Discrimination – Don’t Leave it too Late!
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Discrimination – Don’t Leave it too Late!


Discrimination – Don’t Leave it too Late!

Discrimination, or “protection system coordination” is an important aspect of an electrical system design that is often overlooked. The ideal time to complete a discrimination study is before ordering the switchboard and panel board circuit breakers. However the switchboards and panelboards are generally selected based on cost with no regard to the circuit breaker characteristics. Accordingly discrimination problems generally get solved retroactively rather than pro-actively.

If all parties involved with the electrical system design and implementation are cognizant of potential discrimination problems, and review their work with this in mind, most discrimination problems can be overcome with minimum impact on either schedules or equipment costs. The intent of the discrimination study is to demonstrate that:

  • Electrical equipment is satisfactorily protected by the system protection equipment (circuit breakers, relays etc)
  • The protective devices have compatible characteristics to ensure that protective equipment closest to a fault, operate first
  • Electrical equipment not involved with the fault, remain in service.

    There is an argument that the switchboard manufacturer is in the best position to undertake the discrimination study. However as the work involves the complete electrical system, it can equally be argued that it must be addressed by the overall electrical systems design engineer. There is no doubt that the switchboard manufacturer and also the panel board supplier must be aware of the discrimination requirements, however the systems designer is responsible for the overall protective features of the system so he should coordinate the work or at least, approve the study results. The discrimination study is closely related to the system short-circuit study and electrical load analysis.

    The load analysis is used to determine how many generators and what equipment is running under specific operating conditions. The short-circuit study is used to determine the level of short-circuit current at specific parts of the electrical system. Information from these studies must be considered when addressing discrimination or considering the relevance of a lack of discrimination. It should be understood that most methods of short-circuit current calculation over estimate the probable short-circuit current and this may lead to false conclusions being drawn from the discrimination curves.


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