The Unnoticed Relays That Protect Everything

In high-tension (HT) electrical systems, failure is rarely dramatic at the start.

There is no immediate explosion, no visible warning. What exists first is something far quieter — an unprotected condition, an unchecked assumption, a missing layer of design thinking.

When a fault occurs, the outcome depends less on the size of the equipment and more on the design. In our experience of reviewing HT panels across industrial and commercial installations, the most common risk noticed is incomplete protection philosophy, right from the design phase.

At the centre of those decisions are protection relays.

They do not carry power.

They do not attract attention.

Yet they decide whether a fault is contained, escalated, or catastrophic.

Why “Basic Protection” Is Rarely Enough

Most HT panels include standard overcurrent, earth fault, and overvoltage protection. While these are essential, they represent only the minimum requirement. They do not curb anomalies.

A robust protection philosophy anticipates how systems behave under stress — during abnormal voltage conditions, phase disturbances, control power failures, or repeated breaker operations. This is where specialised relays play a decisive role.

In this blog we touch-base upon 7 key relays that define a safe HT system.

Self-Powered IDMT Earth Fault Relay (ANSI 51)

This relay operates on an inverse definite minimum time (IDMT) characteristic. In simple terms, the higher the earth fault current, the faster the relay trips. Low-level faults allow longer clearing times, while severe faults are isolated rapidly.

The advantage lies in selectivity and damage control. By coordinating trip time with fault severity, the relay prevents unnecessary outages while ensuring serious faults do not escalate into cable damage, insulation failure, or fire.

Being self-powered, it also remains functional even when auxiliary supplies are compromised.

Anti-Pumping Relay (ANSI 94 or 52)

During abnormal conditions, breakers can receive repeated close and trip commands due to control logic errors or unstable signals. This repeated operation known as pumping, causes mechanical wear, contact damage, and premature breaker failure.

The anti-pumping relay prevents this by ensuring the breaker completes one full open-close cycle before responding to another command. It protects the breaker not from electrical faults, but from operational cruelty.

This relay significantly extends breaker life and prevents hidden mechanical degradation.

Lockout Relay (ANSI 86)

The lockout relay is a critical safety device. Once it operates, it locks the system out of service and requires manual intervention for restoration.

Its purpose is to prevent unsafe re-energisation after a fault. By forcing inspection and corrective action, the relay ensures faults are not reset blindly or reintroduced into the system.

In environments where continuity pressure is high, the lockout relay acts as a disciplined barrier.

Trip Circuit Supervision Relay (ANSI 74)

A protection system is only effective if it can trip when required. The trip circuit supervision relay continuously monitors the health of the tripping path — auxiliary supply availability, wiring continuity, and trip coil condition.

If any part of the circuit fails, the relay raises an alarm before a fault occurs. This proactive monitoring prevents situations where a fault is detected but the breaker fails to open due to control circuit failure.

It activates before any scenario that can lead to catastrophic outcomes.

Phase Failure / Phase Reversal Relay (ANSI 47)

Phase abnormalities are among the most damaging and least visible electrical issues. Loss of a phase or phase sequence reversal can cause motors to overheat, rotate incorrectly, or suffer mechanical stress.

The phase failure / phase reversal relay detects these conditions and trips the supply. It is particularly critical for rotating equipment, compressors, pumps, and HVAC systems, where damage may accumulate silently over time.

Under Voltage and Over Voltage Relays (ANSI 27 & 59)

Voltage outside permissible limits stresses insulation, disrupts control systems, and increases the likelihood of nuisance tripping or equipment degradation.

Under and overvoltage relays monitor supply conditions and isolate the system when voltage deviates beyond defined thresholds. This protects both power equipment and sensitive downstream loads from prolonged exposure to unstable supply conditions.

Master Trip Relay (ANSI 86)

The master trip relay acts as the central decision point in the protection scheme. It receives signals from multiple protective devices and initiates a coordinated shutdown.

Instead of fragmented responses, the system trips decisively and predictably. This coordination prevents partial shutdowns, cascading faults, and ambiguous system states during emergencies.

The Often-Ignored Foundation: CTs and PTs

Even the most advanced relay scheme fails if current transformers (CTs) and potential transformers (PTs) are incorrectly selected.

CTs must be chosen based on fault levels, accuracy class, and burden to ensure relays sense faults accurately. PTs must match voltage levels and protection requirements to provide reliable input under both normal and fault conditions.

Mismatched CTs or PTs create false confidence — relays appear installed, but their decisions are compromised.

A Real-World Design Insight

During a recent HT breaker panel review done by Qpro, several of these critical relays were found missing. The system was operational and assumed safe. In reality, it was exposed to avoidable risks that would only surface during a fault event.

Protection gaps rarely reveal themselves during normal operation. They reveal themselves when it is too late.

Designing for Reliability, Not Assumptions

As cities expand and electrical loads increase, HT systems operate closer to their limits. In this environment, safety is no longer about installing larger equipment—it is about designing smarter protection systems.

At Qpro, protection design is treated as a system behaviour exercise, informed by years of real-world experience. Every relay, transformer, and logic path is chosen to answer the question:

How will the system behave when something goes wrong?

If that question has not been revisited in your HT design, it may be time to look deeper — not to add equipment, but to add clarity.

If you wish to ensure, your designs think right and they do it with clarity — reach-out to explore how a protection-first design review can reduce your risk exposure before they become incidents.