How does Grounding a Circuit Protect us

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johncons

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According to this video, the voltage drop protects us. Thought you could not use the earth as a safety conductor? Confused...

 
He doesn't really know what he's talking about. Of course the math is correct, but his assumptions about source impedance are ridiculously high. Of course during a fault the voltage will be dragged down to some degree but generally not any significant amount from a human shock perspective.
 
Arrrg!
First he uses an obsolete voltage, 110V
And too much focus on ohms law
What is this guys you tube channel?
And his wrong about grounding.
 
Welcome to the forum.

Replace what he's calling earth with a grounding conductor, and he's actually explaining why we don't get shocked by touching the grounded (neutral) conductor.
 
What protects us is "bonding".
"Grounding" is nothing more than making an earth reference to a point on the electrical system being grounded. For most multiwire AC systems NEC requires the "neutral conductor" to be what is grounded when a neutral conductor is present in the system.

Bonding protects us by connecting a conductor that is not used for normal current carrying functions to the grounded conductor and connecting that conductor to non current carrying components in the system, bonding all such non current carrying items together to effectively make them all same potential and with low resistance so that should an ungrounded conductor become faulted to it, high level of current will flow allowing overcurrent protective devices to react to said high current and open the ungrounded conductor. There is time factor in there as well as voltage division going on that still can leave some shock potential, but the low resistance does help assure it will only last for very short time.

I did not watch entire video only first couple minutes or so, but one critical protection thing not mentioned in what I did watch is he had no overcurrent protection device in his circuit. Sure impedance of the source will lessen voltage you will be exposed to, but that impedance is generally much much lower than the 10 ohms he used for an example. On top of that earth resistance is much higher - leaving a person in situation like he had drawn early in the video subject to near full supply voltage if the motor case were subjected to a ground fault. He needed to draw a solid bonding conductor (EGC) from the grounded conductor of the source and insert an overcurrent device in the ungrounded conductor to have a good drawing of what should be done.
 
LarryFine said:
Welcome to the forum.

Replace what he's calling earth with a grounding conductor, and he's actually explaining why we don't get shocked by touching the grounded (neutral) conductor.
Click to expand...
why we generally don't, there is resistance in that conductor and if it is carrying current there is some voltage drop in it, usually low enough you can't feel anything but get the right conditions and it can be very dangerous, this why we connect the grounding conductors at the service or near the source and do not carry any neutral current on them during normal operations.
 
tom baker said:
Arrrg!
First he uses an obsolete voltage, 110V
And too much focus on ohms law
What is this guys you tube channel?
And his wrong about grounding.
Click to expand...

120 is the supply voltage. 110 is the utilization voltage taking losses into account.
 
kwired said:
What protects us is "bonding".
"Grounding" is nothing more than making an earth reference to a point on the electrical system being grounded. For most multiwire AC systems NEC requires the "neutral conductor" to be what is grounded when a neutral conductor is present in the system.

Bonding protects us by connecting a conductor that is not used for normal current carrying functions to the grounded conductor and connecting that conductor to non current carrying components in the system, bonding all such non current carrying items together to effectively make them all same potential and with low resistance so that should an ungrounded conductor become faulted to it, high level of current will flow allowing overcurrent protective devices to react to said high current and open the ungrounded conductor. There is time factor in there as well as voltage division going on that still can leave some shock potential, but the low resistance does help assure it will only last for very short time.

I did not watch entire video only first couple minutes or so, but one critical protection thing not mentioned in what I did watch is he had no overcurrent protection device in his circuit. Sure impedance of the source will lessen voltage you will be exposed to, but that impedance is generally much much lower than the 10 ohms he used for an example. On top of that earth resistance is much higher - leaving a person in situation like he had drawn early in the video subject to near full supply voltage if the motor case were subjected to a ground fault. He needed to draw a solid bonding conductor (EGC) from the grounded conductor of the source and insert an overcurrent device in the ungrounded conductor to have a good drawing of what should be done.
Click to expand...

IEEE standards on grounding quite frequently show a diagram of a person standing on soil and run the calculation that way because that is the environment in an outdoor substation or for a utility. We can even do ground potential rise calculations.

The key is that the would be victim is about 1,000 ohms worst case (resistance through the epidermis, the inner skin layer, not just surface). So it’s a voltage divider and matters a great deal because you need a current of 100 mA or more at 5 seconds and quite a bit more at 100 milliseconds to induce fibrillation. So at 100 mA we’d need around 100 V and nearly a dead short and a long (over 5 seconds) trip time to induce fibrillation, assuming zero impedance through the bonding. In many cases with high resistance grounding even medium voltage is nonfatal and I know several victims who have “tested” this.
 
paulengr said:
IEEE standards on grounding quite frequently show a diagram of a person standing on soil and run the calculation that way because that is the environment in an outdoor substation or for a utility. We can even do ground potential rise calculations.

The key is that the would be victim is about 1,000 ohms worst case (resistance through the epidermis, the inner skin layer, not just surface). So it’s a voltage divider and matters a great deal because you need a current of 100 mA or more at 5 seconds and quite a bit more at 100 milliseconds to induce fibrillation. So at 100 mA we’d need around 100 V and nearly a dead short and a long (over 5 seconds) trip time to induce fibrillation, assuming zero impedance through the bonding. In many cases with high resistance grounding even medium voltage is nonfatal and I know several victims who have “tested” this.
Click to expand...
With high resistance grounding though, the place where you would be subjected to any significant voltage during a fault would be if you were able to contact the conductor between grounding resistor and the neutral of the source. You are otherwise in much similar situation as the "bird on a wire" as long as you can't contact something of different potential.
 
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