230.95(A)

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charlie b

charlie b

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The installer asked my co-worker for the settings of a service breaker?s ground fault trip. This is a concept that I do not fully grasp. 230.95 requires this protection in certain systems, and 230.95(A) gives the maximum settings. My questions are,

1. Exactly what type of failure would cause this device to trip? What I am looking for is the current flow path.

2. Why is it acceptable to allow almost 1200 amps to flow along that path without initiating a trip, and why is it acceptable for as much as 3000 amps to flow for an entire second, before initiating a trip?
 
From my understanding the requirement for GFP is to limit the available ground fault current that can be imposed upon the EGC's in a system, as we know even conduit and EMT tubing and there connections has their limit, in systems over 150 volts to ground, connections in these raceways can fail catastrophically when exposed to very high energy current and GFP's should be design to limit this to a safe level, but also will require coordination with any down stream OCPD's although this part I have not seen any requirement for, I have seen the results.

Now I'm going to sit back and learn also as others chime in.:happyyes:
 
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Charlie,

My understanding is that 480 volt systems of 1000 amperes or more can have a ground fault of enough energy to damage the equipment without the fault current going into the instantaneous trip range of the overcurrent protective device.

Also check out the definition of equipment ground fault protection in Article 100 of the NEC.

Chris
 
Let me expound upon my question (or upon my confusion). Consider the scenario in which a wire inside a single phase motor breaks free and comes into contact with the case. There are now two paths for current to flow:

(1) From the breaker, via the phase conductor, into and through the motor, then back to the panel via the grounded conductor. This is the normal current path, and it might still function despite the fault.

(2) From the breaker, via the phase conductor, into the motor, via the broken wire to the case, to the point of connection of the EGC within the motor, via the EGC back to the source. This is the path of the "fault current," the high value of which causes the breaker’s instantaneous trip device to actuate, thereby terminating the event.

Now, can someone use a similar description for the functioning of the “ground fault protection for equipment” device? Exactly what would have to fail, and in what manner, for current to flow via what path(s), in order to actuate this device?
 
Option 1 is off the table as GFP does not watch that.


Picture an ungrounded conductor faulting to a raceway the conductor and raceway could burn back toward the source if it were not for the GFP. The arc would add enough impedance to keep a standard breaker or fuse from opening
 
Let's take a 2000 amp 480 volt switch gear with a 2000 amp main service disconnecting means. We have a ground fault with the bussing in the switch gear with a phase to ground fault. The ground fault is not a bolted fault but an arcing fault which would be a much lower fault current event. This arcing fault would have enough energy to destroy the switchgear but the amount of fault current flowing through the arcing fault would not be sufficient to get into the breakers instantaneous trip range. This would result in the switchgear being totally destroyed without the 2000 amp breaker ever tripping due to the lower fault current.

This was what was explained to me by Phil Simmons a few years ago at an NFPA 70E course that I took from him.

Chris
 
Charlie,
I believe that the high ground fault trip settins are an attempt to get some selective coordination and still protect the main equipment from an arcing burn down. The GFP will see the ground fault current, but the branch circuit breaker will also see it. If there is enough current flow the branch circuit OCPD will be in its instantaneous range and open the circuit before the GFP device does. If the branch device does not open the circuit and the ground fault current exceeds the GFP trip settings the GFP device will open the circuit.
A correctly installed GFP device will see the current from any downstream ground faults. If the settings are left at their factory minimum a ground fault on a 20 amp branch circuit may cause the GFP device to open.
 
Is planet Earth a player in this device's operation? Do you measure current returning to the source via the GEC? In my mind, the notion of a "line to ground fault" involves a transmission line that is knocked down by a falling tree. I can't imagine how an ungrounded conductor within a building's distribution system can come into contact with dirt. Am I overthinking this one?
 
charlie b said:
The installer asked my co-worker for the settings of a service breaker?s ground fault trip. This is a concept that I do not fully grasp. 230.95 requires this protection in certain systems, and 230.95(A) gives the maximum settings. My questions are,

1. Exactly what type of failure would cause this device to trip? What I am looking for is the current flow path.

2. Why is it acceptable to allow almost 1200 amps to flow along that path without initiating a trip, and why is it acceptable for as much as 3000 amps to flow for an entire second, before initiating a trip?
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1. Depends on the type of GFP. Ground return, zero sequence (my preference) and residual. Generally it different ways to "see" current going where it shouldn't be going.

2. Because the equipment is designed to fail safe, not catastrophically blast apart below those values. It will generally stay within its enclosure. It will likely need repalcement.

Generally, the ground fault current trip is often set about 20% of the trip rating, but needs to be coordinated with downstream devices and upstream if you know. When I work on large systems, I set the 230.95 GFP at 1200A and then work my way to lower values as I go lower into the distribution so I get selectivity.
 
charlie b said:
Is planet Earth a player in this device's operation? Do you measure current returning to the source via the GEC? In my mind, the notion of a "line to ground fault" involves a transmission line that is knocked down by a falling tree. I can't imagine how an ungrounded conductor within a building's distribution system can come into contact with dirt. Am I overthinking this one?
Click to expand...

I think you are over thinking this Charlie.:)

The earth is not a player in this devices operation. When I stated a ground fault I was talking about an arcing fault from an ungrounded phase conductor to a grounded and bonded piece of metal such as the case of the switchgear.

The GFP is designed to operate at a lower fault current then the instantaneous trip threshold of the normal overcurrent protective device. The GFP is installed to protect the service equipment from being damaged from a higher impedance arcing fault which may not have enough fault current flowing between the phase conductor and the grounded and bonded equipment to enter the instantaneous trip range of the overcurrent protective device.

Chris
 
charlie b said:
Is planet Earth a player in this device's operation? Do you measure current returning to the source via the GEC? In my mind, the notion of a "line to ground fault" involves a transmission line that is knocked down by a falling tree. I can't imagine how an ungrounded conductor within a building's distribution system can come into contact with dirt. Am I overthinking this one?
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Thank you NEC code panel for the terms Ground, Grounding, and Grounded.

The function of the GFP is the same as for a GFCI, other than the trip point. A fault is sensed when an imbalance of current exists between the normal current carrying conductors of the circuit. At the service entrance this typically only occurs when current is flowing on the 'grounding' conductor.

Back in the 60's it was noticed that on 480Y/277V systems, it was relatively easy to have 277V arcing currents of 1200A burn down buildings instead of burning themselves open.
 
charlie b said:
Is planet Earth a player in this device's operation? Do you measure current returning to the source via the GEC? In my mind, the notion of a "line to ground fault" involves a transmission line that is knocked down by a falling tree. I can't imagine how an ungrounded conductor within a building's distribution system can come into contact with dirt. Am I overthinking this one?
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You are overthinking the part of having a phase in contact with dirt. With under 600 volt systems you will usually have a hard time getting more than 5 amps to flow in this case.

raider1 said:
I think you are over thinking this Charlie.:)

The earth is not a player in this devices operation. When I stated a ground fault I was talking about an arcing fault from an ungrounded phase conductor to a grounded and bonded piece of metal such as the case of the switchgear.

The GFP is designed to operate at a lower fault current then the instantaneous trip threshold of the normal overcurrent protective device. The GFP is installed to protect the service equipment from being damaged from a higher impedance arcing fault which may not have enough fault current flowing between the phase conductor and the grounded and bonded equipment to enter the instantaneous trip range of the overcurrent protective device.

Chris
Click to expand...


:thumbsup:
 
charlie b said:
Let me expound upon my question (or upon my confusion). Consider the scenario in which a wire inside a single phase motor breaks free and comes into contact with the case. There are now two paths for current to flow:

(1) From the breaker, via the phase conductor, into and through the motor, then back to the panel via the grounded conductor. This is the normal current path, and it might still function despite the fault.

(2) From the breaker, via the phase conductor, into the motor, via the broken wire to the case, to the point of connection of the EGC within the motor, via the EGC back to the source. This is the path of the "fault current," the high value of which causes the breaker’s instantaneous trip device to actuate, thereby terminating the event.

Now, can someone use a similar description for the functioning of the “ground fault protection for equipment” device? Exactly what would have to fail, and in what manner, for current to flow via what path(s), in order to actuate this device?
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A similar description for the ground fault path for a GFPE function is exactly the same as you describe in #2.

The only difference is where or how the current is measured. Instead of measuring the current on a phase conductor, GFPE is measuring an unbalance in the phase currents, or it measures the current flowing on the EGC.

As others have explained, the GFPE setting is usually set lower than the breakers instantenous setting. So the GFPE will normally trip before the breakers instantenous function on a line-ground short.

However, in some cases I think the Code would allow you to set the GFPE setting higher than the breakers instantaneous setting. (An 800A service could have a 2400A inst. trip point, with the GFPE set at the code max. of 3000A.) In that case, I beileve the GFPE is basically a back up in case the breaker's instantenous function fails to trip when it should.

That may explain why the code lets the GFPE settings be so high - in most cases the instaneous trip has already been designed to protect the conductors per the NEC.
 
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steve66 said:
However, in some cases I think the Code would allow you to set the GFPE setting higher than the breakers instantaneous setting. (An 800A service could have a 2400A inst. trip point, with the GFPE set at the code max. of 3000A.) In that case, I beileve the GFPE is basically a back up in case the breaker's instantenous function fails to trip when it should.

That may explain why the code lets the GFPE settings be so high - in most cases the instaneous trip has already been designed to protect the conductors per the NEC.
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The code maximum setting for GFPE is 1200A.
It was chosen basically due to evidence of fires caused by 277V to Ground faults not being cleared by standard protective devices. Back in the early 70's there were not as many fires associated with services 1200A and smaller as there were for larger ones.
 
jim dungar said:
The code maximum setting for GFPE is 1200A.
It was chosen basically due to evidence of fires caused by 277V to Ground faults not being cleared by standard protective devices. Back in the early 70's there were not as many fires associated with services 1200A and smaller as there were for larger ones.
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Yes, but isn't the limit allowed to be as high as 3000 amps for faults that last less than a second? Isn't that the setting that would normally trip before a circuit breakers instantaneous setting?
 
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