Before unloading any flammable liquid tank, what essential step should be taken?

Grounding the trailer before unloading flammable liquids is critical because it helps prevent static electricity buildup, which can ignite flammable vapors. When flammable liquids are transferred, especially if they are being poured or moved quickly, static electricity can generate due to friction. If the trailer is not grounded, this static charge can accumulate to a dangerous level.

By grounding the trailer, you provide a safe pathway for any static electricity to discharge harmlessly to the ground, significantly reducing the risk of a fire or explosion during the unloading process. This step is considered best practice in handling hazardous materials and is a crucial safety measure mandated by regulations.

The other options, while they might have their own relevance in different contexts, do not directly address the primary safety concern associated with the unloading of flammable liquids. For instance, notifying local authorities may be necessary in case of an emergency or specific local regulations, but it does not mitigate immediate risks during unloading. Checking the weather can be important for overall safety but does not directly prevent static discharge. Sealing the manhole could be relevant in preventing leaks but does not address the immediate risk of static electricity during the unloading process.