It's not a split phase system in US residential power, it is a center tapped 240V single phase system. Split phase systems have historically had a 45-90 degree phase difference between the split phases. The US system, depending on which wire lead you take as your reference, has a 0, or a 180 degree, phase difference.
The reason it is done this way, is for safety. The center-tap of the mains transformer is grounded to earth, as is the neutral, and service entrance panel grounds. This way, if the power company installed grounding system is working properly, the highest voltage that any residential customer could accidentally encounter would be 120V to ground. (It is not an accident to go mucking around inside of a 240V range/dryer socket, or the service panel!) As it has been noted, if a US based system is defective, people can get hurt. The same is true for the European system. Back in the dark ages of ~220V electrical distribution systems in Europe, the reaping due to unintentional grounding of a ~220V wire was so common and extreme, whole house ground fault interrupters were mandated for all residential/small business power systems therein. And, in so far as properly functioning GFI protectors are in use, and can be maintained, they have been wildly successful! I will have to leave discussions of which system is better/safer/ cheaper/more reliable, for another time and forum...preferably one where there is beer and loud music. -Chuck Harris Gregory Maxwell wrote: > On Mon, Jan 2, 2017 at 4:49 AM, Bill Byrom <[email protected]> wrote: >> Most US homes and small businesses are powered by what is commonly called a >> "split-phase" 240 V feed. The final distribution system transformer has a >> 240 V >> center-tapped secondary. The center tap is grounded, and three wires are fed >> to >> the building (actually it might be up to around 6 houses): (1) Leg L1 or >> phase A >> (red wire) -- This wire will measure 120 V to the neutral or 240 V to Leg >> L2. >> (2) Neutral (white wire) -- This wire is grounded at the distribution system >> and >> at the service entrance to the building. (3) Leg L2 phase B (black wire) -- >> This >> wire will measure 120 V to the neutral or 240 V to Leg L1. > > When someone here previously mentioned observing high voltage, one possible > cause > for this in this common "split-phase" configuration is that if the neutral > wire > is overloaded, damaged, poorly connected, or otherwise has high resistance, > the > voltage on the two legs will swing wildly and in opposite directions > depending on > load. > > So, e.g. if you put a 1kw load on L1 while L2 is nearly unloaded then perhaps > L1s > voltage drops to 108v while L2 rises to 132v. > > The reason for this is that, e.g. imagine that the neutral were removed > completely > you would effectively be connecting your appliances in a parallel-series > circuit > (all on L1 in parallel, all on L2 in parallel, the both in series) across the > 240v > feed. > > I've had issues with neutrals several times in the past, and in one instance, > temporarily dealt with it by moving as much of the load to 240v as I could, > manually balancing the remaining loads, and then using a digital multi-meter > to > dynamically control some additional load to keep the voltage sane on each > side. > > I think the fact that you can end up with a much higher voltages at the > outlet if > the neutral has problems is one of the more unfortunate properties of the > split-phase approach. _______________________________________________ > time-nuts > mailing list -- [email protected] To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the > instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
