3 phase Service Question

[JknSD]

I have a pump house that has no problems but the voltage/amperage is a mystery to me.
The voltage reads as follows
A to Ground, 480
B to ground, Zero
C to ground, 480.
A to C, 480
A to B 480
B to C 480

The current read as follows under load.
A 18.0
B 18.2
C 18

The utility has verified that it is fed correctly.

In a 3 phase how can A to C be the same as the nominal voltage?
I suspect the B phase slight increase in current comes from the step down trans for the control voltage.


What phase configuration am I looking at?

Any help would be great.
Thanks

 

[augie47]

You have a 3 phase system, no neutral, with one phase either intentionally or accidentally grounded.
Such supplies are not real common.
If it was designed to be an ungrounded system and installed per Code you should have some type of monitoring system to denote a grounded phase. Such systems were designed to allow the system to continue to operate if there was a phase fault so corrective action could be taken.
If its a system with one phase intentionally grounded, as noted, its somewhat rare but allowed under Code.

 

[iwire]

What phase configuration am I looking at?

Corner grounded delta, phase B is connected to ground which in this case is how it should be.

To the motor it is just 3 phase 480 line to line, the voltage from line to ground does not mater.

 

[Ingenieur]

Corner grounded delta, as others said, either intentional or faulted
In 3 phase ac should always be the same as ab and bc

do you have a 240:120 xfmr for control/lighting/receptacles?

 

[Tshering]

Interesting! I didn't know that utilities still use this type of service configuration.
It is called Three Phase Two Wire Corner-Grounded Delta service. This configuration is used mainly to save wiring costs as it requires only two insulated conductors instead of three conductors that are typically used in three phase service entrance. [h=3][/h]

 

[Smart $]

Although for a certain class of molded case circuit breakers, this Square D data bulletin provides a decent assessment of using a Grounded B Phase System.

http://static.schneider-electric.us...n/Molded Case Circuit Breakers/2700DB0202.pdf

 

[iwire]

Interesting! I didn't know that utilities still use this type of service configuration.
It is called Three Phase Two Wire Corner-Grounded Delta service. This configuration is used mainly to save wiring costs as it requires only two insulated conductors instead of three conductors that are typically used in three phase service entrance. [h=3][/h]

It's not about saving the cost of one insulated conductor.

It's about getting 230 volts 3 phase instead of 208 volts in places where there are no 120 volt loads.

 

[Tshering]

Yes I have seen it at my customer's facilities for 3-phase 240V systems, some with high leg delta [208V at B phase (240sin60 or sqrt(240^2-120^2))] and some without high leg like you mentioned. But I haven't seen one for 3-phase 480V system.

It's not about saving the cost of one insulated conductor.

It's about getting 230 volts 3 phase instead of 208 volts in places where there are no 120 volt loads.

 

[don_resqcapt19]

Interesting! I didn't know that utilities still use this type of service configuration.
It is called Three Phase Two Wire Corner-Grounded Delta service. This configuration is used mainly to save wiring costs as it requires only two insulated conductors instead of three conductors that are typically used in three phase service entrance.

While the utility can use an un-insulated grounded conductor, you cannot use one on the load side of the service equipment. You treat this grounded conductor like any other grounded conductor. It must be white or gray and must not be connected to ground on the load side of the service disconnect.
Also I have never heard it called a "Two-Wire" system.

 

[ggunn]

Interesting! I didn't know that utilities still use this type of service configuration.
It is called Three Phase Two Wire Corner-Grounded Delta service. This configuration is used mainly to save wiring costs as it requires only two insulated conductors instead of three conductors that are typically used in three phase service entrance.

Not two wire, but three. That nomenclature counts the grounded conductor.