Bob, The reason is quite simple. If a fault (say, a short to the case) occurs in a piece of equipment that is grounded per the NEC, the fault current- which may be 100 amperes or more- flows from the service through the circuit protection device (fuse or circuit breaker) to the fault, and returns via the equipment grounding conductor (green wire) to the service. When the green wire is in close proximity to the hot wire (in the same conduit or cable jacket), the impedance to the fault is as low as it can possibly be- ensuring the immediate functioning of the circuit protection device. If the fault return path were to follow a circular route- as it would if through a path not in close proximity to the supply conductor- the impedance will ALWAYS be much greater, and can possibly be high enough to limit the fault current to a value near the rating of the circuit protective device. For example, if the impedance of the ground return path limited the fault current to 25 amperes on a 20 ampere branch circuit, the circuit may "cook" for several minutes before tripping. Keep in mind that a fancy copper strip running around the shack may have a DC resistance that is a fraction of one ohm, but its AC impedance may be many ohms.
When the electrical installation is within steel conduit, the problem becomes much worse if the grounding conductor follows a different path from the hot and neutral conductors. When a fault occurs, the presence of the steel conduit creates a solenoid and the impedance increases significantly. Case in point: About 20 years ago, I was rewiring a very old theater that was then almost 50 years old. When I got around to working on the house lights, I found that an audible hum could be heard when the dimming rheostats were in a certain position. The hum seemed to come from everywhere, making it difficult to pinpoint the source. On a hunch, I used my trusty Simpson clamp ammeter to test the conduits up in the rafters. Surprise! One of the conduits had almost 15 amperes flowing through it, meaning that there was a fault in the house light circuit, but the impedance of the grounding circuit limited the current to less than the circuit breaker rating. While pulling out the ancient TW wires, I found one that was skinned bare by being pulled through an unreamed conduit, and had shorted to the conduit. Had this circuit been properly grounded, the circuit breaker would have tripped instantly; instead, a dangerous electrical fault persisted, and in a place of assembly, that is unacceptable. The NEC includes many references to minimizing the impedance of a fault current path, but the NEC Handbook includes detailed explanations of the rationale. For example, Article 250.24(C)(1) states, "This [grounding] conductor shall be routed with the phase conductors..." Article 250.32(B)(1) states, "An equipment grounding conductor as described in 250.118 shall be run with the supply conductors..." It takes a leap to assume that it's okay to depart from this consistent mantra. An electrical inspector's primary concern is ensuring the safety of the public and of structures occupied by the public. I will admit that few inspectors will perform a thorough inspection of a mountaintop radio or cellular site, because the public's safety is not much of an issue there. I also understand why an electrician- regardless of how experienced he or she may be- will probably never object to performing an installation that is not fully compliant with the NEC. The owner will probably perceive that electrician as a troublemaker, leading to lost business. The electrician can always say that he followed the plans exactly, and is therefore not likely to be held accountable. I have worked with many electrical inspectors who don't sweat the small stuff, but also with many who are very thorough and would never allow "creative grounding" methods. As an IAEI/ICBO Certified Electrical Inspector myself, I have been in the business long enough to know that a telecommunications site wired and grounded in accordance with the NEC will work properly and be safe. As for the currency of R56, I do not know how often it is updated. I do know that its authors are aware of the NEC conflicts and are planning to reconcile those issues in a future edition. 73, Eric Lemmon WB6FLY -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Bob M. Sent: Thursday, April 09, 2009 1:29 AM To: [email protected] Subject: RE: [Repeater-Builder] Re: Motorola R56 grounding I question the validity of your statement that separating the grounding conductor "would greatly increase the impedance of the grounding system" It might do this depending on the route the ground wire takes, but if the installation had a 4 inch ground foil running around the perimeter of the building and every grounding conductor ran from an outlet to this ground foil, wouldn't that REDUCE the impedance and possibly offer better ground conduction? Seems to me that a long #12 ground wire, from the outlet all the way back to the service panel, would have a higher impedance. I agree with the rest of the paragraph below however. How often is R56 updated? Does R56 mention somewhere that local regs take precedence, or that NFPA codes supercede R56? Is R56 just a thorough collection of installation guidelines and recommended practices? I've never seen a copy so I'm asking merely for my own education. Bob M. ====== --- On Wed, 4/8/09, Eric Lemmon <[email protected] <mailto:wb6fly%40verizon.net> > wrote: > From: Eric Lemmon <[email protected] <mailto:wb6fly%40verizon.net> > > Subject: RE: [Repeater-Builder] Re: Motorola R56 grounding > To: [email protected] <mailto:Repeater-Builder%40yahoogroups.com> > Date: Wednesday, April 8, 2009, 10:04 PM > Martin, > > Sure! The National Electrical Code (NEC) requires > that the equipment > grounding conductor (green or bare wire) must always follow > the same route > and wireway used by the supply and return conductors. > There must never be > any deviation from this basic requirement. The three > wires (hot, neutral, > and ground) feeding every receptacle must always run > together, but the R56 > manual proposes that the grounding conductors of some > "technical" > receptacles shall follow a path separate from the hot and > neutral > conductors. That is not allowed by the NEC, since > that would greatly > increase the impedance of the grounding system and thereby > reduce the > protection of the circuit against faults. Also, the > NEC requires that the > system grounding conductors, equipment grounding > conductors, and lightning > protection grounding conductors must ultimately be bonded > together to create > ONE grounding system. The R56 manual proposes a > scheme that creates > separate grounding circuits that can create dangerous > voltages on some > circuits if a fault occurs on another circuit. > Despite some really creative > schemes to create separate grounding paths, such schemes > are not allowed by > the NEC or by state electrical codes based upon the NEC. > > Readers following this thread should be aware that the NEC > is updated every > three years, and becomes law as each state or commonwealth > ratifies it > through legislative action. The current edition of > the National Electrical > Code, NFPA 70, is the 2008 edition. > > 73, Eric Lemmon WB6FLY
