Generator transfer switch burnt and destroyed

Incident Investigation

Generator transfer switch burnt and destroyed

January 4, 2020


Reference Number:

II-975771-2020 (#16527)

Location: Denman Island

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



Qty injuries: 0

Injury description: NA

Injury rating: None


Damage description: Generator transfer switch burnt and destroyed

Damage rating: Minor

Incident rating: Minor

Incident overview

On January 4, 2020 residents of a dwelling smelled burning and heard arcing coming from inside the generator transfer switch; located in the pantry off the kitchen. The residents turned main breakers off at the main panel, the sub-panel and at the generator. They removed the cover of the transfer switch and used a fire extinguisher to put out the fire within it.

Investigation Conclusions

Site, system and components


· Main Utility service 240V 200A combination panel

· Sub-panel 100A 240V combination panel (emergency power )

· 22KW 100A 240V Generac generator

· 100A 240V automatic generator transfer switch

· 1/0 aluminum conductors connecting equipment

Emergency Panel Significant loads:

· Baseboard heat - two 1250W, two 750W

· Electric Boiler for in-floor heat – 6000W

Normal Operation:

The generator transfer panel has 3 sets of conductors terminated on 3 sets of lugs, one in from the utility, one in from the generator and one out to the emergency panel. Upon a utility power failure the generator automatically starts and the transfer switch changes position from utility power to automatically feed the emergency panel with generator power.

Failure scenario(s)

The dwellings electrical system was operating on utility power when the homeowner arrived. This is a vacation property and was cold inside prompting the owner to turn on the baseboard heat in addition to the electric boiler. The load although within normal ranges was likely too much for the failing terminations at the transfer switch output. It is likely these terminations weren’t torqued to manufactures specifications at time of install in 2018 and deteriorated over time with varying loads. The additional load of the baseboards, that were not normally used, was likely enough to push the already jeopardized aluminum conductor terminations to heat and melt completely off the termination points.

Facts and evidence

Homeowner Interview:

· The owner said she had just arrived the morning of the incident and because the house was cold turned on four baseboard heaters. The electric boiler in-floor heating system had already been running as her son had arrived earlier. She said they don’t normally use the baseboard heaters. A few hours later they smelled burning and heard arcing coming from the transfer panel. Smoke was coming from the transfer panel so they turned off the main generator breaker and the main panel utility breaker. They took the cover off the transfer panel and extinguished the fire.

· The owner said none of the breakers had tripped.

Safety Officer Observation:

· Transfer panel destroyed inside

· Two 1/0 Al conductor’s and lugs that supply the emergency panel where burnt completely off at the transfer switch

· Observed labels on baseboard heaters and boiler for wattage

· The set of terminations coming from the generator are beside the output terminations that burnt off. They were still intact and when checked with a screw driver, they were found to be loose; being able to hand turn each one another 5 quarter turns before it began to feel snug.

· Observed the label on a new identical transfer switch specifying 275 in- lbs torque

Causes and contributing factors

The cause of this incident was very likely due to the overheating of aluminum conductor terminations. It is likely these terminations were not torqued to manufacturer specifications at the time of installation in 2018. The loose terminations were likely already failing from over-heating under normal loads over the past two years. The 6000W electric boiler in-floor heating system was already operating. The 4000 W of baseboard heat was turned on just prior to the incident and was likely the extra current and heat needed to melt the lugs and conductor ends completely off.

It is probable the conductors terminated at the failure point were installed similarly to the other field installed terminations within the switch; and these were found to be loose.

Burnt Transfer Switch – 1/0 AL E-panel conductors burnt off


Bottom lugs on transfer panel (for E-panel supply) burnt off. Top lugs (generator input) found to be loose.


Same as above – note bottom lugs


Torque specifications