How Does a Standard Breaker Respond to Electrical Fault?

Electrical Technology

How Does an Ordinary Circuit Breaker Operates During Short Circuit, Overload & Ground Faults?

When a standard circuit breaker detects a fault, such as an overload or short circuit, it responds by interrupting the current flow. The breaker “trips” to prevent excess current from damaging the circuit, overheating the wires, or causing a fire. This protective action helps ensure the safety and integrity of the electrical system.

A standard circuit breaker responds to both short circuits and overloads by cutting off the current to protect the circuit:

Short Circuit:

When a short circuit occurs, a sudden surge of current flows through the circuit. The breaker detects this sharp increase in current and trips almost instantly, stopping the flow to prevent potential damage, overheating, or fire.

Overload:

During an overload, the current gradually exceeds the circuit’s safe capacity due to the connection of too many devices or a high-demand appliance. The breaker senses this excess current over a short period, heats up, and eventually trips to protect the wiring from overheating and causing damage.

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How a Standard Circuit Breaker Operates During Electric Faults?

A typical circuit breaker cannot detect ground faults or the absence of a neutral. Instead, a standard breaker only provides protection against short circuits and overloads. This is why the National Electrical Code (NEC) mandates the use of Ground Fault Circuit Interrupter (GFCI) breakers for proper protection of both devices and personnel.

Let’s look at how a standard breaker behaves under normal and fault conditions in the following circuit examples.

Normal Condition

In the following diagram, a lighting circuit is controlled and protected by a 15-amp breaker, supplied with 120V from a 120V/240V main panel.

Since there is no fault in the circuit, everything operates normally, and the light glows as expected.

Short Circuit / Overload Condition

Now, suppose a short circuit or overload occurs, such as when the hot wire contacts the metallic body of a device (e.g., the bulb holder). In this case, fault current finds a path back to the source via the ground wire. The ground wire is bonded to the neutral wire at the main panel, allowing circuit completion.

Because the resistance of the ground wire is very low, a large amount of current (up to 600 amps) flows through the circuit. This high short-circuit current causes an overload. The internal mechanism of the breaker detects this excess current and responds by tripping. The 15-amp breaker then disconnects the circuit from the main power supply, protecting both the device and personnel from harm.

Fault Detection and Tripping

As shown in the diagram below, the 15-amp breaker trips after detecting fault current that exceeds its rated capacity. This disconnects the circuit from the main supply, offering protection against overloads and short circuits.

Standard breaker in Ground Faults:

As discussed above, ordinary circuit breakers do not provide protection against ground faults, where electricity unintentionally flows to the ground, or against broken neutral conditions, which can present significant safety risks. In these situations, current may flow through the source (in the case of a lost neutral) or to the ground (in the case of a ground fault), potentially completing the circuit through unintended pathways.

This can result in current flowing through improper channels, including the neutral and ground conductors. Additionally, any exposed metal parts in the circuit may become energized, potentially carrying dangerous voltages of 72V or 120V, creating a serious hazard.

To address this issue, a GFCI breaker should be used instead of a standard breaker to ensure safety in the event of ground faults.

Good to Know:

A standard circuit breaker is designed to protect against overloads and short circuits. However, standard breakers do not protect against ground faults, or against broken neutral conditions, which can create additional safety risks. The National Electrical Code (NEC) mandates the use of Ground Fault Circuit Interrupters (GFCIs) which are specifically designed to detect ground faults and provide enhanced protection.

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