ECN Forum Forums General Topics of the Trade General Discussion Area Grounded or ungrounded system?

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

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

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.

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.

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.

So is there a simple way to test if we have a grounded or ungrounded system?

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.

It's good to see you here Dereck. Welcome aboard.

Ryan_J would that be 618?

The same.


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Plus he's a great guy.

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