Most protective relays are electromechanical, which work by electromagnetic attraction and induction. Overcurrent Relays are a subcategory of protective relays. : 4 The first protective relays were electromagnetic devices, relying on coils operating on moving parts to provide detection of abnormal operating conditions such as. Protective Relay Definition: A protective relay is an automatic device that senses abnormal conditions in electrical circuits and triggers actions to isolate faults. SSR) or their specific function (Time, Protection, or Signal). This prevents damage to equipment, reduces downtime, and safeguards. Protective relays can be classified based on their operating principle, construction, or function: 1. SEL time-domain technology.