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When to use I²R, V²/R or VI while calculating Electric Power?

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I came across a question that was asked to me quite a couple of time: When to use I²R, V²/R or VI while calculating power? At first, the question looks simple and as if it needs no formal explanation but that's not the case. Use of any of them will give the correct result provided that you apply it correctly. Correct application is possible only when you understand the type of circuit you are dealing with. Another important point is to use the one which is going to take less time and help you calculate easily. All of the three equations tell the same thing but their usage is subject to the type of problem you're trying to solve. Let us understand when, how an what to use while calculating power in an electrical circuit.
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Can metering current transformer(CT) be used as Protection CT?

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Current Transformer is an important instrument used in power stations and Sub-stations for stepping down the line current to a lower level. These are used mainly for metering or measurement purpose and for protection purpose. Lower current levels allow us to use low range ammeters to safely measure the respective current in the line. On the other hand, there are current transformers used for actuating Relay which in turn activates circuit breakers to separate the faulty line from the system. These type of CTs are called Protection type CT. Now, can we use a protection type CT for metering purpose and vice-versa? If not then why? Let's understand...

 

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