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 Separately Derived System (SDS) Common Grounding Electrode Conductor?
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| Author Information | Topic: Separately Derived System (SDS) Common Grounding Electrode Conductor? |
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posted August 16, 2002 at 11:07 AM
Is a copper bus bar (¼” x 4” x 30” or larger), with a listed 2-hole compression connectors, bolted to the bus bar considered a “splice or joint” as described by NEC 2002 Article 250-30(A)(3)(b)? Background To Question: I’m an electrical engineer for a large telephone company, and the question has come up for discussion among other engineers within the company after one local jurisdiction insisted we change the Grounding Electrode Conductor (GEC) termination point on the isolation transformers. Our equipment offices are large (up to 200,000 sq. ft.), and contain numerous SDS such as battery plants, UPS’s, stand-by generators, and isolation and/or step-down transformers. In addition to the required NEC grounding for AC electrical service, we install an open air wire ground system to supplement the ACEG system. It is used to frame ground DC powered systems, provide a signal reference ground, provide dedicated lightning discharge paths, and finally reference SDS to the grounding electrodes. Basically it is a common grounding electrode conductor as defined NEC 2002 Article 250-30(A)(2) &(3). It is constructed by placing a large copper bus bar on an outside wall called Office Principle Ground Point (OPGP) located near but adjacent to the AC service entrance switchgear. From the OPGP a dedicated 500 MCM cable is routed to every grounding electrode available such as: · AC service entrance switchgear cabinet. From the OPGP we then run a dedicated 500 MCM cable to individual ground bars located throughout the facility in each equipment room in a radial (parallel/star) fashion. Each equipment room is equipped with two ground bus bars. One located 7 feet above floor line for DC operated equipment, and one underneath the raised floor for AC operated equipment. Each of the ground bars is also supplemented with another connection to building steel if available via 2/0 AWG conductor. All the bus bar-to-bus bar terminations use listed 2” on center, 5/8” two-hole, tongue irreversible compression connectors with ½” bolts. Here is the point finally. We connect all the SDS to the ground bars instead of building steel if available (necessitated by large number of SDS devices). One local inspector objected and required us to connect all are PDU’s (Power Distribution Unit; a self contained isolation transformer with branch circuit distribution built in) directly to building steel rather then the bus bar two feet away and obviously bonded to the building steel coulomb via 2/0 AWG. This has caused some discussion within the company as this is a fairly common practice within the telecom industry and I would like to hear some discussion from other PE’s out there familiar with the subject and the industry. IP: 63.52.128.57 |
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posted August 16, 2002 at 02:20 PM
First of all, I'm not an engineer but I do have some oppinions on this subject, which due to a busy day I can't post at the moment. But, I must ask, what was the local AHJ doing in this building in the first place? See article 90.2(B)(4) Roger IP: 209.95.81.147 |
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posted August 16, 2002 at 03:19 PM
Roger, The company is not registared as a utility, therefore not exempt from local building codes. Only telcos with monopolies get utility status. Yout comments are welcome. Thanks Dereck IP: 63.52.129.3 |
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posted August 16, 2002 at 03:43 PM
I've done design work on cellular towers and their associated communications huts in the past. These installations have several ground bars like the ones you have mentioned. I also believe it's a common practice to have several ground bars installed in data centers. However, it sounds like he is referring to Art. 800-40(b)1 and Art. 250-81(b). I would ask the inspector for his reasoning behind his decision. IP: 216.237.15.126 |
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posted August 16, 2002 at 04:21 PM
Dereck, I see no technical issues with your grounding installation, and if I was the AHJ, I would accept the installation. It does appear that even though your installation is superior to the code required mimimum, that there are violations. The code in 250.32(A) and 250.64(C) require that the common grounding electrode conductor be continuous between the grounding electrode and the taps to the individual SDSs. He also may be objecting because the GEC does not go directly to the local building steel. Again, I see no reason to reject, but the code would permit him to reject. It would also permit him to accept using 90.4. Don IP: 64.133.97.147 |
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posted August 16, 2002 at 04:33 PM
Robbie, I do not think the inspector was questioning outside cables entering the facility. We use a cable vault to bring all outside services into the building. It has a ground bar welded to the ground ring and structural steel. All sheaths and protectors are terminated in vault. The bars under the raised floors are used to bond the PDU's Xo lead, and to reference a signal reference grid (SRG an 8' x 8' wire grid) used to bond equipment frames in lin-ups. These bus bars have a dedicated 500 MCM cable going directly to the ground electrode, and a supplemental 2/0 AWG going to the closest building steel if availible. The AHJ objected to bonding the PDU Xo lead to the bus bar. He wanted it directly to bulding steel. He determined the bus bar was a splice. He wanted it cad welded to the bus with all other cables welded or moved and pressure clamped or bolted to building steel. We did not argue, downtown Chicago, opted to move it to building steel with two-hole bolted lug. Go figure! Thanks, all comments welcomed. Any inspectors out there willing to take a shot at this? IP: 63.52.197.73 |
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posted August 16, 2002 at 04:53 PM
Don, I appreciate the feed back, good comments. The continuous grounding electrode is the question at hand. Do the ground bar-to-ground bar connections constitute a joint or splice. Irreversible crimps (like “H” or “C” taps and terminal connecters) and thermal welds are not considered splices or joints. We use two-hole terminal lugs and tag the cables with brass DO NOT DISCONNECT tags. In addition we bond the bus bars to the nearest building steel if available with a 2/0 AWG bolted to bar and either welded to building steel, or bolted like at bus bar. You are right he objected because it did not go to building steel first, although the building steel was connected to the bus bar 3 feet away. We use the bus bars to accommodate the large number of SDS in a given area. Thanks Dereck IP: 63.52.197.73 |
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posted August 16, 2002 at 06:53 PM
Are the power systems designated SDS suppling the premises wiring or specific equipment? IP: 216.66.117.117 |
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posted August 16, 2002 at 07:41 PM
Dereck, There is an exception for splicing bus bars used as grounding electrode conductors in 250.64(C, but I don't see any exception that permits you to go from the grounding electrodes with wire, connect to OPGP and the switch back to wire to go to the local grounding bars and then to the SDSs. I don't see this as causing any safety problems in the installation, because from a safety standpoint the connection of a SDS within a building to a grounding electrode is not very important. The main bonding jumper is the most important safety related connection for this system. Don Don IP: 64.12.96.73 |
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posted August 16, 2002 at 08:55 PM
quote: Specific equipment like routers, optical transmission, frame relay, and other such data processing equipment. Its a data center and switching & transmission offices. The specific PDU's are fed from 300KVA UPS. The PDU's are strategically located in equipment line-ups to keep distribution circuits short as possilbe to minimize voltage drop, and single point grounded. The PDU's have self-contained modular branch circuit distribution, power monitor, alarms, etc. Its high end computer grade stuff! IP: 63.52.151.23 |
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posted August 16, 2002 at 09:39 PM
Dereck, first of all, your first paragraph of whether this connection is a joint or a splice. This is a mechanical connection no different than the connection inside a panel or transformer. The connectors you describe are listed for the purpose. Article 250.30(A)(3)(b)says by listed connections or exothermal welding. Now your terminology of isolated transformers. Are you talking about true isolated transformers in the sense of single winding ungrounded control sources, or some other ungrounded configuration. The batteries would be isolated by their very nature. Generators could possibly be isolated or seperatly derived, depending on how they are connected through the transfer switch and EGC bonding. Now the SDS point, I hope I am interpreting Steve right, in that per article 100 Seperately Derived System your down line transformers would be equipment wiring and not A Premises Wiring System. Good point Steve. Now every item down line of the service entrance would be connected by EGC the same as say, a receptacle. Dereck, as I said said before I'm no engineer. Wecome to the war Dereck. Roger
[This message has been edited by rp.deas@verizon.net (edited August 16, 2002).] IP: 67.240.155.51 |
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posted August 17, 2002 at 07:49 AM
quote: LOL Roger! I'm trying to relate art 100 SDS, 200.3 & art 647 ( if applicable here) . It would seem 'single point' grounding would be superior ( per Don) as opposed to a 'compliant' system of notable gauss.... would not the power quality consultants allude to similar installs? Thus this becomes somewhat definitional, especially given the (informative, thanks) threads here of late.... ~Steve IP: 216.66.117.117 |
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posted August 17, 2002 at 12:27 PM
Roger, You are correct that the taps can be made to the grounding electrode connector using listed connections to a bus bar that is used as the grounding electrode connector. The code problem in this installation is that the grounding electrode conductor itself is made up of both wires and bus bars. This does not appear to meet the rule in 250.64(C). I would still accept the installation as being superior to what is required by the NEC, but the original question was about violations of the letter of the code and I do see this as a violation. Don IP: 64.12.96.73 |
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posted August 17, 2002 at 02:14 PM
Roger , the transformers are being fed from 300 KVA UPS systems. They are 480 VAC delta input and 208Y/120 VAC output. We call them PDU’s because they are self-contained ferroresonant isolation transformers with branch circuit distribution, power monitor, and alarming. They are used to power data processing equipment found in a data or switch center. They are located strategiacally in the equipment line-ups to keep branch circuit wiring under 40 feet. Batteries are 4000 AH @ 540 VDC. They float (not grounded), are fused on both polarities, and fine. Generators are operated either 480 delta or 13.2KV delta, depends on mega watt requirement. No neutral to worry about. Frame bonded to ground ring and building steel most of the time unless on located on top of high rise building. Anyway, I think I got my answer. The PDU’s are feeding specific equipment. Looks like its just a local problem, not national. THX Dereck IP: 63.52.131.23 |
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posted August 17, 2002 at 10:04 PM
quote: Steve, specific equipment. IP: 63.52.150.105 |
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posted August 17, 2002 at 11:36 PM
As a good friend of mine says, it sounds like we are trying to pick fly sh-- out of pepper. IP: 208.0.86.210 |
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posted August 18, 2002 at 08:10 AM
the very nature of the NEC rkeis...... and somewhat a rather large vectorborne defecation given the shady definition of SDS IP: 216.66.117.117 |
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posted August 18, 2002 at 08:34 PM
Don, I agree that per the the NEC there are violations, but I side with you that this is a superior installation to the minimum. I have thought about this for the last couple of days, and do have some concern as to this grounding being such a surround around personel. Dereck,has anyone actually used a guass meter through out the facility to monitor this installation periodically. Although this is still out to the jury, we have been notified that low radiation due to EMF is present. This will probably be the next asbestos or lead lawsuits. Roger [This message has been edited by rp.deas@verizon.net (edited August 18, 2002).] IP: 67.240.72.177 |
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posted August 18, 2002 at 08:48 PM
Roger, I don't really see any parallel path for grounded conductor current that would create the EMF. Don IP: 64.12.101.167 |
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posted August 19, 2002 at 12:30 PM
Roger, no a gauss mete has not been used. All ground conductors are periodically checked with a clamp-on amp meter to check for any AC or DC current present. If there is, corrective action is taken. Some offices even have CT’s strategically located on ground conductors to monitor/record any current and raise alarms if any develops. We go to great pains to ensure everything is single point grounded from the power sources, and diligent checks to make sure the are no devices connected between neutral and ground on AC systems, or between battery return and ground on 48 VDC systems. All AC circuits are dedicated, distributed in rigid metal conduit, with an EGC and oversized neutral conductor. High quality stuff. THX Dereck IP: 63.52.197.101 |
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