Sunday, October 21, 2018


What is CT or Current Transformer in Electrical Distribution System

The CT in abbreviation of Current Transformer is a type of instrument transformer that used to measure the current in AC or Alternating Current systems, taking the reduced amount from it’s secondary winding which is proportional of primary winding that being to measured.

In the low voltage system where current flow limit within 100A, is possible to measure directly or whole current metering system. For low voltage system current more than 100A and all high voltage system, current measured by using CT. Actually CT takes a reduced portion of current which is proportional to flowing total current in the load circuit.

The instrumental transformer CT is required for low voltage high amount or high voltage system current measuring because of conventional measuring instrument is not practical to design to measure in high voltage or high current directly.

So, Current Transformer’s working principle is not different from ordinary conventional transformer. Current Transformer just reduce the high voltage current or low voltage high current to a much lower value that possible measure by a conventional ammeter.

Current Transformer and Traditional Transformer

Traditional transformer basically formed with a fixed core and primary & secondary winding, both primary and secondary winding have multiple numbers of turns.

In Current Transformer secondary winding contains many numbers of turns , but primary have only one turn of winding, means in primary side main current carrying conductor works as single turn primary winding. some times CT’s cores are split type which allows to open and put into main conductor without interrupting the  circuit.
Current Transformer
Difference Between Conventional and Current Transformer

How Calculate CT Ratio

Generally we know the CT or current transformer used to step-down the current level to monitor the flow of current. We also know that standard current measuring instrument have a limitation to measure up to a certain level of current. Consequently CT uses to step down the circuit current level and that is way we need to calculate CT ratio.

CT ratio actually expressed in terms of rated primary and secondary currents. To calculate the CT ratio we need to know the primary and secondary turns number of the CT. Note that often the primary turn is only one, practically the main current carrying conductor uses as CT primary turn.

Let us consider the following example to calculate the CT ratio to measure the actual current from the measured current.
  1. we know primary turn of CT is one, so for our example we can assume primary turn is 1;
  2. We need to find the total turns in secondary of the CT. We assume the secondary turn is 100;
  3. Now we can find the voltage ratio of the CT. The voltage ratio is the proportional of the ratio between the primary and secondary turns. So, voltage ratio is 1:100
  4. We know the CT ratio is inverse the voltage ratio. So, our calculated CT ratio is 100:1, means current step down 100 times, in other way if primary current is 100 ampere then CT secondary current is 1 ampere.
Using equation in above example we can show, Secondary current of the transformer: Is = Ip (Np/Ns) = Ip (1/100)
if primary current is 100 ampere, then Is = 100(1/100)= 1 ampere.

General Properties of CT Ratio

Generally CT primary current Ip and secondary current Is are standardize as per IEC 60044 and ANSI.  According to IEC 60044-1, the primary current series 1, 1.25, 1.5, 2, 2.5, 3, 4, 5,  6, 7.5; similarly according to ANSI the primary currents for single Ratio CT's Ip = 10, 15, 25, 40, 50, 75, 100, 200, 300, 400, 600, 800, 1200, 1500, 2000, 3000, 4000, 5000, 6000, 8000, 12000A.

According IEC the secondary current can be 0.5, 1 , 2 or 5A and according ANSI the secondary current is always 5A.

Protection CT and Measurement CT in Electrical System

We already know that CT are generally used to measure the high currents, so the parameter CT ratio is important to understand a CT that gives the ratio of primary and secondary currents.

But also, CT can be use to protect an electrical installation from overcurrent or short circuit current by using a protection relay.

As per function of protection or measurement CT or Current Transformer may be two major groups:
  1. Protection CT or Protection Current Transformer
  2. Measurement CT or Measurement Current Transformer

Protection CT

means protect electrical circuit if overcurrent or short circuit occurs. CT operate to protection relay to protect the circuit before reach the short circuit current over the rated current. As per IEC standard protection CT defined as class P, class PX, class TP.

Measurement CT

means to measure current of the circuit. Measurement CT mainly used to metering system for electricity billing purpose. According to IEC the standard accuracy classes  are class 0.2, 0.5, 1, 3 and 5.

Split Core and Solid Core CT in Electrical Distribution System

As per core construction CT may defined as split-core or core opening type and solid-core or fixed core type.

Split-core provides the facility of connect the CT to the line without interrupting, splitting the core opening and placing over the current carrying conductor. This type of CT is suitable for temporary CT connection or re-assembling system.

There are also flexible type core CT which is known as Coil CT, is more simplified in use where others solid and split core are difficult to use.

In case of solid-core CT installation, the power line must be disconnected and then main current carrying conductor placed through the CT window. This type of CT is suitable for new installation and permanent connection.

CT Connection Procedure

How to connect CT safely? Typically the CT primary has only one turn where the secondary may much turns depending on the amount of current carrying through it. Primary never has more than a few turns. So, the conductor or a bus go through the CT window that makes primary connection.


CT secondary connection is much important and must be ensure the safety. It’s notable that,  when CT secondary is closed and current presence in the primary side then secondary coil generate a back EMF to the primary magnetizing force; but if the secondary side is open in presence of primary current, the EMF is removed and extremely high voltage may produces in secondary due to primary magnetizing forces. This high voltage may cause of danger for human or CT. That is way CT secondary always shorted by ampere meter is recommended.

So, before use CT (Current Transformer) in electrical distribution system should confirm the ratio, specification and connection procedure.
current transformer calculation


  1. Is CT transformer or equipment?

    1. I think both, first it is transformer, then equipment.
      the output of this transformer is uses as signal, not as power.

  2. Very Helpful for me.
    Thank you so much.

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    In order to identify such weaknesses in proper time, the status of the insulation must be monitored continuously.


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