250.81
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Re: 250.81
Bennie, generation plants and substations use multi-point grounding exclusively. They are built out of a grid using 2/0 AWG or larger cable to control such voltage potentail problems from fault currents called "Surface Voltage Gradient" or "Step-Rise Potential".
A 10 milli-ohm connection would be entirely unacceptable level. The connection requirement in power plants and substations is 10 micro-ohms or less which exothermic welding, or irreversible compression connections can only achieve.
Common mode currents run high in generation plants and sub-stations. It doesn?t matter, as the impedance of the grid is so low at power frequencies no meaningful voltage is developed except at fault current levels. The objective is to keep voltage within safe limits under a fault condition.
Telemetry, signal, and control circuits are single point grounded at the power source, which could be a rectifier, UPS, transformer or any other SDS, and are grounded/bonded (which ever term you prefer) to the nearest point of the grid in a multi-point ground system. This prevents common mode current from entering these circuits. It doesn?t matter what the voltage is at the reference point as long as all downstream devices use the same reference point and isolated from incidental contact with ground after the reference point or N-G bond?.
[ May 04, 2003, 03:40 PM: Message edited by: dereckbc ]
Bennie, generation plants and substations use multi-point grounding exclusively. They are built out of a grid using 2/0 AWG or larger cable to control such voltage potentail problems from fault currents called "Surface Voltage Gradient" or "Step-Rise Potential".
A 10 milli-ohm connection would be entirely unacceptable level. The connection requirement in power plants and substations is 10 micro-ohms or less which exothermic welding, or irreversible compression connections can only achieve.
Common mode currents run high in generation plants and sub-stations. It doesn?t matter, as the impedance of the grid is so low at power frequencies no meaningful voltage is developed except at fault current levels. The objective is to keep voltage within safe limits under a fault condition.
Telemetry, signal, and control circuits are single point grounded at the power source, which could be a rectifier, UPS, transformer or any other SDS, and are grounded/bonded (which ever term you prefer) to the nearest point of the grid in a multi-point ground system. This prevents common mode current from entering these circuits. It doesn?t matter what the voltage is at the reference point as long as all downstream devices use the same reference point and isolated from incidental contact with ground after the reference point or N-G bond?.
[ May 04, 2003, 03:40 PM: Message edited by: dereckbc ]
Re: 250.81
Dereck: I am a bit familiar with ground grids. Multiple connection to the grid do not produce much voltage difference due to the low impedance. The ground electrodes are bonded to act as one point.
I ran 5 miles of 4/0 cu at the research station at the South Pole, to fabricate a ground plane.
Dereck: I am a bit familiar with ground grids. Multiple connection to the grid do not produce much voltage difference due to the low impedance. The ground electrodes are bonded to act as one point.
I ran 5 miles of 4/0 cu at the research station at the South Pole, to fabricate a ground plane.
Re: 250.81
Bennie, I thought you had experience with grids from our past converstations, that is why I wonder why you object to multi-point grounding. If done correctly it is an excellent method, and in most circumstances the only method that can be implemented.
Glad to hear you are feeling better, get out and enjoy life. Good luck! Dereck
Bennie, I thought you had experience with grids from our past converstations, that is why I wonder why you object to multi-point grounding. If done correctly it is an excellent method, and in most circumstances the only method that can be implemented.
Glad to hear you are feeling better, get out and enjoy life. Good luck! Dereck
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