Underground power 600v grounding

[petersonra]

Every medium voltage distribution system I have ever worked on has some type of grounding connection at every pole or every 500' or so.
I recently had do groundrods for a POCO transformer vault I was setting for them.
I am no expert but I'd look at that NEC 110.10

110.10 has nothing to do with grounding electrodes of any type.

 

[tortuga]

110.10 has nothing to do with grounding electrodes of any type.

Your right, it just says the OCPD has to be able to clear a fault.
Say that hand hole is submerged in salt water and a splice fails to ground.
Is the OCPD 800' away going to trip?

 

[tortuga]

If I were the state I would want one.

It's Your Money: Death By Electrocution Costs Honolulu $550K

www.civilbeat.org

 

[petersonra]

Your right, it just says the OCPD has to be able to clear a fault.
Say that hand hole is submerged in salt water and a splice fails to ground.
Is the OCPD 800' away going to trip?

how would a ground rod help in the case?

 

[Hv&Lv]

Sorry, not required IMO

 

[tortuga]

I would think a groundrod would help the fault current get back to the OCPD and clear it.
Say you had 600V 60A OCPD ground fault 800' away, breaker would never trip in my opinion.
But like I said I am no expert.

 

[petersonra]

I would think a groundrod would help the fault current get back to the OCPD and clear it.
Say you had 600V 60A OCPD ground fault 800' away, breaker would never trip in my opinion.
But like I said I am no expert.

how would a ground rod trip it if the EGC was broken?

Suppose one had a ground fault. That would be 347 V to N/G. To trip a 60 A OCPD reliably would require maybe 100 Amps. That would be about 3.5 Ohms. The chance of getting a 3.5 Ohms connection through a single ground rod at each end is nil.

 

[tortuga]

how would a ground rod trip it if the EGC was broken?

Who said EGC is broken?
Ground rod is bonded to ecg with cad weld in my scenario.

Suppose one had a ground fault. That would be 347 V to N/G. To trip a 60 A OCPD reliably would require maybe 100 Amps. That would be about 3.5 Ohms. The chance of getting a 3.5 Ohms connection through a single ground rod at each end is nil.

I was figuring about 8 ohms 'fall of potential' is 6 inches away, energized hot gets chewed bare by a rat or some critter
Pull box fills up with salty water (OK probably not Florida).
Then somebodies dog walks over, gets zapped
I guess you guys are all in agreement the NEC does not require ground fault protection of a feeder?

 

[Cjmccarthy]

The code does not require the ground rod.

I have never seen ground rods installed at manholes, even on large medium voltage distribution systems. If I am doing there would not be a ground rod.

Thank you Don, I get that there is no code to follow, I found out that this is what the DOT wants. We have a single phase 600v circuit with a ground wire installed along with the 2 hot conductors for a few thousand feet and we can't pull the conductor in one shot, so we end up having splices. when it calls for a 2/0 awg wire we also pull a 2/0 ground wire. knowing we are going a couple thousand feet with multiple splices between points, does installing a ground rod and terminating the ground wire to the rod instead of splicing the ground wire through, serve a purpose?

 

[tortuga]

A 600V street lighting system that gets to be a ½ mile long is a essentially going to behave like a transmission line and the public safety issue is not really addressed other than in 110.10 of the NEC because they are not that common.

You basically run into the situation where a 600V breaker may not be able to properly operate in a fault condition.

The most common feeder / transmission line faults are a single-phase faulting to ground.
Case law clearly indicates FDOT would be found liable if a person encountered a hazardous voltage on the ground around one of the pull boxes from such a ground fault, regardless of if the NEC requires it.
I linked several law cases above, where an plaintiff has been killed from a energized man hole.
I believe FDOTS engineers need to do their due diligence to ensure the public is protected from a ground fault along that feeder.

 

[petersonra]

A 600V street lighting system that gets to be a ½ mile long is a essentially going to behave like a transmission line and the public safety issue is not really addressed other than in 110.10 of the NEC because they are not that common.

You basically run into the situation where a 600V breaker may not be able to properly operate in a fault condition.

The most common feeder / transmission line faults are a single-phase faulting to ground.
Case law clearly indicates FDOT would be found liable if a person encountered a hazardous voltage on the ground around one of the pull boxes from such a ground fault, regardless of if the NEC requires it.
I linked several law cases above, where an plaintiff has been killed from a energized man hole.
I believe FDOTS engineers need to do their due diligence to ensure the public is protected from a ground fault along that feeder.

I am not convinced that a ground rod at each manhole is the answer though. They need to make sure the ocpd can trip under normal fault conditions. Maybe a GFCI needs to be added.

 

[tortuga]

Agreed a real electrical engineer should look the protection scheme over, a typical GFCI that we use is only good for a few hundred wire feet.
Digital relays or 'protective relays' offer several methods for detecting ground faults and the utility I have worked with of uses them
Some methods for detecting ground faults I have seen are:

• Differential
• Directional Ground Overcurrent
• Directional Negative-Sequence Overcurrent
• Mho Ground Distance
• Quadrilateral Ground Distance

I wonder if they are using something like that?
The line is usually modeled in some fancy software but they would probably require some kinda Infinite Half Line Ladder Network, thus ground rods at various points.
Like I said I am no expert in that stuff.