neutral or hot wire touch ground

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jaggedben

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Northern California
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Solar and Energy Storage Installer
As we all know, hot can touch ground or grounded metal, spark, melt metal, and sometimes not trip the breaker.

Agreed. The short has to be low impedance, and last long enough to raise the temperature of the breaker. Often momentary contact doesn't do this, and arc flash interrupts the contact before the breaker heats up.

I may be wrong but it seems to happen more often the longer the run back to the breaker, is it possible that resistance of wire affects breaker tripping?

It is possible, because as resistance is greater current becomes less. This is basic theory, Ohms law, V=IR. On the other hand, the resistance of copper wire is low enough that the length of run should have only a marginal effect on whether a breaker trips, except for very long runs.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Is this summary correct?

1) Hot wire touches Hot wires: Short circuit --> Breaker will trip

2) Hot wire touches Neutral wires: Short circuit --> Breaker will trip

3) Hot wire touches Equipment ground wires (or metal enclosure): Ground Fault --> Breaker will trip

4) Neutral wire touches Equipment ground wires (or metal enclosure): Ground Fault --> Only breaker with ground fault protection feature will trip (GFCI breaker or Electronic breaker with ground fault relay/protection)

Item #4: Per NEC, neutral wire and Equipment ground wires should not bonded together except at service disconnect or separate derived system, etc.

You have basically got it correct, notwithstanding my comment just above. I would only add that the 'metal enclosures' you refer to must be properly grounded for 3 and 4 to be absolutely true.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
One thing I'll add to the discussion is that when neutral touches ground when a load is engaged, the current in the "touch" is limited by the impedance of the load, but when a hot conductor touches neutral or ground, the current in the short is limited only by the OCPD on the circuit, which takes a finite amount of time to react. In that instant the current is limited only by the impedance of the conductors and transformers all the way back to the generation point.

Not that I am advocating this, but a low budget igniter for flashpods I have seen used in some theatrical productions is a direct short to ground through a single strand of copper wire taken from lamp cord. The line to the short is controlled by a simple pushbutton switch and the arc flash from the disintegrating strand of wire ignites the powder in the flashpod. It happens so fast that the OCPD doesn't have time to react.
 
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K8MHZ

Senior Member
Location
Michigan. It's a beautiful peninsula, I've looked
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Electrician
As we all know, hot can touch ground or grounded metal, spark, melt metal, and sometimes not trip the breaker. I may be wrong but it seems to happen more often the longer the run back to the breaker, is it possible that resistance of wire affects breaker tripping?

And many of us have cut a live romex, ruined our Kleins, cussed and then, assuming that the breaker must have tripped, done it again.

So any lab experiments inspired by this thread should be carried out with caution, or not at all.

Agreed.

I had to troubleshoot a dead circiut on a new addition. Turns out the drywaller cut through some live romex with a sawzall using a metal blade. It was on an AFCI breaker. It never tripped the breaker. When we finally found the romex, there was beads of melted copper where once existed solid wire.

I have seen 15 amp breakers carry 115 amps and not trip. True 115 amps, enough to drop the voltage in the house to the point of severely dimming the lights. The cause of the 115 amps was a dead short in an outside light, hot and neutral connected with a wire nut. The resistance of the wiring limited the current to 115 amps, but the breaker still should have tripped almost instantly.

I know an inspector that carries around a pocket knife with about 1/2 inch melted off the end of the blade. He was about 15 feet from the 15 amp breaker that didn't trip when he slipped and shorted out a hot and a neutral with his knife. The blade welded itself to the wires and finally fused open when the tip melted off.

All the above were on CH breakers. Nonetheless, I wouldn't experiment with breakers of any kind by intentionally shorting them out.
 
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readydave8

re member
Location
Clarkesville, Georgia
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electrician
Agreed. The short has to be low impedance, and last long enough to raise the temperature of the breaker. Often momentary contact doesn't do this, and arc flash interrupts the contact before the breaker heats up.



It is possible, because as resistance is greater current becomes less. This is basic theory, Ohms law, V=IR. On the other hand, the resistance of copper wire is low enough that the length of run should have only a marginal effect on whether a breaker trips, except for very long runs.
Like maybe 200' 12/2? (400' round trip).
 

jaggedben

Senior Member
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Solar and Energy Storage Installer
Like maybe 200' 12/2? (400' round trip).

According to the NEC Chapter 9 Table 9, the AC resistance for 12 awg is 2.0 ohms per 1000 feet. At 400' it would be two fifths of that, or .8 ohms. 120volts / .8ohms = 150amps

Splices and terminations would increase the resistance, but no, I don't think 400' of wire is the reason that the 15 or 20 amp breaker on that circuit didn't trip. Probably the short duration and large resistance of the short itself is the reason.
 

kwired

Electron manager
Location
NE Nebraska
According to the NEC Chapter 9 Table 9, the AC resistance for 12 awg is 2.0 ohms per 1000 feet. At 400' it would be two fifths of that, or .8 ohms. 120volts / .8ohms = 150amps

Splices and terminations would increase the resistance, but no, I don't think 400' of wire is the reason that the 15 or 20 amp breaker on that circuit didn't trip. Probably the short duration and large resistance of the short itself is the reason.

Limiting the current to 150 amps will result in slower trip time vs having a shorter circuit length and maybe the current is able to reach 5000 amps don't you think?

I run into this all the time with high inrush starting current equipment - especially with QO breakers. Plug chop saw into outlet next to panel, it will trip the breaker upon starting frequently. Plug same saw into same outlet but with 50 feet of extension cord and it never trips. The additional conductor added enough impedance to limit the current.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Not according to the NEC.

Table 8 lists DC resistance at 75 C. Table 9 lists AC resistance and reactance at 60 hz, three phase, three single conductors in conduit. (Not tubing nor cable) at 75 C.

The tables show different values of resistance at AC and DC.
Hmmm. I see that. That will take some looking into. I was taught that only inductance and capacitance have time dependent values.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Hmmm. I see that. That will take some looking into. I was taught that only inductance and capacitance have time dependent values.

Well.

Resistance itself is indeed independent of time and is therefore the same for DC and AC, but skin effect starts to rear its ugly head even at 60Hz for large diameter conductors, which reduces their effective cross sectional area and therefore increases their resistance.

Thank you for pointing that out, and now I've got to go fix a couple of things.
 
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K8MHZ

Senior Member
Location
Michigan. It's a beautiful peninsula, I've looked
Occupation
Electrician
Hmmm. I see that. That will take some looking into. I was taught that only inductance and capacitance have time dependent values.

That is true, but DC and AC will vary due to skin effect and proximity to other conductors with a difference in phasing. I think the latter may have something to due with eddy currents.

Note that the values in Table 9 are specific to three phase, three conductors in conduit. At 60 Hz, skin effect is almost negligible, but in reality so are the differences between Table 8 and Table 9. You will also note that the differences increase as the diameter of the conductor increases. I attribute that to skin effect.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
We must have been posting at the same time.

Great minds, perhaps?

:D

I'd like to think that.

A question, though. Ohms per unit distance makes sense to me, but Table 9 is labeled "Ohms to Neutral" per 1000' and per km; what does that mean?
 

dkarst

Senior Member
Location
Minnesota
I'd like to think that.

A question, though. Ohms per unit distance makes sense to me, but Table 9 is labeled "Ohms to Neutral" per 1000' and per km; what does that mean?

I think because table 9 was probably developed from the IEEE-141 voltage drop method where Vd = IR cos Φ + IX sin Φ, and specifically that is the line-neutral drop, therefore has to be multiplied by 2 for 1Φ and by √3 for 3Φ to determine the line-line drop. R and X must therefore be the ohms-neutral values.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Limiting the current to 150 amps will result in slower trip time vs having a shorter circuit length and maybe the current is able to reach 5000 amps don't you think?

Sure, good point. Circuit length will affect the trip time. I think we are still dealing with fractions of a second here, though, no?

I run into this all the time with high inrush starting current equipment - especially with QO breakers. Plug chop saw into outlet next to panel, it will trip the breaker upon starting frequently. Plug same saw into same outlet but with 50 feet of extension cord and it never trips. The additional conductor added enough impedance to limit the current.

I don't doubt you. Note also that extension cords are often 16 awg. Adding smaller conductor at the same time as you add length is a double whammy.
 

kwired

Electron manager
Location
NE Nebraska
Sure, good point. Circuit length will affect the trip time. I think we are still dealing with fractions of a second here, though, no?

That will all depend on what the instantaneous trip level is. A 50 amp breaker will hold for quite some time with 150 amps flowing as compared to a 15 amp breaker. 5000 amps of current and you will need more than a hand held stopwatch to check the time of either one.
 
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