Power Grounding
Power Grounding
(OP)
By code and in common practice, is it necessary to have the same ground carried through from the source to the load?
The scenario is this, a 15KV/600V solidly grounded transformer, 600V power is distributed and protected by breakers. One segment of a 600V circuit travels some distance on single conductor overhead lines and doesn't have a ground wire. Eventually the overhead lines terminates at a service entrance at the new location and from that point the circuit is grounded again by being bonded to the building ground grid.
Will the breaker provide the proper protection since the grounding isn't continous from source to load?
What if this was a high resistance grounded transformer does it make a difference?
The scenario is this, a 15KV/600V solidly grounded transformer, 600V power is distributed and protected by breakers. One segment of a 600V circuit travels some distance on single conductor overhead lines and doesn't have a ground wire. Eventually the overhead lines terminates at a service entrance at the new location and from that point the circuit is grounded again by being bonded to the building ground grid.
Will the breaker provide the proper protection since the grounding isn't continous from source to load?
What if this was a high resistance grounded transformer does it make a difference?






RE: Power Grounding
It is dangerous to not have a ground wire or a grounded neutral running the full length of the circuit.
RE: Power Grounding
NEC art.250.2 Definitions
Effective Ground-Fault Current Path.” An intentionally
constructed, low-impedance electrically conductive path
designed and intended to carry current under ground-fault
conditions from the point of a ground fault on a wiring
system to the electrical supply source and that facilitates
the operation of the overcurrent protective device or
ground-fault detectors on high-impedance grounded systems.”
and art.250.24(C):
250.24 Grounding Service-Supplied Alternating-
Current Systems. (C) Grounded Conductor Brought to Service Equipment.
“....the grounded conductor(s) shall be routed with the ungrounded conductors to each service disconnecting means and shall be connected to each disconnecting means grounded conductor(s) terminal or bus”
RE: Power Grounding
RE: Power Grounding
I am trying to the understand the need for this bond between the grounds when the grounds are installed properly.
RE: Power Grounding
RE: Power Grounding
RE: Power Grounding
Why won't the overcurrent protection work? Wouldn't the magnitude of fault current be the same in either cases (with or without the bonding between the two earth grounds, see picture above) and the o/c will trip because of excessive current.
Why would the magnitude of the fault current be different if the bonding between the two earth grounds is missing (see picture above)? Everything is still properly grounded to earth potential, just not bonded together.
RE: Power Grounding
RE: Power Grounding
Adding the others comments:
Each of the earth to grounding connection has impedance - They are not a zero ohms connecting to earth.
Consider if each connection is the NEC mythical 25 ohms. That is the resistance between the rod you drove and the earth, as measured by say "Fall of Potential" method. So, two rods, one at the xfm and one a few hundred feet away at the equipment. Each has a resistance to 25 ohms to earth. For a fault current, returning to the xfm neutral, that is 25 + 25 = 50 ohms (the eath part is considered zero). So, 346V to neutral, through 50 ohms, gives about 7A.
But wait, our grounding systems are much better, each end is only 5 ohms to earth. Okay that's 10 ohms series and 346V gives 35A.
The earth connection is not going to trip anything.
[quote]What if this was a high resistance grounded transformer does it make a difference?{/quote]
Yes, but you still need a bonding conductor.
Recommended reading:
IEEE 141, Electrical Power Distribution fir Industrial Plants, section 7 on grounding
IEEE 142 Grounding of Industrial and Commercial Power Systems
ice
Harmless flakes working together can unleash an avalanche of destruction
RE: Power Grounding
The grounding conductor in the supply cable presents Z=0.061+j0.087 ohm.
The grounding impedance in absence of grounding conductor will be (1+0.5) +j0.2077 ohm.
the reactance of 0.2077 ohm calculated as per Carson formula for 100 ohm.m earth resistivity [depth approx. 1000 m [330 ft.].
The impedance in first case is 0.0938 and in second case 1.514 ohm [16 times more].
Neglecting the live conductor and the transformer impedance for 240 V supply voltage:
in the first case Ish=240/0.0938=2558 A in second case 240/1.514=158.5 A.
RE: Power Grounding
RE: Power Grounding
So in a ground fault sitution, you rely the electrical path created by the bonding wires which the fault current will travel on in order to return to the source (ie transformer). This low resistance short circuit (even lower in resistance than the path to earth ground) will cause enough current flow and trip your overcurrent protection device.
Does this sound right?
If that's the case, what does the grounding electrodes provide in terms of ground fault protection?
RE: Power Grounding
2. Although much bigger than bonding (PE) conductor impedance, the impedance of grounding electrodes is additional parallel path to fault current, hence fault current in line is bigger than without grounding electrodes, resulting in shorter fault clear time.
You can achieve efficient GF protection with just grounding electrodes installed (without bonding conductor), but logic and settings are different in that case, and RCDs are usually required. In IEC world: just earthing electrodes - TT earthing system, bonding (PE or PEN) conductor, with or without grounding electrodes - TN earthing system.
RE: Power Grounding
If in this condition a new live-to-ground fault occurs this will be short-circuit case and the system protection has to act and isolate the fault in time.
Since the touch, step [and GPR] depend on current through ground value a grounding wire-as static wire on overhead line , cable shield or other grounded conductor-will create a parallel way to the grounding grid or electrodes and will reduce the current flowing through the ground and hence the GPR [and touch and step potential].