Which of the following is NOT a main type of electrical distribution system?

The correct answer identifies "Spiral systems" as not being a main type of electrical distribution system. In electrical distribution, several well-established configurations exist to deliver power efficiently and reliably.

Radial systems are one of the simplest forms, where power is distributed from a single source through a series of branches, making them straightforward but sometimes less reliable because if one part fails, the downstream customers may lose power.

Loop systems create a closed-loop configuration, providing multiple pathways for power to reach consumers. This redundancy enhances reliability, as power can be rerouted in case of a fault along one path.

Network systems are complex and interlinked, offering the highest reliability through extensive interconnections that ensure a stable supply by allowing multiple sources to feed into the network, which is particularly useful in densely populated areas.

In contrast, spiral systems do not represent any known standard configuration in electrical distribution. Thus, identifying "Spiral systems" as not a main type is accurate, given the established categories of radial, loop, and network systems in the field of electrical distribution.