ECN Forum Forums General Topics of the Trade Non-US Electrical Systems & Trades US EARTHING SYSTEMS

Hi Paul

By my name you can gather i live in Cornwall,,

if you ask for any alteration on a service down here in the sticks,you now seem to automatically receive a PME supply,,

not sure why the electric companies are changing over to PME from TT..saying that though every pole round here now seems to be earth staked and bonded to neutral..

just a foot note though,, went to a pair of cottages to carry out a test,, TT supply.. found no earth reading on the earth rods,, these had been rewired not more than 5 years previous.. the RCDs just didnt operate,, had to put three rods per house to achive a suitable earth reading,, i dont belive the earth reading would have been reduced to such a level in that time.. i belive the last spark just knocked them in and hoped for the best..

Are you sure about the TT supply for petrol stations?? all the ones i have come across are PME

I am slowly getting used to UK systems. I have this question: I assume TT was used before GFCIs were invented. How was a fault cleared back then, or was everything on a 3 amp fuse?

A study done in 5 midwestern states here showed a ground rod impedance average of about 113 ohms. What do you typically measure on yours?

Karl

Yes, TT has been employed for a considerable time, using protection in the form of an ELCB (Earth Leakage Circuit Breaker).

There were early types of current-balance ELCBs, which would trip on an imbalance of about 500mA or so. In more recent times these were made more sensitive, and until a few years ago 100mA was pretty much the norm. These days, 30mA is considered standard, so they still don't offer quite the same level of protection as the typical American GFCI. As the typical device now feeds an entire house, any attempt to increase sensitivity further would lead to problems of nuisance tripping.

The committees here have an unfortunate habit of messing around with terminology for no particular reason, and the current-operated ELCB has been renamed twice in comparatively recent times. First it became an RCCB (Residual Current Circuit Breaker), then more recently still the name was changed to RCD (Residual Current Device). At least one manucturer appears to have resisted this latest change and still uses RCCB on some products.

We also had a device known as a voltage-operated ELCB, which inserted a solenoid trip coil between the house grounding and the local rod.

These devices are obsolete now, but were very common in rural residential properties up until the 1970s. There are still quite a lot in service in my area.

Click here to see how they were wired.


[This message has been edited by pauluk (edited 08-29-2003).]

Old TT systems solely relied upon the 6A or 10A fuse in the distribution panel.
In the 1950ies old installations were usually upgraded by stringing a red wire from the water pipe to 1 or 2 kitchen receptacles, just across the wall, using the metal water pipe (back then usually lead or in newer work copper) as a ground rod. New wiring back then used a real ground rod and grounded receptacles everywhere. Residential RCDs came up along with MCBs in the mid 1970ies, usually 500 or 100mA.

Thanks for the clear descriptions. In USA I often see many amps flowing from the neutral/ground bond and on to the water pipes, to complete the circuit by way of the neighbor's pipe and neutral. So you save yourself a lot of problems by bonding only at the Tformer.

Of course we are allowed to insert an insulating spacer in the water pipe 10' outside the building. This will force the neutral current to travel in the neutral conductor. This is not a common practice, however, and sometimes meets some resistance (mental impedance).

Karl

lyle,
What a question you ask!.
This very topic has got me into trouble more than once.
Just ask Bill.

Paulcornwall,
PME not permitted on petrol filling stations-Institute of Petroleum. Reason, diverted neutral current through metal work associated with underground tanks could be explosion risk.Where you come across such a situation you should advise client to convert to local TNS or TT by employing locally grounded Isolating transformer.
Karl,
Typical ground resistance where I live (Foothills of the Mourne Mountains in Ireland)is in the order of many hundreds of ohms! Sometimes more money is spent on the earthing system than on all the other parts of an installation.

Sorry, I forgot about the impedance question.

Impedance values aren't too bad around here, I can often register 50 or 60 ohms. But I know that in some parts of the country it can be difficult to get values anything like that low. It's these difficult areas which were the places in which PME was originally employed.

Karl,
One point you may not be aware of is that the typical residential ground rod in Britain is generally only about 4 ft. these days, as opposed to the 8 ft. rods used in America.

Tex,
Did Austria really rely on just a 10A OCPD and a local rod to clear an earth fault? The loop would need to be 22 ohms or less even to get 10A flowing, nevermind enough to open the fuse in a reasonable period of time.

What sort of grounding systems were used way back?

Yep, really, at least in residential wiring. I read that out of a 1958 DIY book.
Grounding methods largely depended on the PoCo, some of them have been using TN-C-S for ages (maybe even TN-C, I'm still waiting for an answer on that question), others solely used TT until 1998, like Wienstrom, even in urban Vienna. I think the ground rods were either driven in the basement or put into the concrete foundation. The oldest RCD I've ever seen was at a school that focused on electrical engineering. It was a _huge_ standalone box on the wall, feeding a single receptacle. Maybe the size of a shoe box.

I can't imagine trying to use TT with no sort of ground-fault protection beyond a 10A fuse! (Regulars will know that I don't care for the TT system at the best of times.)

If the loop impedance isn't very low, then current will flow back via the ground but the whole grounding system of the building will float up to near 220/240V. The ground rod might just as well not be there at all.

In fact you'd be better off with no grounding -- At least then it would be only the frame of the faulty appliance/device which gets energized, not everything that's bonded to it throughout the house.



[This message has been edited by pauluk (edited 09-02-2003).]