A while back I tried to describe the various earthing/grounding methods used in Britain. I've since sent some rough sketches of the different arrngements to Bill and he's kindly scanned them to post.
For those interested, I'll explain one at a time with the aid of the diagrams.
Diagram #1 link: https://www.electrical-contractor.net/forum/international/UK_OldUrban1.gif

This is the old urban distribution system using armored undergound cables. The neutral is grounded only at the xfmr, and the house ground is via the cable armor back to the same point.

I've shown all the usual components at the house service, but for simplicity I've omitted the isolators and fuses that would be at the xfmr.


[This message has been edited by pauluk (edited 10-13-2001).]

Fascinating.

I note the main disconnect also breaks the neutral.

As this is "Old Urban", for the Distribution Panel Earth bonding...is the bond made to the gas pipe system if that is the only piping in the dwelling? Is there an additional ground created by driven or buried ground rods or something like 'em?

Is the underground service cable armor in direct contact with earth, and if so, what is it constructed of?

So many questions, so little time.

Al

Hi Al,

Yes, the main switch on the distribution panel for single-phase always opens both hot and neutral (the hot is colloquially called the "live" here). A 3-phase service to a commercial building would be arranged the same way (with 3-phase meter and 3 main fuses of course), except in that case the main switch is three-pole and DOESN'T open the neutral.

By the way, the neutral link shown in the service fuse block (ahead of the meter) is a bolted terminal; it isn't broken by normal removal of the main service fuse.

Everything up to and including the meter is the property of the utility and is sealed.

Bonding is normally to whatever services are present - water and/or gas, although in some older houses (pre-WWII) it's not unusual to find no bonding in place.

Naturally the bonding to buried metallic pipes creates parallel ground paths, but most of any ground fault current would flow directly through the cable armor back to the xfmr.

It's not usual to have any separate ground rod with these systems; it's usually just as drawn. There's nothing in our "code" (IEE Regs.) to prevent the use of a ground rod as additional protection, but I've never seen one of these systems with it.

The older armored cables had a lead sheath and were then wrapped in some sort of hemp/bitumen covering. I've never worked on one directly (only utility employees can install or replace service cables) so I'm not sure of the exact composition. The later types are a steel-wire armor with a similar covering.

No new systems of this type have been installed for years, but the modern replacements for feeders are steel-wire armored with a PVC outer sheath.

So. . .

Let's see if I can ask this clearly,

In a pre-WWII house with no metal water, gas, etc. systems to bond to, and with the metal armor of the service cable effectively isolated from contact to earth by its covering, is the only earth connection at the transformer?

And the bonding between the neutral and earth, does it only occur at the transformer?

Is the color black reserved for use as a neutral?

Al

If no bonding is made to the water/gas services then yes, the metallic path via the armor back to the sub-station will be the only path for fault currents.
Even if direct bonding isn't present, however, it's quite likely than some other parallel earth path will exist. For example, if there's an immersion heater for providing hot water, then the ground wire from the dist. panel to the heating element will ground the copper cylinder, and on an old plumbing system with all metallic couplings, this will quite likely provide an indirect ground path through the incoming water supply pipe.

Under this system, the neutral is linked to ground ONLY at the star point of the xfmr. From there on it's kept separate.

Yes, black is reserved for use as neutral in British wiring. For anyone who hasn't followed the color code topics elsewhere, I'll post a separate summary of U.K. colors in this area.



[This message has been edited by pauluk (edited 10-13-2001).]

Bill has now got the links to the other diagrams in place (Thanks Bill!) so here's the next one.

Diagram #2 link: https://www.electrical-contractor.net/forum/international/UK_PME.gif

Until comparatively recently, this was the least common system, but I've put in it now as it will probably seem quite familiar because it's the closest we have to the American arrangement.

This system has been in use in some rural areas since the 1930s and is known as PME (Protective Multiple Earthing). The neutral has ground rods connected at regular intervals along its route (hence the name).

Being rural, the lines are overhead, and service drops to houses are a pair of single cables terminated at eaves level on to two porcelain insulators. The neutral overhead drop is often bare.

As with all other systems, the distribution panel has separate neutral and earth busbars, but the main earthing lead is bonded to the neutral at the service entrance, as shown.

Compared to the other systems, the IEE Wiring regulations specify larger size cross-bonding cables when PME is used.

More on the remaining systems later.

Paul,
Would you mind if I copied these sketches in AutoCAD and sent them to Scott for posting in color?

Nick,

I don't mind at all - Be my guest!

They're just rough sketches I made to ilustrate the various systems used here. There are two more to go yet; I'll post the links and brief descriptons later tonight.

Diagram #3 link: https://www.electrical-contractor.net/forum/international/UK_RCD-GFI.gif

This is the system that was installed in most rural areas with overhead lines. Once again, the neutral is grounded ONLY at the xfmr star-point.

The house ground is by way of a local rod, and there is no neutral-to-ground bond at the house.
Because of the relatively high loop impedance with this system, even a direct line-to-ground short will not usually result in enough current to blow a fuse, so a whole-house or main GFI is installed.

[Note on British terminology:]
The original term for a GFI was "current-operated ELCB" or "current-balance ELCB," where ELCB=Earth Leakage Circuit Breaker.
In more recent years this became first an RCCB = Residual Current Circuit Breaker, and now RCD = Residual Current Device.
[End note.]

On older systems the main GFI was usually a 500mA type, but the modern RCD is normally 30 or 100mA trip.

The GFI-type breaker was used on more up-market installations, but until the early 1970s the majority of residential installations used the cheaper "voltage-operated ELCB."

I've left this one until last, as it's probably going to seem very strange from an American perspective.

Diagram #4 shows how this works: https://www.electrical-contractor.net/forum/international/UK_ELCB.gif

Instead of the earth busbar being connected straight to the ground rod, it goes via the tripping coil in the breaker. The C/B is designed so that it will trip before the voltage on the earth busbar can exceed about 40V.

Paul,

In these diagrams I notice Fusing before the meters. Where is this located? Is this accessible to the consumer/electrician? Your diagrams seem to place it at the house and I was wondering if they (Power Co.) have a problem with what we call "Theft of Service" (stealing electricity)?

Bill

Bill,

The main service fuse is located on the board right next to the meter. This board is mounted inside the house in all older installations, or in a meter cabinet outside for some new homes.

Although accessible to everyone, the fuse carrier is sealed by the power co., as is the meter case and terminal cover (meters here aren't the plug-in type).

The short leads between fuse block & meter are clearly visible to the meter reader, so any tampering here would be spotted fairly easily.

I think the most vulnerable area is the cable before the fuse in homes with an overhead service. The overhead lines terminate on a bracket at roof level, then two double-insulated single cables run from there down to the service block.

In some homes these cables are actually routed through the roof space. Unlike most (All?) American services, none of this stuff is enclosed in conduit or raceway, so it wouldn't be very difficult for someone to tap the cables in his attic. Chances are that such a tap wouldn't be discovered for years.

I've never seen aything from the utilities about illegal taps being a big problem. Maybe they just rely on the fact that the majority of people would be too scared to interfere with hot 240V cables.

Paul,

I once saw something like you were describing. A customer had just moved into a house previously owned by a utility worker. The SE cable outside passed through a small overhang on it's way down to the Meter (outside). I found signs in the attic that the previous guy had tapped into the cables as they passed through the overhand and soldered 40 Amp fuses to the wires and continued down to an Electric Stove. The fuses were hanging in mid-air! Definitely a 'Kodak Moment' I may go back there again just to take a picture sometime. (it wasn't live anymore and customer was going to remove it .. yeah, maybe)

Bill

Ah well... At least the illegal tap had fuses, even if the method of installation left something to be desired!

I remember seeing a TV documentary some years ago about the poor "barrio" districts in parts of India. The houses (shacks) didn't have any official supply, so many people just ran small flexible cords out of their windows and with the aid of a ladder wrapped them around the overhead lines running along the street.

They said the PoCo came along and ripped them all down every couple of months or so, but within a week most would be back, even though many people got electrocuted while performing this task.

The picture that sticks in my mind was the guy balanced up the pole connecting his wires (bare-handed) while working around all the other unofficial "service drops" already there. There must have been about 25 at the one pole.

O.K. folks, here's the final installment of this saga: Modern trends.

The diagrams represent the basic systems as they were installed and all of the systems shown are still in use in various areas of the country.

Since the 1980s however, one or two trends have been emerging clearly.

First, main RCDs (GFIs) have become much more common. In part this is due to the demise of the voltage-operated ELCB (diagram #4) which has not been made for about 25 years. When grounding is by way of a local rod only (#3), the GFI is just about essential because of the high loop impedance.

On all older installations the ELCB (of either type) was always a separate unit, as shown. Many of the latest distribution panels are modular, and can be fitted with either a simple double-pole main switch or a GFI. So in modern panels the GFI may be separate or may be incorporated in place of the main switch.

We're also seeing a main GFI employed more and more often with the urban and PME systems during rewires. Some panels have a split busbar so that one or two circuits (usually lights) are tapped off before the GFI.

The second main trend is toward the overhead systems going to PME. The power companies have been installing the extra ground rods so that customers have the option of grounding to the neutral if required. A lineman for this region (Eastern Energy) told me recently that all overhead supplies in this region are now equipped for PME.

Of course, most of the houses were wired before PME, so they still have their rods and main GFIs (or more likely voltage ELCBs) for grounding, but it means that when rewired grounding to the neutral would now be allowed.

There have been a few other gradual changes as well, such as the introduction of outside meter cabinets on new and refurbished homes. When these are installed in rural overhead service areas, the utilities like to use underground feeder from the cabinet to the nearest pole.

This cable is now usually a central live with concentric neutral, PVC sheathed. When overhead service drops are replaced to an inside meter, this same cable is used in place of the original two singles.

Paul,
I sent the first two drawings to Scott. Please proof read when posted.
Nick

O.K. Nick, will do.

Where are they being posted: A new thread or to replace the existing links in the above messages?

Sorry about the delay guys!

I'll convert them tonight and post them in the "Tech Reference" area, with linked discussion thread in this forum area.

All this is of course - "Pending any conversion problems"

Scott SET