grounding power wiring was re a question about vac outlet wiring there has been quite a bit of discussion about house wiring and grounding practices here a few points need to be clarified the equipment grounding conductor green green with a yellow stripe bare or the metal sheath or pipe of some wiring methods is used as a safety ground to carry fault currents back to the circuit breaker panel and to limit the voltage on the metal case of utilization equipment or other metal objects it should never except for a few exceptions to be discussed later carry the normal operating current of a connected load some equipment has filters in the power supply which may cause some slight current flow through the grounding conductor much communications or audio equipment is sensitive to noise or slight voltages on the grounding conductor and may require special wiring of the grounding conductors to provide reliable operation orange outlets are often used for this with insulated grounding conductors wired back to the panel box and in many cases back to the service anyone installing such a system should read both the section on grounding in the national electric code and publications on installing quiet isolated ground systems the code requires the insulated grounding conductors green wires to run with the current carrying conductors back to the panel box and if required back all the way to the service entrance where it is bonded to the service ground water pipe or rod many of these systems are installed illegally or unsafely where they do not provide a safe ground or a quiet ground or either the grounded conductor of a circuit often called the neutral which is referred to in the code as the identified conductor and is supposed to be white or natural grey this conductor is supposed to be connected to ground in most electrical systems at a single point generally at the service entrance panel this connection is through the main bonding jumper in many household service panels the main bonding jumper is actually a bonding screw which attaches the neutral busbar to the case of the panel the grounded conductor neutral is generally a current carrying conductor in the case of a volt circuit it is one of the two conductors completing the circuit from the panel to the load device since the grounded conductor neutral is only connected to the grounding conductor bare or green at the service entrance if the load is any distance from the service and draws any significant current there will be a small but measurable voltage between the grounded and grounding conductors at the load under normal operating conditions if you should incorrectly connect the grounded neutral conductor to the grounding conductor at the load some of the neutral current will flow instead through the grounding conductor since there will now be current flowing through the grounding conductor it will also no longer be quite at ground potential at the load end if the load equipment has a metal case which is connected to the grounding conductor through the u ground plug the metal case is now also no longer quite at ground potential the difference under normal non short circuit conditions may be only a few tenths of a volt but it could also be a volt or two this normally does not present a shock hazard however if you let the metal case of the grounded equipment come into contact with an independently grounded object such as a water or gas pipe a radiator a metal air conditioning duct or such part of the neutral current will try to flow through this aalternate ground path if the contact is not solid you will get a significant arc a low voltage but possibly moderate current arc under the wrong conditions this arcing could start a fire it is possible in some cases that the sneak ground current could also flow through a wire of inadequate size causing it to overheat with the incorrect non single point grounding of the neutral if there is a short circuit from hot to neutral the high short circuit current which may flow will cause a much higher voltage on the grounding conductor which increases the possibility for shock or fire also if you incorrectly multiply connect the neutral and ground the voltage on the ground system is seen as noise bu computer or audio equipment often causing malfunction i have spent some hours tracking down such shorts in technical facilities where they were inducing severe hum into equipment the neutral is usually bonded to the ground at the distribution transformer as well as at the service entrance of each dwelling this is done primarily for lightning protection so that induced lightning currents have a short path back to ground and also to assure that the currents drawn by shorts to grounded objects like pipes draw enough current to trip circuit breakers or blow fuses quickly the bad side of this is that not all the neutral current from the dwelling goes through the neutral wire back to the transformer some of it flows through the grounding electrode water pipe etc this may cause corrosion in the pipes and possibly in things like underground fuel oil tanks and it may also cause measurable ac magnetic fields due to the large loop between the hot conductors in the service and the neutral current in the water pipe and ground there are those who feel these fields may be unhealthy don t flame me on this i m just telling you where the field comes from not it s health effect as far as i m concerned the jury is still out on this note that the bonding jumper is only installed at the main panel not at any sub distribution panels this is one reason why it is illegal to run service entrance cable with the sheath used as a neutral to a sub panel you must have a seperate insulated conductor for the neutral the sheath can be used in this application only as the grounding conductor if the neutral is bonded to the grounding conductor in the sub panel say by forgetting to remove the bonding screw all the grounding conductors of the loads on that panel will be above ground with the possible problems listed above the code makes exceptions for ranges and dryers as well as feeds from one building to another in the cases of the range and dryer the neutral may be used as the equipment ground under certain conditions instead of a seperate wire every time the code is revised these exceptions come up for review these exceptions were in fact the first required safety grounds in the days before u ground outlets and such the appliance manufacturers don t want to have to redesign their ranges and driers and the contractors don t want to have to run four wire cable with four fairly heavy expensive wires in place of three wire to the appliances no question it would be safer with seperate neutrals to the stove but the neutral current is low for most burner settings since most current is in the volt hots except at some low settings the wires are large gauge and there are few reported cases of injury or damage so far the exceptions have survived in the case of feeds between buildings it s primarily for lightning protection people doing wiring should be aware what is and what isn t a legal grounding conductor obviously the bare wire in romex with ground is anywhere there is a green wire installed such as in a portable cord that is a good grounding conductor the sheath of bx clamped in bx connectors in metal boxes is a legal grounding conductor in the us bx has an aluminum band run under the steel sheath to lower the resistance of the sheath you can just cut this aluminum band off at the ends you don t have to bond it to anything it does its job by touching every turn of the bx sheath conduit or emt thinwall tubing is generally a legal grounding conductor but may require a bonding locknut where it enters a box or panel particularly for larger pipes greenfield looks like big bx but you pull your own wires in the empty sheath after you run it is not a legal grounding conductor as it doesn t have the aluminum band to bond it and the spiral steel has too much resistance and inductance you have to run a seperate green grounding conductor inside the greenfield wiremold is also not a legal grounding conductor as the paint on the boxes often prevents good contact and the feed to the wiremold extension is often from a box in the wall that may not be well connected to the first wiremold box i have personally discovered cases where the entire run of wiremold and the cases of everything plugged into all the outlets on the run were hot with volts why do i get a shock every time i touch my computer and the radiator here in the office because there was no ground wire in the wiremold and one of the outlets had shorted to the edge of the wiremold box you must run a ground wire back in the wiremold from the outlets at least to the first box in the original wiring conduit bx etc where you can bond the wire to the box with a screw bnding clip or whatever on another issue while you should ground the green wire lug on gcfi outlets when ever there is a place to ground them it is legal in the nec to use them without a ground if no ground is available it is better to have the protection of the ground fault interrupter than no protection if you don t install it the interrupter doesn t depend on the ground to trip it is desirable to connect the ground if available because if the ground is connected the interrupter will trip as soon as a faulty device is plugged in whereas without the ground it will not trip until someone or something provides a ground path for those questioning the legal use of ungrounded gcfi s read in the nec d exception this is the code my code is in the city but i know the rule hasn t changed it might be renumbered though we have only touched the surface concerning grounding there is much more to this subject but most of you have fallen asleep by now john john h schmidt p e internet schmidt auvax adelphi edu technical director wbau phone days adelphi university evenings garden city new york fax
