Fire and Crossarms

Wood Crossarm Performance in Wildfires

The intensity of wildfires today not only pose risks to utility poles, but also to crossarms at the top of poles. Other risks at pole tops include fires caused by transformers or shorts in electric lines.

Alternative materials for crossarms such as composite fiberglass have been promoted for their fire resistance. However, recent testing shows such claims are unsupported and underscores wood's remarkable performance when subjected to fire.

Fire and Crossarms

The Western Fire Center in 2022 tested both wood and composite fiberglass crossarms. Using proposed ASTM standard pole tests adapted for crossarms, sample arms of each material were exposed to a combination of heat sources. To simulate real-world conditions, the crossarms were attached to a pole stub and had 300-lb. weights suspended from each end.

In the first test, crossarms were exposed to radiant heat panels for 5 minutes, followed by exposure to a convective flame burner for an additional 5 minutes. The second test exposed the crossarms only to the convective flame burner for a total of 10 minutes. In both tests, after the radiant heat and flame exposures, the crossarms were allowed to continue to burn and self-extinguish for up to 20 minutes.

Results of the testing showed conclusively the superior fire performance of wood vs. fiberglass. In tests using both radiant panels and the flame burner, the composite fiberglass crossarm collapsed after nearly 6 minutes. Tests using only the flame burner yielded similar results, with the fiberglass arm failing after 5 minutes and 10 seconds.

Wood crossarms in both tests remained intact for the full 30 minutes. Preservative-treated wood crossarms and treated wood crossarms coated with a fire retardant were tested with similar results. The wood in the crossarm samples showed charring, but continued to hold the suspended weights with little change from pre-fire conditions.

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