Minimum vertical height of 25 kV bushing

[dup18]

Hello,
We have a fenced in substation that is being installed at a mine site. The maximum three phase voltage in the substation is 25 kV phase to phase. The substation is completely fenced in with a 6 foot chain link fence and 1 foot of 3 stranded barbed wire. The entrance gate is locked at all times, and "Danger" signs are posted on all four sides. There is an oil filled transformer (25 kV to 4.16 kV) in the substation. My question is "What is the minimum vertical height requirement to the exposed terminal of the 25 kV bushing"?
Also, please list the reference.
Thanks,
Podo

 

[bacon4life]

The external fence is to keep the general public out. However, even inside the substation there are requirements for protecting personal from inadvertently encountering high voltage. There are many kinds of "qualified" employees such as gardeners, engineers, and communications technicians who may need to be inside the substation, but who are not qualified to interact with HV equipment.

For substations owned utilities in the USA, NESC Table 124-1 2006 lists 2.82 meters to live parts energized at 25 kV. Live parts includes anything not grounded, so it is 2.82 m to the base of the bushing, not to the terminal pad. If the transformer is shorter than this, there are various other guarding/fencing options.

 

[stevenal]

I'll disagree with bacon4life here. 2.82 is the clearance to live parts energized at 25 kV. Assuming the transformer tank is grounded, The base of the bushing just above the porcelain to metal interface is at an indeterminate potential somewhere between ground and 25 kV. This is spelled out in rule 124 A 3, and the minimum clearance 2.60 m. Note that the scope of the NESC includes "similar systems" as well as utility owned systems.

If you don't have the required 2.82/2.60 clearance, you can guard the transformer per rule 124. The guard would be in addition to the perimeter fence, requiring two separate actions to breach.

 

[bacon4life]

Stevenal-You are correct that the based of the insulators must be at least 2.6 meters, and that each indeterminate voltage portion should guarded based on the maximum voltage that may be present on that portion. I had been thinking of a substation I went into where the original bus height design was based on the energized conductor clearance requirements, but the design didn't include the indeterminate voltage portions of the vertical insulators.