Grounding of CT's on a new transformer. Where should the ground connection be located?

[bdn2004]

We got our new oil-filled transformer that came with CT's that are each wired out to terminals on the top of the transformer. From there they are wired to a control panel at ground level.
Looking inside the terminal well of a CT on the top of the transformer, and per the transformer drawing, one side of each CT is grounded up there to the case of the transformer.

If these are left in place it seems it would put these CT's in a 3-phase wye configuration.....Is the intent of this, or is this wrong? We would typically ground the CT's at the control panel. Do you have to ground both sides of a CT when de-energized and performing maintenance?

 

[collies99]

Assumming that primary curremt is flowing, closing the shorting terminal block, provides a low impedance alternate path in parralel to the relay circuit. There is minimal current at the relay (kirchoffs law) but some exist. The isolating terminal blocks lets you work on the CT circuit while the CT is shorted and isolated and energized if you had to. No real danger, but do block protection during the process else unbalance current could be interpreted as a ground fsult.

Shorted CT will still produce voltage evrn shorted but very minimal depending on what the circuit burden is for a given current. Shorted at .1 ohms st 5 amps gives you .5V at the terminls and at 100 amps gives you 50V during faults at max rating.

cheers

 

[collies99]

Oops bsd math 100 Amp ewuates to 10V into .1 ohm of course, brain fart.

 

[waross]

I like your shorting arrangement but don't like the terminals. There are too many places where removing a lead from a shorted terminal may accidentally disconnect a lead from an unshorted CT.
Also, I would lose the last set of switched terminals market N, A, B, and C.
I just don't like this design philosophy. There are too many places where a small oversight may result in an open circuit CT.
Note: conventional power faults are the result of a low impedance path causing a high current. Fuses are used to interrupt the fault current.
CT faults are the result of a high impedance path causing a high voltage. A short circuit always makes the secondary of a loaded CT safe. (Assuming a good CT and some faulted CTs also.)
There are a lot of similarities between CT action with an open secondary and a series lighting circuit with an open circuit. A lamp that normally runs on about 50 or 60 Volts may jump to several thousand volts if a lamp fails and the circuit opens. Automatic shorting buttons may be used to short circuit a lamp socket and remove the high voltage. The shorting device also keeps the other lamps in the circuit lit.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[collies99]

Agreed, those isolating terminal block is an accident waiting for a chance to happen as noted earlier.