Short circuit starts fire in electrical fitting

Reference number
II-747816-2018 (#8735)

Incident Date September 14, 2018

Location: Tie Lake, BC

Regulated industry sector: Electrical - Low voltage electrical system (30V to 750V)

Impact

Injury

Qty injuries: 0

Injury description: No injuries

Injury rating: None

Damage

Damage description: Fire damage to approximately 3m x 2m area of exterior of home. Service cable completely destroyed by prolonged short circuit.

Damage rating: Moderate

Incident rating: Moderate

Incident overview

A fire started in an electrical fitting as a result of a short circuit. The fire damaged the exterior of a home, forcing its inhabitants to reside elsewhere while the damage is repaired. The short circuit travelled along the service cable for approximately 4.5m before it was disconnected. (See Photo 1)

Investigation Conclusions

Site, system and components

The service entrance cable runs from the a consumer owned pole underground to the main panel of the dwelling to supply power to the home. To accommodate a sharp bend in the conductors to enter the home, a metallic L shaped fitting (called an LB) is attached to the cable, and connected to the panelboard via a metallic raceway.

Failure scenario(s)

A short circuit developed in the LB fitting which led to a large release of thermal energy. The energy of the short circuit ignited the conductor insulation which connected additional material to the short circuit, significantly increasing the thermal energy released.

Facts and evidence

Pictures of the failed and damaged components are attached below.

Conversation with Electrical contractor – the service cable was unearthed by this contractor under an installation permit and was found to be damaged for about 4.5 m underground away from the house. This implies the short circuit continued for some time, consuming the cable as it progressed toward the meter (see photo 3).

Conversation with Neighbours – fire was localized to the area around the service entrance fitting and cable. Family was evacuated and uninjured. Fire was extinguished by fire department.

Examination of LB fitting – a hole has been burned into the fitting in the lower portion. The cable is severely damaged from the LB fitting toward the meter base. (see photo 2)

Examination of panelboard – while there is some evidence of heat damage on the conductors at the panelboard, the panel itself was largely spared from damage. The conductors are in fair condition, until the point where they would have been inside the LB fitting, where they are completely destroyed.

Examination of the damaged area – There are no other likely sources in or around this area of the home that could have provided the energy to start this fire.

Causes and contributing factors

It is likely that he hole in the LB indicates the hottest point in the short circuit and subsequent release of thermal energy. This supports the conclusion that the fire was caused by a fault in the LB fitting, which likely led to a short circuit and a large release of thermal energy (see photo 4). Electrical and thermal energy continued to be released at the short circuit, which travelled along the service cable towards the source of electrical energy – the meter base. This is further supported by the fact that the cable was damaged for some distance underground.

 

Damaged area overview

Photo 1: Damaged area overview. Fire damage directly above area of service entrance cable.

 

Interior of LB fitting

Photo 2: Interior of LB fitting. Note relative direction of severity of damage from top to bottom. The hottest area of the short circuit is towards the meter, which was connected to the bottom opening, and away from the panelboard, which was connected to the top opening.

 

 

Photo 3: A

Remnant of aluminium

and other materials from the service cable, unearthed from the ground. The aluminum conductors that were once part of the service cable have been completely fused by the short circuit.

 

Service entrance cable

Photo 4: Service entrance cable protruding from panelboard. Note relative lack of damage of upper portion of cable. This cable would likely have been inside the LB at the time of the incident. This supports the conclusion that this incident was not caused by a short circuit in the panelboard.