testing a 3 phase motor for fault loop impedence

[seamie]

Hi All,

Im new to this site and this is my first post.


Can someone give me advice on doing a fault loop impendence test on a 3 phase motor?

 

[markythesparky]

I presume you are checking for earth loop at the motor connection itself?
Take the connection cover off and clip one lead in to the earth terminal, one to the neutral terminal and then test to each phase in turn. You need to take all readings.
I'm not sure if this sounds like I'm taking the mick but it is that simple unless I've misunderstood.

 

[johnboy6083]

You will have to be careful taking a Zs at a motor, as it usually means its running!

Common practice is Zs to the starter or isolator, then R1+R2 to the motor.

 

[markythesparky]

Agreed

 

[Richard Burns]

An earth fault loop impedance test is performed on a circuit before a piece of equipment, not in the equipment itself.

 

[FrankUK]

Unless he means an IR test on the motor?

 

[johnboy6083]

He mentioned a fault loop impedance test, he forgot the earth bit ;-)

 

[eganelectrical]

You will have to be careful taking a Zs at a motor, as it usually means its running!

Common practice is Zs to the starter or isolator, then R1+R2 to the motor.

Just add them both then yeah?

 

[eganelectrical]

An earth fault loop impedance test is performed on a circuit before a piece of equipment, not in the equipment itself.

What about sockets? You can plug meter in??

 

[eganelectrical]

An earth fault loop impedance test is performed on a circuit before a piece of equipment, not in the equipment itself.

Or are you meaning it shouldn't be done through an appliance as a socket would be an accessory right?

 

[Richard Burns]

Yes, the 3 phase motor would not be connected if you were doing an EFLI. You are testing the circuit not the appliance.
Do as johnboy6083 said Zs at isolator plus R1+R2 from isolator to motor connections.

 

[Hawk81]

I presume you are checking for earth loop at the motor connection itself?
Take the connection cover off and clip one lead in to the earth terminal, one to the neutral terminal and then test to each phase in turn. You need to take all readings.
I'm not sure if this sounds like I'm taking the mick but it is that simple unless I've misunderstood.

Most 3 phase motors don't have neutrals.

 

[johnboy6083]

Very true!
also on a system with star delta, auto transformer or resistance starting, a reading will have to be taken through this side of the starter.
on VSD's and soft starters, the drive protects the motor from short circuit and overload, and following manufacturers info regarding cable size and length is sufficient.
i still take R1&R2 as I like to ensure all phases are of equal resistance, and it's then on record, but strictly not needed!

 

[sparkymalarky]

On the panels that are made in our workshop and sent to site, a 'test' MCB is included using the same size of panel wiring as other circuits to create a negligible impedance. Then once all the appropiate 'dead' tests can be done on the motor circuits, they can be connected to the test MCB and and earth fault loop impedance can be taken without affecting the sensitive gear etc. Obviously take your measured Zs and compare it to the MCB that will be permanently supplying your motor circuit.

You must still measure this Zs/EFLI at the isolator with the isolator OFF. Then measure R1+R2 from the isolator to the motor terminals and add the two together.

 

[seamie]

I presume you are checking for earth loop at the motor connection itself?
Take the connection cover off and clip one lead in to the earth terminal, one to the neutral terminal and then test to each phase in turn. You need to take all readings.
I'm not sure if this sounds like I'm taking the mick but it is that simple unless I've misunderstood.

Hi markythesparky. Thanks for your reply but there is no neutral at this motor...

 

[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.