What is an auxiliary relay?
An auxiliary relay is a simple electrical device that works like a smart switch in electrical substations. It takes one input signal and creates multiple output signals. Think of it like a light switch that controls not just one light, but many lights and fans at the same time.
What is the function of an auxiliary relay?
The primary function is to multiply control signals and provide electrical isolation. When a protective relay detects a fault, it has limited contacts. The auxiliary relay takes that single signal and distributes it to multiple devices simultaneously—tripping circuit breakers, activating alarms, sending signals to computers, and starting recording equipment. It also protects sensitive electronic equipment from high currents and voltage spikes.
Why Do We Need Auxiliary Relays?
More Contacts: One input controls many outputs. A protective relay with one contact can control 10 different things through an auxiliary relay providing 8-15 contacts.
Equipment Protection: Modern digital relays are expensive and sensitive. Auxiliary relays act as a safety barrier against electrical spikes and high currents.
Heavy Load Handling: Circuit breaker trip coils need high current. Auxiliary relays handle these loads, protecting delicate protective relay contacts.
Contact Flexibility: They offer normally open (NO), normally closed (NC), and changeover contacts for flexible circuit design.
What is the purpose of auxiliary contacts?
Auxiliary contacts serve multiple purposes. They allow one control action to trigger multiple operations simultaneously. When closing a circuit breaker, auxiliary contacts send signals to indicator lights, update computer systems, and enable protection functions at the same time. They provide feedback about equipment status and create safety interlocking conditions that prevent dangerous operations.
What Are the Three Main Types of Relays?
In electrical systems, the three main relay categories are
- Protective Relays: Detect faults like overcurrent or overvoltage and send trip signals.
- Auxiliary Relays: Multiply contacts and provide isolation between protective devices and control circuits.
- Control Relays: Manage operation sequences of motors, contactors, and other equipment.
Types of Auxiliary Relays
Standard Auxiliary Relays
These common types work in 10-15 milliseconds with 4-7 contacts. Electricians use them for everyday signaling, contact multiplication, and control functions in AC Distribution Boards (ACDB) and DC Distribution Boards (DCDB).
Fast Auxiliary Relays
These operate in just 4 milliseconds for critical protection of busbars, generators, and important feeders where every millisecond counts.
High Burden Relays
In long cable runs, electrical noise causes false signals. High burden relays need more energy to operate, ignoring noise while responding to real control signals.
Auxiliary AC Relay
Most substation auxiliary relays use DC voltage from station batteries. However, AC auxiliary relays (110V AC, 220V AC, or 415V AC) are common in industrial plants and motor control centers. They work the same way but connect directly to AC power without needing DC systems. Substations prefer DC types because battery power remains available during AC power failures.
Where Are Auxiliary Relays Used?
In AC Distribution Boards (ACDB)
- Checking incoming power availability
- Automatic switching between main and backup power
- Phase presence indication
- Motor starter and heating equipment control
- Preventing dangerous parallel operation of power sources
In DC Distribution Boards (DCDB)
- Battery charger status monitoring
- Low/high voltage alarms
- Safe DC power distribution to control circuits
- Earth fault warnings
- Fuse failure indication
- Battery bank switching
- Load shedding during low voltage
Circuit Breaker Control
Auxiliary relays receive protective relay signals and use multiple contacts to trip both breaker coils simultaneously, activate alarms, stop auto-reclosing, and send fault information to control rooms.
Safety Interlocking
Auxiliary relays prevent dangerous operations like closing breakers with open disconnectors by checking equipment positions before allowing operations.
How to Test Auxiliary Relays
Before Testing
Check physically for damage, loose screws, or dirt. Ensure secure mounting and tight wires.
Setting Up
Remove the relay from its case. Connect the DC power supply, voltmeter, and timer.
Pickup Test
Start with zero voltage. Slowly increase voltage and record when the relay operates (pickup voltage). Good relays pick up at 70-85% of rated voltage. For a 110V relay, that’s 77V-93V. Measure operating time.
Dropout Test
After pickup, slowly reduce voltage. Record dropout voltage when contacts return to normal. Calculate Reset Ratio:
Reset Ratio = (Dropout Voltage ÷ Pickup Voltage) × 100%
Example: If pickup is 80V and dropout is 40V, reset ratio = 50%. The normal range is 10-60%.
Coil and Contact Checks
Measure coil resistance with a multimeter and compare to specifications. Test each contact with a continuity tester—good contacts show less than 20 milliohms resistance. Record all results, including pickup voltage, dropout voltage, reset ratio, coil resistance, contact resistance, and operating times.
Common Problems and Quick Fixes
Relay Won’t Work: Check voltage at relay terminals. Measure coil resistance. Look for loose connections. Verify correct voltage rating.
Works Sometimes: Clean contacts with contact cleaner spray. Check for binding. Ensure a steady power supply voltage.
Contacts Stick: Add arc suppression devices to inductive loads. Verify contact ratings exceed switching current. Use double contact relays for critical applications.
Slow Operation: Measure voltage during operation for voltage drop. Check spring tension. Verify the correct relay speed type for the application.
What is the Difference Between a Contactor and an Auxiliary Relay?
Contactors switch high-power loads like motors and heaters. They have heavy-duty contacts rated for tens or hundreds of amperes with 3-4 main power contacts plus 2-4 small auxiliary contacts. Their job is power switching.
Auxiliary relays handle control and signaling circuits, not power circuits. They have many contacts (4-15), but each is rated for low current (5-10 amperes). Their job is signal distribution.
Think of it this way: A contactor is a heavy-duty power switch. An auxiliary relay is a signal distributor. Often, an auxiliary relay controls a contactor—the relay provides the signal, and the contactor does the heavy switching.
Choosing the Right Relay
Match coil voltage to your system (48V, 110V, 125V, or 220V DC). Count needed contacts and types. Verify contact ratings exceed switching current. Use general-purpose relays for normal functions and high-speed for critical protection. Consider temperature, humidity, and vibration. Ensure IEC and IEEE standards compliance.
Need Help with Auxiliary Relays?
Testing Issues: Check test equipment connections and voltage polarity. Review manufacturer datasheets for specifications.
Selection Problems: Contact manufacturers with your voltage, contact quantity, environment, and current requirements.
Installation Questions: Follow wiring diagrams carefully. Coil terminals are marked A1/A2. Use proper wire sizes with tight connections.
Troubleshooting: Check supply voltage, measure coil resistance, and test contact continuity. Most problems are loose connections, wrong voltage, or worn contacts.
Conclusion
Auxiliary relays are essential devices in substations and distribution boards. Their reliability, electrical isolation, and simplicity make them irreplaceable even with modern digital technology. For electricians and testing engineers, understanding auxiliary relay selection, testing, and maintenance is a fundamental skill for safe and reliable power systems.