What is the purpose of this? At a local light rail station

Typical fence bonding at a substation. Probably not needed and likely a carry over from high voltage utility substations where it is part of the step and touch potential elimination.
 
Light rail is typically single-wire-earth-return, technically speaking, (although the tracks provide a lot of conductivity for the 'earth'). That's probably why you see more stringent bonding specs on these projects. Train movements might create measurable voltage differences to unbonded stuff (like NEV, except not technically a neutral here).
 
Flicker Index said:
But why? Isn't the physical conductivity from the hardware enough? I don't know why this gate needs a jumper to frame.
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electrocution hazards arise when there are two different voltage potentials. assuming that pole right in front of the gate is a high-voltage power pole, there could be large magnetic fields surrounding that metal fence/gate.

the only way to ensure that there is not a voltage difference (electrocution hazard) between the fence and the gate, is to positively bond them together. this way, when someone reached out and grabs the gate while grabbing the fence, there is an established equipotential plane, and no electrocution hazard.

if you think this is a lot, check out the equipotential bonding requirements in agricultural buildings in art 547. it's gets crazy. the NEC cares more about cows and pigs than it does our own children.
 
jaggedben said:
Light rail is typically single-wire-earth-return, technically speaking, (although the tracks provide a lot of conductivity for the 'earth').
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That's backwards- typically the tracks are the intended return but being imperfect conductors on imperfect insulators (wood ties) they leak to the dirt. Earth return is definitely not desired (this was a known issue pretty much since the origin of electric railroads), in newer installations the running rails are insulated from the ties to lessen the leakage.

Fun Fact-
Older lines of the London Underground do not use the running track as a return, they use a 4th rail - the 3rd rail (outside the track) is +420 to 500v and the 4th (between the tracks) is -210 to 250v. Also, those traction current rails are high-resistance grounded. (Other nuances of the system omitted.)

Master Nater said:
the NEC cares more about cows and pigs than it does our own children.
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'tain't just the NEC, compare the laws of animal abuse vs human abuse.
 
zbang said:
That's backwards- typically the tracks are the intended return but being imperfect conductors on imperfect insulators (wood ties) they leak to the dirt. ...
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Yeah, I was being a bit tounge in cheek. The imperfect isolation from dirt is what I was getting at in terms of why more conservative bonding might be in specs.
 
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