High Voltage Electric Power Equipment

High Voltage Electric Power Equipment

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Why the CT secondary should not be kept open?

High voltage Senior Executive Technology R&D at Crompton Greaves Ltd

  • Comment (61)
  • April 26, 2012
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  • Muhammad

    Muhammad A.

    Field Service Engineer at Arab Tech EST

    If the CT secondary is kept open it will burn out .that why we make it short.

  • Dean B.


    Dean B.

    Senior Training Specialist at AVO Training Institute

    A CT is a conductor wound around an iron core. This is similar to wrapping wire and around a nail. If you were to pass current through the wire, (similar to CT primary current) the nail acts like an electromagnet that can be used to pickup metal shavings. The metal shavings can be compared to the burden of a CT).
    Applying more current through the wire will result in "picking up more metal shavings", (up to a point, where the iron of the nail is saturated and cannot pick up any more shavings).
    IMPORTANT: the nail would "TRY" to do more work. Now apply this line of thought to a CT, with some minor changes.
    The construction of a CT is basically the same: it has an iron core (shaped like a donut) with wire wrapped around it. This can be compared to the wire around a nail. It also has a conductor through the center of this wire-wound donut, (similar to the nail).

    Suppose you have a CT with a ratio of 50:1, meaning that 50 amps of primary current would give 5 amps of secondary current.
    Step One: connect two ends of the secondary winding to an AC amp meter.
    Step Two: Place a conductor through the center of the iron core.
    Step Three: Apply 50 amps of current through the center conductor. (This is the Primary Current)
    Step Four: Observe the secondary current reading on the amp meter. It should read approximately 5 Amps. NOTE: the amp meter itself is the only "burden" placed on the secondary of the CT. "Burden" is basically the opposition to current flow in the secondary of the CT, (resistance). An amp meter basically offers no resistance.
    Step Five: With a volt meter, read the voltage drop across the terminals of the amp meter. The voltage drop should be "zero", if the terminals are not loose.
    At this time the following observation can be made: With 50 amps of primary current, the CT produces 5 amps of secondary current, and with basically no burden, the CT does so with very little effort .
    Now, let’s change the secondary circuit up a little:
    What if one of the connects of the amp meter was a little bit loose, such that it offered 5 ohms of resistance (just as an example). Here's what happens: the CT will need to "TRY HARDER" to "push" the same 5 amps of current through the secondary circuit because of the increased burden.
    READ THE VOLTAGE AGAIN: Use a voltmeter and read the voltage drop across the amp meter, just as before. With 5 amps of current flowing through a circuit containing 5 ohms of resistance, ohms law says that (E = I x R) or E = 5 Amps x 5 Ohms = 25 Volts.
    If this connection continues to get worse, the resistance would continue to increase, and the CT would need to "TRY HARDER" to push 5 amps. If the resistance increases to 20 ohms and the CT produces 5 amps of current there would be a voltage drop of 100 Volts, (5 amps x 20 ohms).Under these conditions, rapid overheating of the CT winding would cause it to fail.
    Your original concern was: WHY THE CT SECONDARY SHOULD NOT BE KEPT OPEN?
    Since CTs consist of a significant amount of iron, this means that they can do quite a bit of work. If you compare it the "Nail", this means that it could pick up a hell-of-a-lot of metal shavings. So, if you ever began to open up the CT secondary circuit, "what is happening"? The resistance is going toward "Infinity". If there is enough iron in the CT, for example, to push 5 amps through this "open circuit" when the resistance reaches 100 ohms, the voltage drop across the open circuit is 500V.
    SAFETY: If a technician accidently opens up a CT circuit and quickly grabs the ends of the leads, he has put himself in series with a CT that is "TRYING HARD” to push 5 amps of current through the body technician. The technician is now the CT burden. The voltage produced is more than likely "fatal". There have been many similar incidents involving CT secondary circuits that were opened up by accident and resulted in someone being fatally electrocuted.

  • Milt K.


    Milt K.

    Managing Senior Principal Engineer - Automation/Control & HV Electrical at American Bureau of Shipping

    Ameer - In a typical configuration, the primary of a CT is a wire, cable or bus bar. The current in the primary is not in any way controlled by the CT, it is determined by the characteristics of the load. As an example if we had a 1000 to 5 CT and the primary current is 1000 amps, then the secondary current is 5 amps. If the burden (load) in the scondary circut is one ohm, then the voltage across the secondary would be V = 5 amp x 1 ohm = 5 volts. If the secondary load in 10 ohms, then the voltage across the secondary would be V = 5 amp x 10 ohm = 50 volts. We can easily see that if we carry the exercise further, such that the secondary load is 1000 ohm (we are approaching an open circut here) that the voltage across the secondary would be 5000 volts and it keeps going up. This is one reason why we don't open circuit the secondary of a CT.

  • Ismail E.


    Ismail E.

    Field P&C / Commissioning Engineer at Hydro One

    As the previous posters mentioned, its a major personnel and equipment safety concern. Depending on the voltage level on the primary circuit you will get a dangerous voltage build up on the secondary circuit which can electrocute some one touching the secondary links. If the voltage levels are high enough arcing will start across the links. I personally saw a situation where an electrician open circuited a live ct on a 115 kv cct and it caused the cables to melt. Apparently, there is a possibilty that the CT could explode if left open circuited for long enough.

  • Arnold S.


    Arnold S.


    The other reason is that the CT has an excitation high impendance;when the secondary is open the current that was supposed to pass through the load will be forced to flow thruogh the high impendance.The flow of this current will require a high voltage to flow thru this high impendance.The voltage required to push this current will appear at the secondary open terminials and is normally in kilovolts.

  • Wilfredo M.


    Wilfredo M.

    Sr. Electrical Design Engineer at Al Jaber Energy Services

    I agree all of the above comments....thanks for your comments guys...

  • K A R.

    K A

    K A R.


    Dear Dean
    It has been a concept well explained for beginners to know the minutes of CT.Thanks a lot.
    Thanks Milt also very well explained.


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