Mastering Protection Relays: Complete Guide for Every Industry, Panel & Use Case
As our homes, industries, buildings, and power grids become smarter and more electrified, the need for safety, uptime, and energy efficiency has never been more urgent. At the heart of this transformation lies an unsung hero: the protection relay.
In this all-in-one guide, we take you from the basics of where protection relays are used, how they work inside electrical panels, how to choose the right type, what accessories support them, and how KEW’s product lineup brings it all together. We’ll explain everything simply – so even someone new to electrical systems can grasp their importance.
Real-Life Applications: Where Are Protection Relays Used?
Protection relays are used anywhere electricity flows — from homes and factories to solar farms and smart cities. Let’s explore some common real-world examples:
Power Distribution (Substations, Transformers): Relays protect transformers and feeders from overcurrent and earth faults. Without them, a single fault can damage equipment or shut down a whole area.
Industrial Plants (Motors & Generators): Factories run heavy motors and generators. If these machines draw too much current or get overheated, protection relays immediately cut power to avoid fires or machine failure.
Commercial Buildings (HVAC, Lifts, Panels): In malls, offices, and hospitals, relays monitor electrical panels. They ensure that equipment like elevators and air conditioners shut off safely during faults.
Renewables (Solar & Wind Inverters): In solar power systems, relays protect inverters from voltage fluctuations or grid faults. They also help synchronize energy with the main grid.
Smart Grids & Automation: Modern grids use intelligent relays with communication ports (like RS485) to connect with BMS/SCADA systems for remote monitoring, logging, and control.
Inside a Panel: Where Exactly Are Protection Relays Placed?
Picture a standard electrical panel. You will see circuit breakers, energy meters, fuses, and wiring. Now, imagine inserting a protection relay into this system. Where does it fit?
Just after the energy meter and before the circuit breaker
It takes input signals from CTs (Current Transformers)
It continuously monitors the current or voltage levels
The moment it detects abnormal behavior (like earth leakage or overcurrent), it sends a trip signal to the breaker to disconnect the load
KEW designs relays that are panel-friendly:
Compact design for DIN-rail mounting
LED indicators to show live status
Test/reset buttons for maintenance
CT/CBCT-compatible for flexible installations
By combining KEW Energy Meters + Protection Relays, you get a complete solution: monitor, measure, protect — all in one synchronized system.
Types of Protection Relays: What Does Each One Do?
Every type of relay serves a different purpose. Let’s break it down:
Earth Leakage Relay (ELR): Detects small current leakages to ground (usually below 1A). These leaks can cause electric shocks or fires. ELRs trip the breaker quickly to stop danger.
Overcurrent Relay (OCR): Trips the circuit if current exceeds a safe limit (e.g., due to short-circuit or motor overload). Used for general protection in panels and feeders.
Voltage Relay: Monitors over-voltage and under-voltage. Trips the system to protect sensitive devices like PLCs, servers, and electronic boards.
Frequency Relay: Detects abnormal frequency conditions in power supply, mostly used in generator and solar systems.
Reverse Power Relay: Prevents generator/motor from backfeeding into the grid. Used in solar-diesel hybrid systems.
Accessories That Make the Relay Work
A protection relay alone cannot operate. It needs input & output accessories to sense and act. Here’s what completes the system:
CTs (Current Transformers): Reduce large currents to small, measurable signals (e.g., 100A to 5A) so relays can read them.
CBCT (Core Balance CT): Detects earth leakage current by summing all conductor currents. Required for ELR operation.
Trip Coil: Electromagnetic device fitted in the breaker. Gets signal from relay and physically opens the circuit.
LED Indicators / Buzzers: Visual or audible alerts during faults. Mounted on panels.
RS-485 / Modbus Communication Module: Enables remote monitoring and control via SCADA or BMS.
What Are CTs (Current Transformers)? - Turning High Currents into Safe Signals
Current Transformers, or CTs, are essential for monitoring large electrical currents safely. Imagine trying to directly measure 1000 amps with sensitive electronics — you’d destroy them. CTs scale these large values down to 5A or 1A, making them usable by protection relays and meters.
How CTs Work – The Step-Down Magic
CTs act like transformers: the main conductor (carrying 100A, 500A, etc.) is the primary.
The CT’s internal coils generate a proportional secondary current (e.g., 5A).
This scaled-down signal is sent to the protection relay, which uses it to monitor and take action.
Where to Install & How to Wire CTs
Installed around the phase conductor.
The secondary terminals (S1, S2) connect to the relay input.
Important: Never leave the CT output open while energized — it can generate dangerous voltages.
How Relays Use CT Signals
You configure the CT ratio in the relay (e.g., 500:5).
The relay internally calculates actual current.
If the measured current exceeds set limits (e.g., 120%), the relay sends a trip signal to shut the system.
Where CTs Are Used
Overcurrent Protection (OCR)
Motor overload detection
Feeder circuits in panels
All general current sensing applications
What is CBCT (Core Balance Current Transformer)? - The Brain Behind Earth Leakage Detection
CBCTs are specialized transformers used for detecting earth leakage currents — the tiny, dangerous currents that flow to ground during insulation failures or accidental contact.
CBCT Principle — How It Spots a Fault You Can’t See
All active conductors (R, Y, B, Neutral) pass through one circular CBCT core.
In healthy circuits, the sum of all currents = 0.
If there’s a leakage (say 300mA to earth), this balance breaks.
The CBCT detects this unbalanced current and sends a signal to the Earth Leakage Relay (ELR).
Why CBCT is Different from CT
| Feature | CT | CBCT |
|---|---|---|
| Senses | Line Current | Leakage Current |
| Used with | OCR, Meters | ELR |
| Input | Single conductor | All phase conductors + neutral |
| Output Trigger | Overload/Short circuit | Earth fault |
How to Wire CBCT with Earth Leakage Relay
CBCT secondary output is connected to the ELR’s ‘I+’ and ‘I–’ terminals.
Relay monitors this input continuously.
When leakage current > set threshold (e.g., 100mA), relay trips.
CBCT Settings in the ELR
Set Leakage Current Threshold: (e.g., 30mA for human safety or 300mA for fire protection).
Set Time Delay: (0.3–2 sec depending on sensitivity).
Configure Reset Logic: Manual or Auto reset.
Where CBCTs Are Used
ELRs in industrial and commercial panels
Solar inverter panels for DC/AC earth fault detection
Motor control centers where insulation failures are common
Hospitals, malls, residential towers for human safety
Field Tips for Electricians & Panel Designers
Always match CT/CBCT ratings with your load and relay range.
Never keep CT secondary open — short it during maintenance.
Use shielded cables for CBCT wiring to avoid false trips due to noise.
KEW relays are compatible with both CTs and CBCTs and allow easy parameter programming from the front panel or remotely via Modbus.
How to Select the Right Protection Relay?
Not sure which relay to use? Follow this step-by-step method:
Identify Load Type: Motor, inverter, transformer, general feeder?
Determine Fault Type: Earth leakage? Overcurrent? Overvoltage?
Choose Voltage Class: Is it 230V, 415V, or 11kV?
Time Delay Needed? Instant trip or delayed response?
Communication? Need RS485/Modbus?
Space Limitations? Choose compact relays if panel space is tight.
Conclusion: KEW’s Promise for Safer, Smarter Systems
Protection relays are the foundation of modern electrical safety. They don’t just save machines — they save lives, reduce downtime, and protect your investments. With KEW’s future-ready range of relays, meters, and accessories, you get more than just a product — you get a full protection ecosystem.
