Structural Steel in contact with earth

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George Stolz

George Stolz

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Has anyone ever seen these two methods exercised in the field, either naturally or by an electrician?
2011 said:
250.52(A)(2) Metal Frame of the Building or Structure. The metal frame of the building or structure that is connected to the earth by one or more of the following methods:
(1) At least one structural metal member that is in direct contact with the earth for 3.0 m (10 ft) or more, with or without concrete encasement.
(2) Hold-down bolts securing the structural steel column that are connected to a concrete-encased electrode that complies with 250.52(A)(3) and is located in the support footing or foundation. The hold-down bolts shall be connected to the concrete-encased electrode by welding, exothermic welding, the usual steel tie wires,
or other approved means.
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So as I understand it, the steel beam is not an electrode unless (one scenario) the hold down bolts are connected to a CEE. Now a CEE only needs to be a #4 but we would have to bond the steel based on 250.66. Is that correct or am I missing something. Seems a bit odd.
 
Dennis Alwon said:
So as I understand it, the steel beam is not an electrode unless (one scenario) the hold down bolts are connected to a CEE. Now a CEE only needs to be a #4 but we would have to bond the steel based on 250.66. Is that correct or am I missing something. Seems a bit odd.
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I think you are right. It does sound screwy, but it seems to work out that way.
 
Dennis Alwon said:
So as I understand it, the steel beam is not an electrode unless (one scenario) the hold down bolts are connected to a CEE. Now a CEE only needs to be a #4 but we would have to bond the steel based on 250.66. Is that correct or am I missing something. Seems a bit odd.
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jumper said:
I think you are right. It does sound screwy, but it seems to work out that way.
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The steel beam is a low-resistance ground-fault current return path compared to earth and electrode path back to source. Lower resistance paths will handle more current... bigger wire necessary :happyyes:
 
Smart $ said:
The steel beam is a low-resistance ground-fault current return path compared to earth and electrode path back to source. Lower resistance paths will handle more current... bigger wire necessary :happyyes:
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This is one of those green sky blue grass moments, isn't it. :huh:
 
Smart $ said:
:huh: Not from my perspective... (and I'm not color blind :blink:)

(Alternate version: I do not correlate the quip or its implication :?)
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With grounding and bonding, it's easy to say something entirely different than what you are trying to say. I think you just did.

Smart $ said:
The steel beam is a low-resistance ground-fault current return path compared to earth and electrode path back to source. Lower resistance paths will handle more current... bigger wire necessary :happyyes:
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We are talking about grounding electrodes. Grounding electrodes are not designed for ground-fault current return paths, so I don't think you said what you meant to say.

Jumper and Dennis said that even though the Steel uses the CEE as a surrogate earthing connection, we need a full sized GEC to the Steel per code. That is correct, but only because code says so. There is no technical reason I am aware of that would require a Steel electrode conductor to be larger than it's ultimate output, the CEE.

Remember, we're not doing the steel any favors by using it as a grounding electrode. The Steel is doing the service a favor by serving as a path for "limiting the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operation." (250.4(A)(1)).
 
George Stolz said:
With grounding and bonding, it's easy to say something entirely different than what you are trying to say. I think you just did.


We are talking about grounding electrodes. Grounding electrodes are not designed for ground-fault current return paths, so I don't think you said what you meant to say.

Jumper and Dennis said that even though the Steel uses the CEE as a surrogate earthing connection, we need a full sized GEC to the Steel per code. That is correct, but only because code says so. There is no technical reason I am aware of that would require a Steel electrode conductor to be larger than it's ultimate output, the CEE.

Remember, we're not doing the steel any favors by using it as a grounding electrode. The Steel is doing the service a favor by serving as a path for "limiting the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operation." (250.4(A)(1)).
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Perhaps... but that is not the case here. I said exactly what I meant to say. The part you are not seeing in this scenario is that the metal frame of a structure and non-current-carrying metal parts are required to be bonded to the service ground for the purpose of being a ground-fault-current return path. It makes no difference whether the metal frame is used as an electrode or not. [ref: 250.104(C) and (D)(2)]
 
Smart $ said:
Perhaps... but that is not the case here. I said exactly what I meant to say. The part you are not seeing in this scenario is that the metal frame of a structure and non-current-carrying metal parts are required to be bonded to the service ground for the purpose of being a ground-fault-current return path. It makes no difference whether the metal frame is used as an electrode or not. [ref: 250.104(C) and (D)(2)]
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I have never understood why the bonding conductor to the steel structure or interior water piping (where neither are used as a grounding electrode) is based on the size of the service entrance conductors. I understand that they are thinking about fault current, but in my opinion the bonding should be based on the size of the circuit that might possibly fault to these items. In most cases that would be the largest branch or feeder OCPD in the system. A conductor sized based on that OCPD and Table 250.122 should do the job. Also given the issues with the remote return path inductive reactance limiting the current on the remote path, even a conductor sized based on T250.122 would be larger than technically necessary.
 
don_resqcapt19 said:
I have never understood why the bonding conductor to the steel structure or interior water piping (where neither are used as a grounding electrode) is based on the size of the service entrance conductors. I understand that they are thinking about fault current, but in my opinion the bonding should be based on the size of the circuit that might possibly fault to these items. In most cases that would be the largest branch or feeder OCPD in the system. A conductor sized based on that OCPD and Table 250.122 should do the job. Also given the issues with the remote return path inductive reactance limiting the current on the remote path, even a conductor sized based on T250.122 would be larger than technically necessary.
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I recall you mentioning the "remote return path inductive reactance limiting the current" in prior threads I've read. I am aware of inductive reactance increasing when conductors are separated but I have not run across any "bona fide" documentation that discusses the effect for fault current. I did a crude calculation of the ratio of increase at 1,000 times the spacing between conductors and it only came out to roughly 7.5 to 1. At a million times, 14.3 to 1, and roughly an increase of 7 times for each power of 10^3 increase in spacing. I was using the cable reactance formula on this page (link) for my calculation basis. Being that inductive reactance for the normal conductors is generally considered insignificant, at what point does it become significant for fault current?

Do you have a link to any documentation on this aspect of the electrical phenomenon... preferably brief, I'm only seeking a general understanding?
 
Smart $ said:
I recall you mentioning the "remote return path inductive reactance limiting the current" in prior threads I've read. I am aware of inductive reactance increasing when conductors are separated but I have not run across any "bona fide" documentation that discusses the effect for fault current. I did a crude calculation of the ratio of increase at 1,000 times the spacing between conductors and it only came out to roughly 7.5 to 1. At a million times, 14.3 to 1, and roughly an increase of 7 times for each power of 10^3 increase in spacing. I was using the cable reactance formula on this page (link) for my calculation basis. Being that inductive reactance for the normal conductors is generally considered insignificant, at what point does it become significant for fault current?

Do you have a link to any documentation on this aspect of the electrical phenomenon... preferably brief, I'm only seeking a general understanding?
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Not really. It is what I have read in a number of places. There is some information on this thread on eng-tips.com.
 
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