Should Metal Cross Arms in Overhead Poles be Grounded

M

Milton Huei

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US
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Electrical Engineer
I've seen many overhead poles between 13kV and 25kV, most of them use metallic cross arms, some connected to ground, some not.

Since sometimes -most times- the ground soil resistivity parameters around poles i've measured are really not that good (over 1000 Ohms.m typically), i'm quite concerned about connecting those cross arms to ground, and I would prefer to leave them isolated to ground (allowed by 250.110 for >2.5m), or use another less conductive material for the cross arms. So, in case of a fault where those cross arms get energized, that voltage doesn't get transmitted to the terrain where the unsafe ground are.
Exceptions would be lightning arresters, which of course should be kept with their grounding, so I let the cross-arms or other metal hardware at overhead poles be isolated.

Is my concern valid?
Should that cross arm be better isolated from ground and far from users, and reserve ground only for lightning arresters?
Or should that cross arms be grounded so in case of faults any protection at HV substation level get tripped?

Thanks in advance.
 
I don't think I have ever seen a metal cross arm around here...either wood or fiberglass.
I any rules about this subject would be found in the National Electrical Safety Manual, published by IEEE. That is the standard that the utilities use.
Even though the NEC is trying, it does not do a very good job with medium voltage systems.
 
don_resqcapt19 said:
I don't think I have ever seen a metal cross arm around here...either wood or fiberglass.
I any rules about this subject would be found in the National Electrical Safety Manual, published by IEEE. That is the standard that the utilities use.
Even though the NEC is trying, it does not do a very good job with medium voltage systems.
Click to expand...
I have never seen a metal cross arm either. I have seen some that appear to be some sort of composite, fiberglass I assume. We do have metal "arms" (not sure what the correct term is), for a single conductor. I have never seen them bonded.
 
Depending on what state you are in you either have a state level utility design code like in California it is the General Orders (GO 95, 128, etc.) or the NESC. The general orders are loosely based on the NESC (IEEE C2).

1) Steel cross arm on a steel pole
2) Steel cross arm on a wood pole
3) Steel cross arm on a fiber glass or non wood / non steel pole

In all cases they bond the pins for the insulators to the arm. That is true on wood arms above a certain KV level. Workers climbing and needing a equipotential section from his toes to his hands or suitable isolation. Isolation is not used as the voltages get higher because of how much larger the flash over distance is in free air. At a certain point an outage is required to do the work. The required shed counts and separations wouldn't make sense for someone on the pole needing to reach the equipment. There exist non climbable poles where this can be changed but all wood poles are considered climbable.

Bonding practices change based on the utility / state.

The bonding on the arm does not do much on clearing faults. Though it is argued by communication utilities and power utilities that in states where they bond communication on the poles with LV power or MV power grounds, the communications "ground bed" increases clearing time. The down side is where grounds get cut by transients their can be some gradients between sections of climbable space.

They use small wire, like #6 or #8. It isn't doing much in clearing since it would burn through. It's design is equipotential for worker safety. At least that is my understanding from the meetings.

Fault clearing for utilities is a very different set of practices. That can be more dependent on the voltage system and the substations grounding methods. I don't know if crossarm bonding has any bearing on that.
 
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