Why Doesn’t a Standard Breaker Protract Against Ground Faults?

A broken neutral in a circuit with a standard breaker poses a shock hazard because the breaker does not monitor or protect the neutral wire. The internal mechanism of a standard breaker is not designed to detect ground-fault currents during operation. Standard circuit breakers are engineered to protect against overloads and short circuits, not ground faults.

Standard breakers monitor current in the hot wire and trip if the current exceeds the breaker’s rating—typically due to an overload or short circuit. However, with a broken neutral, fault current may return to the source through the ground wire. This occurs because the ground and neutral terminal bars are bonded in the main panel.

Consequently, a current lower than the breaker’s rated capacity can flow through the circuit in an unintended path. Since no excessive current travels through the hot wire, the breaker does not detect a fault and remains closed. As a result, parts of the circuit stay energized, creating a hidden shock risk that the breaker does not address.

The most common faults in an electric circuit are as follows:
Overloads and Short Circuits

Standard breakers react to excessive current caused by overloads or direct short circuits (high-current faults where current flows directly from hot to neutral or hot to hot). These conditions create a current surge, which the breaker detects and trips to prevent damage.

Ground Faults

A ground fault occurs when current leaks from the hot wire to a grounded surface, bypassing the neutral wire (e.g., due to a broken neutral or a live wire contacting a metal appliance case or wet surface). Ground faults may not generate the high current surges required to trip a standard breaker, especially if only a small amount of current leaks to ground. This leakage can create severe shock hazards without reaching the breaker’s trip threshold.

How Does a Standard Breaker Respond to a Short Circuit or Ground Fault?

Let’s examine how a standard breaker behaves and reacts to short circuits or ground faults in a circuit, as illustrated below.

Consider this example: In a 120V/240V main panel, a lighting circuit is controlled and protected by a 15-amp standard breaker on a 120V supply, and the neutral connection is lost.

As shown in the figure, if the neutral bar in the main panel is unavailable, return current tries to flow back to the neutral bar. Since the neutral bar is bonded to the ground bar, the current’s only path back to the source (typically the transformer) is through the ground wire. This forms a circuit, allowing approximately 2.4 amps of fault current to flow. The light bulb may still emit a dim glow.

This 2.4-amp fault current is well below the breaker’s 15-amp rating, so it does not trip. Consequently, the circuit presents a shock hazard, as all metal components—including equipment enclosures, metal raceways, and the metallic bodies of connected devices—become energized with approximately 72V AC.

Now, consider another scenario where the neutral is lost and the hot wire contacts the metallic body of the device, creating a "double fault." In this case, the light is off due to the absence of load resistance. As shown in the figure, a fault current of approximately 4 amps flows through the ground conductor back to the source.

Again, all metal components in the circuit become energized at 120V AC. This 4-amp fault current remains below the breaker’s 15-amp threshold, so the breaker does not trip. If an operator touches the equipment enclosure, metal raceway, or device’s metallic body, they risk a severe electric shock.

To mitigate these hazards, a GFCI (Ground Fault Circuit Interrupter) breaker is recommended over a standard breaker. GFCI breakers are engineered to detect ground faults and trip in dangerous scenarios—including those caused by a broken neutral—ensuring safer operation.