Short circuit/ Ground Fault - Electrician Talk - Professional Electrical Contractors Forum
 Electrician Talk - Professional Electrical Contractors Forum Short circuit/ Ground Fault
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 02-12-2019, 02:44 PM #1 Member     Join Date: Oct 2018 Location: Indiana Posts: 98 Rewards Points: 6 Lets say a 120v circuit feeding a recipticle had the hot wire come loose and touch the grounded junction box. I understand that creates a path of no resistance because there's no longer a load and that causes the breaker to trip but what exactly happens. Is there a major current spike going back to the breaker and that causes it to trip? Or does the current take the path to ground back to the panel and where does it go from there? Where does the current actually enter the earth at? Difference between this and if the hot came off and touched the neutral?
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 02-12-2019, 02:49 PM #2 Senior Member   Join Date: Aug 2013 Location: Northeast USA Posts: 16,902 Rewards Points: 13,015 Do the math - voltage pushing current over extremely low resistance (metal) causes the current to go abnormally high, which trips the breaker. The current is trying to return to the source, not the earth. It's always flowing in a closed loop between the source and the load. Sometimes the earth could be a parallel path but the current is not automatically seeking earth, it's seeking to follow all available paths back to the source. The connection between all metallic paths and the neutral is made at the service disconnect, wherever that may be. This bonding jumper is what completes the path back to the source on the grounding system. positron likes this.
02-12-2019, 02:57 PM   #3
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Quote:
 Originally Posted by Maintenance2 Lets say a 120v circuit feeding a recipticle had the hot wire come loose and touch the grounded junction box. I understand that creates a path of no resistance because there's no longer a load and that causes the breaker to trip but what exactly happens. Is there a major current spike going back to the breaker and that causes it to trip? Or does the current take the path to ground back to the panel and where does it go from there? Where does the current actually enter the earth at? Difference between this and if the hot came off and touched the neutral?
The current doesn't enter the Earth. The current flows back thru the grounding system (in this case the ground wire in the cable), to the panel in which it flows thru the main bonding jumper and to the neutral, which returns to the transformer.

The only reason the current flows is because the neutral is bonded to ground at the main panel (and a few other places, but that might be too high resistance to trip the breaker).

02-12-2019, 04:48 PM   #4
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Quote:
 Originally Posted by Maintenance2 Lets say a 120v circuit feeding a recipticle had the hot wire come loose and touch the grounded junction box. I understand that creates a path of no resistance because there's no longer a load and that causes the breaker to trip but what exactly happens. Is there a major current spike going back to the breaker and that causes it to trip? Or does the current take the path to ground back to the panel and where does it go from there? Where does the current actually enter the earth at? Difference between this and if the hot came off and touched the neutral?
You seem to struggle with the concept of current.

Lets say electricity is water and wires are hose pipe.

Voltage is the pressure in the hose pipe. Notice that the hose pipe has pressure even when you have not opened the sprayer on the end of the hose.

Current is the flow. if you press the sprayer then water will flow through the pipe.

Ok now going back to a short circuit between a live and neutral/ground (technically the same as they are joined at some point).

A breaker is magnetic/thermal. During normal use something in the breaker warms up. At a set temperature (max amps rating or above) the part that heats moves and unlatches the breaker (this is why you have to turn a tripped breaker off before on so you can prime the spring that is held by the part that moves). I know this is not technical but different breakers have different ways to do this. (anyway thats the thermal side).
The magnetic side is simply a coil that when a huge amount of current travels through the loop cause a magnetic pin to be pulled and releases the breaker spring causing it to trip. This can be at 10000 times the named breaker rating.
Thermal side of the breaker is designed to be slow reacting and may go for quite a while before tripping (30amps on a 20amp breaker). Magnetic side is practically instant as should be able to clear the fault (trip) with in 1 cycle (60th of a second or less at 20,000 amps)

Google is smarter than me and can probably explain it better with pictures.

02-12-2019, 04:49 PM   #5
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Quote:
 Originally Posted by Maintenance2 Lets say a 120v circuit feeding a recipticle had the hot wire come loose and touch the grounded junction box. I understand that creates a path of no resistance because there's no longer a load and that causes the breaker to trip but what exactly happens. Is there a major current spike going back to the breaker and that causes it to trip? Or does the current take the path to ground back to the panel and where does it go from there? Where does the current actually enter the earth at? Difference between this and if the hot came off and touched the neutral?

YES.

The current goes back to the panel, and then returns to the transformer where it originated from.

Most of the current returns via the neutral line, some how ever may go via the earth. From there it will return to the transformer, which is grounded, so in essence it returns to the same spot as current via the neutral line, it just takes a different path.

Nothing.
All three previous answers, would apply with a neutral to hot fault, just like a ground to hot fault.

02-12-2019, 05:12 PM   #6
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Quote:
 Originally Posted by Maintenance2 Lets say a 120v circuit feeding a recipticle had the hot wire come loose and touch the grounded junction box. I understand that creates a path of LOW resistance because the RESISTANCE OF THE CIRCUIT CHANGES AND THAT RESULTS IN HIGH CURRENT that causes the breaker to trip but what exactly happens. Is there a major current spike going back to the breaker and that causes it to trip? Or does the current take the path to ground back to the panel and where does it go from there? Where does the current actually enter the earth at? Difference between this and if the hot came off and touched the neutral?
1. The current always returns to the source be it a transformer, generator, UPS or utility transformer.
2. The circuit breaker must supply the current through the low resistance fault (which is still a LOAD). When the current exceeds the rating of the circuit breaker ZTime Current Curve (TCC) the circuit breaker will trip.
3. Assuming a standard 20 amp circuit breaker the current typically would be in the range of 160-200 amps for an instantaneous trip.
4. Earth has nothing to do with anything in this instance GROUNDING and BONDING have everything to do with a low resistance (IMPEDANCE) path for an instantaneous operation of the circuit breaker.
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Last edited by brian john; 02-12-2019 at 05:14 PM.

02-12-2019, 06:34 PM   #7
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Quote:
 Originally Posted by Maintenance2 Lets say a 120v circuit feeding a recipticle had the hot wire come loose and touch the grounded junction box. I understand that creates a path of no resistance because there's no longer a load and that causes the breaker to trip but what exactly happens. Is there a major current spike going back to the breaker and that causes it to trip? Or does the current take the path to ground back to the panel and where does it go from there? Where does the current actually enter the earth at? Difference between this and if the hot came off and touched the neutral?
Repeat this to yourself until you scream it in your sleep: Current only flows in complete circuits.

At the main service, the green ground wire of your circuit is connected to the neutral. So a hot touching a grounded box is basically the same as the hot touching the neutral. A huge current will flow in a complete circuit and trip the breaker. The circuit is from the transformer, through the hot conductor, through the ground wire, through the ground-neutral connection (called the Main Bonding Jumper), through the neutral, and back to the transformer.

 02-12-2019, 06:50 PM #8 Senior Member     Join Date: Oct 2015 Location: South East US Posts: 6,566 Rewards Points: 836 This may help. Link below has other pics... https://www.google.com/search?client...24.4EV-F3e5Nn4 __________________ EARTH FIRST! We'll strip-mine the other planets later. To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts. Popcorn munching forum observer "The supreme art of war is to subdue the enemy without fighting." Sun Tzu
 02-12-2019, 07:15 PM #9 Senior Member     Join Date: Feb 2018 Location: Minnesota Posts: 181 Rewards Points: 22 What is a recipticle? Sent from my iPhone using Tapatalk
02-12-2019, 07:22 PM   #10
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Quote:
 Originally Posted by CoolWill Repeat this to yourself until you scream it in your sleep: Current only flows in complete circuits.
what about lightning, static, and galvanic reaction?
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02-12-2019, 07:37 PM   #11
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Quote:
 Originally Posted by Wiresmith what about lightning, static,
A capacitive circuit is still a circuit.

Quote:
 and galvanic reaction?
There is a complete circuit if you look close enough.

02-12-2019, 07:51 PM   #12
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Quote:
 Originally Posted by CoolWill A capacitive circuit is still a circuit. There is a complete circuit if you look close enough.
so i have a pile of extra electrons over here in this pile(sky, person, steel) and barely any over here in this pile (earth, couch, copper) and the electrons move to even out when i create a path between them, i only see one line and one flow direction. by circuit i think circle (rectangle, whatever, but no ends)

i'm just trying to screw with you, as you probably know. its a semantics argument. your statement was a good enough general statement i'm just being an ass
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 02-12-2019, 09:52 PM #13 Junior Member     Join Date: Jan 2018 Location: Rockies Posts: 13 Rewards Points: 4 Good thread. Recap: In order for current to flow, it needs a closed circuit which ultimately originates back to the transformer, generator, powerplant. I have a couple more questions to help me understand: For example, there is a transformer 1/4 mile away from a powerpole that has a breaker panel mounted on it. There is a hot conductor ran from a breaker to a recepticle next to the panel, and then the neutral on the recepticle was ran to a ground rod at the base of that powerpole. A lamp is then plugged into the recepticle. The lamp may illuminate a bit depending on conductivity of soil (depending on geology, moisture, salts, etc) being able to close the circuit back to the transformer. But the OCPD should trip from a thermal threshold. Correct? Are POCO transformers always bonded to buried grounding electrode? And if the transformer is tapped correctly, it's N is at the same potential as Earth before it's bonded? Thanks A electrically charged fence only operates properly if the grounding has an allowable resistance (soil moisture, geology, etc), to allow the soils to act as the GEC to complete the circuit. I understand how this functions when powered from 120V or a battery, but I'm having a hard time picturing how/why photovoltaic cells need a closed circuit.
02-13-2019, 05:24 PM   #14
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stand:

For example, there is a transformer 1/4 mile away from a power pole that has a breaker panel mounted on it. There is a hot conductor ran from a breaker to a receptacle next to the panel, and then the neutral on the receptacle was ran to a ground rod at the base of that power pole. A lamp is then plugged into the receptacle. The lamp may illuminate a bit depending on the conductivity of soil (depending on geology, moisture, salts, etc) being able to close the circuit back to the transformer. But the OCPD should trip from a thermal threshold. Correct?

More than likely the lamp will not draw enough current to produce light.

No light, no watts, no heat, no thermal trip no way the CB is going to trip.

Quote:
 Are POCO transformers always bonded to buried grounding electrode? And if the transformer is tapped correctly, it's N is at the same potential as Earth before it's bonded?
Before it is bonded NO, before it is bonded it "floats" it is an ungrounded system.

Thanks

Quote:
 A electrically charged fence only operates properly if the grounding has an allowable resistance (soil moisture, geology, etc), to allow the soils to act as the GEC to complete the circuit. I understand how this functions when powered from 120V or a battery, but I'm having a hard time picturing how/why photovoltaic cells need a closed circuit.
Think of a battery..you need to connect to the Positive and Negative terminals to operate your flashlight, CORRECT?
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02-13-2019, 06:04 PM   #15
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Quote:
 Originally Posted by positron The lamp may illuminate a bit depending on conductivity of soil (depending on geology, moisture, salts, etc) being able to close the circuit back to the transformer. But the OCPD should trip from a thermal threshold. Correct?
The ground itself has considerable resistance. So in this circuit you have the light in series with the ground resistance. You have more resistance, so less current.

Generally ground resistance alone is high enough that a short to the dirt won't clear a fault. And of course the light has enough resistance that the breaker wouldn't trip with the lamp alone. So the breaker DEFINITELY won't trip with the light AND the ground resistance.

I like this Mike Holt video:

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02-13-2019, 06:14 PM   #16
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Quote:
 Originally Posted by splatz The ground itself has considerable resistance. So in this circuit you have the light in series with the ground resistance. You have more resistance, so less current. Generally ground resistance alone is high enough that a short to the dirt won't clear a fault. And of course the light has enough resistance that the breaker wouldn't trip with the lamp alone. So the breaker DEFINITELY won't trip with the light AND the ground resistance. I like this Mike Holt video:
Yeah, he's so cute in his completely staged and unnecessary hard hat.

 02-13-2019, 06:26 PM #17 Light Bender     Join Date: Feb 2009 Location: Canada Posts: 5,734 Rewards Points: 8,168 A few times I have found abandoned wires feeding into the ground that were drawing current without shorting. They were all from customers complaining about high hydro bills. Usually an old shed feed or something drawing 9 or 10 amps 24/7.
02-14-2019, 07:55 PM   #18
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Quote:
 Originally Posted by splatz The ground itself has considerable resistance. So in this circuit you have the light in series with the ground resistance. You have more resistance, so less current.
Septic tank inspection?
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