Thanks for the explanation. I was certainly thinking lower resistance with the 4-inch copper ground but didn't think the impedance at 60 Hz would be that much different. I can definitely see the coupling problem with conduit and I believe conduit is required in commercial installations.
I bet a lot more people would follow R56 if it was more freely available and not shrouded in secrecy and cost. Bob M. ====== --- On Thu, 4/9/09, Eric Lemmon <[email protected]> wrote: > From: Eric Lemmon <[email protected]> > Subject: RE: [Repeater-Builder] Re: Motorola R56 grounding > To: [email protected] > Date: Thursday, April 9, 2009, 10:51 PM > 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
