andy pugh wrote: > On 11 November 2011 09:25, Peter Blodow <[email protected]> wrote: > > >> In Germany (and most european countries, i think), any electrician >> installing a three phase supply without neutral and protective earth >> would be kicked out of business by law for life time instantly. >> > > The US has a VARIETY of 3-phase systems. The 120/208 Wye system is intended for office building and apartment buildings, and you get 208 3-phase for larger loads like elevators and air conditioning, and 120 normal lamps, computers, appliances, etc. A problem is 240 V appliances may not run well on 208.
For shops, industrial buildings and such, they often have 120/208 for the office space and 240 Delta for the shop. There are 3 ways to do Delta. There is balanced Delta, where each hot wire has the same voltage to ground. A neutral is not provided by the mains transformer, as it is a fully-floating Delta winding. They provide a small balance transformer, and if it draws any significant current it trips the whole transformer off-line with a ground fault indication. There are two OTHER systems still seen, but generally not installed anymore. There is corner-grounded-delta, where one of the hots is grounded. This gives you 240 V between any of the mains, and is often implemented with only two single phase transformers, so it is often called open delta. One advantage is you get to use residential 2-pole breakers. Another older system that has an extra advantage is center-grounded-delta, where one center-tapped residential transformer is used. This transformer gives you 120/240 V single phase power for office and computer items. A second single phase transformer forms an open delta system, for the 3-phase loads. In this system, no phase wire is neutral, and the system is unbalanced relative to ground. The motors don't care, of course, as long as all the hot-hot voltages are the same. Our building at work has 408 delta, 240 delta, and 120/208 delta, and the breaker panels fill an entire wall of the utility room. (Mostly the 480 breakers, they are really big.) ------------------------------------------------------------------------------ RSA(R) Conference 2012 Save $700 by Nov 18 Register now http://p.sf.net/sfu/rsa-sfdev2dev1 _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
