A discrimination study in an electrical system is an analysis aimed at ensuring that protective devices (such as circuit breakers, fuses, and protection relays) operate in a coordinated manner. The main objective of this engineering work is to ensure that in the event of a fault the protective device closest to the fault will operate first. This means that it isolates the smallest part of the electrical system as possible, only the affected section of the system and it can leave the rest of the electrical installation online. By doing this it minimizes disruption as the fewest number of circuits go without power.
What is the aim of a discrimination study?
It is aiming to help design an electrical system where only the device nearest to the fault opens. If multiple devices open, then it should be designed so that it has the smallest effect on the system as possible. The design should prevent upstream devices from tripping unnecessarily.
What information is required for a discrimination study? And how is it carried out?
The information required for us to complete a discrimination study include information about the electrical system, including single-line diagrams, load details, and protective device specifications such as current ratings and part numbers.
We the produce a model of the electrical system using specialized software. The software is programmed with part numbers and circuit breaker overload and short circuit curves and these can be analysed by one of our engineers.
After we have received the information and inputted the characteristics, we can analyse the system with the electrical software.
This ensures that devices operation time can be compared against the current specifications. The types of protective devices that are put in to the software include circuit breakers, fuses, protection relays, and conductors which may have a fault current withstand value that can be evaluated, such as cable. After the analysis is completed, a report is provided to the customer with the overlapping curves showing the discrimination of the system, and recommendations can be made to improve its selectivity.
Benefits of Discrimination and Selectivity
As required by AS3000, the electrical system should accommodate discrimination as far as practical, especially for essential loads.
Having an adequately selective system improved reliability as it ensures that only the affected part of the system is isolated during a fault and minimizes downtime and maintains system integrity. It prevents potentially unnecessary tripping and wear on upstream devices to extend their life by reducing stress and operational frequency. A discrimination study is crucial for the effective and safe operation of electrical systems, particularly in complex installations like industrial plants, commercial buildings, and utility networks.
What is the difference between selectivity and discrimination?
