Delta Grounded B phase Transformer

[Red Wiggler]

We are working with an indoor switchboard that is wired to receive a 3 phase 4160 volt Delta feed. There is a 4160 / ? (I am assuming 240 volt) CPT (Control Power Transformer) that is fed from a fused leg on "B" Phase. What I don't see is another phase feeding the primary of the CPT.

Is there anyway that a transformer would function with just one leg on the primary side of this CPT? The secondary looks like it has one phase and a "Neutral" going out as control power.

Any help?

 

[mivey]

There must be two conductors on the primary side. Either a grounded conductor or an ungrounded conductor. I would hope they did not use the ECG as a conductor.

add: A transformer will work with only one ungrounded conductor, but you still need a second conductor.

 

[Red Wiggler]

Delta Ground

Delta Ground

I see a "Ground Wire" landed on a terminal, but because of the configuration of the transformer and its position inside of the gear I am having problems see what is "Marked" on the terminal where the ground wire is terminated.

If a fused "B" phase is terminated onto the line side (H1) of the transformer, and a "Ground Wire" is terminated on the other H2 terminal, then the transformer could potentially be at a 2400 volt primary. The secondary voltage could possibly be 240 volts, on a (T1) terminal with a potential to ground of 120 volts.

Is this possible?

 

[mivey]

You could have 2400 to ground on the primary. On the secondary, you could have 240 to ground if one side of the winding is grounded or you could have 120/240 if you have a center-tap (or whatever the appropriate secondary voltage level would be given the winding ratio). The grounding point on the secondary would determine if you have 120 & 240 to ground or two 120 volt legs to ground.

 

[LarryFine]

It could be an autotransformer.

 

[mivey]

It could be an autotransformer.

Yep, I forgot about that.

 

[Smart $]

It could be an autotransformer.

Yep, I forgot about that.

Yeh, but you'd still need two supply conductors, for which a grounding conductor should not be one of them.

 

[LarryFine]

Yeh, but you'd still need two supply conductors, for which a grounding conductor should not be one of them.

I dig. I was just pointing out the possibility that there may be three connecting conductors instead of four.

 

[SG-1]

I see a "Ground Wire" landed on a terminal, but because of the configuration of the transformer and its position inside of the gear I am having problems see what is "Marked" on the terminal where the ground wire is terminated.

If a fused "B" phase is terminated onto the line side (H1) of the transformer, and a "Ground Wire" is terminated on the other H2 terminal, then the transformer could potentially be at a 2400 volt primary. The secondary voltage could possibly be 240 volts, on a (T1) terminal with a potential to ground of 120 volts.

Is this possible?

Yes. Mivey answered your question about the secondary voltage very well. IMHO.

If the terminal you cannot see appears to be near the base of the CPT, then it is most likely H2. I cannot speak directly about your switchboard, but in switchgear it is very seldom any type of primary neutral is even present let alone avilable for use. Only the 3 ungrounded phase conductors & a ground bus is inside the big grey box. Anytime a L-G PT or CPT is desired the ground bus is the only option for connection to H2. I would expect it is used to ground the transformer frame or tray also. Probably a bare conductor to boot. The transformer may have a Nameplate/Schematic diagram somewhere. A zoom lens on a point & shoot might capture it.