While driving some nearby rural backroads to take these pictures , I came across something else of interest:
I've driven past this transformer several times before, and at first glance it doesn't appear out of the ordinary. But this time I just happened to look up a little more closely and......... Wait! HOLD EVERYTHING! Did I see what I think I just saw? I had to stop and investigate!
The incoming HV line is a typical short 2-wire spur from the nearby 11kV delta network, and it's quite normal to see a spur like this feeding a single-phase transformer to provide a 2-wire 240V service to one or two isolated houses.
But this LV strung along the poles to the nearby homes has 3 wires:
Yep, definitely three secondary terminals on that transformer and two pole-mounted LV fuses:
I can't think of anything else this could be but a 240/480V system (houses tapped for 2-wire 240V as usual).
A 3-wire network at 200/400 to 250/500V was common for the old DC systems used in the older parts of some towns years ago, but I never knew that 1-ph 3-w AC was ever employed here.
Trumpy and I had a little discussion about this one in the chat area yesterday.
The input to the transformer is definitely single-phase only. We don't distribute neutrals with our HV, so all xfmr primaries are wired phase-to-phase, whether a single-phase spur or 3-ph delta.
We even discussed the possibility of a Scott arrangement inside the can to provide two phases of 240/415V service, but it seems highly unlikely to me that the PoCo would go to that much trouble. As far as I can tell, there are only normal residential 2-wire 240V services tapped from this LV system anyway.
The 240/480V 1-ph 3-w system still seems the most likely bet, even if unusual by British standards.
By any chance is it feeding a timed lighting circuit ? or perhaps it was in the past? Or even traffic signals, back-lit signs? or something to do with the telecom's network ? Which might have a seperately fused supply ? Or may have had in the past..
Or could it be some kind of ground connection back to the transformer?
[This message has been edited by djk (edited 06-19-2004).]
I remember reading that the 240/480V system was used for farms long ago in the UK and 480V motors were available at the time. I don't remember who wrote it, only that it was a Brit who seemed trustworthy :-)
Thanks for that C-H. I guess that not being a lineman I'd just never noticed one of these systems before.
There are quite a lot of rural networks where a 480, sorry 460, volt supply is taken. I suspect there are still quite a few farms or small rural businesses around with 480 volt motors fitted.
The main reason for this type of supply was cost. Many rural areas weren't electrified until the 50's and 60's. At that time loads were thought to be fairly static with an average admd (after diversity maximum demand) of about 2kva. It was cheaper to run single phase 11kv and three wire lv, rather than a full three phase system.
A 2 wire lv system will have a significantly greater voltage drop for a given load than a three wire system. If the transformer sits right alongside just two or three cottages, no problem, use 2wire, but if the cottages are spread out over a few hundred yards, then volt drop problems have to be taken into consideration.
That does indeed describe the area where this transformer is located. It's not the typical one or two houses standing on their own, but a very rural backroad where there are maybe a couple of dozen houses spread over a few hundred yards, plus a couple of farm buildings as well.
Next time I'm out that way I think I'll take a closer look and see if any of those farms look like they have a 3-wire service to them.
By any chance is it feeding a timed lighting circuit ? or perhaps it was in the past? Or even traffic signals, back-lit signs? or something to do with the telecom's network ?
Sorry to disappoint, but no.......
Nearest street lights are in Stalham, several miles away. As for traffic lights, I had to stop and think about that one. I think the nearest set are in North Walsham, nearly 10 miles distant!
[This message has been edited by pauluk (edited 06-20-2004).]
Hi this arrangement is not just for places out in the sticks there is exactly the same arrrangement just outside of thatcham in berkshire it is only about half a mile from the local 132kv sub. the transformer looks very rusty and must be very ancient by now.
I know of such a system feeding a set of holiday bungalows on the Isle of Wight. The bungalows were fed with 3 wires along the backs with alternate bungalows being fed with one side and neutral. At first I wondered if it was 2 phases of a 3 phase system. However I later got access to the switch room which has big labels "Danger 480 volts".