Ground Fault.

Miguel c

Member
Location
Barranquilla colombia
Occupation
Bilingual agent
Mike I understand that a breaker should trip under 3 different circumstances. 1 Overcurrent 2 Short circuit 3 Ground fault. Analyzing the code at 240 seccion I realized that if a ground fault occurs the breaker should trip because the metal enclosure of the main panel has a very low resistance that will cause a huge amount of current to flow through the EGC back to the source. Now lets change the scenario, suppose that a hot wire makes contact with the metal part of an appliance within the electrical installation and the protective overcurrent device is a 40 amps breaker ( MCB) and the resistance of that specific metal part is 10 ohms. Ohms law says I = V/R 120 V / 10 Ω = 12 amps. the question is, Will that breaker trip or not? I think it should not trip, then how do we clear that fault? Does the fact the ECG is connected to the neutral wire result in a short circuit in the event a hot wire touched the metal enclosure of an (equipment) appliance?
 

Dsg319

Senior Member
Location
West Virginia
Occupation
Journeyman Electrician
Mike I understand that a breaker should trip under 3 different circumstances. 1 Overcurrent 2 Short circuit 3 Ground fault. Analyzing the code at 240 seccion I realized that if a ground fault occurs the breaker should trip because the metal enclosure of the main panel has a very low resistance that will cause a huge amount of current to flow through the EGC back to the source. Now lets change the scenario, suppose that a hot wire makes contact with the metal part of an appliance within the electrical installation and the protective overcurrent device is a 40 amps breaker ( MCB) and the resistance of that specific metal part is 10 ohms. Ohms law says I = V/R 120 V / 10 Ω = 12 amps. the question is, Will that breaker trip or not? I think it should not trip, then how do we clear that fault? Does the fact the ECG is connected to the neutral wire result in a short circuit in the event a hot wire touched the metal enclosure of an (equipment) appliance?
From my experience (short) Mike doesn’t actively answer questions in the forum. But plenty of educated people here to help you with your questions.

(MY take on it) In your example a 40amp breaker in the event of a ground fault, and there is no low impedance fault current return path, your example states a path of 10ohms resulting in 12amps. Unless the circuit has some kind of ground fault protection. The device will remain operating.

If you have a high resistance path back to the source where in such example it does not exceed the OCPD rating it will not trip, unless gfci protected. Then the GFCI device looks for current imbalance between the phase conductor and neutral. So any current say over 5ma returning to the source anyway other than the branch circuit neutral the device will open.

Your second question yes. The fact that the EGC is bonded back to the service neutral has absolutely everything to do with the low impedance path back the source and opening the OCPD. In essence with your example a hot wire touching a metallic case bonded to the EGC, yes it creates a short circuit/ground fault.
 
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don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
The metal parts of equipment are required to be connected to the Equipment Grounding Conductor, so the fault path should not be close to 10 ohms. The EGCs are sized based on the size of the upstream Overcurrent Protective Device to flow enough current to quickly open the OCPD with a ground fault.
 

Fred B

Senior Member
Location
Upstate, NY
Occupation
Electrician
This issue has been noted a lot in reference to remote lighting (parking lot, street lights) that have no ground wire back to source. Even if they have driven 3 rods per pole, if there was a short to the metal pole it would not clear the fault, and people have been injured. Mike does reference this issue in his training materials and videos. One mention of a person's dog being killed by peeing on a pole, no ground conductor (EGC) only a ground rod. If memory serves, their initial attempt to "fix" was to drive another rod, which did nothing to fix it, and I believe another injury did occur.
 

Dsg319

Senior Member
Location
West Virginia
Occupation
Journeyman Electrician
This issue has been noted a lot in reference to remote lighting (parking lot, street lights) that have no ground wire back to source. Even if they have driven 3 rods per pole, if there was a short to the metal pole it would not clear the fault, and people have been injured. Mike does reference this issue in his training materials and videos. One mention of a person's dog being killed by peeing on a pole, no ground conductor (EGC) only a ground rod. If memory serves, their initial attempt to "fix" was to drive another rod, which did nothing to fix it, and I believe another injury did occur.
I believe you are correct on their attempt to fix. Love his material. So simple and easy to understand.

Recently got his bonding and grounding 2020 book and get to see all the graphics, which is nice.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Now lets change the scenario, suppose that a hot wire makes contact with the metal part of an appliance within the electrical installation and the protective overcurrent device is a 40 amps breaker ( MCB) and the resistance of that specific metal part is 10 ohms.
This points out exactly why EGC conductors and pathways are so important, and why conductor upsizing triggers EGC upsizing. 10 ohms is way too high for grounding.

Does the fact the ECG is connected to the neutral wire result in a short circuit in the event a hot wire touched the metal enclosure of an (equipment) appliance?
Yes. The idea is for that contact to mimic a line-to-neutral fault.
 
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