Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!samsung!zaphod.mps.ohio-state.edu!lavaca.uh.edu!menudo.uh.edu!nuchat!steve From: steve@nuchat.sccsi.com (Steve Nuchia) Newsgroups: sci.electronics Subject: Re: ANOTHER house wiring question (this one's basic) Message-ID: <1991Jan16.035358.28312@nuchat.sccsi.com> Date: 16 Jan 91 03:53:58 GMT References: <1948@umriscc.isc.umr.edu> Organization: South Coast Computing Services, Inc. Houston Lines: 165 In <1948@umriscc.isc.umr.edu> robf@mcs213k.cs.umr.edu (Rob Fugina) writes: >Ground and neutral are exactly the same thing. If you were to trace the And more dangerous speculation. It probably doesn't need to be said here, but I'll feel better if I say it anyway: If you don't know that you know what you're doing, don't play around with your wiring. There. I do feel better. Now, I'll lay it all out for reference. This isn't electronics, but maybe it will enlighten some, and help others avoid being lit up :-) Power wiring is always discussed in nominal RMS, peak is of interest only when engineering insulation and protection systems. 99.999% of all wiring is done using pre-engineered components which have their voltage and current ratings tabulated in nominal RMS terms in catalogs and handbooks and such. Residential service is normally provided from overhead or buried high voltage distribution lines through a transformer serving one or several subscribers. The distribution is often a 3-phase Y, but rural branches may have only one power conductor plus the ground return. Several other arrangements can be used to suit the power company's needs. Distribution lines are typically in the 15 - 45 kV range. The distribution circuit is protected by overcurrent devices and ground-fault detection devices at the substation, but it takes one heck of a fault to open those. The subscriber transformer primary is often wired from one phase to neutral on the distribution side, but phase-to-phase is sometimes used. If you have three-phase service (normally the case only for industrial and large commercial customers) the transformer primary is attached to all three distribution phases (and maybe neutral). Autotransformer configurations are also used, I believe. The residential transformer secondary is a single phase 240 volt center tapped winding. The center tap is coupled at the pole to a ground rod and to the service drop neutral conductor. This positively limits the voltage on the service cables, relative to ground, to a few times the nominal service voltage. It also ensures that a core short will draw enough current to open an upstream protection device. The three wire single phase service entrance cable passes through the meter and into the customer's distribution panel. There the neutral is again bonded to grounds, including the frame if it is a metal building. This limits the voltage present on the interior wiring relative to ground -- if the service were not gounded the whole circuit would float. The rules for what should and should not be grounded, and where, and how, are complex. Not terribly technical, but lots of cases and exceptions have to be dealt with. In general, neutral and ground are bonded at the service entrance and kept separate throughout the rest of the interior wiring system. The cable system from the transformer to the meter, together with the transformer secondary winding, is capable of delivering some calculable short circuit current. There should be an overcurrent protection device somewhere in the circuit capable of interrupting that much current. It is typically in the 10-20 thousand amp range, and typical residential breaker box panels should have a main breaker or fuse rated to handle the maximum allowble short circuit current for a residential service drop. If the overcurrent protection device is a fuse there should also be a master disconnect switch. The distribution panel supplies the two "phases" to the branch breakers in an alternating pattern. 120V branch circuits are assigned essentially at random to the two sides of the supply, 240V circuits use dual breakers straddling the two phases. Due to I^2R losses it is best to balance the loads as much as possible, but people seldom bother. Many breaker boxes are arranged in such a way that only certain positions will accept a 240V circuit, or it will only work in certain positions. If you ever have to work inside a breaker box you should be able to puzzle out its configuration rules just by looking at it. Note that the dual breakers for 240 circuits (and tripples for 3-phase) are not just two single breakers with their handles wired together. The trip mechanisms are connected internally so they open both sets of contacts together more reliably than can be achieved by tying the handles together. Examining the interior wiring plant as a circuit, we have several loops carying varying amounts of current. Many of those loops involve the neutral conductor, some are balanced 240V loads. The neutral current on any given 120V branch circuit is the same as the supply current. The overall current in the service neutral is the difference of the currents in the two supply wires. Interior wiring is normally designed with voltage drops in the wires of no more than a couple of volts, so the neutral at an outlet is normally very near the ground potential at that outlet. Ok. Now it get interesting. Power system engineers earn their feed by saving money in normal operation and saving lives and property in fault conditions. An electrical distribution system with a fault looks very little like the nice little circuit model you might think it should resemble. Lets take three fault scenarios. Consider an open neutral in a branch circuit. That neutral conductor, on the load side of the fault, now carries 120V with the load in series limiting the current. If that condutor is exposed downstream of the fault a life-threatening hazard exists. It bears repeating that the minimum lethal dose of electric current is in the 20 mA range. A dangling neutral with a 100W bulb in series can supply 50 times that (ignoring the fact that the filament is cold). A few volts between the cold water supply and the drain pipe can kill a person in the bath tub. Consider a fault to neutral -- a common scenario. Hundreds of amps flow through a circuit designed for 15, and you get tens of volts dropped in the neutral. Not only resistive voltage drop needs to be considered -- there is a considerable inductance in the loop. Now if there are multiple connections between the neutral and various "ground" points there will be currents flowing in those grounds, which have non-negligable resistance. different pieces of the ground network will be at different potentials, and unless one controls the geometry carefully (no one does), those pieces may be close enough together for people to short themselves between them. Yes, the overcurrent protection device will open, but it will take at least a few milliseconds and possibly several hundred. Consider a fault to the chassis of an appliance. Nah, lets leave off with the above. In a ground fault scenario it doesn't make much difference how the neutral is wired. That's why we have ground fault detectors. By the way, in addition to the ground fault receptacles now available (and required for bathrooms) you can get ground fault circuit breakers for most kinds of panels. Pricey ($40ish) but it might come in handy sometime. You can also get breakers with remote control inputs or (and?) alarm outputs. Get creative. The important thing to remember is that you have to forget the idea that a wire is a node. Wires are distributed resistive+inductive circuit elements with several potential failure modes. And ground is a network, not a node. It's a lot like RF. If any of the above comes as a surprise, you should probably limit your electrical handy-man efforts to replacing dead parts in existing circuits and maybe cook-booking low current (<= 20 A) branch circuits. Find and *use* a cookbook though, don't try to wing it. And don't believe the salescritters at the hardware store either. Some know what they're talking about, but unless you know what they're talking about you can't tell them apart from the ones who don't. If you want to do something that is beyond your certain knowledge, get out the phone book and call some of the small (ie, one-man) electricians. Chances are you can find one who will agree to check your work (and/or plans). You save money and still have the comfort of a journeyman's OK. Do I need to add that you shouldn't expect to get his time for free? Finally, I've probably messed up some of the above. With probability 1, I'm misusing some technical terms. I'm not interested in being corrected about trivial things, though substantive corrections are most welcome. This is not a professional opinion. You get what you pay for. Be careful out there. -- Steve Nuchia South Coast Computing Services (713) 964-2462 "Could we find tools that would teach their own use, we should have discovered something truly beyond price." Socrates, in Plato's Republic