Inexpensive fence suitable for 34.5 to 5kV substation

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mshields

mshields

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Location
Boston, MA
Just put out CD's consisting of a new double ended 5MVA 34.5 to 4160V substation, somewhat unique in that all components are pad mounted. i.e. no open lattice work to contend with with all interconnecting cable run underground in conduit.

A contractor has pointed out that the chain link fence used in part to enclose this substation is not acceptable/code compliant owing to it not being contiguous/bondable.

a) is this true
b) is it applicable given that all of our equipment is pad mounted.
c) if I need to specify another type of fence, what kind of fence is required?

Thanks,

Mike
 
All Padmounted and Enclosed?

All Padmounted and Enclosed?

If all of the equipment is padmounted, there are no exposed bushings or conductors, if equipment is secured from unauthorized access (locked), and all surfaces that someone can make contact with are grounded, there is no need for a fence. To mitigate step and touch potentials, I would provide a ground mat under the in the area around the equipment.
 
beanland said:
If all of the equipment is pad mounted, there are no exposed bushings or conductors, if equipment is secured from unauthorized access (locked), and all surfaces that someone can make contact with are grounded, there is no need for a fence. To mitigate step and touch potentials, I would provide a ground mat under the in the area around the equipment.
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When I have done a EPG under and around a sub stations we tapered the mat deeper as we passed the fence boundaries to keep from having a step potential at the edge of the mat, at one time this was a common practice, is it still being done like this on MV subs?
 
The substation fence needs to be grounded with earth mat under fence gate. Whether fence is separately grounded or connected to the ground grid is a design issue.
 
Ground Mat

Ground Mat

On a typical flat fenced substation, the ground grid extends ~3 feet outside the fence and gate swing. Since the goal is touch potential mitigation, a person should not be able to touch the fence and stand outside the ground mat. The steepest potential gradients are right at the edge of the ground mat.
 
If there are no exposed live parts the NEC does not require the fence to be grounded.
250.194 Grounding and Bonding of Fences and Other Metal Structures
Metallic fences enclosing, and other metal structures in or surrounding, a substation with exposed electrical conductors and equipment shall be grounded and bonded to limit step, touch, and transfer voltages.
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This is a new section in the 2014 code.
 
It is not clear what advantage an enclosed substation has over exposed one to exclude fence grounding in case of ground fault and so is the new code rule.
 
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