# NEC question #109 Corner grounded delta load



## EngrinSF (Mar 11, 2018)

Hello,

I came across this question on the NEC practice exam :

A 3 phase 3 wire ungrounded 13.2 kV (phase to phase) wye connected source is connected to a balanced delta load that is grounded on CORNER A. The voltage measured between corner B and the ground is most nearly :

The answer is 13.2 kV.

Can someone please elaborate on this answer and if there is any good material I can refer to on this topic. Also how would the values change when the source is grounded ?

Thank you!


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## supra33202 (Mar 12, 2018)

I also need help on this one.

Could you confirm my diagram (attachment) is correct?

Thanks!


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## JohnMdd (Mar 17, 2018)

Ignore the grounded corner. It doesn't change anything.

The question could be asked like this, without the confusing mention of a grounded corner:

"A 13.2kv (L-L) wye source is connected to a balanced delta load. What is the L-L voltage on the delta load from A to B?"

Supra, your drawing would be fine except that you have drawn a ground on two corners of the delta, on both A and B. Erase the ground on B. Now you're just measuring the voltage from A to B, which is the L-L voltage of the system.

There's really nothing to it.


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## EngrinSF (Mar 17, 2018)

JohnMdd said:


> Ignore the grounded corner. It doesn't change anything.
> 
> The question could be asked like this, without the confusing mention of a grounded corner:
> 
> ...


Thank you for the reply!

The answer to your question would just be 13.2 kV right.


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## rg1 (Mar 18, 2018)

There may not be ready reference for these questions answering or explaining specifically. This comes under the subject of circuit theory of EE degree course and you have to make an understanding from there.  Let me try to explain if it makes sense.

When a system is grounded you have a reference of ground and so you can measure a voltage from ground to any of the phases or between the phases. A man touching a phase conductor with feet on ground will be subjected to phase to ground voltage. A voltmeter too will read the same. Now in such a grounded system the other corner ( another Phase conductor) if grounded will create a fault.

Coming to ungrounded system as there is no reference to ground theoretically if a voltmeter is connected between ground and a phase, should not give any reading because one end of the voltmeter is  as good as hanging in air. So I can say that a person touching his hand to a phase with feet on ground in a ungrounded system should not get a shock.(Practically because of capacitance between ground and phase makes the Voltmeter read some Voltage and the man should measure the capacitance or confirm with voltmeter before attempting). So first connection to ground is not a fault here and the voltages will remain same as were before touching the ground. Now making another phase connection to ground will create a fault in this case.


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