What is the meaning of Discrimination (Selectivity) in Electrical Systems, and what are its types?

Selectivity in Electrical Systems:

In addition to the common understanding of discrimination in the social and workplace context, the concept of selectivity also plays a crucial role in electrical systems, particularly with circuit breakers. There are two main aspects to consider in electrical discrimination: current discrimination and time discrimination.

Current Discrimination:

• Total Discrimination: In electrical terms, total discrimination occurs when a circuit breaker, let's call it D2, opens for all fault current values, from overloads up to the non-resistive short circuit current, while another breaker, D1, remains closed.
• Partial Discrimination: Partial discrimination is observed when the above condition is not respected up to the full short circuit current but only to a lesser value known as the selectivity limit currents.
• No Discrimination: In the event of a fault, both circuit breakers D1 and D2 open, indicating a lack of discrimination.

Time Discrimination:

In addition to current discrimination, time discrimination is another vital aspect of electrical systems. It involves the precise coordination of response times between different circuit breakers to ensure that the closest breaker to the fault responds faster, minimizing nuisance tripping and improving system reliability.

Time discrimination operates as follows:

• Circuit breaker D2, for example, is set to respond faster than D1. In the event of a fault, D2 will trip before D1, isolating the fault closer to the source while D1 remains operational.
• This coordination of response times is crucial in preventing unnecessary disruptions in power distribution. For example, during a temporary overload, only the closest breaker will respond, allowing the system to continue functioning without interruptions.

The significance of time discrimination cannot be understated. It plays a critical role in maintaining a stable power supply, reducing downtime, and preventing power outages. When a well-designed time discrimination system is in place, it enhances system reliability and minimizes the impact of transient faults.

Conclusion:

Total and partial discrimination, as well as time discrimination, are integral to preventing system-wide disruptions. Achieving discrimination, however, is not without its challenges, and it requires meticulous planning, equipment selection, and ongoing maintenance.