Final answer:
The screw shell of all sign lampholders supplied by a grounded circuit must be connected to the neutral wire to ensure safety and effective grounding.
Step-by-step explanation:
The screw shell of all sign lampholders supplied by a grounded circuit shall be connected to the neutral conductor. Grounding a conductor ensures that it is at zero volts by connecting it to the earth using a conductor. This grounding process provides a safe, low-resistance path directly to earth, which ensures that any electrical charge can flow freely to and from the earth's unlimited reservoir, minimizing the risk of electrical shocks and allowing for safe interaction with electrical appliances and fixtures. The neutral wire, which is typically insulated with white, acts both as the return path for electric current to complete the circuit and provides an additional safety measure by being at zero volts relative to the earth.