240volt delta center leg

[ndc81167]

I was just on a service call in a old bulding that had 240volt 3phase delta
and on the center leg "B" read nothing on my votlage meter but on A AND C
legs it read 240 volts to ground,,, and this 3 phase config went into a tramsformer which stepped it down to 120/208 3 phase,,,my question is
why didnt i see the voltage on my meter on the center leg ??

 

[iwire]

What you had was likely a corner grounded delta service.

In your case phase B was intentionally bonded to ground.

It is also possible but less likely it was a un-grounded delta system that has a ground fault in it.

 

[ndc81167]

so you can get 120/208 3phase coming out of the transformer
from only 2 legs of 240 ...

 

[LarryFine]

Bob is very likely correct. If it's intentionally grounded, that phase should be bonded as a neutral would be in a typical service.

If you can find a fused disconnect, there should be a 'dummy' fuse or maybe even a piece of copper pipe in the grounded-phase space.

It's also possible, and today it would be required, that the grounded phase is colored as a neutral conductor.

 

[infinity]

If you can find a fused disconnect, there should be a 'dummy' fuse or maybe even a piece of copper pipe in the grounded-phase space.

I have seen the copper pipe thing done. That cannot be code compliant.

 

[LarryFine]

I have seen the copper pipe thing done. That cannot be code compliant.

Fusing the intentionally-grounded conductor would definitely not be compliant.

 

[iwire]

so you can get 120/208 3phase coming out of the transformer
from only 2 legs of 240 ...

No.

What you have is 3 legs of 240 measured phase to phase it just happens that one of these 3 legs is grounded.

 

[ndc81167]

but then why cant my meter see the 240v when i put 1 test lead on the "B"leg
and the other test lead on a grounding lug

 

[hardworkingstiff]

but then why cant my meter see the 240v when i put 1 test lead on the "B"leg
and the other test lead on a grounding lug

Somewhere upstream from where you were testing the "B" phase was grounded. So you are testing from ground to ground.

 

[iwire]

but then why cant my meter see the 240v when i put 1 test lead on the "B"leg
and the other test lead on a grounding lug

The meter can not read a voltage phase B to ground because phase B is attached to ground.

The fact that phase B is grounded does not prevent it from still being phase B of a 3 phase system.

If you put you meter

A to B you Will have 240

A to C you will have 240

B to C you will have 240.

Forget about A, B or C to ground, that has no bearing on the operation of 3 phase delta loads

The 208 volt transformer being supplied is connected phase to phase, it is not connected phase to ground.

 

[roger]

Along with Iwires earlier post

but then why cant my meter see the 240v when i put 1 test lead on the "B"leg
and the other test lead on a grounding lug

For the same reason that you will read zero volts when putting both leads of a meter on the same wire of any system.

Since this B phase conductor and earth are common to each other, there is no difference of potential.

Roger

 

[LarryFine]

but then why cant my meter see the 240v when i put 1 test lead on the "B"leg
and the other test lead on a grounding lug

For the same reason that you will read zero volts when putting both leads of a meter on the same wire of any system.

Since this B phase conductor and earth are common to each other, there is no difference of potential.

Better analogy: connecting a meter between the grounded neutral and the EGC of a system or circuit.

 

[kbsparky]

Better analogy: connecting a meter between the grounded neutral and the EGC of a system or circuit.

Depending on how far from the service point you are, and how much load the neutral conductor is carrying, your readings might be as high as several volts ... :grin:

 

[iwire]

Depending on how far from the service point you are, and how much load the neutral conductor is carrying, your readings might be as high as several volts ... :grin:

That is true and it also applies to the grounded leg of a Delta system.

 

[roger]

Better analogy: connecting a meter between the grounded neutral and the EGC of a system or circuit.

Larry, although your analogy will give the same reading, it's important to note that the grounded B phase is not a "neutral" in this system.

Roger