Reliable protection systems are integral to electricity networks whether they be within industrial, distribution or transmission networks. The requirements of safety, minimisation of damage to faulted plant, minimisation of consequential damage to healthy plant and maintenance of a stable and secure supply of electricity are paramount to all protection designs. A reliable protection scheme needs to be both dependable and secure. A dependable scheme thus has a very high level of certainty of operating correctly for all faults within its zone of coverage. Conversely, a secure scheme has a very high level of certainty of not operating incorrectly under any circumstances, especially in the presence of faults and disturbances external to its zone of coverage. These two requirements of dependability and security are thus conflicting.
To meet both of these requirements, the protection systems need, simultaneously, to be coordinated, fast operating, secure and dependable. The provision of these conflicting requirements is fundamental to the National Electricity Rules (NER) which regulates the performance of protection systems and ensures inter-regional power flows are not constrained and that power system stability is maintained. Thus the task, not only of the protection engineer, but of all engineers and technicians associated with power systems, is to ensure that planning, design, coordination, implementation, commissioning, operation and maintenance of protection systems optimises both the dependability and security of these schemes.