What common arrangements are made inthe US re the provision of supplies to domestic and commercial premises? I am aware that there are a number of "funny wonders" but lets say if it were a new supply. Paul, might you have diagrams?
I'd say the most common arrangement is TN-C-S or PME, i.e. the neutral is bonded to earth at several points, the final bond being at the main distribution panel. Each of these systems has to have a local ground rod as well (unlike some older Austrian installations that were only "earthed" to the neutral) TT systems like used in many rural areas of Europe are illegal. In simple words, at the subscriber's customer unit the incoming neutral, the earthing (or grounding) elctrode conductor and all branch circuit neutrals as well as abonding jumper to the earth bus bar go to the nutral bus bar.
Re: US EARTHING SYSTEMS#138294 08/27/0311:02 AM08/27/0311:02 AM
Yes, the TN-C-S system is standard for 120/240V 3-wire residential systems, and for 120/208 and 277/480V 3-phase commercial.
The neutral busbar in an American panel has provision for a bonding screw to connect it to the metal casing. At the main panel this bond is put in place, and both the incoming supply neutral and the GEC (Grounding Electrode Conductor) are connected to the neutral bar. As Ragner has said, both neutral and ground wires from circuit cables can be landed on this busbar.
Where a unit is employed as a sub-panel, the grounding bond has to be removed and separate earth and neutral busbars employed. The feeder from the main panel also needs to be 4 (or 5) wire to keep the neutral and ground separate.
As you can see from this, the only major differences between this and British PME are that in the U.K. we have the bond placed before the meter and keep neutral and earth separate in the main panel, and that the American system has a local ground rod as well. The latter is quite reasonable when you consider that in a typical American neighborhood there will be a larger number of smaller transformers feeding the houses, instead of the one big xfmr with a 3-ph/4-w network that's typical here.
The other main point to note is that range and dryer outlets fed from the main panel were not requirted to have a separate EGC, but the frames could be grounded to the neutral (i.e. a 3-wire connection from panel to receptacle, with the white serving as both neutral for the 120V loads and as a protective earth). Ranges and dryers fed from a sub-panel still required a 4-wire system.
This system has now been outlaws under the 2002 NEC, and all new range and dryer outlets must be 4-wire.
I don't have any diagrams I can post easily, but maybe one of our other members can point you toward something.
Re: US EARTHING SYSTEMS#138295 08/27/0311:29 AM08/27/0311:29 AM
Pauluk, that was a good description. The UK system avoids some net current problems we have here, and to me it makes more sense to keep the neutral and ground (earth) separate until the Tformer.
Instead, with neutral bonded to "ground" at the main panel, if there is a metal water supply system, some of the neutral may return to the Tformer by way of the water pipe to a neighbor's house and back their neutral. Once again, if impedances differ, neutral can be shunted all over the neighborhood, creating the magnetic fields I am called on to deal with.
Of course the rod to earth does not enter into this circulation since the impedance to earth is relatively so high.
Another problem is that the bonding screw you spoke of is sometimes turned in in a subpanel out of ignorance, thus putting neutral on many earthing paths throughout the house, and in commercial buildings all over the place. Best to keep the two separate until the Tformer, but the US is stuck with its system.
Re: US EARTHING SYSTEMS#138296 08/27/0311:36 AM08/27/0311:36 AM
I forgot to say that the local ground rod for the building is there only for lightning protection (if such exists) or for accidental high voltage surges from storms.
However, many US electricians carry in their heads the belief that it plays the main or sole role in clearing a fault (opening a breaker or fuse) in the case of a wiring fault. Hence there are many serious discussions about how many rods to use, and how to measure their impedance, etc. It has a lot to do with the multiple meanings of the word "ground", which some are trying to correct in the Code (NEC).
Re: US EARTHING SYSTEMS#138297 08/27/0311:52 AM08/27/0311:52 AM
The underground systems (now designated TN-S) as originally installed in most urban areas did indeed have a ground on the neutral only at the star point of the distribution transformer. The ground-fault path was by way of the outer armor on the distribution cables.
Our PME system (otherwise known as TN-C-S) can give rise to similar problems of parallel neutral paths as in the States due to bonding to all water services, gas pipes, etc.
The TT system, very common in rural areas, does actually rely on the earth as the ground-fault path (and would thus not be allowed under the NEC). The relatively high impedance limits fault currents, but necessitates the use of an RCD (GFI) for all circuits in the house.
Re: US EARTHING SYSTEMS#138298 08/27/0304:28 PM08/27/0304:28 PM
Karl, Confused about function of earth rod at building. Would have thought that primary purpose was to keep neutral at or about earth potential. Function of star point connection to earth at xtrfmr may well serve to limit voltages impressed from primary through capacitive coupling.
Re: US EARTHING SYSTEMS#138299 08/27/0305:52 PM08/27/0305:52 PM
Pauluk, yes the GFCI provides the needed safety. We can't use that on the service cable here due to the water-pipe alternative for neutral. The GFCI would trip constantly.
Lyledunn, yes, the ground rods of the system as a whole keep the voltage at a more or less constant relation to earth, but this does not rely on any particular rod; the rod at the building would not be needed as the others are doing their job. Of course, neutral is ony at ground potential when no current is flowing, but since 30% or more of primary neutral flows back to the substation by way of the earth, there is always a small neutral/earth potential, sometimes negative.
Did I muddy those waters? If so, set table on fire.
Re: US EARTHING SYSTEMS#138300 08/27/0305:57 PM08/27/0305:57 PM
Current on the pipes isn't a problem with TN-S and TT systems, but it's a definite possibility with PME/TN-C-S where the pipes are bonded to the neutral.
Newer water/gas installations use PVC service lines these days, which obviously reduces the current flow, but there are still many old systems with metallic pipes criss-crossing a neighborhood.
The possibility of parallel neutral paths via pipework is why the PME system is not permitted by our code in certain situations, e.g. at gas ("petrol") stations.
Originally, the majority of distribution in Britain was TN-S in urban areas and TT in rural areas. The neutral was grounded only at the transformer.
The TN-C-S/PME system has been in use since the 1930s, but was only employed in areas where soil conditions made it difficult to get a sufficiently low resistance ground connection. The PoCo had to install multiple ground rods along the route and actually get the permission of the Secretary of State for Energy before they would allow customers to ground their systems to the neutral.
In recent years, however, there has been a move toward converting all the main LV distribution lines to PME by installing the extra ground rods. In the area served by my local PoCo, the conversion is complete, so all customers can switch to TN-C-S if they wish, although I'd say that 95% of the rural homes in this area are still TT.