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I am reading a book on electrical practices and I've come across to a statement that says it is safe practice to ground the secondary winding of a transformer.

In general, why does grounding one terminal of a secondary winding not cause a ground fault? If, for example we hook up a load to the secondary winding side, what prevents the current from going to the path of least resistance which is the grounded point of the secondary, and ignore the high resistance load?

Especially since here in our ship, our generators wye neutral is connected to our ship's hull, and grounding the secondary winding means connecting it to the ship's hull. Why doesn't this cause a ground fault?

Edit: Addded photo.

Edit 2: I apologize for the over simplification of the drawing, let me stress that the diagram is only for the sake of simplicity and that we have the necessary circuit protection in between the generators, switchboard, transformers, and load. The ground fault current is also limited by the NER in this case, which in the drawing there is a mistake wherein if you interpret the grounding on the secondary winding it would immediately make electrical contact to the ships hull.

enter image description here

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    \$\begingroup\$ Try drawing the loop the current would take in that case. Where would it go? \$\endgroup\$
    – Hearth
    Oct 6, 2022 at 12:39
  • \$\begingroup\$ Welcome! That entirely depends on how it's connected. What transformer, where? If the secondary is floating and not connected anywhere else then not much will happen if you ground one terminal of the secondary. Please draw a schematic of the particular case you are asking about! \$\endgroup\$
    – winny
    Oct 6, 2022 at 12:40
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    \$\begingroup\$ Draw the circuit and look at the path the current takes. Remember that current flows in a loop (or multiple loops in parallel). Also remember that current does not only follow "path of least resistance" - current will follow all the paths it can follow, whit the amount of current depending on the resistance of the path. \$\endgroup\$
    – brhans
    Oct 6, 2022 at 12:40
  • \$\begingroup\$ What type of transformer? What voltage is your switchboard at? \$\endgroup\$ Oct 6, 2022 at 19:46
  • \$\begingroup\$ @StainlessSteelRat current transformer, 440V switchboard. \$\endgroup\$ Oct 7, 2022 at 3:51

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You write that at time (2) on your graph,

b is at highest potential, why does this not cause the current to flow from point b to ground? Isn't it effectively shorting one line to ground at the highest reverse peak voltage at the negative cycle of AC?

Since point b is connected to the ground, point b is always at ground potential and no current flows between b and the ground.

On the other hand, point a will rise above and fall below ground potential as the AC cycle proceeds. Specifically, the highest potential that point a will reach is approximately ground plus 311 V, and the lowest potential that it will reach is approximately ground minus 311 V. At no time will point b be at a potential different from ground potential.

(This analysis assumes that the capacitance between the secondary side of the circuit and the ground is zero. In fact, the capacitance is nonzero, but it's so tiny that it's negligible.)

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  • Current flows in loops.

  • It won't flow into ground/earth because there is no conduction loop involved.

  • Current will flow into the load.

Faults are only faults if they are designated as such. For instance, your ship's generator's ancillary equipment will recognize a ground fault if any of the live wires connect to ground but, given that it's highly likely that your generator's neutral is bonded to the ships hull, you wouldn't regard a neutral to ground/earth/chassis connection as a fault (and neither should it be regarded as a fault).

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  • \$\begingroup\$ Hello Ive added a photo of the diagram at question. Since our fault monitoring system checks for the potential between a single line and ground(ships hull), why is it that when voltage is at its highest peak in point b of the secondary not cause a zero potential between line and ground? I am thinking since its an isolation transformer, current would only flow from a to b strictly and not to the ground connection(assume that the ground connection is tapped directly to point b), however, I dont understand this behavior if we take into consideration that current flows from high to low potential \$\endgroup\$ Oct 7, 2022 at 3:47
  • \$\begingroup\$ So why "it is safe practice to ground the secondary winding of a transformer" when current won't flow into ground through human body for example? \$\endgroup\$
    – ufok
    Oct 7, 2022 at 5:49
  • \$\begingroup\$ @ReubenAnthonyRenes I don't know what you mean here --> a zero potential between line and ground <-- which line and ground? \$\endgroup\$
    – Andy aka
    Oct 7, 2022 at 9:13
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    \$\begingroup\$ @ufok that isn't the question raised by the op. That is a statement by the op and, when you consider all the options of grounding or not grounding, the safest option is to ground and fuse for maximum safety. There are answers on this site about that subject such as this one. \$\endgroup\$
    – Andy aka
    Oct 7, 2022 at 9:15
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A tad strange a 450V switchboard has a neutral connected to the hull. This is usually done with higher voltages. Any ground fault, as in a hard or incindental connection of a mains line to the hull could black out the ship.

Most 450V switchboards have an insulated neutral. This allows for the ship to have a single ground fault, which will not interrupt the flow of power to the loads, i.e. black out the ship.

A single ground fault means ship hull is energized, but much like birds on bare wires, the crew is not at direct risk. But ground faults must be cleared because a second ground fault on another power line could black out the ship or shock personnel if they make contact with circuitry. So proper PPE is required to protect personnel. A ground fault and a person makes contact with a second circuit, but has insulated shoes means there is no where for the current to go. Again, birds on wires.

Ground fault detection circuitry is required to detect ground faults. Transformers magnetically isolate secondary side from primary side, so secondary side needs ground fault detection circuitry also. This is a clue to the answer. I suggest you stop here and think about your question and this paragraph.

The power system needs to be monitored, so instrument transformers step mains voltages and currents down to lower voltages (voltage transformer - 450V:120V) and currents (current transformer - rated current:1A or 5A) to create dead switchboards to protect personnel. Again, transformers provide magnetic isolation, so to detect ground faults, the secondary side of voltage transformers and current transformers is grounded to the ships hull.

This applies to an insulated neutral or a neutral grounded at the switchboard (but without hull return - load is balanced at switchboard so no current flows through hull).

The major difference being (in your case), any ground fault will trip breakers or blow fuses, with hopefully proper protection discrimination to isolate the fault to the proper sub-circuit and not black out the ship. Easier to detect, but any ground fault could put ship at risk if sailing in a channel, close to land or other vessels.

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  • \$\begingroup\$ I apologize for the over simplification of the drawing, let me stress out that the diagram is only for the sake of simplicity and that we have the necessary circuit protection in between the generators, switchboard, transformers, and load. \$\endgroup\$ Oct 7, 2022 at 6:19
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A transformer with a primary and a secondary winding as you drew without a connection between those windings isolates the primary circuit from the secondary circuit.

No current will flow between those two circuits, whether they are grounded or not. The two ground connections are only a line but a closed loop is needed to let a current flow. Of course a voltage or current source within the loop is needed to let a current flow. In a loop without a source the current is zero.

A ground fault detector uses a special current transformer with two primary and one secondary windings. This transformer is used to get the difference between the currents through the phase and the neutral conductors. If this difference is bigger than a few milliampere, a ground fault is detected and the phase connection to the load is switched of.

Grounding of the transformer is done in the same way on the ship and on Earth surface, detection of a ground fault works the same way. If not all current through the phase conductor returns through the neutral conductor, a ground fault is present.

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  • \$\begingroup\$ I think you misunderstood my question. I am aware that a ground fault in the secondary of an isolation transformer is not reflected as a ground fault in the primary. My question is that how tapping the ground to the same terminal as the other end of the secondary winding(hence point b, refer to diagram) does not cause a ground fault at the second half of the Voltage peak in the sine voltage at point 'b'. Since I am thinking that assuming a voltage peak of 240 for the sake of argument, will cause a ground fault current since you are basically shorting 1 line to ground. \$\endgroup\$ Oct 7, 2022 at 6:26
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It's an interesting question, makes you wonder. The primary of the transformer is across line-to-line (from the generator); its secondary is across line (transformer pin)-to-neutral (generator neutral).

As long as the three-phase generator is perfectly balanced, no current flows into or out of the generator neutral (from the generator's point of view) - hence it is a perfect reference point for the transformer whose secondary induced voltage (via transformer turns ratio) is free to excurse positive and negative about it.

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