Grounding/OCPD

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alixenos

alixenos

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Electrical Design Engineer
Happy New year!!

I'm trying to learn article 250, and it mentions in its beginning that grounding facilitates the operation of the OCPD.

My question is: How can grounding facilitates the operation of the OCDP devices ? my understanding is that grounding is to clear un wanted surge in voltage, while the OCPD is to detect if there is an a current overload/leakage/fault so that the device opens the circuit.

whats the relationship between grounding an electrical system and OCPD, and what about if the system if ungrounded? how to facilitate the operation of such devices in the absence of grounding.


Thank you
 
alixenos said:
Happy New year!!

I'm trying to learn article 250, and it mentions in its beginning that grounding facilitates the operation of the OCPD.

My question is: How can grounding facilitates the operation of the OCDP devices ? my understanding is that grounding is to clear un wanted surge in voltage, while the OCPD is to detect if there is an a current overload/leakage/fault so that the device opens the circuit.

whats the relationship between grounding an electrical system and OCPD, and what about if the system if ungrounded? how to facilitate the operation of such devices in the absence of grounding.


Thank you
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Where does it say that? The connection to earth has nothing to do with the fault clearing path and the operation of the OCPDs.
 
alixenos said:
I'm trying to learn article 250, and it mentions in its beginning that grounding facilitates the operation of the OCPD.
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250.4(A)(5) seems to say just the opposite.

250.4(A)(5) Effective Ground-Fault Current Path.
Electrical equipment and wiring and other electrically conductive material likely to become energized shall be installed in a manner that creates a low-impedance circuit facilitating the operation of the overcurrent device or ground detector for high-impedance grounded systems. It shall be capable of safely carrying the maximum ground-fault current likely to be imposed on it from any point on the wiring system where a ground fault may occur to the electrical supply source. The earth shall not be considered as an effective ground-fault current path.
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It's the equipment grounding conductor that facilitates clearing of line to ground faults by providing a low impedance path for the fault current to return to the transformer neutral. The return path through the earth generally has much higher impedance, so it can't be relied upon for fault clearing.
 
Eddie702 said:
I was thoroughly stuck when studying the code back when I got my license 40 years ago and article 250 made no sense to me until someone clarified the meaning of "grounded" and "grounding" conductors.
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Yes, there's the difference between grounded and grounding and then there's the difference between bonding and grounding. What's even more confusing is that the NEC has historically used the term grounding when they should be using the term bonding. There have been hundreds of PI's and proposals over the years to correct this but so far the CMP hasn't budged.
 
infinity said:
250.4(A)(5) seems to say just the opposite.
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From your own quote...

250.4(A)(5) Effective Ground-Fault Current Path.
Electrical equipment and wiring and other electrically conductive material likely to become energized shall be installed in a manner that creates a low-impedance circuit facilitating the operation of the overcurrent device or ground detector for high-impedance grounded systems. It shall be capable of safely carrying the maximum ground-fault current likely to be imposed on it from any point on the wiring system where a ground fault may occur to the electrical supply source. The earth shall not be considered as an effective ground-fault current path.
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alixenos said:
Happy New year!!

I'm trying to learn article 250, and it mentions in its beginning that grounding facilitates the operation of the OCPD.
Click to expand...
I'll try to actually address your question...

alixenos said:
My question is: How can grounding facilitates the operation of the OCDP devices ?
Click to expand...

Bonding is what facilitates the operation of an OCPD where the 'ground fault' has a low enough impedance. Grounding does not, except where an overcurrent device happens to also incorporate GFCI or ground-fault protection. Part V of Article 250 covers bonding. Most of the other parts cover grounding.

As roger mentioned above, the NEC confuses the situation by referring to conductors that bond equipment as 'equipment grounding conductors'. In reality EGCs do both, but they help open overcurrent devices by bonding, not grounding. We also talk about a 'ground fault' and 'ground fault current path' even though these things have more to do with bonding than with the earth. These phrases only make a bit of sense because the bonded parts are also supposed to be earthed.

alixenos said:
my understanding is that grounding is to clear un wanted surge in voltage,
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I think this is only barely correct in part. Grounding can provide a path that helps trip ground-fault protection from a high voltage source that accidently contacts lower voltage distribution, and it may provide a path for lightning discharge that mitigates damage. But those aren't exactly the main purposes of grounding. And grounding isn't by itself going to help with a high voltage surge from the intended power source.

alixenos said:
while the OCPD is to detect if there is an a current overload/leakage/fault so that the device opens the circuit.
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Overcurrent devices typically protect against two different things: overload from drawing too much power, or a short circuit. Leakage is something else. Fault is a more general term that could cover leakage detection (such as GFCI) or short circuit.

alixenos said:
whats the relationship between grounding an electrical system and OCPD, and what about if the system if ungrounded? how to facilitate the operation of such devices in the absence of grounding.
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In a grounded system the grounded conductor is also bonded to conductive parts; a low impedance fault from ungrounded conductors to bonded parts will therefore complete the circuit with enough current to open the overcurrent device.
In an ungrounded system that won't happen because grounded and bonded parts are not connected to one side of the circuit. That is why ungrounded systems are typically required to have fancier ground fault detectors.
 
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One thing that should help a lot with this topic is to clarify whether you referring to SYSTEM grounding, or equipment earthing, or equipment bonding. They are each different things which serve different purposes, and without know which one you mean, it is hard to offer much assistance. As Roger said, those three things are often just called "ground" so it is confusing.
 
David Castor said:
It's the equipment grounding conductor that facilitates clearing of line to ground faults by providing a low impedance path for the fault current to return to the transformer neutral. The return path through the earth generally has much higher impedance, so it can't be relied upon for fault clearing.
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IMHO... I believe this to be a half truth. Its not just the EGC, but the EGC in combination with the MBJ, or SBJ and the system Ungrounded conductor. :) :)
 
From your own quote...
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250.4(A)(5) Effective Ground-Fault Current Path.
Electrical equipment and wiring and other electrically conductive material likely to become energized shall be installed in a manner that creates a low-impedance circuit facilitating the operation of the overcurrent device or ground detector for high-impedance grounded systems. It shall be capable of safely carrying the maximum ground-fault current likely to be imposed on it from any point on the wiring system where a ground fault may occur to the electrical supply source. The earth shall not be considered as an effective ground-fault current path.
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250.4(A)(5) is about the effective ground-fault current path....the connection to earth is not a part of the effective ground-fault current path.
 
jaggedben said:
But the inclusion of 'ground' in the terminology is confusing for those who don't know that.
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That is one of the reasons Article 100 is probably the most important article in the code. The following was added to the 2020 code to address the issue that you bring up.
Effective Ground-Fault Current Path.
An intentionally constructed, low-impedance electrically conductive path designed and intended to carry current under ground-fault conditions from the point of a ground fault on a wiring system to the electrical supply source and that facilitates the operation of the overcurrent protective device or ground-fault detectors. (CMP-5)
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Note, contrast that with the definition of :
Ground-Fault Current Path.
An electrically conductive path from the point of a ground fault on a wiring system through normally non–current-carrying conductors, grounded conductors, equipment, or the earth to the electrical supply source. (CMP-5)
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The title of section 250.4(A)(5) is "Effective Ground-Fault Current Path"
 
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