Circuit breakers are safety devices in our power system. They perform key functions of tripping during any fault. So keeping in view their functions, we must observe and test in regular intervals for their long-term operation in the power system to keep them working in safer mode. In this article, we are going to discuss an insulation resistance value test and how we can perform it.
IR Value Test in a Circuit Breaker:
⚙️ Purpose of IR Test
- To check the health of insulation in a circuit breaker.
- To detect insulation degradation due to moisture, contamination, age, or thermal stress.
- To prevent flashovers, equipment failure, or electrical hazards.
There are different types of circuit breakers used in our power distribution grids for protection purposes:
- SF6
- Vacuum Circuit Breaker
- Air Circuit Breaker
We apply insulation resistance test on all types of circuit breakers in two conditions:
- Open Condition
- Closed Condition
In the circuit breaker, one side (Red, Yellow, blue phase) of the circuit breaker is connected to the incoming from the bus bar, and a second side ((Red, Yellow, blue phase) is connected to the motor or feeder directly.
So in case of open condition, the circuit bear is in the off state. In this condition, contactors in the circuit breaker will remain in the open state.
On the other hand, in the closed condition, the contactors in the circuit breaker will remain closed and we can say the circuit breaker is in an ON state.
Voltage Level for Different Circuit Breakers
According to the standard value, we need the value of DC voltage from the IR Testing Kit to apply to different capacities of circuit breakers. Here is a list of different circuit breakers’ capacity voltages and Megger Set voltages.
| Serial No. | Circuit Breaker Voltage Level | Set Voltage |
| 1 | 650 | 500V DC |
| 2 | 1.1kV | 1kV DC |
| 3 | 3.3kV | 2.5kV DC |
| 4 | 6.6Kv and above | 5kV DC |
It means that if we have a 6.6kV circuit breaker, we can apply 5kV DC from the Megger Set to test the insulation resistance. These are standard values and similarly, the concept goes for other voltage cases.
Open Condition in Circuit Breaker
In case of the open condition, we will connect the positive probe of the Megger Set to any one of the incoming phases. Similarly, we will connect the negative probe of the Meggar set to the outgaining phase (the same outgoing phase as the incoming phase).
You can understand the above concept by following the table indication:
| Sr. No. | Incoming Pole-Outgoing Pole | Megger Set IR Value (MΩ) |
| 1 | R-R | From the Megger Output result |
| 2 | Y-Y | From the Megger Output result |
| 3 | B-B | From the Megger Output result |
Here:
R = Red Phase (incoming pole and outgoing pole)
Y = Yellow Phase (incoming pole and outgoing pole)
B = Blue Phase (incoming pole and outgoing pole)
Following is the picture describing the same concept using incoming and outgoing poles in our circuit breaker in case of open condition:
We will assess whether particular pole contacts are in an apart condition.
| Equipment Voltage Level | Acceptable IR Value (General Guideline) |
|---|---|
| LV (<1kV) | > 1 MΩ |
| MV (1kV–33kV) | > 100 MΩ |
| HV (>33kV) | > 200 MΩ – 1000 MΩ (or as per standard) |
Key Tips While Performing IR Test:
Here are the following key points that one must consider while performing the IR Test:
- You must apply appropriate voltage according to the industry standard.
- Compare measure value with standard value to check the required results.
- You can measure the IR value after making the circuit off and getting proper Line Clear from the appropriate authority.
⚠️ Important Considerations
- Temperature and humidity affect IR readings (higher temperature or humidity = lower resistance).
- Allow charging time when testing high-capacitance equipment.
- Perform the test periodically for preventive maintenance.
- After testing, discharge the breaker insulation before reconnecting.
📘 Standards and References
- IEEE Std 43: Guide for Testing Insulation Resistance
- IEC 60060: High-voltage Test Techniques
- NETA MTS: Maintenance Testing Specifications
Conclusion:
The insulation resistance test is important in a circuit breaker to detect the flow of electric current in conductive parts of a circuit breaker. It helps us to measure the leakage current possibilities. In the article, we have studied to procedure of the IR value test using the Megger Set.
