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Tool Talk Discussion Forum

Re: Basic AC Power Systems

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John T

04-26-2013 05:59:32

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If one leg of a floating 3 phase system went to ground,would there be a circuit breaker tripping somewhere? Or not, since there was no reference to ground in the system previously? Did I even say that right?


NO TRIPPING of a circuit breaker somewhere. Theres no voltage with respect to ground on any leg....Theres no circuit breaker between any leg and ground (there can be detectors)


It is still confusing in the high leg system that the voltage between any two legs is 240, and high leg to neutral is still 208, but other legs to neutral are less. I know JMOR explained the math, but that does not 'soak in' very well sometimes either!


Okay there are three transformers connected in DELTA (picture an equilatteral triangle) and the voltage across each of the legs (L1 to L2,,L2 to L3,,L3 to L1) is 240 VAC. In ONE of the transformers (L1 to L3) lets tap in the dead center and run a wire out there for NEUTRAL and lets run that also to Mother earth......HMMMMMMMMM think about it, since that one L1 L3 transformer has a mid point tap HECK its only 120 VAC (1/2 of the full 240) from L1 or L3 to Neutral. Buttttttttt if you go way up there to L2, the Red/High Leg top of triangle, its 120 x Square Root of Three or 208 volts to Neutral. Thats the infamous 120/240 Volt Three Phase FOUR Wire Red/High Leg Delta System.


Also, and I know this has probably been explained a thousand times on here, in 240 single phase: are the two hot legs 180 degrees out of phase with each other?

ANSWER YES In the household 120/240 Volt Single Phase Three Wire System theres only ONE transformer and its 240 Line to Line. However with a tap in the center mid point (Neutral) its only 1/2 or 120 VAC either Line to Neutral yes 180 out of phase from the other

GREAT QUESTIONS, has this and all else I posted making any sense and does it help???? Dont expect to understand what takes libraries to explain and education and years of experience. I cant put that in a few sentences here

John T BSEE, JD Longgggggg retired Electrical Engineer

John T

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