Important: Ensure the secondary of the CT is not left disconnected while the primary supply is on. In this condition high-voltage spikes may be produced in the transformer secondary, often into thousands of volts, sufficient to degrade the transformer insulation.

Current Transformers

CTs should be specified as follows :

  • RATIO: Input / Output Current ratio
  • VA: total burden including pilot wires, see diagram to left
  • CLASS: Accuracy required for operation
  • DIMENSIONS: maximum & minimum limits
  • Specifying a higher VA or CLASS than necessary usually results in a higher cost.
  • The cost generally increases as the CT's internal diameter increases.
  • 1A CTs are usually more expensive than 5A.
  • Fittings such as mounting feet are charged as extras. However, these costs are often more than recovered by the reduction in fitting costs on assembly.
  • Should special accuracies or burdens be required, other than those listed on our standard lists, please contact our Design Engineers who may be able to suit your needs.

In general, the following applies:

  • 0.1 or 0.2 for precision measurements
  • 0.5 for high grade kilowatt hour meters
  • 1.0 for commercial grade kilowatt hour meters
  • 1.0 or 3 for general industrial measurements
  • 3 or 5 for approximate measurements
Burdens (depending on pilot lead length)
  • Moving iron ammeter 1-2VA
  • Moving coil rectifier ammeter 1-2.5VA
  • Electrodynamic instrument 2.5-5VA
  • Maximum demand ammeter 3-6VA
  • Recording ammeter or transducer 1-2.5VA
In addition to the general specification required for CT design, protection CTs require an Accuracy Limit Factor (ALF). This is the multiple of rated current up to which the CT will operate while complying with the accuracy class requirements.

In general the following applies:

  • Instantaneous overcurrent relays & trip coils - 2.5VA Class 10P5
  • Thermal inverse time relays - 7.5VA Class 10P10
  • Low consumption Relay - 2.5VA Class 10P10
  • Inverse definite min. time relays (IDMT) overcurrent - 15VA Class 10P10/15
  • IDMT Earth fault relays with approximate time grading - 15VA Class 10P10
  • IDMT Earth fault relays with phase fault stability or accurate time grading required - 15VA Class 5P10

Class X CTs are special CTs used mainly in balanced protection systems (including restricted earth fault) where the system is sensitively dependent on CT accuracy. Further to the general CT specifications, we would also need to know:

Vkp - Voltage knee point

Io - Maximum magnetising current at Vkp

Rs - Maximum resistance of the secondary winding.

Voltage Transformers (VTs) should be specified as follows:
  • RATIO: input / output voltage ratio
  • TAPPING'S: all taps required on each winding
  • VA: total burden of each winding or tapping
  • FREQUENCY: If different from 50Hz
  • DIMENSIONS : maximum limits
  • MOUNTING: required terminations and mounting
  • TEMP. CLASS : If other than Class B
The following factors can affect Transformer prices:
  • Specifying a higher VA then necessary
  • Rated Voltage
  • Temperature rise other than 80o (40oC ambient)
  • The number of windings required
  • Frequency
  • Regulation lower than those on our listings.

Voltage Transformers


Regulation is the inherent voltage drop due to the impedance of the transformer expressed as a percentage of the output voltage. If %R is the inherent resistive voltage drop and %X is the inherent reactive voltage drop, both expressed as a percentage of the output voltage then:

$$ \%{REG} = \%R + (\frac{\%X}{200})$$

This applies as rated output, unity power factor only.

Intermittent Overload

This is permissible, if of short duration and followed by an underload. If a transformer is subjected to a regular overload current Ii (i.e. contactor inrush current) for ti seconds, followed by a regular underload Is (i.e. contactor seal current) for ts seconds, the equivalent continuous current is:

$$I_{EQ} = \sqrt{\frac{(I_i^2 t_i) + (I_s^2 t_s)}{t_s t_i}}{(Amps)}$$

Multi-Tapped Primary

If the range of primary taps is limited, say within +/-15% of the median voltage, the VA is increased by the factor:

$$0.5 (1 +(\frac{V_{max}}{V_{min}}))$$

This alters if the range is larger than 15% as graded copper sections are required. Please call our design department for more information.

D.C. Loads

The effective VA of a transformer winding supplying D.C. loads depends on the type of rectifier and filter used.

If D.C. values are specified this information must be given.

Auto Transformers

Where electrical isolating is not required, auto transformers may offer considerable advantages in economy & space. The nearer the input/output ratio approaches unity, the more marked the reduction in size.

For a simple step-up or step-down auto transformer:

$$Frame VA = Output VA (\frac{V_{max} - V_{min}}{V_{max}})$$

In general, the following Classes apply:

  • 0.1 or 0.2 for precision instruments
  • 0.5 for high-grade meters
  • 1.0 for commercial-grade meters
  • 1.0 or 3 for general industrial measurements
  • 3 for approximate measurements

In addition to the general specification required for VT design, protection VTs require a Rated Voltage Factor (VF). This is the multiple of rated voltage up to which the VT will operate while complying with the accuracy class standards.