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On, Off or Tripped? Analyzing Circuit Breakers After a Fire


When investigating the origin and cause of a building fire, one of the primary items to examine is the electrical panel, and specifically, the positions of the circuit breakers in the panel. Positions of the breakers post-fire can provide clues as to the origin and cause of the fire. Are all the breakers on? Are all the breakers off? Have any of them tripped? While evaluation of the positions of the circuit breakers in the electrical panel is an important step in any fire investigation, it’s not as straight forward as one might think. Even when circuit breakers are found in the tripped position after a fire, it doesn’t necessarily mean that the fire was caused by electrical activity. Nonetheless, the positions of the circuit breakers in the panel can be used in combination with other evidence and known facts to help piece together how the fire occurred.

Circuit breakers are designed to ’trip’, or disconnect an electrical circuit automatically, when too much current is flowing through the circuit. Circuit breakers can have three positions: on, off, or tripped. Under normal operating conditions, a circuit breaker in the on position permits current to flow through the circuit, and the circuit is said to be ’energized’. When in the on position, every circuit breaker is designed to remain on until a pre-determined threshold is reached (combination of current and time).

When the amount of current flowing through a circuit exceeds the threshold of the circuit breaker, the circuit breaker is designed to trip and disconnect power from the circuit. The tripping function of a circuit breaker happens automatically when a current overload condition occurs. In addition, circuit breakers have an off position so that any circuit in the electrical panel can be de-energized manually. A circuit breaker in the off position has not typically been subject to abnormal conditions; it’s simply been switched off manually.

There are various manufacturers of circuit breakers we typically find in our homes. While many brands have three distinct positions (on, off, and tripped), at least one popular brand of residential circuit breakers, the Cutler-Hammer brand, only has two positions: on and off. When a Cutler-Hammer circuit breaker trips, it goes to the off position. As a consequence, you can’t tell just by looking at it whether or not a Cutler-Hammer circuit breaker was manually switched off or had tripped during a fire. There are ways to determine whether or not the circuit breaker was off or had tripped, but it usually involves x-raying the component or opening its housing.

While circuit breakers are designed to trip under overcurrent conditions, there are other conditions which can cause a circuit breaker to trip. Circuit breakers have both magnetic and thermal thresholds, so they can also trip if their surrounding environment gets too hot. Though far less common, a defective circuit breaker in the panel may have failed to trip in response to an excessive current draw on a circuit. In the case of a house fire, it’s possible that some of the circuit breakers in the electrical panel may trip due to the heat of the fire, not necessarily as the result of an overcurrent condition. Furthermore, tests have found that circuit breakers may also trip if they are exposed to significant mechanical shock. If a circuit breaker is dropped or hit with enough force, it could trip, even if no overcurrent condition is present.

Alternative scenarios must be ruled out when examining an electrical panel. Did someone turn the circuit breakers off after the fire had ignited? Firefighters typically ensure that there is no live power in the building while they are trying to extinguish a fire. In rare cases, some people ’steal’ power from other sources or from their neighbours, so while the circuit breakers in their electrical panel may be in the off position, the home’s electrical circuits may still be energized via extension cords or other illegal wiring from neighbouring buildings. Furthermore, there are instances when fires can start due to an electrical problem that wouldn’t cause the circuit breakers to trip in the first place. High resistance connections and in-line arcing are conditions which would not typically cause a circuit breaker to trip.

The positions of the circuit breakers in an electrical panel can provide important insight into origin and cause investigations. However, it is important to evaluate all the circumstances and possible scenarios which could have led to positioning of the breakers to ensure a thorough analysis of the factors contributing to the origin and cause of a fire.


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