testing a 3 phase motor for fault loop impedence

[seamie]

Thanks all for the replies.... Im going to measure the earth fault loop impedance at the isolator for each phase and add R1+R2 to this reading and document all readings on the test cert. From reading the posts this seams to be the way to go....

 

[sparkymalarky]

Seamie,

Read the bottom paragraph of GN3 Page 76.

John

 

[kodr]

Most 3 phase motors don't have neutrals.

With respect to performing Earth Fault Loop Impedance at the isolator of a 3-phase motor (with no neutral) fed through a VSD I have two queries.


1. Can the test be perfomed with the VSD in the circuit or should it be by passed or perhaps measure EFLI at the incommer of the VSD and calculate R1 + R2 for the remainder of the circuit ?

2. My test kit is a Kewtech KT65 which I believe requires a neutral to perform the test or do I get around this ?

I'm open to ideas. Thanks.

 

[Darkwood]

Kodr... why are you wanting to test the set-up are you doing a EICR if so the testing is the circuit itself not the equipment/controls attached to it... you shouldn't be performing any tests through the VSD, if you are checking the motor i.e. Insulation resistance then ensure the VSD isn't connected. You need to be very careful you dont damage anything if its a control system including the VSD so isolate the load from the circuit and test the circuit only....BS7671 does not cover the machine/motor control system as such although if it was a simple DOL pump on an isolation switch then usually no harm in testing through.

 

[kodr]

BS7671 does not cover the machine/motor control system

True. I'm not including the motor. Isolator in the off position I just want to know if I can perform a EFLI test whist the VSD is connected or should calculate Zs=Ze + (R1+R2), I appreciate VSD are control equipment but what is normal practice to determine EFLI for these circuits.

Also, in cases where I have DOL motor cicuits with no neutral how is test performed without the neutral, my multifunction test kit (Kewtech KT65) requires a neutral.

Thanks.

 

[Farmelectrics]

We used to do a lot of testing on motors we used to use a Clare tester it was a dead test shorted one of the phases to earth and this proved your earth fault path or eli

 

[johnboy6083]

You don't need test results for a motor fed by a VSD. Usually the manufacturers manual will tell you the cable size and max length. Cables feeding motors on VSDs can seem oversized, but it's due to the effects of the changing frequency!
a long lead test would be a good idea, as this would confirm an earth path. It's more important to check the screens, especially on larger motors. I've tested 15A flowing to earth via a screen, and it was brand new, and the engineers were happy.