Building Steel vs Telecommunication Backbone conductor

[Craig D Dunton]

The 2020NEC says in 250.121(B) not to use the building steel as a Equipment Grounding Conductor. In NEC 800.100 it describes and shows a picture of Communication equipment being connected directly with a bonding conductor to the IBT or the Electric Grounding Conductor. The TIA Standards says in 6.3.6.1 the building steel may be used as a equipment bonding conductor subject to local codes which would include the NEC.
It would appear as if we can't use it per the NEC
What do others think?

 

[don_resqcapt19]

The communications bonding conductor is not an Equipment Grounding Conductor, so those rules do not apply.
There is nothing in the NEC that would prohibit you from using the building steel for that purpose.

 

[Craig D Dunton]

The communications bonding conductor is not an Equipment Grounding Conductor, so those rules do not apply.
There is nothing in the NEC that would prohibit you from using the building steel for that purpose.

Don,
Your note below your name says NEC 2017. Is that what you are using or are you using the 2020 NEC? What happens if the data switch you have in the relay rack and it develops a fault and the third prong does not work as designed would it not then become a Equipment Grounding Conductor and then trip the circuit breaker? Also by using the Building steel alone it will create a third prong ground reference and the building steel which might not be at the same reference. Also what about 800.100 B(B)(1) It clearly shows a separate conductor from Communications equipment to the Electric Ground Conductor (GEC)
Then 250.50 also does not list the building steel as a Grounding Electrode System. So if you could give me more information that would be great!
Thanks,
Craig

 

[don_resqcapt19]

Don,
Your note below your name says NEC 2017. Is that what you are using or are you using the 2020 NEC? What happens if the data switch you have in the relay rack and it develops a fault and the third prong does not work as designed would it not then become a Equipment Grounding Conductor and then trip the circuit breaker? Also by using the Building steel alone it will create a third prong ground reference and the building steel which might not be at the same reference. Also what about 800.100 B(B)(1) It clearly shows a separate conductor from Communications equipment to the Electric Ground Conductor (GEC)
Then 250.50 also does not list the building steel as a Grounding Electrode System. So if you could give me more information that would be great!
Thanks,
Craig

I should change the note as I am working off the 2020 code, but it doesn't matter.

If the conductor in question is not run with the power circuit conductors for the equipment, it is not an Equipment Grounding Conductor and is not subject to the rules that apply to EGCs.

The key to the figure you cited, is that is the grounding conductor from the communications circuit protector that is installed on the communications circuit at the point it enters the building. That figure has nothing to do with any communications bonding beyond the communications circuit point of entry into the building.

There should be no potential difference between the steel and the EGC of the power circuit other than under fault conditions. The code requires the steel to be bonded to the electrical system.

While building steel that is not in direct contact with the earth is not a grounding electrode, however 250.68(C)(2) permits the use of the interior building steel as a bonding path between grounding electrodes. This rule is what permits the bonding connection in section 6.3.6.1 of the TIA Standard that you cited.

The conductor in question is not a conductor that is covered by the NEC, and there are no rules in the NEC that cover its installation. You can do it in any manner that you want to.

 

[hbiss]

There should be no potential difference between the steel and the EGC of the power circuit other than under fault conditions. The code requires the steel to be bonded to the electrical system.

My experience has been that that's not always the case. I have found as much as 50 volts between building steel and the ground pin of the receptacle powering equipment. If the equipment has a ground screw it is customary to connect that to building steel and in fact the manufacturer's instructions require it. Unfortunately I have had such communications equipment that had the green wire of the line cord connected internally to the ground screw through a ground trace on the circuit board. I shouldn't need to say what happened.

-Hal

 

[don_resqcapt19]

My experience has been that that's not always the case. I have found as much as 50 volts between building steel and the ground pin of the receptacle powering equipment. If the equipment has a ground screw it is customary to connect that to building steel and in fact the manufacturer's instructions require it. Unfortunately I have had such communications equipment that had the green wire of the line cord connected internally to the ground screw through a ground trace on the circuit board. I shouldn't need to say what happened.

-Hal

What caused that 50 volt difference? That is a very dangerous condition.

 

[hbiss]

I agree. It was intermittent and actually caused the line cord to heat up before the equipment was destroyed. No idea where it was coming from, I was only there for the telecom so it wasn't my job to try and track it down. I do remember that there was a large cell site on the roof and you could hear the power supplies from the offices below.

-Hal

 

[Craig D Dunton]

Don,
I also have found a difference in resistance to ground between the third prong and the building steel. Building steel is not consistently low impedance. So with that condition you could have a difference in potential between the building steel and the third prong. Then what happens when you only have your racks bonded to the building steel and your electronic equipment only had the hot and neutral and there is a fault. Where does the fault current go? I would think it would follow the building steel to ground and trip the overcurrent device. Or I we assuming all equipment is new and that two prong equipment is always double insulated?
I also am thinking this from a telecom perspective.
Craig

 

[don_resqcapt19]

Don,
I also have found a difference in resistance to ground between the third prong and the building steel. Building steel is not consistently low impedance. So with that condition you could have a difference in potential between the building steel and the third prong. Then what happens when you only have your racks bonded to the building steel and your electronic equipment only had the hot and neutral and there is a fault. Where does the fault current go? I would think it would follow the building steel to ground and trip the overcurrent device. Or I we assuming all equipment is new and that two prong equipment is always double insulated?
I also am thinking this from a telecom perspective.
Craig

Unless you have current flow on the building steel the impedance doesn't make any difference. Without current across the impedance there is no voltage drop. In addition the code is not very concerned with a possible high impedance on the building steel as the code permits, in 250.68(C)(2) , the building steel to be used as a grounding electrode conductor.

Again, the only EGC in the the code is the one run with the power supply circuit. Sure there are cases where fault current flows on other paths, but that does not make those other paths in to a conductor that must follow the NEC rules for EGCs. That possibility is one of the reasons that building steel must be bonded to the the electrical grounding system.

The telecom bonding is outside the rules of the NEC and you can make the install pretty much any way you want to without violating the rules in the NEC.

 

[mbrooke]

There can be current on the building steal due to the POCO MGN, 3 wire 120/208-240 appliances and standing neutral to ground faults within the building.

Thus where any part of the system is not 5 wire data loops should be avoided.

 

[zbang]

I also am thinking this from a telecom perspective.

You may be overthinking this. From a tele/data-com perspective, most signal interconnects now are isolated in some fashion- either via fiber or some form of transformer-isolated connection (TP or DAC for Eithernet, etc); metallic interconnects that use a "ground" reference are getting fewer every year and tend to stay in a small area because of that. This suggests that in many modern locations "telecom" or "technical' grounds don't provide any benefit. (In high-RF areas, they're usually needed.)

Then what happens when you only have your racks bonded to the building steel and your electronic equipment only had the hot and neutral and there is a fault. Where does the fault current go?

Down the building steel, of course.... does that equipment have/recommend it's own EGC? If the equipment is designed not to need one, then the likelihood of a line-ground fault is vanishingly small (not zero, but pretty much not relevant).

 

[rnatalie]

My experience has been that that's not always the case. I have found as much as 50 volts between building steel and the ground pin of the receptacle powering equipment.

Yep, and I got to feel quite a tingle grabbing a raceway with one hand and building steel with the other once.

 

[augie47]

Yep, and I got to feel quite a tingle grabbing a raceway with one hand and building steel with the other once.

That would sure make you wonder if the building steel had ever been properly bonded.

 

[zbang]

Sounds like not, and that's a whole different problem.