reading voltages across lines

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inspector23

inspector23

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Temecula, CA
I just finished a test, and I am not sure of the answer to this question.

This concerns a 277/480V panel. What should the volt meter register when I place the leads as follows?

Line 1 to ground
line 2 to ground
line 3 to ground

across phases, ie;

line 1 to line 2
line 2 to line 3
line 1 to line 3


Is is possible to get 120v from any of these lines?

I think the line to ground voltage on each line should read 277V. I also think any two lines would read 480V across them.

I do not believe it is possible to get 120V from this system.

I would really appreciate a speedy response, especially if you know of anywhere I can find this info on the internet. You know, a picture being worth a thousand words and all.

Thanks for your help -

Mike
 
Re: reading voltages across lines

Line 1 to grd-277
Line 2 to grd-277
Line 3 to grd-277
Line to line on any-480

Now that is a normal system, if it is an open delta you may get 0 to ground on one of the phases. You could also posibly get 480 to ground on one of the phases. You will never get 120 on any of those legs unless something is wrong ie-broke nuetral.
 
Re: reading voltages across lines

Chris,
Now that is a normal system, if it is an open delta you may get 0 to ground on one of the phases. You could also posibly get 480 to ground on one of the phases.
Click to expand...
An open delta would not read 0 to ground on one phase. A corner grounded delta would read 0 to ground on one phase. If you have 480 to ground, you have a corner grounded delta and you will have that voltage to ground on two phases.
 
Re: reading voltages across lines

Open delta voltage is a collective voltage.
You really can not measure it with the standard meter we use everyday.
Remove one of the fuses (blown fuse) and see what you get.
 
Re: reading voltages across lines

Big John, I don't understand. What is a collective voltage and why can't you use a standard meter to measure voltage? :confused:
 
Re: reading voltages across lines

Open delta is when only two single phase transformers are used to supply the service. The two secondaries are connected in a delta arrangement like normal and the phases voltages are normal, but the maximum loading will be 57% of the kVA rating because the two have to make up for the one which isn't there.

There is a centertapped delta which commonly has 240 line to line and from the center of the tapped winding (the neutral point) the adjacent lines to neutral will be 120V. This centertapped delta is sometimes an open centertapped delta to serve customers with small 3 phase loads. We have this service to our farm and are just glad to have 3 phase service!

Jim
 
Re: reading voltages across lines

Charlie
I say collective because if you come off of a 240/3phase service, which is a delta 120, 190, 120 if you measure to ground.

If you use the (2) transformers in an open delta to transform to 208 3phase, between phases you will get 208 +/- 5-10% and a motor will run fine.

Now if you try to measure this from phase to ground. You will get 120, 190+/-, 120 Which is not a true 208/3phase system. But collectively it is 208/3phase/ open delta. Keep in mind that a neutral is not needed, only a ground.

If you want a true 208/3phase system, there is no
other option than to use a 240/delta to 208 Wye transformer. But if a neutral is not needed, it can be quite expensive. Using the 2 transformers is a cheaper and more compact method.
An open delta is just a cheaper way to get say a 208/3phase system. Keep in mind that buck and boost transformers are only good for a +/- 10% in the voltage that is applied.

(I think the above is correct, All ears. This is just how I understand.)
Always willing to learn, ears are wide open.
 
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