Subelement A: — Topic :
Question 6A4
Element 6 (Radiotelegraph)Ship's power is generated as 3-phase and is ungrounded. On a delta-wound transformer with 120 VACline-to-line secondary, what is the voltmeter reading from line to ground:
Explanation
An ungrounded delta system has no intentional connection to earth ground. Its voltage relative to ground is determined by parasitic capacitances to ground or by fault conditions.
* **A) Approx 67 volts:** In a perfectly balanced, unfaulted ungrounded delta system, the system's "neutral" point floats due to equal capacitance from each line to ground. The voltage from any line to this floating neutral (and thus to ground) would be the line-to-line voltage divided by the square root of 3 (120V / √3 ≈ 69.28V), which is approximately 67 volts.
* **B) 0 VAC:** If the specific line being measured develops a direct fault to ground, its potential becomes 0 VAC relative to ground.
* **C) 120 VAC:** If *another* phase (not the one being measured) faults to ground, that faulted phase effectively becomes the ground reference. Since the voltage between any two lines in a 120V delta system is 120 VAC, the unfaulted lines will then measure 120 VAC relative to ground.
Since all these scenarios are possible depending on the system's condition, "D) Any of the above" is correct. This characteristic is why ungrounded systems require ground detection systems to identify the first fault.
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