29 Sep-2026

During National Day duty, equipment alarms often come at the worst possible time — before shift handover, in the middle of the night, or when staffing is at its lowest. When the backstage system suddenly reports an abnormal signal, the duty officer's first instinct is to report it to the dispatcher. But practical experience shows that a considerable number of "abnormalities" are not equipment faults at all, but problems in the measurement process. When data is abnormal, retest and investigate first, then decide whether to report. This can avoid a large number of false alarms.

The following summarizes the three most commonly used troubleshooting actions during duty inspections — measuring insulation, checking grounding, and measuring voltage and current — as well as the most common pitfalls on site.

I. Insulation Testing: A Low Megohmmeter Reading Does Not Necessarily Mean the Equipment Is Faulty

When duty personnel encounter abnormalities in motors, cables, or transformers, the first reaction is to test insulation. This is correct, but three things must be done before testing. Missing any one of them will make the reading inaccurate.

Power off, verify de-energized, discharge. If discharge is not thorough, the megohmmeter reading will be too high. More dangerously, residual charge can damage equipment and injure personnel. Testing directly after verifying de-energized, or removing wires without discharging after testing, is a serious violation.

Disconnect other circuits connected to the equipment under test. Testing without disconnecting will cause parallel circuits to pull down the reading. If the measured value is incorrect, do not rush to judge that the equipment is faulty. First check whether there is a parallel branch.

Clean dirt from the surface of the terminals under test. Surface leakage current directly affects the reading, especially in humid weather. If the terminal surface has dust or oil, a low reading does not mean the insulation has actually deteriorated.

How to interpret the results? Do not look at a single reading alone. Compare with historical data and with similar equipment. For example, for the same 10kV cable, if it measured 500MΩ last year and 50MΩ this year, although 50MΩ is still within the acceptable range, the downward trend is obvious and should be noted. If the reading is 0 or close to 0, first check whether the circuit is short-circuited. Do not rush to conclude that the insulation has broken down.

In addition, in humid weather, the reading will be low if shielding is not used. Using the shield terminal of the megohmmeter to eliminate the effect of surface leakage can rule out most weather interference.

II. Grounding Check: If Grounding Resistance Exceeds the Limit, Check the Connection Joints First, Then the Grounding Body

Grounding resistance exceeding the limit is the root cause of many "unfindable but persistent" abnormalities. The standard generally requires less than 4Ω, but what really matters on site is not the number itself, but why it exceeds the limit.

The most common cause of exceeding the limit is not corrosion of the grounding body, but loose or rusted grounding flat steel connection joints. Especially at bolted compression joints, oxidation and loosening over time increase contact resistance. In this case, excavation is not needed. Disassemble the joint, grind it, and retighten it, and the resistance will drop. Prioritize checking connection points, then the grounding body. Only after all connection points have been checked and confirmed to be problem-free should the grounding body itself be considered.

There are two common mistakes during measurement. The four-wire method and the clamp method are suitable for different scenarios. The four-wire method is suitable for independent grounding bodies, while the clamp method is suitable for multi-point grounding systems. Choosing the wrong method can cause large reading deviations. Some duty personnel use a clamp-on grounding resistance tester to measure an independent grounding electrode. The reading may look normal, but it is actually completely unreliable. Test leads that are too long or tangled will affect the measurement result. Try to keep them straight and avoid running them parallel to live conductors. Grounding resistance changes with seasons. In dry autumn and winter, readings tend to be higher. When judging, combine with the seasonal coefficient. Do not panic just because it exceeds the limit.

III. Voltage and Current Measurement: If the Multimeter Reading Fluctuates, Suspect Contact First, Then the Equipment

The multimeter is a must-have tool for duty personnel. Measuring voltage to determine whether a circuit is live and measuring current to determine whether three phases are balanced are basic actions. But when readings are abnormal, a considerable portion is a measurement problem, not an equipment problem.

Whether the test probes are in good contact. Oxidized terminals or loose probes can cause fluctuating readings. As in the example at the beginning, retesting at a different position solved the problem. When readings fluctuate high and low, do not rush to report. Press the probes firmly again and retest at a different measuring point.

Whether the range is selected correctly. Using a high range to measure a small voltage gives insufficient accuracy; using a low range to measure a high voltage may damage the instrument. Especially when measuring DC voltage, the wrong range causes large reading deviations.

The effect of multimeter input impedance. When measuring voltage in high-impedance circuits, an ordinary multimeter does not have high enough input impedance, and the reading will be low. In this case, a high-impedance voltmeter is needed. If the measured voltage does not match the theoretical value, first confirm whether it is a meter problem.

Clamp meters have large errors when measuring small currents. Try to place the conductor in the center of the clamp jaws to avoid surrounding magnetic field interference. If the measured three-phase current is unbalanced, first check whether the clamp jaws are properly closed, then determine whether it is truly unbalanced load.

When readings are abnormal, retest first — change to another meter, change to another position. If the data is still abnormal, then draw a conclusion.

IV. For Uncertain Data, Call the Technical Duty Phone Directly

During holiday duty, if test data is uncertain or you are unsure whether an equipment abnormality should be reported, you can directly contact a technical engineer for online assistance.

Technical Duty Phone: 027-86861986 (Wuhan KEDE Zhongwei Electric Co., Ltd.)

Working days 8:30–17:30, duty personnel are on duty during holidays.

Wuhan KEDE Zhongwei Electric Co., Ltd.
September 29, 2026