Not difficult to explain Redsy, perhaps Scott can be imposed upon for a schematic.
The transformer (called "Buckets", "Pots" and a host of other things by crews) are simply constant current variety. Typically they provide 20 amps steady current to the series circuit, open circuit voltage can be 2200.
The shunts are there on the other side of the individual lighting transformer (that is there to limit the amount of current that an individual light may draw)so that a single light fixture failing will not put the entire circuit in the dark.
Airports use this type of lighting for the simple reason that all lights are the same intensity at all times, works that way on the street lighting as well.
In other words, no light is brighter than the one beside it.
Simple system, tough to get your mind around. Your system can "shoot grounds" and still work perfectly well while being damned dangerous, wierd, huh?
If you're foolish enough to pull a ground wire in a pipe with your ungrounded conductors, your not gonna like what you get, even though it sure feels like you are complying with the code, it makes the situation extremely dangerous. That's about the worst thing that can go wrong.
And you also feel like a complete Dumb A** when you roll up to a transformer at 2 am, throw an amprobe on it and say "Must be working, it's got a 20 amp load on it" DOH, of course it's got a load on it, if it were shorted completely out and burning it would IT'S A CONSTANT CURRENT TRANSFORMER!!!
Not that this ever happened to me, of course..
#31138 - 11/12/0308:15 AMRe: Street light supplies
If not 240 or 208, almost never 120 volt, to much voltage drop and the conductors get to large.
I figured that 120V might give voltage-drop problems with the length of runs involved, but the guy with the street PC seems to have 120 available at that lamp post (unless he had his computer set up for 240V operation).
A higher voltage system with a xfmr every so many poles to provide a 120 utility outlet sounds a reasonable solution.
#31139 - 11/12/0309:56 AMRe: Street light supplies
just to add to the series lighting set up employed on street lighting, although i have never seen this for myself on street lights, as george corron rightly said most airports use this system.At first on airports a parallel setup was used but you can imagine the problems involved with volt drop,when supplying an approach circuit 900m long!!on modern day systems a constant current regulator is used to supply series circuits usually wired in double insulated 4mm2 cable single core cable, giving a constant output of 6.6 or 12amps.ccrs consist of a thysistor(for diffrent brilliancy settings(like a dimmer) for diffrent times of the day/night.)main transformer to isolate series cicuit from earth and many other monitoring circuits eg percentage lamps failiure,open circuits,etc To avoid all lights going down when there is open circuit (lamp outage) this is dealt with by installing isolating transformers (ratio 1/1) at each point where there is a light so there is a continued path via the primary side of t'former for the circuit to be energised even though there is a open circuit on the secondary side of t'former. The main disadvantage to this system is that the transformers are usually in pits under ground or buried in the ground and so are submersed in water a lot of the time leading to a breakdown in insulation on the trans formers which can cause open circuits on the primary side of the isolating transformer.
the main advantages is that there is an equal output at each light fitting (equal current) small power loss and cable size(6mm2)for circuits over three miles long!!! control overall out put to each light(dimmer)
when there is a good solid earth fault on an isolating transformer the ccr will regulate its current as normal until there is a second fault introduced on to the circuit (second transformer or joint break down) even when there is a second fault the circuit current will run down to ground at first fault and back up to circuit at second fault leaving a bank of outages between faults. ps sorry went off subject a touch presume street light series circuits would be wired to a more simplified but same way to those in airports.
#31140 - 11/13/0304:13 AMRe: Street light supplies
In my area, most of the residential street lighting is 120v. It is run straight from the pole or padmound down the easement between houses to the light poles. The ones I've seen burried are 12-2 or 10-2 with an in-line fuse. No ground. Aerial drops are as small as they think will survive the distance.
#31141 - 12/19/0312:50 PMRe: Street light supplies
In Hiawatha, KS we currently use a 2300 volt 6.6 amp series street lighting system that was originally installed in the early 1900's. This lighting system is part of our city's historical preservation efforts and we would hate to see it have to be replaced. We have had problems when trying to locate bulbs for our existing system. Does anyone have any suggestions on where we can locate these bulbs, or if they are even manufactured any longer?
Our street lighting committee has looked into completely rewiring the system to parallel, but the cost at over $1.3 million seems incredibly high for 320 street lamps.
Any input would be greatly appreciated.
George L. Hiawatha, KS
#31142 - 12/19/0311:45 PMRe: Street light supplies