Final answer:
An ESD strap should not be used when working on hardware that is plugged in or powered, as it poses a risk of electric shock to the technician.
Step-by-step explanation:
One situation in which you would not use an ESD strap when servicing hardware is when you are working on a system that's plugged in or powered. An antistatic or ESD (Electrostatic Discharge) strap is designed to prevent static electricity from building up and discharging into sensitive electronic components, which can cause damage. However, when a device is plugged into an electrical outlet, there's a risk of electric shock. In this case, using an ESD strap could provide a continuous path to ground for electrical current, which could be dangerous to the technician. Therefore, it's critical to ensure that all equipment is unplugged and powered down before attaching an ESD strap and starting work. If you're in an environment where the ground connection isn't guaranteed or is unsafe, other ESD safety precautions must be used instead.