No Short circuit to ground ?

K8MHZ

Senior Member
So what if it's shorted to ground?

Thank you a million times for showing us that it's bonding, not grounding that makes things safe.
 

mivey

Senior Member
The electricity is trying to get back to the source, not the earth. In using the earth (or other weak path) as a path back to the source, there is a lot of impedance that limits the current flow. When someone contacts that weak path and provides an alternate path that has less impedance, they get more of the current flow.

If the earth at that point had a low enough impedance path to the source, it would have tripped the breaker, allowed enough current to melt the wire, etc.
 

Ropeadope

Member
Can you explain this mathmaticaly ?

I see short circuits to ground, all the time, I've even seen a utility feed phase contact the side of a panel, and explode. Here you have a 120V circuit in contact with the ground, and no short, or arc, or burning of wires.

In a resister, you have something controlling the amount of current. In this case, I would think the resister (the light pole) would be very large, and let an infinite amount of current pass through. Or at least enought to burn the wire up feeding this.
 

Little Bill

Moderator
Staff member
Can you explain this mathmaticaly ?

I see short circuits to ground, all the time, I've even seen a utility feed phase contact the side of a panel, and explode. Here you have a 120V circuit in contact with the ground, and no short, or arc, or burning of wires.

In a resister, you have something controlling the amount of current. In this case, I would think the resister (the light pole) would be very large, and let an infinite amount of current pass through. Or at least enought to burn the wire up feeding this.
If the resistance of the earth around the fault were around 25ohms (which I doubt it is that low) and if the voltage was 120, then 120/25=4.8A, which would not trip a 15A breaker. But it would injure or kill someone.
 

jumper

Senior Member
Simple ohms law. If You connected 120v to a 12# wire on a 20 amp breaker to a ground rod with 25 ohms of resistance, how much current would flow?

4.8 amps. Breaker ain't gonna trip.


Little Bill types faster than me.
 

K8MHZ

Senior Member
Can you explain this mathmaticaly ?

I see short circuits to ground, all the time, I've even seen a utility feed phase contact the side of a panel, and explode. Here you have a 120V circuit in contact with the ground, and no short, or arc, or burning of wires.

In a resister, you have something controlling the amount of current. In this case, I would think the resister (the light pole) would be very large, and let an infinite amount of current pass through. Or at least enought to burn the wire up feeding this.
Before you even think about trying to understand the math, you need to accept the fact that the electricity is NOT seeking a path to ground. It is seeking any and all paths to back to the source. The earth and stuff sticking in it are in parallel with the intentional connection back to the source, usually the neutral. If the intentional connection becomes compromised and get some extra impedance, the earth is now charged with the return, pun intended. If the earth presents 100 ohms of impedance back to the source to 199 of the 120 volts (the other managing to get through the neutral) 1.99 amps will flow through it. That is not even close to being able to trip a breaker. But it is enough to make a 400 watt light bulb light and is also about 10 times over what is need for a fatal shock.
 

Ropeadope

Member
Simple ohms law. If You connected 120v to a 12# wire on a 20 amp breaker to a ground rod with 25 ohms of resistance, how much current would flow?

4.8 amps. Breaker ain't gonna trip.


Little Bill types faster than me.
Thx, I really just found here, a great explaination, as to why

Then why is it called an ?Equipment Grounding Conductor (EGC)? in the NEC if it?s primary purpose is to ?bond? things together? Simple answer: tradition. It?s always been called that, and the terms in the NEC have served to confuse people for a long time. Proposals have been made to change the term, and progress has been made, but the EGC continues to hold it?s misnomer.

Electricity does not seek the path of least resistance to the earth. It seeks all available paths back to it?s source, in proportion to their resistance. The reason that a person gets shocked when touching an ungrounded conductor and the earth is because the neutral of the system is repeatedly connected to earth in a grounded electrical system. The earth becomes part of a return path to the transformer ? it?s part of one route back to the source; the earth is not the destination for the electricity.

Driving a ground rod to ?ground? any electrical equipment does not provide the low-resistance path required to trip breakers. Driving a ground rod, or using a Ufer, or a metal water pipe is not a substitute for an EGC. A ground rod with 25 ohms to earth will allow almost five amps to escape the system into the earth when directly energized from a 120V source. Five amps will never trip a 15A or 20A breaker, and in the meantime everything bonded to this ground rod will be energized to 120V.
 

K8MHZ

Senior Member
Thx, I really just found here, a great explaination, as to why

Then why is it called an ?Equipment Grounding Conductor (EGC)? in the NEC if it?s primary purpose is to ?bond? things together? Simple answer: tradition. It?s always been called that, and the terms in the NEC have served to confuse people for a long time. Proposals have been made to change the term, and progress has been made, but the EGC continues to hold it?s misnomer.

Electricity does not seek the path of least resistance to the earth. It seeks all available paths back to it?s source, in proportion to their resistance. The reason that a person gets shocked when touching an ungrounded conductor and the earth is because the neutral of the system is repeatedly connected to earth in a grounded electrical system. The earth becomes part of a return path to the transformer ? it?s part of one route back to the source; the earth is not the destination for the electricity.

Driving a ground rod to ?ground? any electrical equipment does not provide the low-resistance path required to trip breakers. Driving a ground rod, or using a Ufer, or a metal water pipe is not a substitute for an EGC. A ground rod with 25 ohms to earth will allow almost five amps to escape the system into the earth when directly energized from a 120V source. Five amps will never trip a 15A or 20A breaker, and in the meantime everything bonded to this ground rod will be energized to 120V.
Well, I was close.

;)
 

Ropeadope

Member
Well, I was close.

;)

Thx, You were !!
I was in class, and I remember the instructor saying, that electricity is trying to get back to the generator, and uses earth to do so if no other path was available.

But the BONDING really explained the reason as to why. Interesting stuff, anyway. Also shows, how a dangerous a situation could become, when a Hot, comes in contact, with a ungrounded surface, and the resistance is low.
 
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