STANDARDS AND RATINGS - CURRENT TRANSFORMERS

Current Ratings / Accuracy Ratings

Voltage Ratings

Current Ratings:
Most Polycast current transformers are available as single, double or multi-ratio units. Five amp secondaries are standard, but one amp secondaries are available for most styles. Maximum and minimum primary ratings depend upon the style selected.

Accuracy Ratings:
For most applications the simplicity and high mechanical withstand capability of window type and bar type CTs makes them preferable for most applications. Wound primary CTs are recommended where very high accuracy at lower ratios (5-5A through 300-5A) is required.

Testing:
Polycast testing capabilities include polarity, ratio and phase angle, saturation, temperature rise, open circuit, induced, power factor,
hipot, impulse, partial discharge, low temperature withstand and other
physical and mechanical testing.

All Polycast current transformers are individually tested.
Routine and type test reports are available on request.

Polycast
Standard
Designation
Nom.
Voltage
Class kV
Low
Freq.
Dry
kV
Impulse
kV Crest
1.2x 50us
Chopped
Wave
kV Crest
( us)
0.6 0.7 4 10 12 (1.0) 
05 5.0 19 60 69 (1.5)
08 8.7 26 75 88 (1.6)
15 15.0L 34 95 110 (2.0)
18 15.0H 36 110 130 (2.0)
25 27.5L 50 125 145 (3.0)
28 27.5H 60 150 175 (3.0)

The above values are based upon, and are intended to meet the most stringent requirements of, the following standards: ANSI/IEEE C57.13, CSA CAN3-C13 and IEC 185

MATERIAL STANDARDS:

  • Cores are grain-oriented silicon steel (M4 or higher), annealed after forming, to provide maximum accuracy.
  • Core insulation is electrical grade pressboard and kraft paper, or epoxy.
  • Secondary coils are 200C rated copper magnet wire with silver soldered connections to terminals or leads, and high quality electrical grade kraft or nomex interlayer insulation.
  • Primaries (where provided) are electrolytic copper, electroplated with silver.
  • Cast indoor units are manufactured from bis-A epoxy.
  • Cast outdoor units are manufactured of cycloaliphatic epoxy, and are complete with weatherproof terminal boxes.
  • All CTs are nameplated and have prominent polarity markings.

Some Standard Terminology:

Turns Ratio  the ratio of the secondary turns to one.
Current Ratio  the ratio of the primary current to the secondary current in amps.
TCF  The correction for the overall error due to both ratio and phase angle error for a
 specified primary circuit power factor.
RCF   Ratio Correction Factor is the ratio of the true ratio to the marked ratio
 (excitation & other losses result in ratio error).
Rated Burden  The maximum load which may be placed on the CT secondary without causing
 an error greater than that allowed by the stated accuracy (in ohms impedance, e.g.
 B0.1, B0.9, B2 or corresponding volt-ampere values of 2.5, 22.5, 50 VA).
Accuracy Class
Metering
Relaying
 Metering Accuracy = the TCF shall be within specified limits at 10%, 100%, and
 CCRFx100% at a given power factor with a specified burden. Relaying Accuracy
 = the composite error shall not exceed a specified percentage error at a specified
 secondary terminal voltage based on a maximum fault level of secondary current.
CCRF  Continuous Current Rating Factor is the factor by which the rated current of the
 CT can be multiplied to obtain the maximum continuous current that the CT can
 carry without exceeding the temperature rise or accuracy requirements.
1 Sec Thermal
Withstand
 The maximum rms symmetrical primary current that can be carried for one
 second with the secondary short circuited without exceeding the limiting
 temperature.
Mechanical
Withstand
 The maximum rms asymmetrical primary current that a CT can carry with the
 secondary short circuited without any damage which would render it incapable of
 meeting other standard accuracy and transformation requirements.

SPECIAL CONSIDERATIONS:

  • Current Transformers should never have primary current injected with the secondaries open-circuited. This is especially true at higher current ratios (1200-5A and above) where open circuit voltage may exceed 3500 volts (OC standard test voltage limit).
  • The test standard and any special requirements for each current transformer ordered should be specified on the purchase order.

Because the conditions of use of any current transformer is beyond the control of the supplier, it is always the responsibility of the user to ensure that the current/voltage ratings and the accuracy of the transformer used is adequate for their application.

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