250.4(2)

P

Pinnie

Senior Member
Location
Ohio
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Commercial Electrician
“(2) Grounding of Electrical Equipment. Normally non current-carrying conductive materials enclosing electrical conductors or equipment, or forming part of such equipment, shall be connected to earth so as to limit the voltage to ground on these materials.”

When the code says limit the voltage to ground what does that mean?
 
Also why does 240(A)(1) mention line surges but 240(B)(1) does not. Are line surges not an issue with ungrounded systems?
 
Pinnie said:
“(2) Grounding of Electrical Equipment. Normally non current-carrying conductive materials enclosing electrical conductors or equipment, or forming part of such equipment, shall be connected to earth so as to limit the voltage to ground on these materials.”

When the code says limit the voltage to ground what does that mean?
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You want earth and your conduit to be "bonded" so they are at the same potential. If the conduit (for example) was floating, it could develop a potential relative to earth from leakage currents, high impedance faults, or capacitive coupling.

Pinnie said:
Also why does 240(A)(1) mention line surges but 240(B)(1) does not. Are line surges not an issue with ungrounded systems?
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Note that (A)(1) is about System grounding. This is when you take one conductor and connect it to earth to make that conductor stay close to earth potential. Despite what the code says, I dont see how system grounding would reduce "line surges". So then (B)(1) is talking about connecting electrical equipment to earth. Doing so, or not, would not influence "line surges" (regardless of whether its a grounded or ungrounded system).
 
electrofelon said:
If the conduit (for example) was floating, it could develop a potential relative to earth from leakage currents, high impedance faults, or capacitive coupling.
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Could you explain each scenario (leakage currents, high impedance faults, capacitive coupling) and why the potential could become dangerous or “non-limited”
electrofelon said:
Note that (A)(1) is about System grounding. This is when you take one conductor and connect it to earth to make that conductor stay close to earth potential. Despite what the code says, I dont see how system grounding would reduce "line surges". So then (B)(1) is talking about connecting electrical equipment to earth. Doing so, or not, would not influence "line surges" (regardless of whether its a grounded or ungrounded system).
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I see. I didn’t notice the different headings. In an ungrounded system we don’t ground the system, we ground the equipment.
 
Pinnie said:
Could you explain each scenario (leakage currents, high impedance faults, capacitive coupling) and why the potential could become dangerous or “non-limited”
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We have all this electrical stuff and in the real world insulation systems are never perfect. For example if you have heard of the issues with ranges and AC equipment tripping a class A GFCI because there is too much leakage current. You can also have insulation failures that are not a total failure but allow a small amount of current, but not enough to trip a breaker. For example recently we had a water heater that seemed totally fine, but we disconnected the EGC while redoing something (water line was non metallic) and the water heater became energized and I got shocked. I didnt measure it, but it was probably only 10's of milliamps, not nearly enough to trip the breaker, but with everything bonded, that shunts out the fault and the potential drops to nil (We replaced the unit). For the most part I think of connecting equipment to earth as just bonding, you want everything conductive (earth is kinda an exception as its a poor conductor but it is everywhere) connected together. The NEC and many people think of the earth as some magical thing that sucks up charges and solves all these problems, but its really just trying to tie everything together which can help equalize potential and thus reduce shock hazards.

Pinnie said:
I see. I didn’t notice the different headings. In an ungrounded system we don’t ground the system, we ground the equipment.
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(y)
 
And I just want to be clear that in my water heater example, just connecting the water heater to an isolated ground rod would not do anything to mitigate the raised potential of the unit. In that case it is bonding back to the source and to the other conduits and conductive things in the building in the vicinity of the water heater. I was more trying to give an example of a high impedance fault.
 
I guess I’m thinking about the codes reasoning/intent for grounding systems.

1. Voltage imposed by lightning.
(This I get as the electrons want to get to the earth when nearby lightning strikes.)

2.Line surges
(Not so sure on. I think I’ve heard of say there’s a fault on the utility side or capacitive switching)

3.Unintentional contact voltage lines
(I’m guessing if you need to clear a fault in a SDS)
 
Pinnie said:
I guess I’m thinking about the codes reasoning/intent for grounding systems.

1. Voltage imposed by lightning.
(This I get as the electrons want to get to the earth when nearby lightning strikes.)

2.Line surges
(Not so sure on. I think I’ve heard of say there’s a fault on the utility side or capacitive switching)

3.Unintentional contact voltage lines
(I’m guessing if you need to clear a fault in a SDS)
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System grounding is just a system topology. Grounded and ungrounded have advantages and disadvantages. System grounding determines insulation levels on conductors, locations of OCPD's, and how faults are handled. I am skeptical that system grounding really does much for the things you (and the NEC) mention.
 
Pinnie said:
2.Line surges
(Not so sure on. I think I’ve heard of say there’s a fault on the utility side or capacitive switching)
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The NEC is probably not worried about the utility type voltage surges most electricians think of.

There is a condition involving ungrounded systems where an arcing fault can cause resonance, with the coupling capacitance, resulting in a surge of near double the L-L voltage. This waz a big deal in the early 40s as industry quickly expanded. It is one of the reasons the NEC require ground detection on ungrounded systems.
 
jim dungar said:
The NEC is probably not worried about the utility type voltage surges most electricians think of.

There is a condition involving ungrounded systems where an arcing fault can cause resonance, with the coupling capacitance, resulting in a surge of near double the L-L voltage. This waz a big deal in the early 40s as industry quickly expanded. It is one of the reasons the NEC require ground detection on ungrounded systems.
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can you explain resonance and coupling capacitance?
 
Imagine that some fault event is occurring periodically. This event is charging a natural capacitance in the system. Now if the timing is right, another charging event can occur before the previous charge dissipates. This is like having two charged capacitors in series, which doubles the voltage of either alone. I'm sure someone else could explain it more eloquently.
 
Pinnie said:
can you explain resonance and coupling capacitance?
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This can be some advanced mathematics usually covered in engineering courses.
Go with CoolWill's explanation in post 11.

Coupling capacitance exists everywhere, it is a component of leakage current.
 
This might help with understanding the effects of a restriking fault on a ungrounded (or * NOT* earth bonded) system.


Resonance is a fun term that just means it is oscillatory with a excitation that might match or consist of the natural frequency of the system. You can look at bridges that match the natural frequency and mimic waves on the ocean.

Like jumping on a trampoline, if you stop jumping you will ultimately come to rest. That is considered dampen by gravity. If you keep up the energy you can keep jumping making it resemble a sine wave where the zero crossing is half way to the full height of the jump. But when someone "double" jumps you, you get a little more energy in your jump and fly a little higher. They match your timing to provide extra excitation to the "system". A bad double jump could take some of the energy out from under you and result in less of a jump.
 
Elect117 said:
This might help with understanding the effects of a restriking fault on a ungrounded (or * NOT* earth bonded) system.


Resonance is a fun term that just means it is oscillatory with a excitation that might match or consist of the natural frequency of the system. You can look at bridges that match the natural frequency and mimic waves on the ocean.

Like jumping on a trampoline, if you stop jumping you will ultimately come to rest. That is considered dampen by gravity. If you keep up the energy you can keep jumping making it resemble a sine wave where the zero crossing is half way to the full height of the jump. But when someone "double" jumps you, you get a little more energy in your jump and fly a little higher. They match your timing to provide extra excitation to the "system". A bad double jump could take some of the energy out from under you and result in less of a jump.
Click to expand...
A bad double jump ended with me have a broken ankle when I was 12.
 
Elect117 said:
This might help with understanding the effects of a restriking fault on a ungrounded (or * NOT* earth bonded) system.


Resonance is a fun term that just means it is oscillatory with a excitation that might match or consist of the natural frequency of the system. You can look at bridges that match the natural frequency and mimic waves on the ocean.

Like jumping on a trampoline, if you stop jumping you will ultimately come to rest. That is considered dampen by gravity. If you keep up the energy you can keep jumping making it resemble a sine wave where the zero crossing is half way to the full height of the jump. But when someone "double" jumps you, you get a little more energy in your jump and fly a little higher. They match your timing to provide extra excitation to the "system". A bad double jump could take some of the energy out from under you and result in less of a jump.
Click to expand...
Wow that’s a great explanation. Electrons are bound to physics as well.
 
Btw I didn't have time earlier, but I had a moment to dig it up and here is a more detailed description of the advantages and disadvantage of grounded and ungrounded systems, post #21:

What will the voltage be?

The reason for my, as to why we ground the electrical system. If I understand it correctly, If the system is not grounded the voltage could be any value, even though capacitive, there is a chance of getting shocked. Now, if the system is grounded the voltage value is stable at 120V and for sure...
forums.mikeholt.com
 
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