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gs99

05:34PM | 12/07/02
Member Since: 11/18/02
30 lifetime posts
Dana1028, Here’s the converse question - If all the equipment grounding conductors are installed properly, but the grounding electrode conductor and/or the grounding electrode are NOT installed properly (or have degraded performance for some reason), what happens if a fault occurs (excluding situations where GFCIs provide protection)?

Since the grounding bus and neutral bus are bonded in the service panel, would the transformer’s earth ground provide any protection?
Thanks
Gs99

tdhorne

06:18AM | 12/08/02
Member Since: 09/01/02
31 lifetime posts
quote:
Dana1028, Here’s the converse question - If all the equipment grounding conductors are installed properly, but the grounding electrode conductor and/or the grounding electrode are NOT installed properly (or have degraded performance for some reason), what happens if a fault occurs (excluding situations where GFCIs provide protection)?

Since the grounding bus and neutral bus are bonded in the service panel, would the transformer’s earth ground provide any protection?
Thanks
Gs99


The purpose of the Grounding Electrode system is to "limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operation." You will notice that there is nothing in there about clearing faults. The code's language does not leave the matter there however. It goes on to say that "The earth shall not be used as the sole equipment grounding conductor or effective ground-fault current path." Section 250.4 (a) (3) lays out that fault current clearing is to be accomplished by bonding rather than by grounding.

So the answer to your first question is that the fault would be cleared by the Over Current Protective Device (OCPD). The circuit breaker or fuse would open the circuit in the normal manner.

The question you did not ask which I am going to answer anyway is what would happen with one of the situations that the whole grounding system is installed to cope with. In the event of a lightning strike, power cross, transformer puncture (primary to secondary fault), or a line surge the voltage within your homes electrical system would rise to the voltage of the unwanted event. Since the insulation of the conductors in the homes wiring is only rated at 600 volts the wiring system could be damaged and arcing to other conductive structure in your home could release sufficient energy to kindle a structure fire.

As to whether the utility transformer grounding electrode would provide any protection it would provide some but in my experience not much. I own a clamp on grounding electrode impedance tester. I have gotten into the habit of checking the impedance of the transformer electrode. I have yet to find one on a residential service transformer that is lower than fifty ohms.

In my un humble opinion the key to avoiding surge and spike damage to your home's wiring system and the appliances that are supplied from it is to install a low impedance grounding system and make sure that every wire carried service that enters your home is connected to it.
--
Tom

250.4 General Requirements for Grounding and Bonding.
The following general requirements identify what grounding and bonding of electrical systems are required to accomplish. The prescriptive methods contained in Article 250 shall be followed to comply with the performance requirements of this section.
(A) Grounded Systems.
(1) Electrical System Grounding. Electrical systems that are grounded shall be connected to earth in a manner that will limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operation.
(2) Grounding of Electrical Equipment. 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.
(3) Bonding of Electrical Equipment. Non–current-carrying conductive materials enclosing electrical conductors or equipment, or forming part of such equipment, shall be connected together and to the electrical supply source in a manner that establishes an effective ground-fault current path.
(4) Bonding of Electrically Conductive Materials and Other Equipment. Electrically conductive materials that are likely to become energized shall be connected together and to the electrical supply source in a manner that establishes an effective ground-fault current path.
(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 permanent, low-impedance circuit 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 used as the sole equipment grounding conductor or effective ground-fault current path.

[This message has been edited by tdhorne (edited December 08, 2002).]

gs99

05:52AM | 12/12/02
Member Since: 11/18/02
30 lifetime posts
tdhorne,
Thanks Tom. I had been viewing circuit breakers only for their protection against overloads.
But they trip also when faults / short circuits occur.

The breaker’s internal parts react in a similar way in all “overcurrent” events. I understand that some breakers have a bimetal part (similar to thermostat sensing temperature) that reacts to overloads, moving the breaker’s position to OFF. There is also an electromagnet that reacts even faster to faults and short circuits. I assume the reactions are based on amount of current the breaker senses. Perhaps some breakers work in other ways.

Are you saying that properly connected Grounding Electrode Conductors and Grounding Electrodes are not needed for circuit breakers to work?

What is a “low impedance grounding system” that you recommend?

Gs99

tdhorne

06:46AM | 12/14/02
Member Since: 09/01/02
31 lifetime posts
quote:
tdhorne,
Thanks Tom. I had been viewing circuit breakers only for their protection against overloads.
But they trip also when faults / short circuits occur.

The breaker’s internal parts react in a similar way in all “overcurrent” events. I understand that some breakers have a bimetal part (similar to thermostat sensing temperature) that reacts to overloads, moving the breaker’s position to OFF. There is also an electromagnet that reacts even faster to faults and short circuits. I assume the reactions are based on amount of current the breaker senses. Perhaps some breakers work in other ways.

Are you saying that properly connected Grounding Electrode Conductors and Grounding Electrodes are not needed for circuit breakers to work?

What is a “low impedance grounding system” that you recommend?

Gs99


Yes that is exactly what I am saying. The conductors that the code calls Equipment Grounding Conductors (EGCs) are actually bonding conductors. They bond the non current carrying parts of the electric system back to the utility neutral at the neutral bonding point. The Service Entry Conductor Neutral is terminated to the grounded Conductor buss bar in the service disconnecting means. That buss bar is bonded to the service disconnecting means enclosure. The EGCs in most homes are terminated to that same buss bar but in larger services they terminate to a buss bar that is mounted directly to the service disconnecting means enclosure without insulating mountings in order to make the only connection between the two buss bars the main bonding jumper. That allows the use of a current detecting coil around the main bonding jumper so that a large ground fault can be more quickly detected and the overcurrent device operation can be accelerated by the use of a shunt trip in the main breaker. This equipment is called Ground Fault Protection of Equipment (GFP).

Notice that the current path is from the transformer winding terminal Xa, one of the ungrounded (energized) service entry conductors, main breaker, one of the ungrounded buss bars, branch circuit breaker, ungrounded (energized) conductor, the fault (unintended contact with a conductive enclosure), equipment bonding pathway that the code calls the EGC, main bonding jumper (the screw or strap that bonds the grounded conductor buss bar to the enclosure), through the utility's neutral, to the X0 of the transformer which is the other end of the winding in which the current was produced.

You will observe that the current does not pass through the Grounding Electrode Conductor or the grounding electrode system to any great degree. You will also notice that there is no load in this unintended fault pathway. If the fault is low impedance, which is generally called a bolted fault, the magnetic element of the breaker opens it very quickly. If the fault is high impedance, such as an arcing fault, the current may continue to flow causing equipment damage or generating heat to ignite a fire unless the breaker is an Arc Fault Circuit Interrupter (AFCI) or the service is equipped with GFP.
--
Tom

gs99

09:43AM | 12/14/02
Member Since: 11/18/02
30 lifetime posts
tdhorne,
Thanks for clarifying that, and the listing of objects in the current path.

In an earlier post I had asked if the transformer’s earth ground would provide any protection. It appears I was on the right path (neutral conductor to the transformer), but evidently the utility’s separate grounding electrode (if available) is not important. For those persons who have the NEC 2002 Handbook, see Exhibit 250.1.

What is the Ground Fault Protection of Equipment (GFP) you mentioned; any web links to an existing product?

Gs99

tdhorne

05:21PM | 12/16/02
Member Since: 09/01/02
31 lifetime posts
quote:
tdhorne,
Thanks for clarifying that, and the listing of objects in the current path.

In an earlier post I had asked if the transformer’s earth ground would provide any protection. It appears I was on the right path (neutral conductor to the transformer), but evidently the utility’s separate grounding electrode (if available) is not important. For those persons who have the NEC 2002 Handbook, see Exhibit 250.1.

What is the Ground Fault Protection of Equipment (GFP) you mentioned; any web links to an existing product?

Gs99


Ground Fault Protection of Equipment (GFP) is a set of devices installed on a wye connected service larger than 800 amperes with a voltage greater than 150 volts to ground to prevent the destructive burn downs that can occur due to high impedance arcing faults in such services. Here is a link to one approach to this problem. http://www.squared.com/us/products/circuitb.nsf/07a0210021262d45862564b5006e4f84/947cdfc0a7f701208525699e007102b4/$FILE/0600DB0002.pdf
Another approach is to monitor the main bonding jumper for current flow but such currents can be caused by stray neutral return currents from other services.
--
Tom

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