rating and amount of overload, the breaker will trip, disconnecting the voltage source from the load. If a short circuit occurs, the electromagnetic sensor responds immediately to the fault current and disconnects the circuit.
DC Circuit Breakers
A DC circuit breaker is an OCPD that protects electrical devices operating with DC and contains additional arc-extinguishing measures.
DC circuit breakers are a relatively new technology to most homeowners since most devices used in a house work with AC and AC circuit breakers. General AC circuit breakers for the home are rated to interrupt above 6 kA. Some manufacturers produce circuit breakers that are dual-rated for both AC/DC from 48 VDC to 125 VDC. DC circuit breakers are used with 24 VDC to 48 VDC programmable logic controllers (PLCs) and in wind power applications.
Though AC and DC breakers appear similar in form and function, internally they operate very differently. During an overload, the internal contacts of both AC and DC circuit breakers separate to protect the circuit. However, as the contacts pull apart from each other, an arc will form as the current jumps across the air gap created. Contact arcing is an electrical arc that occurs when opening and closing circuit breakers. See Figure 7. As the arc continues to jump across the air gap, the current will continue to flow through the circuit. These arcs must be extinguished quickly.
Figure 7. Contact arcing is an electrical arc that occurs when opening and closing circuit breakers.
The ways in which AC and DC breakers are designed to extinguish the arc are very different and this is why AC and DC breakers are not interchangeable. Only breakers that are labeled as DC rated should be used for DC applications.
An AC-rated breaker should never be used in a DC circuit. AC circuit breakers are not designed to handle the problems of arcing associated with DC. DC circuit breakers include additional arc-extinguishing measures to dissipate the electrical arc when opening and closing and elongate the device lifetime.
DC Arc Suppression
DC arcs are considered the most difficult to extinguish because the continuous DC supply causes current to flow constantly and with great stability across a much wider gap than an AC supply of equal voltage, often shown in metrics such as peak value and RMS.
To reduce arcing in DC circuits, the switching mechanism must be such that the contacts separate rapidly and with enough of an air gap to extinguish the arc as soon as possible when opening. When DC contacts are being closed, it is necessary that the contacts move together as quickly as possible to prevent some of the same problems encountered in opening them. If a circuit breaker is DC rated, it will be indicated on the breaker by the manufacturers.
Figure 8. Some circuit breakers are rated AC/DC. This information will be made clear on the manufacturer’s label.
It is worthwhile to mention that when a short circuit occurs across the terminals of a DC circuit, the current increases from the operating current to the short-circuit current depending on the resistance and the inductance of the short-circuited loop.
Some types of circuit breakers are rated AC/DC for use with either type of application. This information will be stated on the manufacturer’s label.
AC Arc Suppression
An AC arc self-extinguishes when the set of contacts opens. An AC supply has a voltage that reverses its polarity 120 times a second when operated on a 60 Hz line frequency. The alternation allows the arc to have a maximum duration of no more than a half-cycle.
The AC current reaches zero 60 times each second. See Figure 8. When AC reaches zero, no current flows, and therefore the arc is extinguished.
Figure 9. When AC current reaches zero, no current flows, and therefore the arc is extinguished.
Circuit Breakers as OCPDs
A circuit breaker is an overcurrent protection device with a mechanical mechanism that can automatically open a circuit when a short circuit or overload occurs. Circuit breakers use two principles of operation to protect the circuit: thermal and magnetic.
Thermal circuit breakers consist of a heating element and a mechanical latching mechanism. The heating element is usually a bimetallic strip that heats up when current flows through it.
Magnetic circuit breakers use an electromagnet to detect an overcurrent condition. Most magnetic circuit breakers contain both thermal and magnetic components. While the magnetic components protect the circuit against high overload current or short-circuit currents, the thermal components protect the circuit against a constant overload current that is not of sufficient level to activate the magnetic components.
A DC circuit breaker is used to protect electrical devices that operate with direct current (DC) and contains additional arc-extinguishing measures. DC circuit breakers are a relatively new technology and used in EV charging stations, photovoltaics, and battery storage systems, as well as industrial DC distribution networks.