According to NEC guidelines, how must structures' electrical systems be designed concerning bonding?

The design of electrical systems in structures according to NEC (National Electrical Code) guidelines mandates that all metal systems must be bonded together. This requirement is crucial for safety, as bonding helps to ensure that all conductive components within a structure—such as metal pipes, frames, and electrical enclosures—are electrically connected. This connection prevents differences in electrical potential between metal parts, which could pose a shock hazard.

By bonding these metal systems, the risk of electrical shock during a fault condition is significantly reduced because it allows fault currents to safely travel back to the ground, triggering protective devices like circuit breakers to operate quickly. This interconnectedness is essential in promoting a safe working environment by minimizing the risk of electrocution and equipment damage, ensuring that any stray electrical currents have a safe path to ground.

The other options focus on isolating systems or allowing for independent paths, which do not align with the objectives of bonding, making them less adequate in addressing safety needs tied to electrical systems.