Protection Grading and Discrimination Studies


Power Testing Ltd (PTL) are undertaking works for several large industrial sites to deliver power system reinforcement and upgrades.  A component of these works requires studying the existing system and protection scheme.  PSC have carried out these system studies, reviewed the existing protection scheme and recommended updated settings where required.


During fault conditions, protection schemes rely on suitable grading margin to ensure coordination and discrimination between different devices.  If incorrect settings are applied the consequences can range from nuisance tripping of circuit breakers to unsafe operating conditions due to failure to operate.  Suitable grading margin has to be achieved between all protection devices from the Distribution Network Operator (DNO) to the LV distribution boards taking into consideration the expected short circuit fault currents, the rated system currents and the minimum allowable grading margins between relays.


PSC developed system models for the industrial units and performed load flow and fault studies to determine the rated currents and short circuit fault currents along each potential fault path.  Existing protective devices and protection settings were collected​ during a site audit and PSC developed appropriate relay models for protection studies in either PowerFactory or AmTech ProtechHV.

PSC reviewed the time-overcurrent curves for the existing settings indicated some insufficient grading margins and therefore potential for protection mal-operation. Alternative protection settings were proposed to ensure suitable discrimination between all protection devices for different operating scenarios and fault conditions.

time-overcurrent curves


The assessment identified the cases where there were inadequate protection settings and indicated the potential risks during fault conditions. The revised settings ensured minimum protection operating times, reducing risk to equipment and personnel whilst also maintaining having sufficient discrimination to maintain site operations.  The new protection settings and time-overcurrent curves demonstrated that safe discrimination exists in the complete range of short circuit currents for overcurrent and earth fault events expected at the site.


Transmission and Distribution



PSC have completed a number of detailed reports for PTL and always in agreed time scales. All PSC staff are extremely helpful and flexible which ensures PTL offer our end users accurate and detailed reports. It is a pleasure working with such competent designers.

Chris Williamson

Head of Projects, Power Testing Ltd