The range within which an electrical protection device can detect a fault is termed as the device’s zone of protection.
A short circuit fault occurs when a phase is connected to another phase or to ground via a low-impedance path. A short circuit results in the production of very high current magnitudes that can cause severe damage to equipment. They also pose threat to personnel working on power systems.
The primary focus of power engineers is to design a system in such a way that all short circuit faults are quickly and accurately detected, and or cleared or reliably remove from the system.
Also see: Zone Overreach and Under reach
Electrical protection devices such as relays, fuses, and circuit breakers are employed for this purpose. Most protective devices detect faults within a certain proximity. Such a limited range is vital since it prevents multiple devices from operating simultaneously. The range within which a device can detect a fault is known as its zone of protection. Power Engineers when devising protection systems are focused to include every portion of the system within a zone of protection. Any portion of the power system that is not included within a protection zone, remains unprotected and a fault happening in such portion will not be cleared.
Practically, it is desirable that protection zones overlap. The overlapping of zones increases the overall reliability of system, since a fault occurring in the protection of the system will be detected by multiple devices. However, it is important that only one device should operate at a time. This can be achieved by varying the time characteristics of protection devices. Another major advantage that comes with overlapping zones is backup protection. The application of multiple devices being aware of the fault, the backup protection is available in case the primary protection device is unable to clear the fault.
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The figure below illustrates the overlapping of Zones on a power system: