John McHarry wrote
In the USA, the fourth wire is called the "MGN" ("multi-grounded neutral").
And yes, for safety reasons, distribution primary is (almost always) wye connected. Consider:
- If one phase (say, phase X) of a wye-connected distribution circuit fails, all phase-X customers have no power, but phase-Y and phase-Z customers are not affected.
- In one phase (say, phase X) of a delta-connected distribution circuit fails, the phase-X conductor downstream from the point of failure becomes a floating bus. Customers across Y-Z are not affected, but X-Y customers are suddenly in series with X-Z customers. Big voltage drop. Unhappy customers.
I wrote "almost always" above because I've heard of at least one case of delta-connected distribution. Some fellow in New Jersey posted a story about it here on T-D several years ago -- apparently, he was one of the customers with half-voltage. Maybe he'll post his story again.
Single-phase primary distribution is quite common on residential side streets in the USA.
Not necessarily. A separate line is sometimes used in situations where several mercury-vapor (or sodium-vapor) lamps are installed in a relatively small area (say, a city block or an interstate interchange).
The lamps are wired in series, and each lampholder has a cutout circuit that shorts the circuit across a failed lamp. Presumably, there are current-limiting devices somewhere in the circuit that keep the total current constant.
Far more commonly, however, streetlights of any species (incandescent, mercury, or sodium) are simply wired across any conveniently-available
115-volt secondary distribution circuits -- the same circuits that feed nearby residences and small commercial customers. Each streetlight has its own photocell (at dusk, the lights come on at different times). A burned-out lamp simply goes out, but it doesn't affect any other streetlight. A photocell may short out (or get covered with bird poop), in which case the streetlight burns all day.Of course, there will be a dedicated 115-volt secondary distribution conductor pair ("duplex") for any streetlight that happens to be on a pole that wouldn't otherwise require secondary distribution.
Other pole-mounted 115-volt loads are wired in the same manner: traffic signals, pedestrian lighting, CATV power supplies, seasonal decorations.
The primary windings may be delta-connected, but the secondary windings are (in my experience) always wye connected. But I suppose a power company could provide a delta connection to a customer that specifically requested (and paid for) it.
Why would ambient air temp affect load balance? I would think that load balance would be determined by the customer's instantaneous demand.
Are wye-delta and delta-wye transformations ever used in secondary distribution circuits? The ones I've seen were always in primary distribution or sub-transmission.
Neal McLain