Grounding a floating bridge

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loren

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I am the electrical inspector on the Hood Canal Floating bridge. The electrical distribution equipment is in the middle of the bridge. The 12470 Volt 200 amp line feeding it has a 1/0 ground wire sized for it.

The secondary of the transformer is 480/277 1200 amps.

Will my primary grounding wire be properly sized to handle any fault to ground? Primary and Secondary?
 
Re: Grounding a floating bridge

You need to determine the available primary fault current from the utiltity. Is the primary AL or CU. When you say ground wire do you mean the primary neutral?
If memory serves 1/0 AL is good for 6800 amps.
 
Re: Grounding a floating bridge

3 phase conductors and a ground wire used as equipment because it is in PVC and the primary neutral.

1000 KVA #2 feeder 1/0 neutral ground. The distance from the Pole Switch and the transformer is .65 Miles. 12470/480-277 xfmr.
 
Re: Grounding a floating bridge

By Loren: Will my primary grounding wire be properly sized to handle any fault to ground? Primary and Secondary?
Click to expand...
I hope you know that any fault on the load side of a transformer does not fault all the way back to the service feeding the primary. All fault current returns back to source which will always be the transformer feeding that circuit which faulted. so if the primary EGC is sized for the feeder feeding the transformer then it will carry any load placed upon this EGC by the transformer. the faulted secondary current to primary current will have the same turn ratio that the primary to secondary voltage has. A short circuit is nothing more than a oved sized load and can be figured as such. If you have a 1200 amp fault on the secondary then you would have a 46.1 amp fault on the primary. If you are only using a 1200 amp @277/480 the primary feed is sized about 4 times over what the load could ever draw. to produce even a 200 amp fault on the primary would require a 5196 amps fault on the secondary. I would say you are well within range with only a 183.6 voltage drop accros the 1/0 from a 12470 volt primary. I hope this transformer is fused at little closer then 200 amps on the primary?
 
Re: Grounding a floating bridge

I thank you for the repling to this question. I still wonder where the equipment grounding conductor from the antenna's or anything that may be struck by lightning. Will this still be large enough?

Every bit of grounding will have to travel on that wire.
 
Re: Grounding a floating bridge

Loren -- what is your 480/277V distribution setup?

On 480Y/277, every circuit breaker (including a main or a feeder CB) with a trip rating of 1000A or more must have ground fault protection.
 
Re: Grounding a floating bridge

That is correct. We will have a straight 480 system even though the 277 will be present we will not be using it.

All the breakers 125 and above will have ground fault.

It is the outside ground current that is my concern now. Others have verified that the ground is sufficient for the service.

To date lighting has never hit the bridge. However we are adding communication, cctv, radar units to the highest point on the bridge.
We get very little lighting in our area of the Olympic peninsula. But all it will take is once. I just want to make sure that we have a very low impedance path to ground. All I can do is inspect what they put in. And I can't find anything in the NEC to cover this.
 
Re: Grounding a floating bridge

Lightning protection is not really a NEC issue other then what the NEC does require at a service. NFPA 780 is the code you would need but if it hasn't been adopted by you state or local government then it can't be enforced. But what is in the NEC is the requirement of bonding to all available grounding electrodes(250.50) which since a bridge is a metal structure (unless it's wood) it would be one of the best electrodes for that and required to be used as a grounding electrode, also the antenna tower lead in wires are also required to be bonded to the same electrode system. Lightning wants to get to Earth unlike our AC current which wants to return to source. The EGC that is ran with the circuit conductors are only for fault current on the primary side of the transformer including the case of the transformer. Now you have to follow the requirements of 250.30 once you get to the other side of the transformer as you now have a separately derived ac system which the X0 needs to be bonded and also connected to the same grounding electrode system as the primary. So with the conductors you have there shouldn't be a problem as long as the electrode system is install as required.

After looking at what this bridge is I would see if there will be any grounding electrode connection points on the concrete pontoons. They would serve as a great electrode if there was a way to connect to any reenforcement steel in them and or to the sheet pile wall. I would think that all these sections would be required to be connected together to prevent any voltage potential between them?

[ July 17, 2005, 03:21 AM: Message edited by: hurk27 ]
 
Re: Grounding a floating bridge

In case someone want's to see it:

19HoodBridge.jpg


And more info can be found HERE

[ July 17, 2005, 03:30 AM: Message edited by: hurk27 ]
 
Re: Grounding a floating bridge

They didn't think of that when they built the section of the bridge that will be staying. Everything is connected to the EGC. The pontoons that the road way is set on are concrete. They are held together with cables that pass through plastic tube. The cables have threaded ends and the a nut and washer are installed on each side. The nuts are tightened and the pontoons pull together. They do not attach to rebar at all.
I believe that there is a circulating current running through the roadway rebar to the column rebar and then through the pontoons top deck. The roadway has a expansion joint made of rubber at every pontoon joint During the recent widening the rebar that was exposed could draw a good arch. (I was told)
Is the concrete good enough? The pontoons set in salt water.
NEC 821.10 directs everything back to the Grounding electrode. That would be over half a mile in each direction. (The total bridge length is 1.5 miles. Approx. 1.3 if floating.) The ECG will have to do.
I would think that the ECG well be good enough.
 
Re: Grounding a floating bridge

Loren send me a private message with your email address I will email you an article I wrote on traffic signal radio and antenna installations for the IMSA Journal.
To determine what would be allowed for the antenna grounding conductor, read thru 810.21, if its not listed there then it can not be used.
One of the bridge maint techs, Joe, owes me a tour.
Tom in Bremerton
 
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