Question about UK distribution transformers and fusing requirements

| I don't want to be accused of being one of those | my-country's-system-is-best Americans. That's why I like to learn how | it is done in other countries and what is the reasoning behind | different practices.

That's a good approach. That way you know _specifically_ what is wrong with all those other systems :-)

IMHO, everyone got it all wrong.

Distribution is 3-phase in the UK (and all of the rest of Europe that I know of). The final stepdown transformer will be a delta primary and a star/wye secondary, thus converting a 3-phase 3-wire medium voltage supply to a 3-phase 4-wire low voltage supply.

Sorry, don't know the details, but I'm pretty sure the circuit breakers are always ganged across all 3 phases. Low voltage (230/400V) supply lines are separately fused.

11kV, standardised. It's also the standardised supply voltage for industry which wants higher than the normal 230/400V.

Next higher distribution voltage is 33kV, but I don't know if this is ever used to directly feed low voltage network.

There are 3 types of supply: TN-S -- a separate safety ground is run TN-C-S -- safety ground is combined with the neutral TT -- no safety ground is run (old rural supplies).

Yes.

Hello Beachcomber

You have had some good answers from Andrew Gabriel. I will try and fill the gaps.

Practice varies. In urban areas where transformers are ground mounted and >

200 kVA the transformer will be protected by either a circuit-breaker, or a 3 phase fuse-switch, which will have a trip all phases feature.

In rural areas where the transformers are smaller, and pole mounted there is frequently no protection local to the transformer on the high voltage side of it. Transformers tend to be protected in groups using fuses, and auto-reclosers. There are a variety of policies with different authorities, and situations, and it is difficult to go further on a news group.

Manchester City Centre is 6.6kv (norweb)

John:

Thanks for some good information. I was just wondering if the overseas transformer connections were anything like of seen here in North America. The typical pole mounted transformer is fed from a high-side primary and a neutral (always mounted in a lower position on the pole). Usually I will see a lightning arrestor and a fused-disconnect in the primary lead and a single HV bushing going into the transformer. Also, in many cases there is a Ground (Earth) wire running down the pole either to a rod or some other sort of earthing system.

I was wondering specifically how the primary fusing was handled at the transformer in the British installations with the two hot leads. Are you saying the transformers are not fused locally?

Beachcomber

Never an HV neutral

There may be lightning arrestors fitted on each phase at the pole particularly if it is at the end of an overhead line. All pole mounted transformers are fitted with arc gaps, which will flashover if there is an overvoltage on the line. When they flash over it is important that the circuit-breaker or auto-recloser protecting the line trips quickly to interrupt the power follow current before the bushings are damaged.

Yes !

Many years ago they used to be, but the fuses often blew in large numbers during lightning storms. So a policy of "Group Fusing" and auto-reclosing was developed, and this works very well.

John Rye

I don't want to be accused of being one of those

This is something which interests me as well. Something which does strike me is just how different American and European practice is; I'm talking more about the low-Voltage supply side than medium-Voltage distribution. Also, the American way of doing things does seem to be much more complicated than the European way. I can see certain advantages to both.

If UK practice is anything like Australia, then there are not any pole mounted transformers fed with a hot and a neautral. They are all 3 phase delta input, lots at about 40KVA size. With 3 phase Wye output with grounded neautral and all 3 phase continued to houses in many, not all, cases for Aircon and Ranges.

All houshold aplliances are 230 (was 240) volts and all domestic 3 phase are 400 (was 415) volts There is no confusion like the US 240/208 mixture. The 240 to 230 change is like the UK, a work in progress to harmonise with the rest of the 50 hertz world. In remote areas there is some Single Wire Earth Return distribution.

With Delta input of course there are 3 wires, and in Australia in many cases they are fused at the pole with Drop fuses that can be replaced from the ground by a man with a fibre glass pole.

-- John G

don't forget the UK 480 single phase centre tapped installation.. working with one of these currently... PITA cos you have to use double pole breakers on a 3ph board...looks a right mess.

Complex technological systems evolve from the local needs and culture. No one system is best...

Here is another thought...

Maybe it's just my imagination, but in the recollection of my travels in Switzerland and France, it seems they do a better job of hiding the power poles when compared with rural areas of the USA. I was also wondering if the same techniques apply in the UK.

For example, pick any state in the US, say Kansas or Nebraska in the Midwest. Along every road of any size you will see power lines running along the side of the road and frequently come across high voltage transmission and sub-transmission lines going every which way. In the US, it's pretty much like that in every state. To some it is ugly, to others; it's just part of the American scene, just like an Edward Hopper painting.

During my travels in the more rural parts of France about 10 years ago, I was amazed to see, more often then not, that the power lines seem to blend into the landscape. Occasionally one sees a concrete set of poles with 3 super-thin wires serving some remote farm. The practice of running long lines parallel with the roads seems to be avoided. France is a modern country with 70% nuclear power, yet they seem to do a better job in hiding their electrical infrastructure.

Beachcomber

Somebody was asking about a source for 480V motors in the UK a few days ago, but I've never seen anything like this; where is it used?

I used to buy 480V 3Phase 60Hz IEC type motor on a regular basis from any of the major motor supplies in the UK for istallation on machinery we were selling to the USA. Just send them any email and ask for a quote.

BillB

480 (as 240-0-240V) was used on some remote farms which are fed with only 2 wires from a 3-phase 11kV distribution supply, where a long run for just one customer of a 3-phase 3-wire 11kV distribution would have made the installation significantly more expensive. I don't believe it's installed nowadays, but there are some still around.