New Legislation Increases the Safety of Three-Phase Motors by Improving the Accuracy of Insulation Deterioration Detection

This article looks at the different types of insulation deterioration in three-phase motors, key risk factors, and the new legislation in Japan which is driving improved accuracy in insulation performance monitoring. We’ll also look at key features of one of the leading products on the market used to monitor insulation deterioration.

Traditionally, insulation performance (leakage current, or lo) was used to assess the degree of insulation degradation in three-phase motors and AC servo motors. However, a recent legislative amendment established by the Ministry of Economy, Trade and Industry (METI) in Japan is changing the criterion used to determine insulation performance. The amendment, which will go into effect in spring 2021, includes resistive leakage current (lor) as an acceptable method in determining insulation performance. This new criterion is expected to greatly increase the safety and efficiency of equipment maintenance. This change will not only impact Japan, but rather will have a global impact as Japan continued to be a major source of insulation detection equipment used around the world.

Insulation deterioration and risk factors for three-phase motors

Three-phase motors are commonly used in many moving mechanical devices such as machining equipment, water distribution pumps, and elevators, but the internal insulation resistance decreases over time. A significant drop in insulation resistance can cause equipment failure, resulting in a dangerous situation. There are three types of insulation deterioration risks that can cause a three-phase motor to fail.

  • Grounding fault
  • Phase short circuit
  • Layer short circuit
  • Grounding fault
    • In a grounding fault, the insulation between the electrical circuit and ground is extremely reduced, and an arc or conductor–based connection occurs.Grounding faults can lead to an increased risk of electric shock or equipment damage.
  • Phase short circuit
    • A state in which the lines between two or three of the phases are in contact with no load passing between them.Because this results in a circuit with very little resistance, a large amount of current flows through, increasing the possibility of fire or damage to the motor.
  • Layer short circuit
    • A layer short circuit occurs when contact occurs between the lead portions of the coil, which should be insulated. Such short circuits can result in various problems such as the number of effective coil windings to change, leading to degradations in performance. In addition, because magnetic flux also affects the short-circuited coils, the electromotive force may also be affected, leading to abnormal heating or other problems.

Detecting these risks requires a diagnosis of the motor’s internal insulation, meaning the equipment must be shut down every few months to check the insulation resistance using a resistance tester.

Addition of Ior as Insulation Performance Judgment Criterion per Revision to METI Regulations

Measuring the leakage current makes it possible to measure insulation deterioration even during operation, eliminating the need to stop the operation of a facility. There are three values used to measure the insulation performance of low-voltage circuits.

  • Io:A vector composite of Ior and Ioc leakage current values
  • Ior:Resistive leakage current (dangerous leakage current that can generate heat)
  • Ioc:Capacitive leakage current (safe leakage current that does not generate heat)

Conventional methods based on lo values include loc, a safe leakage current that does not generate heat, posed an operational problem because normal, essentially nonhazardous conditions were often catagorized as dangerous.


Inverters, required components for motor control, have a particularly large Ioc, making it difficult to determine the insulation deterioration inside a motor because a normally safe condition is detected as being dangerous.

The revision to the law, however, will make Ior a criterion for judging insulation performance, which should allow for more accurate judgment of insulation deterioration inside motors.

Only Ior

Moreover, the traditional measurement method did not allow for Ior measurement during operation, only the Io value.

The new measurement method enabled by the legislative changes, however, allows for Ior measurement during operation, opening the door for constant insulation deterioration monitoring even while the facility remains in operation.

Features and Advantages of Insulation Deterioration Monitoring Modules

CONTEC’s insulation deterioration monitoring modules are designed to detect grounding faults, mentioned above as a motor risk factor.
Simultaneous measurement and calculation of the voltage and zero-phase current enable a new measurement method that makes it possible to measure the Ior insulation resistance of three-phase motors and AC servo motors even while they are in operation, a feat that was previously not possible without stopping the equipment.

Insulation deterioration monitoring modules, which consist of a measurement module and a ZCT (zero-phase current transformer), can be easily installed as shown below. The features of insulation deterioration monitoring modules are also listed below.


  • Patented technology(Medes-IO)enabling motor insulation resistance measurement even during operation
  • Insulation resistance calculation from leakage current Ior
  • Insulation resistance measurement on power supply circuit and secondary side of inverter
  • High-precision measurements with reduced hardware and software noise
  • Multiple module stacking on CONPROSYS devices for measuring insulation of multiple motors

Continuous Monitoring with CONTEC’s CPS-MM-LC and M2M Controllers

CONTEC’s CONPROSYS series offers IoT solutions for predicting breakdowns of three-phase motors. The main module in this solution is the CPS-MM-LC insulation deterioration monitoring module.

The CPS-MM-LC module enables easy Ior measurement when used in addition to configurable CONPROSYS series CPU modules. Sending data to the CONPROSYS Cloud Data Service (CDS2) also makes it possible to monitor motors in equipment at remote locations for long-term prediction of insulation failures and equipment breakdown.

The combination of CONTEC’s CONPROSYS series insulation deterioration monitoring module and cloud data service ensures a reliable solution to monitoring insulation deterioration without having to stop the equipment.

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