How to check if wires and cables have damaged insulation layers?

I. Basic Method: Visual and Tactile Inspection (Suitable for Obvious Damage)

This is the most straightforward inspection method, requiring no tools, and focuses primarily on any visual or tactile abnormalities in the insulation layer.

II. Advanced Method: Auxiliary Tool Detection (Suitable for Hidden Damage)

For minor damages that are difficult to detect with the naked eye (such as cracks inside the insulation layer), simple tools must be used to check whether the insulation performance has deteriorated.

III. Key Inspection Areas (High-Frequency Damage Zones)

Insulation layer damage is often concentrated in areas subject to stress, heat, or external interference—key areas that require careful inspection.

Check for appearance abnormalities

Observe whether there are any obvious cracks, notches, or flaking—especially in areas that have been bent for long periods or are located near heat sources (such as the base of plugs or cable bending points), where the insulation layer is prone to cracking due to aging or friction.

Check for discoloration, bulging, or softening: High temperatures, overload conditions, or chemical corrosion can cause the insulation layer to discolor (such as turning black or yellow), bulge (due to internal heat expansion), or feel softened—these are all signs of impending damage.

Pay attention to any foreign object punctures or bite marks: Check the cable surface for traces of sharp objects (such as nails or wire) piercing through, or for gnawing marks left by rodents or insects—these can directly damage the insulation layer.

Tactile Sensory Abnormality

Gently touch the insulation layer with your hand to feel for any bumps, dents, or hard spots: A bump might indicate that an internal conductor has shifted and pushed through the insulation, a hard spot could suggest localized aging and hardening, while a dent may result from external pressure causing damage.

Check the connection between the plug and cable: Gently tug on the cable. If the insulation layer at the base of the plug is loose, peeling off, or if you can see the internal conductors, it indicates that this area has been damaged.

Use a megohmmeter (shake meter) to test insulation resistance.

Applicable scenarios: When suspecting hidden damage to cable insulation (e.g., cables exposed to long-term humid environments), or when verifying the insulation performance of aging cables.

Operating steps:

Disconnect the power at both ends of the cable and ensure there is no voltage remaining.

Connect the "L" terminal of the megohmmeter to the cable conductor, and the "E" terminal to the outer layer of the cable insulation (or the armor layer).

Slowly and steadily turn the megohmmeter (at approximately 120 rpm), then read the insulation resistance value.

Judgment criteria: The insulation resistance of standard low-voltage cables (such as 450/750V) should be ≥0.5 MΩ. If the value is significantly lower than this, it indicates that the insulation layer is damaged or aged, making the cable unsuitable for normal use.

Use a voltage tester or multimeter to detect electrical leakage.

Applicable scenario: When suspecting minor cable damage causing electric leakage (e.g., feeling a slight tingling sensation when touching the device).

Operating steps (voltage tester): Power on the cable, then touch the surface of the cable's insulation layer with the voltage tester. If the tester lights up, it indicates that the insulation layer is damaged and there is an electrical leakage.

Operating steps (multimeter): Set the multimeter to the "AC voltage" range (e.g., 250V scale). Connect one probe to the cable's insulation layer and the other probe to ground (or to the equipment's metal casing). If the display shows a voltage reading (e.g., tens of volts), it indicates that leakage current is present, meaning the insulation layer has been damaged.

Plug/joint base: Frequent plugging and unplugging or bending can lead to fatigue cracking of the insulation layer, making it the most common point of failure.

Cable bending areas—such as the frequently moving parts of device connection cables—can cause the insulation layer to crack over time due to prolonged bending.

Near heat or water sources—such as cables running near radiators or water heaters, or cables located in bathrooms and kitchens—is prone to accelerated aging and damage due to high temperatures and humidity.