Grounding as it applies to over 600 volts

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mbrooke

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Here is the question. Its said ground rods do little in 600 volt and under systems for personal protection and fault clearing, but what about systems over 600 volts? Here a rod is being used to protect a lineman:


https://www.youtube.com/watch?v=G2OtgeDkTLs


Does this hold any truth in what he is saying? Does the bulk of the electricity really choose the rod and not the line man?
 

Tony S

Senior Member
First you have to look how the linesman is earthed or grounded. If he’s clipped on to a wooden pole he’s going to present a high resistance therefore the rod is a better conductor.


There was a lot of what would be the best practice missed out. Such as a rod at each of the poles on the upstream and downstream of the pole you’re working on.
 

mivey

Senior Member
Here is the question. Its said ground rods do little in 600 volt and under systems for personal protection and fault clearing, but what about systems over 600 volts? Here a rod is being used to protect a lineman:


https://www.youtube.com/watch?v=G2OtgeDkTLs


Does this hold any truth in what he is saying? Does the bulk of the electricity really choose the rod and not the line man?
Yes. It is a current divisor circuit. Keep the lineman impedance much higher than the ground impedance.

The same principal is used when you calculate touch potential in a substation. That is why we have gravel on top: to increase the touch path resistance. Keep the gravel clean. I have seen many subs with a lot of dirt and clay tracked in but that gravel should be cleaned or replaced.
 

mbrooke

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Yes. It is a current divisor circuit. Keep the lineman impedance much higher than the ground impedance.

The same principal is used when you calculate touch potential in a substation. That is why we have gravel on top: to increase the touch path resistance. Keep the gravel clean. I have seen many subs with a lot of dirt and clay tracked in but that gravel should be cleaned or replaced.

But what impedances are you looking for? Technically you have two circuits in parallel.
 

mbrooke

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United States
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Technician
First you have to look how the linesman is earthed or grounded. If he’s clipped on to a wooden pole he’s going to present a high resistance therefore the rod is a better conductor.


There was a lot of what would be the best practice missed out. Such as a rod at each of the poles on the upstream and downstream of the pole you’re working on.

Id say so. I've seen linemen put ground rings on the pole to create an "equal" potential zone above the work area.
 

GoldDigger

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Placerville, CA, USA
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Retired PV System Designer
A key distinction is that at medium and high voltages the resistance of an earth electrode, even a simple one like a ground rod, will draw enough current from a fault to trip upstream OCPD.
If rod and man are truly in parallel, then for the time until OCPD operates the man will still be exposed to full line voltage.
Without the rod the man would be exposed to full voltage indefinitely. (Until crispy critter status at worst.)

Sent from my XT1585 using Tapatalk
 

mivey

Senior Member
But what impedances are you looking for? Technically you have two circuits in parallel.
The fact is the worker should be in an equipotential zone where we limit the voltage exposure. The pole where the worker is at should also be bonded. If not, as shown, we have a current divider with the rod rather than with the protective ground cable.

First, we make assumptions about worker impedance and clearing times which lead to the result that a 75 volt or less exposure is the goal (for line work). See substation grounding design guide.

Next, we have to know fault current levels and safety ground cable impedance to properly size the cable, place the cable (that is important for impedance modeling: closer to worker is better), and even control fault current level to keep the exposure voltage below 75 volts.

Then the question is where do we ground? A substation grid is the best, a multi-grounded neutral is next, etc. but the last resort is the driven ground similar to in the video (also driven if we are unsure of the pole/tower ground integrity).

With a great ground like a MGN and proper bonding then the worker is in an equipotential zone and only exposed to the cable voltage drop as discussed. With a driven rod the worker is also in an equipotential zone but the ground needs to have a low enough impedance to trip the protection in an adequate time (and the metal pole/equipement should also be bonded).
 
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