current 150+a. on grounding conductor
- Thread starter 5868
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- Illinois
- Occupation
- retired electrician
Re: current 150+a. on grounding conductor
Is this a grounded system? What size are the OCPDs for the compressors? Are the OCPDs breakers, fuses or motor cirucit protectors?
Don
Is this a grounded system? What size are the OCPDs for the compressors? Are the OCPDs breakers, fuses or motor cirucit protectors?
Don
Re: current 150+a. on grounding conductor
First , Im getting all my info second hand from a plant elect. asking me . It s a grounded system. And since the megger and grounding test were both done at the peckerhead should be good connection there . Don't know ocp. probably 300 + a. Square D cbs. Will try to find out more , After 30 yrs never heard of this ,abreak down even if only during run in a motor should have tripped.
First , Im getting all my info second hand from a plant elect. asking me . It s a grounded system. And since the megger and grounding test were both done at the peckerhead should be good connection there . Don't know ocp. probably 300 + a. Square D cbs. Will try to find out more , After 30 yrs never heard of this ,abreak down even if only during run in a motor should have tripped.
- Location
- Illinois
- Occupation
- retired electrician
Re: current 150+a. on grounding conductor
Don
In a high impedance grounded system the ground fault current is normally limited to less than 20 amps and often to 5 amps.
Don
Re: current 150+a. on grounding conductor
Many facilities such as the one that I work at, disable the trip mechanism totally. This is done because of the need for orderly shutdown. If you totally took one whole MCC bank down, a fire would most definitely occur.
You could weld one leg to the grounding bus, and still not trip. You will get an alarm, two bright phase lights, and the grounded phase light will be out. But until a second phase goes to ground, nothing will trip. Matt
Don,
Many facilities such as the one that I work at, disable the trip mechanism totally. This is done because of the need for orderly shutdown. If you totally took one whole MCC bank down, a fire would most definitely occur.
You could weld one leg to the grounding bus, and still not trip. You will get an alarm, two bright phase lights, and the grounded phase light will be out. But until a second phase goes to ground, nothing will trip. Matt
Re: current 150+a. on grounding conductor
Don,
Do you mean that even with a solid ground fault, the current is normally limited to less than 20 amps because of the high impedance resistor? I think I see your point. You're not saying it will trip, but depending on the size of the resistor, current would be limited to under 20 amps.
Don,
Do you mean that even with a solid ground fault, the current is normally limited to less than 20 amps because of the high impedance resistor? I think I see your point. You're not saying it will trip, but depending on the size of the resistor, current would be limited to under 20 amps.
- Location
- Illinois
- Occupation
- retired electrician
Re: current 150+a. on grounding conductor
Matt,
Yes, on a single ground fault the fault current is limited by the impedance between XO and the grounding and bonding system. In the ones that I have worked on, the impedance was sized to limit the fault current to 5 amps. This system has many of the advantages of both the grounded and ungrounded systems. A single ground fault will not shut the system down just like an ungrounded system. The impedance grounding also prevents the sharp voltage spikes that are commonly caused by ground faults in ungrounded systems. These voltage spikes often are high enough to damage motors.
Don
Matt,
Yes, on a single ground fault the fault current is limited by the impedance between XO and the grounding and bonding system. In the ones that I have worked on, the impedance was sized to limit the fault current to 5 amps. This system has many of the advantages of both the grounded and ungrounded systems. A single ground fault will not shut the system down just like an ungrounded system. The impedance grounding also prevents the sharp voltage spikes that are commonly caused by ground faults in ungrounded systems. These voltage spikes often are high enough to damage motors.
Don
Re: current 150+a. on grounding conductor
Don,
Thank you!
Duh on me!
I guess I had never thought about the impedance limiting the current to a maximum value, but of course that does make perfect sense.
The plant that I work at was wired when they allowed 277 volt loads to be on the high impedance grounding system. Well over half of our ground faults are found in HPS high bay lights. This facility makes OSB (orientated strand board). The process involves miles of hot oil lines running throughout the facility for pressing and drying, as well as hot air ducts used in the drying process for the aspen wafers.
Loss of power to a critical pump or fan would cause an overheating condition, which in turn would most definitely cause oil line rupture or dry wafer storage fire. The hot oil is at a temperature where it would instantly combust when exposed to ambient air. Good thing for backup generators and high impedance grounding systems. Matt
[ November 25, 2003, 12:36 PM: Message edited by: itasca_mn ]
Don,
Thank you!
Duh on me!
I guess I had never thought about the impedance limiting the current to a maximum value, but of course that does make perfect sense.
The plant that I work at was wired when they allowed 277 volt loads to be on the high impedance grounding system. Well over half of our ground faults are found in HPS high bay lights. This facility makes OSB (orientated strand board). The process involves miles of hot oil lines running throughout the facility for pressing and drying, as well as hot air ducts used in the drying process for the aspen wafers.
Loss of power to a critical pump or fan would cause an overheating condition, which in turn would most definitely cause oil line rupture or dry wafer storage fire. The hot oil is at a temperature where it would instantly combust when exposed to ambient air. Good thing for backup generators and high impedance grounding systems. Matt
[ November 25, 2003, 12:36 PM: Message edited by: itasca_mn ]
roger willis
Member
- Location
- Texas
Re: current 150+a. on grounding conductor
Sounds to me like you have an ungrounded delta system with the ground monitor lights.
This is usually what you have in a plant of this type.
Roger
Sounds to me like you have an ungrounded delta system with the ground monitor lights.
This is usually what you have in a plant of this type.
Roger
whrobinson
Member
Re: current 150+a. on grounding conductor
Is the building steel effectively grounded. Is there effective bonding between electrodes and are the transformers grounded to the building steel and is 250.104(4) enforced. The source of current could be from another source. I have seen this before and have found systems improperly grounded and differences of potential between piping systems. In Maryland, a recent death of a steam fitter was due to core drilling through an electrical conduit and the placement of chilled water piping through the sleeve. On first report of a shock hazard, no shock hazard was found. A day later while draining the water from the piping system the steam fitter was killed. The water piping system was not properly bonded per 250.104 NEC.
Is the building steel effectively grounded. Is there effective bonding between electrodes and are the transformers grounded to the building steel and is 250.104(4) enforced. The source of current could be from another source. I have seen this before and have found systems improperly grounded and differences of potential between piping systems. In Maryland, a recent death of a steam fitter was due to core drilling through an electrical conduit and the placement of chilled water piping through the sleeve. On first report of a shock hazard, no shock hazard was found. A day later while draining the water from the piping system the steam fitter was killed. The water piping system was not properly bonded per 250.104 NEC.
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