CT Secondary Earthing 1

[mikelangel78]

Hello,

I need your help regarding with this issue. I'm not an electrical enginnering, but currently i working in this fieid area.

My doubt is: from what i have been reading, the S2 of a Current Transformer should be ground. In that case the current that flows in the secondary will flow to earth in that point (S2)and the circuit will never be closed, because the current never return to S1. If its true, there is a permanent leakedge to earth. How could this work? Peharps this is a huge nonsense, but i have this doubt.

Can somebody help me trying to understand this?

Thanks in advanced
Mike

 

[ScottyUK]

The CT secondary circuit forms a closed loop. The loop is connected to earth as a reference at exactly one location. No current flows to earth, it just provides a means of defining the potential at that point in the circuit. If the secondary loop wasn't earthed then the whole secondary circuit could rise to line voltage.

 

[mikelangel78]

Thank you ScottyUK for your explanation...but i don't understand why the current does not flow to earth, because there are an alternative path in the S2 point (the one connected to ground. A portion of the current should flow in towards S1 direction and some of the current goes to earth in S2. Could you explain me why this doesn'y occur like that?

I know this is my ignorance speaking, but i would like to understand.

Thank you very much.

 

[davidbeach]

Current requires a complete circuit, with only one connection to earth there is not a complete circuit, thus no current flow.

 

[cranky108]

Consiter it a static discharge connection. And that's all it does, discharge static.

 

[mikelangel78]

Leet me e see if i understand:

In the primary the current flows from the P1 to P2 terminal, and in the secundary the current will
start to flow from S1 (phase) through the load/device, and return to the S2, that is connected to earth for safety precautions. The the current will flow from S2 to S1 and the cycle will repeat.

What would happened if instead of connect S2 to ground, we connect S1, without changing the flows in the primary side?

Thank you all in advanced.

 

[davidbeach]

Nothing would change.

 

[mikelangel78]

And if by both of them were connected to earth?

I must confess that this confuse me...

 

[davidbeach]

Both grounded would be bad. The rule is the CT secondary shall be grounded exactly once; no more, no less.

 

[scottf]

Both terminals grounded would form a second current loop that would divide the current between each loop based on the loop impedance.

In practice, if they were both grounded at the CT terminal, it would short-out the CT secondary winding.

 

[mikelangel78]

scottf, this second loop that appears is flowing through the earth, from one side to the other, is that correct?

And another question, to know the correct secondary to be earthed, we must know the flow in the secondary, ie, P1 to P2, S2 should be grounded and if P2 to P1, S1 should be grounded. Is that correct?

Thank you

 

[scottf]

Yes on the first question.

It does not matter which side of the secondary winding is grounded. It is important to only ground one side at one point. The convention is to have the non-polarity terminal (in this case S2) grounded.

 

[mikelangel78]

Ok.

When you say "non-polarity" terminal, what does it mean? Because the wire that is connected between the S2 terminal and the load will have a current flow throught it. Am I right?

PS: sorry if my questions seems stupid

 

[scottf]

If you look on a schematic, there is a polarity dot on one primary terminal (normally P1) and on one secondary terminal (normally S1). So the non-polarity terminal would be S2.

The polarity dot indicates instantaneous polarity/current direction....which means when the current is flowing into the P1 terminal it flows out of the S1 terminal.

 

[davidbeach]

Actually, I've found that that the polarity markings are merely useful for keeping track of which secondary terminal corresponds with which primary terminal. Beyond that point, the proper connection is determined by what you want the relays to see. Sometimes we wind up with the polarity side of the CT wyed (and grounded) with the non-polarity side connected to the polarity of the relay and other times just the opposite. In free-standing breakers, the CT polarity is generally toward the termination and away from the interrupter while in switchgear it is just the opposite with the polarity toward the breaker and the non-polarity toward the terminations. But in both cases it is probably the side of the CT toward the breaker that gets connected in the wye and then grounded; polarity in one case and non-polarity in the other.

 

[cranky108]

The only times that we have the secondary wyed is if we are adding CT's for differentials or ring and breaker and a half schemes, in which case the grounding point is in the relay cabment. But I think that too far from the questions asked.

The convention is to ground the non-polarity side at one point, with a few exceptions. And David is correct about one ground only.

 

[mikelangel78]

Thank you all for your patience. I think i'm clarified by now.

My best regards

 

[oldfieldguy]

Somewhere, ONE terminal of a CT secondary needs to be grounded at ONE point. If the CT is part of a set, then the secondaries can be tied together and the set can be grounded, again at ONE point. The drawings should show this point, and it is expected that everyone who gets to futz around with your CT secondary circuits will understand the need for this ground and will not molest it.

Among less experienced people venturing into system protection and metering, the application of that silly little ground seems to be one of life's larger mysteries.


old field guy

 

[mikelangel78]

Hello again. I would like to understand better this poiny of the secondary of the CT, grounding.

ScottyUK (Electrical)5 Mar 15 13:12
The CT secondary circuit forms a closed loop. The loop is connected to earth as a reference at exactly one location. No current flows to earth, it just provides a means of defining the potential at that point in the circuit. If the secondary loop wasn't earthed then the whole secondary circuit could rise to line voltage.

ScottyUk, could you explain me with it more detail this statement: "(...) If the secondary loop wasn't earthed then the whole secondary circuit could rise to line voltage."

Thank you in advanced.

 

[scottf]

The secondary winding is coupled to the primary winding by capacitive coupling.

Think of the secondary winding at a whole being at some potential...not the potential across the secondary winding, but of the whole winding itself. If one point of the secondary winding is not tied to ground, then that winding could "float up" in potential towards the primary winding's potential.

In all reality, it wouldn't float all the way to the primary winding potential, but it can certainly get high enough to be of serious danger.

So, by tying one end of the secondary winding to ground potential, the maximum voltage that can be present in the secondary circuit is limited to whatever voltage could be developed across the secondary winding itself.