Evaluating a Current Transformer

  1. Big inner diameter; small outer diameter
    A small outer diameter occupies less space and can fit better in cramped places, while a large inner diameter makes it easier to fit onto the load wire.
    Note that the load capacity is not necessarily the determining factor for the size of the CT, as the actual wire size will determine whether it can fit within the inner window of the CT.
  2. Accuracy
    The true test of accuracy is the accuracy when measuring very small currents, the lower the starting current the better; at the same time the lower phase deviation the better, but this technically harder to achieve.
    When the CT is produced, each would have minor deviations from the other which will make them not entirely compatible with one another (especially when it comes to the shift in the phase angle), this is the result of deviations in their material and construction. If their consistency is poor, then it would be necessary to have the meter be calibrated to each CT.
  3. Interchangeability
    Whenever either the meter or the CT needs to be replaced, then both need to be replaced at the same time. In
  4. Current output better than voltage output
    Small current transformers are available with current and voltage outputs types. The voltage output type is susceptible to noise, while the wiring cannot be extended for long distances because of the voltage drop and capacitive effect. In contrast, the current output type is more stable, reliable and is not easily affected by noise. Thus, most industries choose to use 4~20mA signal transmitters.
  5. Multiple primary windings
    When a regular 5A output CT has a small primary such as 30A, 50A or 100A, it is often necessary to loop the load wire twice or more around the CT primary to compensate for the small signal, otherwise the signal is strong enough to make it usable. But small sized dedicated CTs have such sensitivity and accuracy that they need no looping to compensate.
  6. Comprehensive selection to match all types of meters
    In addition to a selection of current ratings, the CTs should also be available in split core. The meter manufacturer should have the right CT to meet every need, the inventory should be stock, all of these will have an effect on the seamlessness of the project implementation.

Special Considerations

  1. Different CT brands are not interchangeable
    Unlike regular CTs, dedicated CTs for meters from different manufacturers cannot be changed with a CT from another manufacturer. Even more so, regular 5A output CTs most definitely cannot be used, otherwise, the meter could be damaged.
  2. Are the same dedicated CT type from the same manufacturer interchangeable?
    This will depend on the consistency and quality of the CT used by the manufacturer. Most won’t, but DAE’s dedicated CTs are of very high quality and can be interchanged among the same type. When the CT are not of consistent quality then the meter will need to be recalibrated together with the CT, if this is not possible then the alternative would be to replace the meter together with the CT.
  3. What about for regular 5A out
    If a regular 5A output CT is already being used then a dedicated 5A CT need to be used, the CT can either be of the solid or split core type. Be sure to use it with the model of meter that accepts the dedicated 5A CT.

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