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Topic Title: Loop impedance measurement through an inverter
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Created On: 16 April 2012 09:05 AM
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 16 April 2012 09:05 AM
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Joined: 25 July 2008

I am looking at doing earth loop impedance measurements on a machine to determine if the fusing on inverter supplies is suitable for EN60204 compliance. E.g. using an installation tester such as Metrel MI 3000. This is a validation of an imported machine manufactured by a third party.

No problem with conventional motors, but what about servo motors and motors powered by inverters?

Any risk of damage to the inverter? I guess not because all the tester does is create a 2 cycle earth fault and measure the current during the fault. Most modern inverters can take that sort of thing in their stride.

Will it get meaningful results anyway?

EN60204 suggests that we should compare loop impedance with expected values. The copper resistance is easy to calculate providing I can identify the conductor size correctly, but in a large machine context the earth wiring may go through a few terminal boxes. Is there any guidance about acceptable impedance for connections?
 16 April 2012 10:14 AM
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Doug, the text in 60204-1 that refers to a comparison with expected values is simply to verify the continuity of the protective bonding circuit and that the resistance is in the right ballpark. Test 2 and A.4 is where you need a precise measurement to ensure the conditions for ADS are achieved.

There's a draft revision to IEC 60204-1 that suggests the fault loop impedance is measured at the input terminals of the inverter, since as you say measurement after the inverter would give meaningless results. There will also be a requirement to verify that the type and setting of the associated protective device is in accordance with the inverter manufacturer's instructions.
 16 April 2012 10:15 AM
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The output of an invertor is not a true ac it is derived from pulsed DC via a 'DC link' in the invertor. Any results obtained (at 50Hz output, other frequencies might make the tester sulk !)) would not really bear any usefulness in relation to the overcurrent protection at the origin.

Empty barrels make the most noise.

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