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Joined: Nov 2001
Posts: 75
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Bill39 Offline OP
Member
If you get voltage readings from phase to ground does this automatically mean you are working on a grounded system? I’m setting up a VFD drive and one of the parameters is to select if it is connected to a grounded system. So how would you classify the system? Here’s what is present:

30KVA delta-delta transformer (buck-n-boost)
Primary is fed from 3 phase 208/120 system.
Secondary is 440VAC 3 phase, no corner or center tap ground (ohmmeter readings are infinity from each phase to ground).
Actual readings on secondary are:
A-B=444V A-GND=239V
A-C= 442V B-GND=271V
B-C=443V C-GND=250V

Actual readings on primary are:
A-B=210V A-GND=121V
A-C=210V B-GND=122V
B-C-209V C-GND=121V

Joined: Apr 2004
Posts: 349
Member
I think if your delta secondary were grounded, you would get predictable readings of 440V, 220V, or 0V phase to ground, depending on the point of ground and point of measurement.

This looks like an ungrounded (floating) system. Your secondary phase to ground readings appear unpredictable and may vary or drift over time.

Radar


There are 10 types of people. Those who know binary, and those who don't.
Joined: Apr 2002
Posts: 2,527
B
Moderator
The described 440V circuit would be considered ungrounded. [The 30kVA transformer may be a motor-drive isolation transformer intended for somewhat higher-harmonic laods, with labeling to reflect such ratings.] Some ø-g voltage unbalance can be expected as a result of insulation-quality variation in the 440V windings or wiring and loads connected to it.

It would be a misapplication to use slash-rated overcurrent devices such as a 480Y/277V circuit breaker or 300V fuses in the 440V circuit. Also, if surge-protective devices intended for grounded-wye service were applied {or integral to a component like a motor drive} were intended for a grounded-wye application, it could experience failure in an “exothermic” mode.

Joined: Aug 2001
Posts: 7,520
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Member
Even with no load on the transformer secondary you could expect to get varying voltage readings to ground due to capacitance.

The measured volotages will also be dependent upon the impedance of the meter you're using to take the readings.

Joined: Oct 2003
Posts: 156
D
Member
The varible/unpredictable voltage is the result of capacitance coupling to ground. The capacitance is formed between the windings of the transformer and case, and between the ungrounded phase conductors and EGC/raceways. A Wye system shorts out this capacitance and stabilizes system voltages with respect to earth.

Joined: Jan 2003
Posts: 4,391
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Moderator
So is there a simple way to test if we have a grounded or ungrounded system?


Bob Badger
Construction & Maintenance Electrician
Massachusetts
Joined: Apr 2002
Posts: 2,527
B
Moderator
The right way to determine system grounding is by reading equipment [transformer] labels and doing a visual inspection of associated grounding. If the ‘originating’ transformer is utility-owned, ask them. It’s important to realize that because no neutral or ground conductor is routed with phase conductors in a circuit, that does not automatically mean the source is ungrounded.

On the North American continent, although the more recent tendency is to use 4-wire grounded-wye systems, there may also be ungrounded, resistance grounded, corner grounded or midcoil-grounded systems, which may be applied to 240-600V systems.

Aside from that, assemble {series!} five 100-watt, 120-volt lamps protected by a pair of 3-amp 600-volt fuses. Carefully connect the string phase-to-ground, and by reading the three phase-to-ground voltages, you can get an idea of ground impedance. If you suspect a grounded or ungrounded system, prove it to yourself through visual inspection.

Joined: Aug 2003
Posts: 1,374
R
Moderator
It's good to see you here Dereck. Welcome aboard.


Ryan Jackson,
Salt Lake City
Joined: Oct 2003
Posts: 156
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Member
Ryan_J would that be 618?

Joined: Aug 2003
Posts: 1,374
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Moderator
The same. [Linked Image]


For those of you who don't know Dereck Campbell, he will be a great asset here. His knowledge of both code and theory and complete and very impressive.

Plus he's a great guy. [Linked Image]

I'm sure you'll find some familiar faces here Dereck, and if you havfen't already, take a look at this thread to see what some of your freinds look like:

https://www.electrical-contractor.net/ubb/Forum1/HTML/002850.html
Then send me or Bill a pic so that you can get added to the list!!!

[This message has been edited by Ryan_J (edited 06-27-2004).]


Ryan Jackson,
Salt Lake City
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