The Vital Role of Relays in HT Panels

In high-tension (HT) power systems, relays are critical components that prevent disasters, such as fires, and ensure the safety of electrical panels. While standard relays like overcurrent, overvoltage, and earth fault devices are widely used, several specialized relays provide vital protection. Below are key devices and their functions:

Self-powered IDMT Earth Fault Relay (51): Trips based on an inverse time characteristic, where trip time decreases as fault current increases, preventing further damage.

Anti-pumping Relay (94 or 52): This relay prevents excessive breaker switching, which can damage equipment, by ensuring that breakers only operate when necessary.

Lockout Relay (86): This relay locks the system out of service after a fault, preventing unsafe restoration before inspection and repair.

Trip Circuit Supervision Relay (74): This relay monitors critical components like auxiliary power sources, sending alerts in case of failure to avoid catastrophic incidents.

Phase Failure/Phase Reversal Relay (47): Detects phase failures or reversals and trips the power source to protect equipment.

Under/Over Voltage Relay (27 & 59): This relay protects the system from voltage fluctuations and tripping circuits if the voltage deviates outside a specified range.

Master Trip Relay (86): Consolidates multiple protective signals to trigger a coordinated shutdown, preventing further damage.

For these relays to function effectively, the correct current transformers (CTs) and potential transformers (PTs) must match the application’s specifications.

A recent review of HT breaker panels revealed the absence of some critical relays. This was promptly addressed, underscoring the importance of comprehensive safety measures. Neglecting these safety features exposes systems to vulnerabilities.

As cities expand and energy demand increases, the safety of electrical systems is paramount. Relays, often unnoticed, are essential in maintaining the reliability and safety of power systems. Proper installation and maintenance are crucial for protecting infrastructure and the millions relying on it.

(Note: ANSI codes in parentheses refer to respective relays.)