"B" phase to ground power source

[steelbar]

Can incoming power as "B" phase to ground cause problems with 110 v circuits, used for computers. How does "B" phase to ground actually work for 480 (3 phase) and 220. Why was this a method of providing power to a production facility.

 

[rbulsara]

Review this and NEC, while someone posts more information.

http://www.patchn.com/deltasys.htm

 

[cuky2000]

An ungrounded system is one in which there is no intentional connection
between the neutral or any phase and ground. Ungrounded system implies that the system is capacitively
coupled to ground. The neutral potential of an ungrounded system under reasonably balanced load
conditions will be close to ground potential because of the capacitance between each phase conductor and
ground. When a line-to-ground fault occurs on an ungrounded system, the total ground fault current is
relatively small, but the voltages to ground potential on the unfaulted phases will be high. If the fault is
sustained, the normal line-to-neutral voltage on the unfaulted phases is increased to the system line-to-line
voltage (i.e. square root of three times the normal line-to-neutral value). This, over a period of time,
breaks down the line-to-neutral insulation and hence results in insulation failure. Ungrounded system
operation is not recommended because of the high probability of failures due to transient overvoltages
caused by restriking ground faults. The remaining various grounding methods can be applied on system
grounding protection depending on technical and economic factors. The one advantage of an ungrounded
system that needs to be mentioned is that it generally can continue to operate under a single line-to-ground
fault without an interruption of power to the loads.

 

[rbulsara]

steelbar:

A brief explanation:

A brief explanation:

The center-tapped delta giving 120V single phase is only possible and available in a system with 240V line-to-line secondary voltage. In a 3-phase system, three 240V single-phase transformers are arranged in delta connection. You get 240V (3 phase) between line to line. One of the transformers is center-tapped and the center tap is also grounded. Voltage between the center-tap and the two lines connected to this unit will be 120V. Convention is to keep this two as A and C phases. The voltage between the center-tap (or the ground) and the B phase will then be 208V (not 120 or 240). That’s why it’s not good for 120V equipment.

Such system is not possible with 480V system (to provide 120V) or any other voltage.

The reason such system is used for a production facility is that it is very convenient and economical way for a utility co to provide both 3 phase and single phase (120V) power with one set of transformers. They have to inventory one type of single-phase transformers and group them to provide 3-phase service. Also in production facilities single phase loads are a very small percentage of the overall 3 phase loads such as motors. Such a system also does not require a intervening step-down transformer for 120V as it will be required in a 480V system. This is all assuming 240V, 3-phase service is adequate for the facility.