Current transformer (CT) Installation Instructions

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Safety

CTs can be installed in systems were lethal voltages are present. CTs should only be installed or maintained by competent individuals. After reviewing the risk for any particular installation all required precautions should be taken.

CTs can produce high voltages on the secondary terminals if power is applied to the primary whilst they are in an open circuit condition. This may pose a hazard and can even damage the CT itself. If a CT must be installed without the secondary wiring connections made, a shorting link should be installed between the secondary connections.

Where connections are made to a wound primary, these should be insulated correctly as they carry the full system voltage and current.

Work with caution and apply common sense, there may be risks other than those detailed above, for instance large CTs can be heavy, high ratio CTs may get uncomfortably hot during operation.

General Installation Instructions

  • Install the CT to the primary connection, on most CTs this means passing the primary conductor through the aperture in the CT. If the CT is installed onto a system that is powered, or there will be a delay before making the secondary connections, install a link of wire too short out the secondary terminals.
    1. Occasionally some CTs require multiple turns of the primary conductor to give the correct ratio, this will be indicated by a label on the side of the CT. If the number of turns of the primary conductor is incorrect then the final ratio will be incorrect.
    2. Some CTs have wound primary connections in this case the primary connections must be connected directly in circuit with the current to be measured.
    3. Multi-Ratio CTs have several secondary connections. If some of these connections are unused they must be left open circuit during normal operation.
  • Ensure the CT is installed such that the P1 primary connection faces the power source
  • For measurement, and most protection applications ensure, that the CT is installed around a single conductor. If a CT is installed with both live and neutral conductors passing through the aperture, then no current will be measured under normal working conditions.
    1. For protection applications where the CT is connected to an Earth leakage relay the neutral conductor and ALL of the live conductors must be passed through the CT aperture, the Earth connection must NOT be passed through the CT.
  • Use a suitable fixing method to secure the CT in the correct position around the primary conductor
  • Do not force the CT over the primary conductor as this could damage the insulation on the CT
  • Make the secondary connections using the shortest usable length of a suitable sized cable. Select the cable diameter in accordance with the required length so as not to over burden the CT
  • Observe the wiring polarities for the specific installation.

Power coming from the source must go through the P1 side first, in order maintain correct polarity.

Split core CTs

Ensure that the mating faces of the CT core are clean and free from damage and debris before mating the two halves of the CT together. To ensure stated performance the sections of core must be properly aligned, and the clamping mechanism is done up tightly.

Protective CTs

It is essential that adequately sized cable is used and that the CT is not overburdened, the cable used for the secondary connections must be capable of supporting the secondary fault current for at least the duration of the fault.

Summation CTs

Summation CTs should be used when it is not practical to have a single high ration CT, summation CTs are typically used with a number of Primary CTs

Summation CTs can only be used across a single phase, they cannot sum across multiple phases. If summation needs to be done on multiple phases, then multiple summation CTs are required.

Polarity must be observed, if a primary CT is wired with the wrong polarity to summation CT its current will be subtracted from the total.

If All of the primary CTs do not all have the same ratio, a special summation CT is required. On installation the Primary CTs must be wired to the correct input terminals on the summation CT or the resulting sum will be incorrect.

Core Balance CTs

A core balance CT is used with an earth leakage relay to detect earth leakage faults. Ensure that the CT is correctly sized for the relay being used. All Live conductors and the neutral conductor must be passed through the CT, the Earth conductor must remain OUTSIDE the CT.

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