What is CT and why we need small value for measurement and protection in electrical circuits ?

What is Current Transformer  ?

Current transformer is one type of instrument transformer which is useful for measuring AC current in any electrical equipment. Current transformer primary is connected in series with the circuit whose current is to be measured and it's secondary is connected to relay,  ammeter,  watt-meter,  etc. This current transformer measures current in primary by producing proportional current in secondary circuit.  All current transformer have particular ratio like 200/1, 400/5 etc upon it's requirement. So for ratio of 200/1, this current transformer will produce 1 ampere in secondary circuit if there is 200 ampere in primary circuit. 

Why we need small scale value for measuring the current in electrical circuits ?

Current transformer is converting high value of current to low value of current and we use low value of current for measuring and protection purpose. We need low value of current because If we will use large value current for measurement and protection, then the instruments like ammeter, wattmeter and relay rating will have to be larger to bear the high current and the  costing of this devices will also be high. Large currents are also not easy to handle without circuit breakers.