# Medium Voltage Grounding Question



## Yohan (Mar 3, 2015)

Had a thought, would the smaller wire being attatched to the braid be for induced currents from the wire itself? 

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## joe-nwt (Mar 28, 2019)

Your ground tail should be at least the same ampacity as the ground braid. When I pigtail 3 braids together, I always use 2/0.


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## joe-nwt (Mar 28, 2019)

Yohan said:


> Had a thought, would the smaller wire being attatched to the braid be for induced currents from the wire itself?
> 
> Sent from my SM-A526U using Tapatalk


Did you isolate the sheilding at one end?


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## Slay301 (Apr 23, 2018)

I use number 6 to a ground rod


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## joe-nwt (Mar 28, 2019)

Slay301 said:


> I use number 6 to a ground rod


Any place I've ever terminated MV there has always been some manner of ground grid installed. Even a pad mount transformer with a simple ring and a few rods. Never seen the shielding grounded to a single rod. Is that common in your area?

Edit: I suppose if you run MV up a pole.


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## Yohan (Mar 3, 2015)

joe-nwt said:


> Did you isolate the sheilding at one end?


It is only grounded on one end and has a separate egc. 

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## Yohan (Mar 3, 2015)

joe-nwt said:


> Any place I've ever terminated MV there has always been some manner of ground grid installed. Even a pad mount transformer with a simple ring and a few rods. Never seen the shielding grounded to a single rod. Is that common in your area?
> 
> Edit: I suppose if you run MV up a pole.


I forgot to mention, its for a motor starter at a chemical plant. We are adding a new compressor into their system. I'm not familiar with their grounding setup their. 

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## paulengr (Oct 8, 2017)

The size really doesn’t matter so #14 is smallest )NEC). Some of 3Ms kits come with something similar to a stainless steel tie wrap with a green #14 wire welded to it instead of the braid.

The shield itself is intentionally designed to have a relatively high impedance. In a concentric neutral cable there are regular cable strands instead of a foil shield that can carry full ground/neutral current. The cable has high losses because the neutral is a fraction of an inch from the current carrying conductors, so power capacitively flows across the gap. In foil shielded cables the foil is designed to intentionally have a high impedance to prevent this. The only thing left is that the drain needs to allow any capacitive current to harmlessly dissipate so that charge doesn’t build up on the shield, which is 1 A or less. So the NEC minimum applies. You could argue for #1 (minimum for 2 kV+) but very little voltage is present on the wire and it is not an effective ground.

There are arguments about whether you need to have a drain on one or both ends. It depends on cable length but I’d suggest grounding both ends is required in some cases and can’t hurt in others. It’s not an effective ground so little chance it will mess up high resistance grounding, even if you have a separate ground grid at each end.


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## Slay301 (Apr 23, 2018)

joe-nwt said:


> Any place I've ever terminated MV there has always been some manner of ground grid installed. Even a pad mount transformer with a simple ring and a few rods. Never seen the shielding grounded to a single rod. Is that common in your area?
> 
> Edit: I suppose if you run MV up a pole.


No my bad usuallly its 4 rods and then the frame


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## joe-nwt (Mar 28, 2019)

Medium Voltage Cable Shield Grounding – Voltage Disturbance


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## Tonedeaf (Nov 26, 2012)

12 is too small, people do stuff wrong the whole career


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## paulengr (Oct 8, 2017)

Tonedeaf said:


> 12 is too small, people do stuff wrong the whole career


Did you read? These are DRAIN wires. So you are saying 3M termination kits are made incorrectly?


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## Tonedeaf (Nov 26, 2012)

paulengr said:


> Did you read? These are DRAIN wires. So you are saying 3M termination kits are made incorrectly?



*12 is too small from the braid to ground it will blow up like a fuse on a fault or lightning strike*....certain POCO's require you to cris cross a splices or elbows with #12 strand this is a drain function ....I always make it the same size at the Braid or the size of the twisted concentric to ground


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## paulengr (Oct 8, 2017)

Tonedeaf said:


> *12 is too small from the braid to ground it will blow up like a fuse on a fault or lightning strike*....certain POCO's require you to cris cross a splices or elbows with #12 strand this is a drain function ....I always make it the same size at the Braid or the size of the twisted concentric to ground


A concentric neutral is completely different. It is effectively a conductor within a conductor. It needs a properly sized conductor since it is a neutral. It also incurs high losses.

A foil or braided shield is NOT an effective ground. It is purposely designed to be a fairly high impedance to minimize losses. It can’t conduct a fault, lightning strike, or whatever. The surge impedance is so high that a fault or lightning is seriously limited. The ground that is run with it carries lightning or fault currents.



https://www.pesicc.org/iccWebSite/subcommittees/E/E04/2001/f2001_landinger.pdf



So foil shields cannot possibly conduct lightning or fault currents and aren’t effective grounds. #12 is overkill when the shield carries at most less than 1% of the current.


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## Tonedeaf (Nov 26, 2012)

paulengr said:


> So foil shields cannot possibly conduct lightning or fault currents and aren’t effective grounds. #12 is overkill when the shield carries at most less than 1% of the current.


realistically a ground fault it going to be 5-15% down the shield..... current is going to path done the tape shield.....current flows like water to the easiest path.....

say you have a ground fault (from whatever) of 400a.....lets say worst case 15% as any good designer should ....that 60 amp trying to flow down a #12.....every seen a fault...the **** can sizzle for minutes before they are cleared.

Even if everything above is an absolute LIE AND FABRICATION. BTW I called my college buddy from PECO protective relaying department for the 5-15%. They use 15%

the *common sense reality of a * #12 or smaller getting damaged and metal fatigue in normal operations. 

have you ever pulled a stubborn ELBOW.....????


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## paulengr (Oct 8, 2017)

Shield losses are a big issue because it results in significant voltage drop and the tape shields in particular have low short circuit ratings. Losses affect ampacity because of thermals and result in cable failures during faults. So they purposely design shields with high shielding current to cable current ratios. Think about what you are suggesting. A15% current loss on a 500 A cable is 150 A. Even with 1 milliohm of resistance that’s 22.5 Watts, and you would need a shield drain of around #3 and you would be carrying 150 A per cable on a ground with a cable that is only around 600 MCM.

If anyone cares to calculate it the formulas are readily available but shield losses are a whole lot less than 15%. This sounds like typical spec engineer MEP BS.



http://www.okonite.com/media/catalog/product/files/EHB_2018.pdf





https://www.pesicc.org/iccWebSite/subcommittees/E/E04/2001/f2001_landinger.pdf



Just because some guy at a power company contradicts cable manufacturers doesn’t make it right. I do work for power plants all the time. They definitely know how to waste money. You wouldn’t believe all kinds of stupid I see.


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## paulengr (Oct 8, 2017)

Also there seems to be some confusion here. It is a SHIELD, not a conductor. It has an impedance…resistance and reactance. The reactance is obviously capacitive. In either case this means we get a small current across the shield flowing to ground. The amount of current depends on the cable voltage. “Fault” conditions mean that the current to the fault load increase but since impedance has not changed shield currents don’t either. Fault currents as a result of physical damage or lightning are somewhat different. The current handling is no longer thermally limited in the same way and hence NEC allows down to #18 as a lower limit near power cables because they are looking at 3 cycle ratings which is an eternity for lightning strikes. Puncturing if insulation in the cable is a real problem and if it has significant exposure to potential surges you MUST install surge protection. I’ve replaced a lot of 35 kV where this was not done. So if it is exposed to faults and lightning like you are describing a burned shield drain wire is not only unlikely but it’s the least of your problems. Sorry that you just have zero maintenance experience and mostly do whatever a guy who is paid to invent ways to charge more money tells you.


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