Why CT Secondary Should Not Be Kept Open?

Why CT Secondary Should Not Be Kept Open?

The electrical power system load current always flows through current transformer primary; irrespective of whether the current transformer is open circuited or connected to burden at its secondary. If CT secondary is open circuited, all the primary current will behave as excitation current, which ultimately produce huge voltage.

    Every current transformer has its own non-linear magnetizing curve, because of which secondary open circuit voltage should be limited by saturation of the core.

    If one can measure the rms voltage across the secondary terminals, he or she will get the value which may not appear to be dangerous.

    As the CT primary current is sinusoidal in nature, it zero 100 times per second (As frequency of the current is 50 Hz). The rate of change of flux at every current zero is not limited by saturation and is high indeed.

    This develops extremely high peaks or pulses of voltage. This high peak of voltage may not be measured by conventional voltmeter.

    But these high peaks of induced voltage may breakdown the CT insulation and may case accident to personnel. The actual open-circuit voltage peak is difficult to measure accurately because of its very short peaks.

That is why CT secondary should not be kept open.

Current Transformer

CT's are categorized as Protection CT, Special Protection 
CT and Measuring CT. Based on this, the CT's are 
classified. Here is the meaning of the CT classes:

Class 5P20: 

The letter 'P' indicates it is a protection CT.

The number 5 indicates the accuracy of the CT. It means 5% secondary current error at 100% primary current. Most common accuracy numbers are 5 and 10.

The number 20 (called accuracy limit factor) indicates that 
the CT will sense the current with the specified accuracy 
even with 20 times of its secondary current flows in the 
secondary. This is required for protection CT, because the 
fault current is high and the CT should be able to sense 
the high fault current accurately to protect the system. 
The common numbers are 10, 15, 20 and 30.

Class PS:

PS is for 'Protection Special'. This class of CT's are used 
for special protection such as differential protection, 
distance protection etc.

Class 1M:

The letter 'M' indicates it is a measuring CT.

The number 1 indicated the accuracy of the CT. The 
measuring CT's should be more accurate than the protection 
CT. The most common accuracy numbers are 0.5 and 1.

Measuring CT's are intended to work in normal and it 
doesn't require working with accuracy beyond the CT 
secondary current rating, so the accuracy limit factor is 
not mentioned here. 

CT 0.2 is having a secondary current error of 0.2% at 100% of primary current and 0.75% error @ 5% of primary current. It is used for standard revenue metering.

CT 0.2s(special) is having a secondary current error of 0.2% at 100% of primary current, 0.35% error @ 5% of primary current and 0.75% error @ 1% of primary current. It is used for Precision revenue metering.

CT 0.5 is having a secondary current error of 0.5% at 100% of primary current and 1.5% error @ 5% of primary current. It is used for standard commercial metering.

CT 0.5s(special) is having a secondary current error of 0.5% at 100% of primary current, 0.75% error @ 5% of primary current and 1.5% error @ 1% of primary current. It is used for Precision commercial metering.

3 phase induction motor numerical