The grounding resistance of rural distribution transformers is frequently overlooked, yet it plays a critical role in ensuring their safe operation. Generally speaking, a lower grounding resistance is preferable, as it minimizes risks associated with electrical faults. For transformers rated at 100 kVA or higher, the grounding resistance should ideally remain under 4 ohms. Transformers below 100 kVA should have a grounding resistance of no more than 10 ohms. However, over time, grounding poles buried underground can degrade due to soil erosion, leading to an increase in resistance. Consequently, it's essential to measure the grounding resistance of transformers periodically to ensure compliance.
In rural areas, the most common method used to measure the grounding resistance of transformers involves an insulation resistance meter. Here’s a detailed description of the process:
**Preparation Before Measurement**
1. **Weather Conditions**: Always perform the measurement during dry weather to avoid inaccuracies caused by moisture.
2. **Safety Measures**: Prior to any measurements, ensure all safety protocols are followed, including disconnecting the transformer from its grounding pole.
3. **Grounding Pin Placement**: Insert two grounding pins, each at least 500 mm long, into the ground at depths of no less than 400 mm. Ensure these pins lie along the same straight line, spaced approximately 20 meters apart. Attach the E, P, and C terminals on the insulation resistance meter to the transformer’s grounding pole and the two grounding pins using specialized wires. The point P should be positioned between the second grounding pin and the transformer’s grounding pole.
**Measurement Process**
1. **Initial Setup**: Position the insulation resistance meter horizontally and adjust the zero knob until the needle rests at zero.
2. **Adjustment and Calibration**: Based on the required grounding resistance and current field conditions, select the appropriate gear on the override switch. Begin turning the meter slowly while adjusting the potentiometer dial to stabilize the needle at zero. Once steady, accelerate to 120 revolutions per minute (rpm), continuing to fine-tune the dial until the pointer stabilizes again. Multiply the dial reading by the magnification factor to determine the transformer’s grounding resistance.
3. **Troubleshooting Higher Resistance**: If the measured value exceeds the required threshold, investigate and address the issue promptly to bring it within acceptable limits.
4. **Handling Lower Resistance Values**: When the measurement falls below 1 ohm, open the connection points on the insulation resistance meter and directly connect the wires to the grounding pole to rule out the effects of wiring and contact resistance.
By following these steps, you can accurately assess the grounding resistance of rural distribution transformers, ensuring both operational safety and regulatory compliance.
2.54mm Pitch
2.54mm Pitch
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