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How Do GFCI and Standard Breakers Respond to Ground Faults?

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How Does a GFCI Breaker Protect Us in Case of a Fault, Compared to Standard Breakers?

An ordinary circuit breaker in the main panel for household appliances protects only in the event of a short circuit or overload. In contrast, a Ground Fault Circuit Interrupter (GFCI) offers protection against both overload (short circuits) and, more importantly, specific ground fault conditions.

In this article, we’ll explain why a standard breaker cannot protect you in the event of a ground fault and why GFCIs are necessary to ensure safety for both devices and personnel.

We’ll explore how standard breakers and GFCIs respond to ground fault currents and why standard breakers may leave you vulnerable to hazards compared to GFCIs.

Table of Contents

Normal Operation

Consider the following sample lighting circuits, which are protected first by a standard circuit breaker and then by a GFCI circuit breaker.

In case of standard breaker, the light circuit is working properly as there is no overload or short circuit fault.

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In case of GFCI, the circuits operate normally as current in both Hot and Neutral is same. Hence, the light is functioning as expected.

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Short Circuit Fault

Consider a lighting circuit protected by a 15-amp standard circuit breaker. During a short circuit (where the hot wire touches the neutral or a metallic part of the device), the low resistance (0.2Ω) of the path allows a high current of approximately 600 amps to flow. This surge trips the 15-amp breaker, disconnecting the circuit from the power supply. This scenario illustrates the correct response of a circuit breaker to a short circuit.

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A GFCI circuit breaker also acts like a standard circuit breaker when a short circuit or overload occurs, but the operation is different. It detects the current difference between Incoming and outgoing current in Hot and Neutral. If both current are not equal, it suddenly trips to disconnect the circuit from the main power supply.

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But what happens during a ground fault or a lost neutral in the main panel when only a standard breaker is used? Let’s examine two scenarios.

Broken Neutral
Broken Neutral & Ground Faults

Broken Neutral

In Case of a Standard Breaker

If the neutral wire breaks in the main panel, current will flow back through the only available path to the transformer to complete the circuit. For example, if the load has a resistance of 20Ω and the ground path to the transformer has 30Ω, a current of 2.4 amps will flow. This low current is insufficient to trip the ordinary 15-20 amp breaker in the main or subpanel.

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Furthermore, all metal parts, including equipment and metal raceways, will be energized at 72 volts. If someone touches these parts while barefoot, they could receive a dangerous shock.

In Case of GFCI

To prevent this hazard, a GFCI breaker detects any current imbalance between the hot and neutral wires and trips, disconnecting the circuit from the main supply to ensure personnel safety.

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Broken Neutral & Ground Fault

In Case of Standard Breaker

Now, consider a worst-case scenario involving both a lost neutral and a ground fault. With no neutral in the main panel, current will seek an alternative path to complete the circuit, as the neutral and ground busbars are bonded in the main panel.

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In this situation, a current of 4 amps will flow through the 30Ω ground path back to the transformer. However, this current is still too low to trip a standard 15 or 20-amp breaker, which is commonly used for household appliances.

Again, all metal parts, including equipment and metal raceways, will be energized at 120 volts, posing a significant shock hazard to anyone who touches them while barefoot.

In Case of GFCI

In this scenario, a GFCI breaker detects the current imbalance caused by the lost neutral and immediately trips, providing protection and safeguarding personnel from potential shock hazards.

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Good to Know:

Standard Circuit Breaker vs. GFCI:

Standard Circuit Breaker: A simple circuit breaker is designed to protect against overloads (too much current) and short circuits. It trips when the current exceeds its rating, but it does not detect ground faults.
GFCI: Provides both ground fault protection and overload protection. It adds a critical feature by responding to even small current leaks that could cause electrocution.

Resources & Tutorials:

How Do GFCI and Standard Breakers Respond to Ground Faults?