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Goin' Down in Flames....
Highwayman
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Discussion Starter · #1 ·
If the transformer secondary was ungrounded (no connection to earth), you would have, in effect, an "isolated" system. There would still be a separate "ground" back to the source for clearing faults, but no earth connection.

So, if you had an "isolated" system, it would (theroretically) not be possible to get electricuted by contacting 1 current-carrying conductor, and earth. Like a bird sitting on a power line. Right? Because the only reason current flows through the human body in that situation, is on the return to the source, through earth, and through the earth ground from the grounded center tap.

Since this is not the way the systems are configured, there must be a reason. I know that limiting stray voltages, and dissipating lightning strikes are 2 of the reasons for ground rods, but what are the other important reasons for earth-ground connections (at the transformer/source)?

Also, is this what the ungrounded, 3-phase, delta set-ups are? Fully "isolated" systems? I know that they have alarms on them to alert you to the presence of a ground fault, but is there any danger from a ground fault, as there is no return to source throught he earth?


Thanks for any education you care to supply.




Delta
 

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I'll go for the ungrounded secondary one, with a caveat. The secondary is ungrounded, and isolated fromt he primary. Not all secondaries are isolated from the primary - some have the commons tied together.

Since it's ungrounded, both terminals of the secondary will float at an undetermined voltage above ground. You can still get zapped, except it can be from either terminal.

If you're perched on a line and not touching anything that would complete some circuit (you're just surrounded by air), you can't get zapped easily. You can get a very slight current you probably wouldn't notice because your body will act as a small capacitor. If it's very high voltage, you'll definitely notice it.
 

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Goin' Down in Flames....
Highwayman
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Discussion Starter · #3 ·
OK.

So, what I'm wondering is, Why did we start grounding in the first place. There must be an overrriding reason to, it would seem, create a new problem, and that is the ability to make a second, parallel path back to the source, through the ground.

I have a pretty good basic understanding of electrical theory, and I've read a great deal on grounding and bonding, and this one eludes me.


I guess my question is, if you didn't ground the center tap of the secondary, and didn't install ground rods, but bonded just like normal, on a standard 120/240v residential electrical system, would you eliminate the possibility of electrocution through standing on wet ground and touching a live wire, and what would be the problem that it would create.
 

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diplomat
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Is it because it's likely (and common in our world) for a hot wire to accidentally make its way to ground?
 

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Goin' Down in Flames....
Highwayman
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Discussion Starter · #6 ·
Is it because it's likely (and common in our world) for a hot wire to accidentally make its way to ground?
The way that fault is cleared is because the ground is coupled to the neutral at the service disco, not because it goes to earth.

So what would happen if the secondary was ungrounded as well?


As when you run a whole house generator. All grounding is done through the generator itself. Back to the source.


Generally every AC power line transformer acts as an isolation transformer, and every step up or down has the potential to form an isolated circuit. However, this isolation would prevent failed devices from blowing fuses when shorted to their ground conductor. The isolation that could be created by each transformer is defeated by always having one leg of the transformers grounded, on both sides of the input and output transformer coils. Power lines also typically ground one specific wire at every pole, to ensure current equalization from pole to pole if a short to ground is occurring.
The part in bold is what I'm trying to understand. Why is that not the case with a GenSet?
 

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Seven-Delta-FortyOne said:
I guess my question is, if you didn't ground the center tap of the secondary, and didn't install ground rods, but bonded just like normal, on a standard 120/240v residential electrical system, would you eliminate the possibility of electrocution through standing on wet ground and touching a live wire, and what would be the problem that it would create.
the ground in the center tap is what gives you the option to acquire 120v instead of 240v, that's why it's there. If there was no ground then you could still get electrocuted because the current would go through you and into the ground.
 

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Capra Aegagrus
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the ground in the center tap is what gives you the option to acquire 120v instead of 240v, that's why it's there.
Not really. It's just as easy (and workable) to ground one end of the winding as to ground the center tap, and you would still have the dual-voltage option.

However, if done that way, any short or leakage to the high side could expose users to 240V rather than 120V. That would make your eyeballs spin considerably faster.

As for portable generators, they are supposed to be grounded also, though most users never do so. Read your manual. :thumbsup: Sure, they work without grounding, but they aren't as safe.

That Wikipedia article lays it out pretty well.
 

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So what would happen if the secondary was ungrounded as well?
As I mentioned before, all terminals will float to an undetermined DC potential, relative to the ground you are standing on. Theoretically, it can charge up to whatever voltage it will support before coronal discharge or insulation leakage puts a cap on the DC voltage. Everything has at least some capacitance, you you're charging a capacitor up to possibly several thousand volts. You can definitely get knocked on your butt under the right circumstances, or killed.
 

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Goin' Down in Flames....
Highwayman
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Discussion Starter · #10 ·
If there was no ground then you could still get electrocuted because the current would go through you and into the ground.
Current will only travel through the ground if the ground is also a parallel return path to the source.



As I mentioned before, all terminals will float to an undetermined DC potential, relative to the ground you are standing on. Theoretically, it can charge up to whatever voltage it will support before coronal discharge or insulation leakage puts a cap on the DC voltage. Everything has at least some capacitance, you you're charging a capacitor up to possibly several thousand volts. You can definitely get knocked on your butt under the right circumstances, or killed.
This makes sense. Thank you.
 

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An ungrounded system is inherently safer than a grounded system in theory. The lack of a current path would mean that contact with energized conductors would not result in receiving a shock. Sounds ideal.

High voltages impressed by a lightning strike or a fallen utility line will mean voltage to ground can be enough to cause fires from insulation breakdowns in an ungrounded system.

Utility distribution systems are ungrounded for continuity of service. If a tree contacts a line a ground detector in the station detects the fault and patrolmen will locate the fault and have it repaired without a loss of service. Same thing for operating rooms and electromagnetic cranes. Faults are monitored and repaired.

In real life there are two additional major problems. First, ungrounded systems rarely stay ungrounded. Due to accident, component failure, or stupidity one conductor can become grounded which then means that contact with the other energized conductors could result in a lethal shock. No maintenance means the fault will not be repaired. Faults on different lines in different locations also means that dangerous ground currents can flow, sometimes between homes.

A grounded system will reduce hazards from lightning and cause OCP to trip on a line to ground fault.

If the system voltage is high enough the capacitance to ground will mean a shock hazard even if system is in good repair and ungrounded. Since DC current will not flow through a capacitor there would be no effect, except that the system is AC so current will flow based on total system capacitance.

The 240/120 system has nothing to do with grounding. It is only 120 volts to ground if the system is grounded through center tap.
 
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