Discuss How do you continuity test a motor circuit to obtain Zs in the Commercial Electrical Advice area at ElectriciansForums.net

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J

Jackson JC

Hi, I can't seem to get my head around how to test a 3 Phase Motor circuit to obtain the value of Zs.

1. After isolation of circuit and locking off u short out the 3 Phases on the motor terminal block with shorting straps.
2. At the Dist Board with a Low Ohm meter you test the resistance between each phase:-
L1/L2, L1/L3, L2/L3, then obtain the average of the 3 then divide by 2 because the cable is half that resistance. The value obtained is R1
3.To get R2 u go between L1,L2 or L3 and CPC.
4. Then you add the Ze at the origin of the installation and add it to Ze+(R1+R2) = Zs


Is this correct?
 
Zs is a live test!

It is earth fault loop of the circuit.

On 3 phase you would connect your meter up to L1 with Live probe, Earth with Neutral and earth probes together and test. Repeat same test on all phases, and your highest reading of the 3 is your Zs. Doing this test will also display PEFC value, this is the one you double for 3 phase, to get your PFC reading. UNDER NO CIRCUMSTANCES CONNECT THE METER ACROSS ANY 2 PHASES. Before attempting any of what i have just wrote, you must look at the Instruction manual for your test instrument, and not take my word for how to connect it to a live circuit. The Zs test does not require connection across phases, but PEFC can be. Unless your instrument is capable of doing this, then dont, see the highlighted red text. Even if you kit is capable, it is still preferable to do it as above and double it. This is also an accepted answer in 2391/4/5.


Cheers............Howard
 
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and your prospective short circuit current (PFCC) is always higher than your prospective earth fault current (PEFC) on 3 phase.....so its that ...that you would note down on the cert...
 
If I remember for the 2391 practical Howard, we were not allowed to do any live tests at the motor itself (safety reasons), it was done with two dead tests. (R1 + R2) to the motor isolator/contactor added to the (R1 + R2) of the supply cable to the motor which in turn the highest readings of which was added to the Ze.
 
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R1 + R2 are dead tests, if you have got these and the Ze can't you work out the Zs?​

Yes of course, Zs = Ze + (R1 + R2)

When I did my 2391 practical though the R1 + R2 had to be done in two parts, ie. the supply cable from the DB to the motor Isolator/starter and then from the outgoing side from the Isolator/starter to the motor terminals.

so you end up something like this Zs = Ze + (Rx1 + Rx2) + (Ry1 + Ry2), where x and y are the highest readings of the two cables respectively (the highest of the three line conductors to the CPC) 6 low ohm resistance tests in total.
 
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Hi Howard, or anyone else, could you explain a step by step approach please, coz i'm still none the wiser.

Kind regards
 
OK,

I will assume before starting that you have safely isolated, locked off, proved dead, and zeroed your meter on low ohms resistance, and that you have your Ze already.

At the DB, temporarily short or link the CPC to each line conductor of the cable that goes to your motor starter in turn, note the highest reading at the incomer of your motor starter measured between each line conductor and the CPC that is linked, We will call this reading (Rx1 + Rx2).
remove your links.

Do the same three tests as above but this time either link or short the CPC either at the motor terminals or at the outgoing side of the starter, so if you have linked at the outgoing side of the starter measure at the motor, alternatively if you have linked at the motor end measure at the outgoing side of the starter.
Take a note of the highest of the three readings. We will call this reading (Ry1 + Ry2).

Add these two readings together, this is the total (R1 + R2) for the motor circuit.

Add this to your Ze reading (taken previously), and this will be the Zs for the motor circuit.
 
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It has to be done this way, as the motor starter or contactor needs to be powered to operate (close), and as we are testing dead it has to be done in two parts because the motor starter will be open, therefore disconnecting the three line conductors to the motor.

If we tried to measure it in one, it would read open circuit.

I hope this helps.
 
OK big boys, cool.
OP is just asking about testing the cables on, I presume, a test rig, for his I&T practical.
 

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