Power Grounding

[zimmerDN]

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?

 

[dpc]

Without a continuous ground/neutral conductor in close proximity to the phase conductors, the zero sequence reactance will be quite high and the resulting ground fault current could be greatly reduced. This is a bad thing. The NEC requires that your 600 V feeder from the transformer contain a suitable grounding conductor that is continuous from the transformer to the service entrance. If this is truly a service entrance cable from a utility, it will contain a grounded neutral conductor. This is bonded to your service entrance neutral and this is bonded to the ground at the service entrance.

It is dangerous to not have a ground wire or a grounded neutral running the full length of the circuit.

 

[7anoter4]

As dpc said:
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”

 

[PZC]

In the actual scenario, an L-G fault to any phase at the load side would result in over voltage of other two healthy phases by a factor of 1.732 if neutral is un-grounded. Therefore, as advised in above threads, the grounding resistance and continuity of neutral conductor from the transformer upto the building load point has to be excellent.

 

[zimmerDN]

So is the bonding wire between the two earth grounds to ensure the potential difference (if any) between the grounds is zero???

I am trying to the understand the need for this bond between the grounds when the grounds are installed properly.

 

[7anoter4]

If this bonding is missing but the equipment grounding wire is grounded the bonded equipment stays at the earth potential in a fault to ground case but the protection does not work and some equipment could get fire.

 

[dpc]

If one phase goes to ground, you need sufficient ground fault current to rapidly clear the overcurrent device. If this doesn't happen, things that should be "grounded" will have voltage on them and represent a shock hazard. Anytime you have to rely on the earth itself to carry fault current, you have a problem.

 

[zimmerDN]

equipment stays at the earth potential in a fault to ground case but the protection does not work and some equipment could get fire.

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.

If one phase goes to ground, you need sufficient ground fault current to rapidly clear the overcurrent device.

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.

 

[dpc]

If one phase is grounded, the fault current path will include the ground conductor. If there is no ground conductor, the current must flow through the earth. The impedance of the earth is much higher than the impedance of the ground conductor. Therefore the ground fault current will be much lower. This low ground fault current will generally not be of sufficient magnitude to trip the phase overcurrent device.

 

[iceworm]

Are you in NEC land? Maybe Canada? Doesn't matter much, the physics doesn't change. But you are looking at different codes for the regulatory requirements. Only reason I ask is 600V is common in Canada - not to common in NEC l;and

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.

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

 

[7anoter4]

Let’s take-as simple example-a four-cores cable of 3*250+1*250 kmcil copper conductors 1000 ft. length and parallel 1 ohm local grounding and 0.5 ohm substation grounding grid.
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.

 

[squeeky]

The idea behind LV over current earth fault protection is to push the fault current up as high as possible to get a fast clearing time. Any impedance will result in the application of the trip curves. Should impedance be high enough, the system cannot draw sufficient current to trip the protection. The earth return is susceptible to weather conitions.

 

[zimmerDN]

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.

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?

 

[MikiBg]

1. Grounding electrodes reduce touch voltage value during the fault.
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.

 

[7anoter4]

In an ungrounded -neutral- system, in some conditions, the system may function with one phase grounded. The equipment and cables insulation has to be elevated in such a manner to withstand the increased ground potential-and the live-to ground potential of the remaining healthy phases.
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].