Which action is least effective in preventing equipment accidents?

Using two-pronged plugs on all electrical devices is the least effective in preventing equipment accidents because two-pronged plugs do not provide the same level of safety that three-pronged plugs offer. Three-pronged plugs are designed with an additional prong that connects to the ground, which helps to prevent electrical shock and ensures that if there's a fault in the device, the electrical current has a safe path to the ground. This grounding feature significantly reduces the risk of electrical shock and equipment malfunction.

In contrast, two-pronged plugs do not have this safety feature, making them more prone to causing electrical accidents if the equipment malfunctions or if there's a short circuit. Therefore, relying solely on two-pronged plugs can result in an increased risk of accidents related to electrical equipment. The other options involve practices like proper equipment maintenance and training staff, both of which are essential for ensuring safety in equipment use.