When does the tractor protection valve automatically close?

The tractor protection valve is designed to safeguard the air brake system of a commercial vehicle by automatically closing under specific conditions. It primarily activates in response to low air pressure in the system, specifically when the pressure drops to a range of 20-45 psi. This closing action serves to prevent air from escaping the trailers and ensures that the vehicle remains under control, as it helps maintain sufficient air pressure necessary for braking.

A low air pressure situation indicates a potential issue that could compromise the braking capability of the vehicle. By automatically closing, the tractor protection valve helps maintain the integrity of the brake system, ensuring that the driver has the ability to stop the tractor and any attached trailers safely.

In contrast, options related to temperature, the vehicle being stationary, or the trailer being disconnected do not trigger the automatic closure of the valve and are therefore not applicable in this context. Each of these circumstances does not directly relate to the operational integrity of the air brake system in the same way that low air pressure does.