Hello. I'm an electrical engineer providing a design for the Phoenix area Light Rail that involves a bridge going over the Salt River. The light rail train itself along with cars and pedestrians will be using this bridge. The train is powered by an overhead cantanary system with medium voltage. There will be many lights on the bridge and at 120V (I did not provide the lighting, only providing the power distribution) which will require larger conductors as normal because of the max 1000' distance. I would of done the lighting at 240V. I've designed many of the passenger stations electrical over the years and we have to bond the metal handrailing with a #2AWG wire to a ground ring system (4/O wire). This bridge also has a pedestrian railing. Why wouldn't I also bond THAT railing as well? Per 250.104(C) - anything that might become energized. I most certainly think it will give the train DC medium voltage power along with all my conductors tied right next to this handrail itself. I'm asking this question because the architect on the project is fighting me on NOT providing this bond wire at every 150' or so for the railing. He's only concerend about aesthetics and I'm concerned about safety. Wouldn't this type of railing bond be required at the bridge and the stations?? Thank you very much for any help or suggestions. I appreciate it!
Metal Handrail Bonding
- Thread starter alcornjr
- Start date
- Location
- New Jersey
- Occupation
- Journeyman Electrician
If you bond the railing to a ground ring is it only connected to the dirt? If so how will the dirt do anything to clear a fault to the rail?
jaggedben
Senior Member
- Location
- Northern California
- Occupation
- Solar and Energy Storage Installer
The handrail must, of course, be bonded somewhere.
That said, from here, your choice of every 150ft seems ... arbitrary, without further info. Are the handrails not to be installed in a manner that would make them electrically continuous beyond that distance? Are the light fixtures with EGCs not metal, or not to be attached to the metal bridge structure in such a manner as to likely create redundant bonding at each fixture? 250.104(C) seems only to require a single bond where structral metal is electrically continuous. It seems like your multiple points of bonding could be required, well advised, or overkill, depending on the details of construction.
I would note that for the lighting the NEC would cover this. But the NEC does not cover railroads, including light rail.
That said, from here, your choice of every 150ft seems ... arbitrary, without further info. Are the handrails not to be installed in a manner that would make them electrically continuous beyond that distance? Are the light fixtures with EGCs not metal, or not to be attached to the metal bridge structure in such a manner as to likely create redundant bonding at each fixture? 250.104(C) seems only to require a single bond where structral metal is electrically continuous. It seems like your multiple points of bonding could be required, well advised, or overkill, depending on the details of construction.
I would note that for the lighting the NEC would cover this. But the NEC does not cover railroads, including light rail.
- Location
- Illinois
- Occupation
- retired electrician
What failure would energize the railing? The lighting system should already have an EGC that would prevent a lighting system failure from energizing the railing.
- Location
- Illinois
- Occupation
- retired electrician
Why?
jaggedben
Senior Member
- Location
- Northern California
- Occupation
- Solar and Energy Storage Installer
I was assuming it's 'likely to be energized' by something or other, but if you want to question that, go ahead. He stated his opinion it is.
Last edited:
- Location
- New Jersey
- Occupation
- Journeyman Electrician
My thoughts as well. The EGC in whatever may energize the railing would also provide a path for any fault to the railing. A ground ring only buried in the earth would not do the same thing.
- Location
- Illinois
- Occupation
- retired electrician
The NEC Style Manual says that "likely to become energized = insulation failure" What insulation failure would energize the handrail?
Thanks for the responses. The bonding at 150' was because of the NEC recommendation of 50m (160'). NEC 250.194 for bonding fences and other metal structures. A station years ago was not bonded at a section of the railing during install. People that were leaning on that part of the handrail when the light rail train came along got a static shock. 25+ years ago an engineer in my firm provided metal light pole lighting in a Tempe park and unknowingly he designed them without any extra bond. There was lights under medium voltage lines (like the train has) and these poles became energized (see previous code reference) and people were getting a good static shock when touching them. That engineer got in a little trouble from the City not bonding these metal poles. Wouldn't this be the same kind of thing? A station has a groud ring around it with ground rods at each end and at every 50' or so. The ground ring is buried 24". The bridge (existing) has rebar all thru it. Bonding to it would provide a better ground I would think given the rebar in concrete would act like a Ufer GND. Then I would bond it to two gnd rods at each end (one on each side of the bridge for a total of 4). The river bed makes it impossible to form the aforementioned ring. I'm thinking the bridge handrail should be bonded similiar to how a station would be. I'm afraid of a person touching anything metal when the train comes along - just like those aformentioned people did when touching metal under a medium voltage line. FYI - the station's metal water fountain, chairs, columns, plant beds - anything metal that a human can touch received a #2AWG bond that connects to the 4/O GND ring that is connected by the system of ground rods buried 24" into the ground. Testing by the electrical contractors during install of these gnd rods must achieve a resistivity of less than 25 Ohms - which the did for the stations. I'm thinking the same kind of thing should be done for a bridge with people on it. I've never designed anything for a bridge previously. Again - Thank you for your help!
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Grounding to protect against static shocks is not necessarily the same as grounding for energized circuits.
Grounding conductors for energized circuits must be able to carry the fault current until a protective device operates, which could be measured in seconds. Grounding conductors for static shock need to be able to carry the current only until the potential difference/charge equalizes, which is usually in milliseconds or less.
The grounding of fences in utility substations is usually for step potential issues associated with open air kV systems.
Grounding conductors for energized circuits must be able to carry the fault current until a protective device operates, which could be measured in seconds. Grounding conductors for static shock need to be able to carry the current only until the potential difference/charge equalizes, which is usually in milliseconds or less.
The grounding of fences in utility substations is usually for step potential issues associated with open air kV systems.