KS C IEC 61557-8

[Insulation resistance fault (Ohmic insulation fault) measurement principle] and (MED-IMD) requirements]

[Insulation resistance failure measurement principle]

1. Add DC measurement voltage (Um) between the system and the protective conductor.

2. The measured voltage generated within the insulation monitoring device (IMD) is applied to the system through a high-ohm coupling resistance (Ri).

3. Because the secondary side of the isolation transformer in the monitoring system is low ohm compared to the DC measurement voltage (Um), the measurement voltage is added to all conductors.

4. When an insulation failure occurs, the measurement circuit between the system and ground is closed, and a small amount of measurement current (Im) flows through the insulation failure circuit.

5. In this case, the leakage capacitance of the system is charged to the DC measurement voltage and does not affect the measurement after a short response.

6. This measured current (Im) becomes the total insulation resistance (RF) of the system based on the following formula.

7. A ㏀ ohmmeter connected in series with the measurement circuit displays the total insulation resistance (RF) of the system.

8. This insulation resistance value is displayed as a ㏀ value on the IMD or in a higher level system, and the alarm relay is activated through an amplification stage to generate an alarm.

9. Alarm response values ​​are different according to the requirements of each standard.

In the case of [medical IMD], the insulation resistance response value (Ran) is 50 kΩ.

10. The insulation fault locator for fault circuit detection must comply with the requirements of KS C IEC 61557-9, Annex A.