Transformer Ground Pads Question

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Flapjack

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USA
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EE
Where did grounding opposite ends of a substation transformer tank originate? Is it just a common utility requirement to ground opposite ends to a grounding electrode?
The NEC requires a low-impedance path to clear a ground fault, but that can be achieved with the ground pad on the tank below the X0 on the secondary.
I thought I recalled seeing a Mike Holt video where he said there is no code that requires the two ground pads on opposite ends of the tank to be grounded, but I can't find it.
 

Smart $

Esteemed Member
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Ohio
Well if "there is no code that requires the two ground pads on opposite ends of the tank to be grounded" you won't find it... but carry on if you wish to find Mike Holt's video.


:angel:
 

Flapjack

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USA
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EE
Well if "there is no code that requires the two ground pads on opposite ends of the tank to be grounded" you won't find it... but carry on if you wish to find Mike Holt's video.


:angel:

Haha, very true. I am not searching for a code reference though, just an explanation of why the two ground pads on opposite ends have become standard.
 
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iwire

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Location
Massachusetts
Haha, very true. I am not searching for a code reference though, just an explanation of why the two ground pads on opposite ends have become standard.

Same reason some EEs specify 'delta grounds' (Three ground rods driven in a triangle pattern)

Same reason the local power company requires us to install one 10' x 3/4" ground rod about 18" off each corner of the cement transformer pad, cad weld bare 4/0 to them making a ring around the pad and then bringing in a 4/0 whip from opposite sides of the ring into the transformer.


Tradition!




OK, honestly I am just an electrician, there may well be good reasons for these types of things but I have yet to see any of those reasons spelled out clearly.
 
Substation grounding is covered by IEEE 80. When designing to IEEE 80, you're looking at ground potential rise, short circuit current available, fault clearing time and their relation to step and touch potentials.

As far as why 2 grounds on opposite corners, other than "that's the way we've always done it", two grounds on opposite corners have a much less likely chance on both being unintentionally disconnected, making the transformer tank a severe touch potential hazard.
 

jim dungar

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...two grounds on opposite corners have a much less likely chance on both being unintentionally disconnected, making the transformer tank a severe touch potential hazard.
IEEE80 says, in section 9.8, "Those facilities that are most likely to supply or carry a high current, such as transformer and circuit breaker tanks, switch frames, and arrester pads, should be connected to the grid with more than one ground lead. The leads should preferably be run in opposite directions to eliminate common mode failure."
 

big john

Senior Member
Location
Portland, ME
..Tthe local power company requires us to install one 10' x 3/4" ground rod about 18" off each corner of the cement transformer pad, cad weld bare 4/0 to them making a ring around the pad and then bringing in a 4/0 whip from opposite sides of the ring into the transformer....
I would wager the ring ground is to attempt to reduce touch potential around the entire parameter of the transformer in the event that the tankwall becomes energized. Distribution grounding is a bit of a different animal than the copper-burial we do for low voltage.
 

Flapjack

Senior Member
Location
USA
Occupation
EE
Follow up question... on a delta-wye substation transformer with full-height air terminal chambers, why would a ground pad not be provided on the primary side? Where would the equipment grounding conductor terminate? It wouldn't seem as clean to route the phase conductors and EGC into the air terminal chamber and the have to route the EGC to one of the ground pads on the exterior.

Or does it have to be requested because the manf assumes the transformer would be used for a service?
 
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