Why current transformer secondary should not be opened.

Current transformers are always used with the secondary windings circuit closed through ammeters, current coils of watt-meters or relay coils. Its secondary winding circuit should not be opened while its primary winding is energized. A violation of this precaution may lead to serious consequences. Here, I am discussing the reasons behind it.
 
As we know, in a power transformer,

  • the current flowing in the primary winding depends upon the current in the secondary winding whereas,
  • in a current transformer current flowing in the primary winding depends upon the current flowing through the line whose current is being measured.

This current is no way controlled by the conditions of the secondary winding circuit of the CT.
 
why current transformer secondary should not be opened
 
Under normal conditions, both primary and secondary windings produce MMF which opposes each other. The primary MMF is slightly more than the secondary MMF and consequently, the resultant MMF is small.
 

This resultant MMF is responsible for the production of flux in the core and as this MMF is small under normal operating conditions, a small voltage is induced in the secondary winding of the CT.
 
If the secondary winding is open-circuited with energized primary, the primary MMF remains the same while the opposing secondary winding MMF reduces to zero.
 
In this condition, the resultant MMF becomes very large. This large MMF produces a large flux in the core till it saturates.
 
This large flux links with secondary winding and induces a high voltage in the secondary winding. This could be dangerous to the transformer insulation and to the person who has opened the circuit.
 
Also, the eddy current and hysteresis losses would be very high under these conditions and due to this the CT may be overheated and damaged.
 
Even it does not occur, the core may become magnetized permanently and this gives considerable ratio and phase angle errors.
 
Mostly, CTs are provided with a switch or short-circuiting link at the secondary winding terminals. If such a link is available, it should always be short-circuited before any change is made in the secondary winding circuit with primary winding energized. 
 
When a CT is used for measurement, its secondary winding can be short-circuited safely since it is practically short-circuited the impedance of the burden (i.e. an ammeter, CC of wattmeter etc.) is very small.
 

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