Final answer:
NEC 406.9 stipulates that only GFCI protected receptacles are allowed in bathtub and shower spaces to prevent electric shock by interrupting the circuit when a leakage current is detected.
Step-by-step explanation:
According to the National Electrical Code (NEC) 406.9, the type of receptacles permitted to be installed in bathtub and shower spaces are GFCI (Ground Fault Circuit Interrupter) protected receptacles. This restriction is in place because GFCIs are designed to prevent electric shock by breaking the circuit when there is a difference in currents between the live/hot and neutral wires, which indicates a leakage current. This leakage could create a hazardous path, and the GFCI will trip if it detects a leakage current greater than 5 mA, which is the accepted maximum harmless level for a shock. GFCIs are crucial in areas where electrical equipment is in close proximity to water, thereby preventing the chance of an electrical shock.