To understand how different protective relays work, it's essential to know these terms. Key terms include: Pick up current. Inverse time delay, on the other hand, depends on the current magnitude so, the higher the current, the shorter the delay. A busbar in a single line diagram and. Protective Relays - Technical Seminar Nov 2016 - Copyright: IEEE 2 Abstract: Protective relays and devices have been developed over 100 years ago to provide “lastline”of defense for the electrical systems. ) based on operating parameter, definite time, inverse time, stepped etc. The rectangular devices are test connection blocks, used for testing and isolation of instrument transformer circuits.