Protection relays are the "decision makers" of an electrical protection scheme — they continuously monitor current, voltage, or other electrical quantities and issue a trip command to the associated circuit breaker when a fault condition is detected. Choosing the right relay type for each protection zone is essential for both equipment safety and supply continuity.
Overcurrent Relays (50/51)
The most common and simplest protection type — an overcurrent relay trips when current exceeds a set threshold, either instantaneously (ANSI code 50) for high fault currents or with a time delay (ANSI code 51, often using an Inverse Definite Minimum Time curve) to allow downstream devices to clear the fault first. Used almost everywhere as a baseline protection layer — feeders, transformers, motors, and cables.
Earth Fault Relays (50N/51N)
A specialized overcurrent relay that monitors the residual (earth fault) current rather than phase current, detecting insulation breakdowns or faults to earth that a standard phase overcurrent relay might miss, especially in systems with high-resistance grounding.
Differential Relays (87)
Differential protection compares current entering and leaving a protected zone (a transformer, generator, busbar, or feeder) — under normal conditions these currents are equal (accounting for turns ratio), but a fault inside the zone causes a mismatch that the relay detects almost instantaneously. This makes differential protection extremely fast and highly selective, since it only responds to faults within its own zone, not external ones.
Distance Relays (21)
Used primarily on transmission and sub-transmission lines, distance relays measure the impedance between the relay location and the fault point (which is proportional to distance along the line) to determine whether a fault falls within its protection zone. Distance protection is typically set up in zones (Zone 1, 2, 3) with increasing reach and time delay, providing both primary and backup protection along a line.
Directional Relays (67)
Used where fault current can flow in either direction (e.g. ring-main or parallel-feeder systems), directional relays add a directional element to overcurrent protection so the relay only trips for faults in a specified direction — critical for correct coordination in meshed or looped networks.
Numerical (Digital) Relays
Modern installations almost universally use numerical relays — microprocessor-based devices that can combine multiple protection functions (overcurrent, earth fault, differential, etc.) in a single unit, along with event recording, communication protocols (IEC 61850, Modbus), and remote monitoring — replacing what used to require several discrete electromechanical relays.
TEKTOW supplies protection relays from major manufacturers along with CTs/PTs to match. Get in touch with your protection scheme requirements for a quote.