Why CT secondary should not be open circuited?

Why CT secondary should not be open

The most important precaution that shall be taken care of while working with the Current Transformer (CT) is that its secondary winding should never be kept open. Either it should be shorted or must be connected in series with a low resistance current coil of Ammeter, Wattmeter, Relays, etc. So In this article, We will discuss in detail what will happen if secondary of CT is kept open.

Why CT secondary should not kept open-circuited?

As we all know, CTs are always used with secondary winding short-circuited with Ammeter and Relays. A precaution that shall always taken care of is: Never open the secondary winding of CT while its primary is energized.

If the secondary of CT is accidentally left open then it will be very dangerous and lead to severe consequences for the person working with it and also it may destroy the equipment. So let’s understand the concepts and actual reason behind this.

To understand this, first of all, We should know the basic difference between working of a Power Transformer and a Current Transformer. In Power Transformer, Primary current is the reflection of secondary current by mmf balance rule N1I1 = N2I2. While in the Current Transformer (CT), the primary current is dependent on the load current. As the CT becomes connected in series with the line. So primary current of CT is constant irrespective of whether the secondary is connected with burden or not.

Read More: Why Transformer is rated in kVA, not in kW?

During normal operation of CT, primary and secondary winding both produce mmf which by Lenze’s low apposes each other. As secondary mmf is less than primary mmf. So the net-working mmf is small. This net-working mmf is also known as the magnetizing mmf of the core of CT.

Now, in case of secondary winding is kept open then obviously secondary current will be zero. While the primary current remains the same. Therefore opposing secondary mmf will be zero. Hence the net-working mmf will be due to primary current only i.e. N1I1 which is very large. This large mmf will produce huge fluxes in the core of CT and will saturate the core. Again, due to the large flux in the core, the flux linkage in secondary winding will be large and hence produce huge voltage across the secondary terminal of CT. This huge voltage across the secondary terminal will be very dangerous and lead to insulation failure. Hence due to the very large secondary voltage, the person working with CT may die due to electrical shock while the primary is energized.

Hence, When the secondary circuit of CT left open, It can result in an extremely high voltage on the secondary terminal of CT, which may pose a very high risk of electrical shock to working personnel and equipment. This condition is also known as “Burden Voltage” or “Excitation Voltage”.

Also because of excessive core flux, the hysteresis and eddy current losses will be very high and the CT will get overheated. Hence the oil of CT will get boil and start to vaporize. Because of the vaporization of oil, the CT housing feels huge pressure and blasts. This blasting of CT leads to fire and smoke. Again, due to smoke and fire, the nearby lines will trip due to an earth fault, which may trip the power station.

In summary, keeping CT secondary circuit closed and properly connected is essential for Safety, Accurate measurements, Equipment Protection and reliable operation of electrical systems.

So in this article, We have discussed all the consequences of what will happen if secondary of CT is left open and how much it will be dangerous for working people as well as the equipment itself. Hope that this article will help you.

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Hey, I'm Satish Gupta an Engineer by profession and blogger by passion. I am writer and founder of this blog, Here I publish contents related to Electrical and Electronics Engineering..

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