Circuit Breakers
- Thread starter scrino
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- Bremerton, Washington
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- Master Electrician
Re: Circuit Breakers
A class A GFCI trips at 4-6 millamps. A 20 amp circuit breaker will trip at 40 amps in several minutes. The GFCI is for personal proteciton, there is a GFPE breaker, it trips at 30-50 ma and is used for equipment protection.
A class A GFCI trips at 4-6 millamps. A 20 amp circuit breaker will trip at 40 amps in several minutes. The GFCI is for personal proteciton, there is a GFPE breaker, it trips at 30-50 ma and is used for equipment protection.
brian john
Senior Member
- Location
- Kilmarnock, Va
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- Retired after 52 years in the trade.
Re: Circuit Breakers
In answer to you first question A circuit breaker will clear a ground fault condition (assuming the system is wired corectly), the problem is the level of fault current required to obtain the trip.
In answer to you first question A circuit breaker will clear a ground fault condition (assuming the system is wired corectly), the problem is the level of fault current required to obtain the trip.
B
bthielen
Guest
Re: Circuit Breakers
My understanding of a GFCI is that it monitors the current on the hot and grounded legs for a difference in excess of 4-6mA. Normal operation is that all load current flows on these two legs only. A regular circuit breaker only trips due to excessive heat caused by excessive current over a given time period. Any current on the hot leg that is not on the grounded leg indicates a ground fault regardless of the level of current. The GFCI is more capable of reacting to even a partial fault condition adding a degree of safety.
Bob
My understanding of a GFCI is that it monitors the current on the hot and grounded legs for a difference in excess of 4-6mA. Normal operation is that all load current flows on these two legs only. A regular circuit breaker only trips due to excessive heat caused by excessive current over a given time period. Any current on the hot leg that is not on the grounded leg indicates a ground fault regardless of the level of current. The GFCI is more capable of reacting to even a partial fault condition adding a degree of safety.
Bob
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: Circuit Breakers
As a ?thought experiment? (DON?T DO THIS AT HOME!!), suppose a person unbent a coat hanger, brought it outside, stuck one end into dirt, and stuck the other end into the ?hot? terminal of an outdoor receptacle. This creates a current path from the source, through the branch circuit breaker, to the receptacle, via the coat hanger into dirt, through planet Earth to the service panel?s grounding electrode, and up the grounding electrode conductor back to the source. I would call that a ?ground fault.? Will the circuit breaker trip? No it will not. The current is too small to trip the breaker, because the resistance of the dirt is too high. Would a GFCI breaker (or an upstream GFCI receptacle) trip? Yes it would. That is because there is current leaving the hot leg and not returning via the neutral leg.
Now let me change the scenario only slightly. Let?s forget the coat hanger. In its place, suppose a person is working outdoors with an electric leaf blower. It is plugged into an extension cord and that is plugged into an outdoor receptacle. The insulation of the extension cord has been damaged and the ground is wet. Current travels from the hot leg of the extension cord to the outside of the extension cord (due to its being wet), along the outside of the wet cord to the hand of the person holding the leaf blower, through the person?s body to her feet and into the dirt. From there it returns to the source in the same way I describe above. I would also call that a ?ground fault.? Will the circuit breaker trip? No it did not, for the reason given above. Would a GFCI breaker (or an upstream GFCI receptacle) trip? It might have, but that receptacle had no such protection. I found this story several years ago in a Chicago area newspaper. The woman?s body was found several hours later by her son.
That is why outdoor receptacles are required to have GFCI protection.
Let?s take a moment to be sure we are using the term ?ground fault? in the same context.
As a ?thought experiment? (DON?T DO THIS AT HOME!!), suppose a person unbent a coat hanger, brought it outside, stuck one end into dirt, and stuck the other end into the ?hot? terminal of an outdoor receptacle. This creates a current path from the source, through the branch circuit breaker, to the receptacle, via the coat hanger into dirt, through planet Earth to the service panel?s grounding electrode, and up the grounding electrode conductor back to the source. I would call that a ?ground fault.? Will the circuit breaker trip? No it will not. The current is too small to trip the breaker, because the resistance of the dirt is too high. Would a GFCI breaker (or an upstream GFCI receptacle) trip? Yes it would. That is because there is current leaving the hot leg and not returning via the neutral leg.
Now let me change the scenario only slightly. Let?s forget the coat hanger. In its place, suppose a person is working outdoors with an electric leaf blower. It is plugged into an extension cord and that is plugged into an outdoor receptacle. The insulation of the extension cord has been damaged and the ground is wet. Current travels from the hot leg of the extension cord to the outside of the extension cord (due to its being wet), along the outside of the wet cord to the hand of the person holding the leaf blower, through the person?s body to her feet and into the dirt. From there it returns to the source in the same way I describe above. I would also call that a ?ground fault.? Will the circuit breaker trip? No it did not, for the reason given above. Would a GFCI breaker (or an upstream GFCI receptacle) trip? It might have, but that receptacle had no such protection. I found this story several years ago in a Chicago area newspaper. The woman?s body was found several hours later by her son.
That is why outdoor receptacles are required to have GFCI protection.
Re: Circuit Breakers
This is the way I look at this:
Even though we would like them to do otherwise the common UL489 circuit beakers are thermal magnetic meaning they respond to both I2t and instantaneous current.
When you referred to ground faults, ground faults are either bolted or arcing faults, either L-L, L-N, or L-G. If bolted they a very high current, a current limited to what is available at the service entrance and the impedance of the current path to the point of the fault. A breaker protecting a circuit which is subjected to a bolted fault will with almost all certainly trip instantaneously by means of its magnetic element. Please note the magnetic pick-up may be as low as 6-7x to as much as 10x an sometimes more than the breaker rating.
Arcing faults may never trip a breaker. They may begin as somewhat of a bolted fault but quickly burn apart to form an arc. As such a current that may start at somewhat of a high value decreased due to the impedance of the arc. As such that current often isn't high enough to trip the breaker instantaneously. The only other option is the breaker to trip on overload by sustaining an overload current for a long enough time to trip the breaker thermally which most likely wouldn't happen.
In order to provide better defense against ground fault electronics sensing is used to monitor L-L and/or L-N currents that service a branch circuit, a circuit of which measure the current going to and from the load. When there is an imbalance sensed the imbalance is considered to be caused by current flowing back to the source through ground.
Class A ground fault is what most of us are familiar with that is provided with ground fault breakers and receptacles. They are designed to trip when fault current to ground exceeds 6ma which is a fraction of the magnetic pick-up value of a comparable TM breaker.
Class B ground fault is 20ma or more. These breakers are commonly 30ma. I can't recall that it is stated in any literature that the AFCI provides Class B protection though.
This is the way I look at this:
Even though we would like them to do otherwise the common UL489 circuit beakers are thermal magnetic meaning they respond to both I2t and instantaneous current.
When you referred to ground faults, ground faults are either bolted or arcing faults, either L-L, L-N, or L-G. If bolted they a very high current, a current limited to what is available at the service entrance and the impedance of the current path to the point of the fault. A breaker protecting a circuit which is subjected to a bolted fault will with almost all certainly trip instantaneously by means of its magnetic element. Please note the magnetic pick-up may be as low as 6-7x to as much as 10x an sometimes more than the breaker rating.
Arcing faults may never trip a breaker. They may begin as somewhat of a bolted fault but quickly burn apart to form an arc. As such a current that may start at somewhat of a high value decreased due to the impedance of the arc. As such that current often isn't high enough to trip the breaker instantaneously. The only other option is the breaker to trip on overload by sustaining an overload current for a long enough time to trip the breaker thermally which most likely wouldn't happen.
In order to provide better defense against ground fault electronics sensing is used to monitor L-L and/or L-N currents that service a branch circuit, a circuit of which measure the current going to and from the load. When there is an imbalance sensed the imbalance is considered to be caused by current flowing back to the source through ground.
Class A ground fault is what most of us are familiar with that is provided with ground fault breakers and receptacles. They are designed to trip when fault current to ground exceeds 6ma which is a fraction of the magnetic pick-up value of a comparable TM breaker.
Class B ground fault is 20ma or more. These breakers are commonly 30ma. I can't recall that it is stated in any literature that the AFCI provides Class B protection though.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: Circuit Breakers
In this sense, ?bolted? means ?in intimate and permanent contact.? The coat hanger example that I described above would qualify as a bolted, L-G fault. I agree that a bolted L-L or L-N fault would result in very high current and immediate trip of the breaker. A bolted L-G would not, or at least would not for systems under 480 volts. Since planet Earth is part of the path that the fault current must take to get back to the source, the current available in a L-G fault from a household 120/240 volt system will not be enough to trip the breaker.
bphgravity
Senior Member
- Location
- Florida
Re: Circuit Breakers
Would a L-L fault technically be a ground-fault? I always considered that a short-circuit. I know it results in the same effects and conditions as far as the OCD is concerned but, what would be the proper used terminolgy for a L-L fault?
Would a L-L fault technically be a ground-fault? I always considered that a short-circuit. I know it results in the same effects and conditions as far as the OCD is concerned but, what would be the proper used terminolgy for a L-L fault?
Re: Circuit Breakers
Bryan, I also like the term "short circuit" and think it would apply in the sense of "shorting" the current path.
I also think "Bolted" is relevant, and is correct IMO.
I don't know about you and the others, but I find myself being more "Electrically Correct" in conversation now than in my earlier career. I would refer to this as a "Line to Line Fault" in lieu of "Short Circuit" if I were discussing this with another trade person while trouble shooting an event.
I miss the simple days. Of course I may be telling my age.
Roger
[ July 14, 2004, 07:26 PM: Message edited by: roger ]
Bryan, I also like the term "short circuit" and think it would apply in the sense of "shorting" the current path.
I also think "Bolted" is relevant, and is correct IMO.
I don't know about you and the others, but I find myself being more "Electrically Correct" in conversation now than in my earlier career. I would refer to this as a "Line to Line Fault" in lieu of "Short Circuit" if I were discussing this with another trade person while trouble shooting an event.
I miss the simple days. Of course I may be telling my age.
Roger
[ July 14, 2004, 07:26 PM: Message edited by: roger ]
electricman2
Senior Member
- Location
- North Carolina
- Occupation
- Retired Electrical Contractor
Re: Circuit Breakers
Me too. I find myself using "conductor" instead of wire, "lamp" instead of bulb, etc. :roll:
Re: Circuit Breakers
You make a good point Charlie. If the current faults to ground it isn't necessarily faulted to an equipment grounding conductor but to an object that it itself is grounded to earth. As such there is a darned good possibility that there wouldn't be a high enough magnitude of current to trip a common TM breaker instantaneously.
To beguin with isn't the maximum allowable resistance of a ground electrode to ground 25ohms? That by itself limits current back through ground not even considering any other factors between the point of the ground fault and the grounding electrode which is something that I overlooked.
You make a good point Charlie. If the current faults to ground it isn't necessarily faulted to an equipment grounding conductor but to an object that it itself is grounded to earth. As such there is a darned good possibility that there wouldn't be a high enough magnitude of current to trip a common TM breaker instantaneously.
To beguin with isn't the maximum allowable resistance of a ground electrode to ground 25ohms? That by itself limits current back through ground not even considering any other factors between the point of the ground fault and the grounding electrode which is something that I overlooked.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: Circuit Breakers
At my desk I have a well-respected textbook by a well-respected author, and its subject is fault analysis. It names the four most common types of faults as follows:
</font>
</font>
You are right, Bryan. An L-L is not a "ground fault."
At my desk I have a well-respected textbook by a well-respected author, and its subject is fault analysis. It names the four most common types of faults as follows:
</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">Single Line to Ground Fault (SLG)</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">Line to Line Fault (L-L)</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">Double Line to Ground Fault (2LG)</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">3 Phase Fault (3 ph)</font>
</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">3 ph - 5%</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">2LG - 10%</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">L-L - 15%</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">SLG - 70%</font>
Re: Circuit Breakers
GFCI type CB's protect personnel by tripping at 6 mA or more (QO-GFI.) As some have said, there are ground fault type CB's with tripping set at 30mA or more (QO-EPD,) for equipment protection such as heat tracing. To complete the picture, no one has mentioned ground fault trip units on large CB's, set at hundreds of amps, used to detect large arcing faults to ground which could be missed by a CB without a ground fault trip function, because the arcing fault just looks like a regular load to the regular trip elements.
Ed MacLaren
Senior Member
Re: Circuit Breakers
These are used in low-voltage, high-current systems to reduce the damage that can result from an arcing ground fault.
For example, an arcing fault of, say 900 amps, will never trip a 1200 amp breaker in a 480 volt service, but a GF relay set for 500 amps will sense this type of fault.
Ed
[ July 16, 2004, 12:09 AM: Message edited by: Ed MacLaren ]
Scrino, as sparkie001 has mentioned, perhaps you were referring to a shunt trip breaker with a ground fault relay and zero-sequence GF sensor.
These are used in low-voltage, high-current systems to reduce the damage that can result from an arcing ground fault.
For example, an arcing fault of, say 900 amps, will never trip a 1200 amp breaker in a 480 volt service, but a GF relay set for 500 amps will sense this type of fault.
Ed
[ July 16, 2004, 12:09 AM: Message edited by: Ed MacLaren ]
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