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This is probably a silly question but I read that for class I equipment I need to perform insulation resistance tests and it must be under 1 megaohm.

My understanding is that one of these tests involve checking the resistance between live and earth connections.

So if my supply is 240V and I have a 24W resistive load (let's say I am a heater). Then the resistance of my load will be about R=V^2/P = 240^2/24 = 2.4k ohm.

So essentially I have 2.4k ohm load between live and Earth. So during the insulation test I will fail because the resistance between live and earth is less than 1 megaohm.

Is this right?

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    \$\begingroup\$ the load is not between live and earth but live and neutral \$\endgroup\$
    – tobalt
    Aug 19, 2021 at 9:11
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    \$\begingroup\$ If your load is between live and earth, you failed the insulation test. \$\endgroup\$
    – Simon B
    Aug 19, 2021 at 10:30
  • \$\begingroup\$ What if I wanted to measure the line voltages with respect to earth. Eg. Have a voltage divider. I suppose I need the voltage divider to have a total resistance of more than 1Meg Ohm? \$\endgroup\$
    – MMmmm
    Aug 19, 2021 at 11:15
  • \$\begingroup\$ You generally don't do insulation testing on live equipment. You isolate the equipment and then apply the test voltage between the L or N to E. \$\endgroup\$
    – Transistor
    Aug 19, 2021 at 14:48

2 Answers 2

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No, it's not right. There will be high impedance from earth to live and earth to neutral, because load will be between live and neutral.

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No, you are not right. You do test resistance between power terminal (whichever) and the protective shield that is connected to the earth.

The tested device shall have the power wires disconnected, so even there is a bond between neutral and earth you do measure the isolation.

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  • \$\begingroup\$ What if I wanted to measure the line voltages with respect to earth. Eg. Have a voltage divider. I suppose I need the voltage divider to have a total resistance of more than 1Meg Ohm? \$\endgroup\$
    – MMmmm
    Aug 19, 2021 at 11:16
  • \$\begingroup\$ Depends on what standard you need to comply to. The standard might specify earth leakage to be less than 1mA in which case you need to design to that requirement. \$\endgroup\$
    – Kartman
    Aug 19, 2021 at 12:00
  • \$\begingroup\$ @MMmmm I think that this could be a new question that is not related to the actual question. For sure it's a bad idea to use a megger on the measuring equipment, but still it measures the voltage between neutral and phase and not the earth. \$\endgroup\$ Aug 19, 2021 at 12:50

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