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Topic Title: Zs of board less than Zs at the end of a circuit?
Topic Summary: Can it happen?
Created On: 09 March 2017 10:12 AM
Status: Read Only
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 09 March 2017 10:12 AM
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Apostolos1983

Posts: 99
Joined: 03 December 2012

Good morning.
In a water treatment works I have a switchboard tagged PDB5 feeding among others some pumps.
We have the Periodic Inspection Tables (PITs) in pdf where you can see that the switchboard has a Zs of 0.04 and the maximum measured EFLI at one motor is 0.02.

I have a senior engineer here that says because the pumps are cross bonded to the metal pipes which are then cross bonded to other pipes etc, the measured Zs at the motor can be less than the Zs of the switchboard.

Now, I do not believe that and I told him I am going to ask in here to get a second opinion.

However good earthing you may have, from the R1+R2 you may succeed lowering R2 to zero. The R1, which is the phase conductor resistance will always be R1 no matter how good earth and bonding you have.

Do I miss something here?
 09 March 2017 11:27 AM
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mapj1

Posts: 9371
Joined: 22 July 2004

yes, it can happen - Zs is measuring a path that is a loop, out down the live and back to the transformer through the CPCs and the earthing, a full length R1 + R2 from the substation if you like.

If the load is earthed via its pipes, then at load the R2 part may be almost zero, if the pipes are fatter than the circuit CPC, so Zs is just the R1 part If you r wiring has T and E for example then the CPC is reduced relative to the live core, so more of R1+R2 is actually R2, so the reduction can be more than half.

-------------------------
regards Mike
 09 March 2017 12:19 PM
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AJJewsbury

Posts: 15838
Joined: 13 August 2003

However good earthing you may have, from the R1+R2 you may succeed lowering R2 to zero. The R1, which is the phase conductor resistance will always be R1 no matter how good earth and bonding you have.

Indeed - to expand on Mike's point, Zdb might be made up of R1= 0.1 and R2=0.3 giving 0.4, while circuit end R1 would be higher - say =0.15 while R2 (including parallel paths) might be 0.05 giving 0.2 (or any number of other possibilities).

Also remember that loop testers can have quite a bit of variation in their readings - especially in low current (no trip) modes.

- Andy.
 09 March 2017 12:22 PM
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Apostolos1983

Posts: 99
Joined: 03 December 2012

Hm...
So indeed it is possible.

Still cannot get my head around it but there are too many votes now to ignore!
:-)
 09 March 2017 01:31 PM
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mapj1

Posts: 9371
Joined: 22 July 2004

If it helps you to see why draw out a full circuit from substation windings to a meter conencted L-E at the motor, passing via fuse and DB, back to substation star point earth mostly via the pipes (but partly in parallel with the CPC and any NE bond if supply is PME)

The resistance of that loop is Zs motor.

Then draw it again, but put the meter at the DB - sure the live path is now a little shorter than it was when we measured at the motor, but the long bit all the way to and from the substation is unchanged. The earth path is now back via the supply CPC, which is also slightly shorter, but the very significant shunting effect of the pipes is now in series with load CPC wire.

Does that help ?

-------------------------
regards Mike
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