The principal functions of an optimal protective system are the following:

- Preventing faults.
- Fault detection.
- Fast isolation of the affected part of the installation, keeping the rest operative.
- Signaling and monitoring the location, nature, and magnitude of the fault.

A protective system that does not comply with these functions can cause damages derived from the electrical fault, serious process interruptions, civil liability for damages to third parties...

An engineering study of selective coordination is fundamental to implement a protective system that isolates only the damaged part of the installation, keeping the rest in service.

Adjustment values are calculated from the short-circuit study, from the limits of the equipment, and from the properties of protective devices. The functions of adjustment values of protective relays are: first, giving a sufficient protection and after, coordinating these protective relays.