Current Transformer Thermal Rating Factor is a number by which the primary load current of CT may be increased over its nameplate rating without exceeding the allowable temperature rise. Thus thermal rating factor is nothing but the overload capacity of current transformer.

Normally thermal rating factor is specified in conjunction with the ambient temperature. The standard ambient reference levels are at 30°C or 55°C. In the manufacturer’s detail, a typical statement for Current Transformer Thermal Rating Factor would be like RF 2.0 at 30°C ambient with RF 1.5 at 55°C ambient . These statements mean that in a 30°C ambient, the CT will safely carry a continuous current of 2 times the nameplate rating and at 55°C ambient, it will carry 1.5 times the nameplate rating.

Thus it is very important to consider the ambient temperature while loading a CT beyond its name plate rating.

Most of the time Manufacturer only specifies Current Transformer Thermal Rating Factor for one ambient temperature like 4.0 at 30°C ambient. Now suppose we want to overload this CT (say of current rating 400 / 5 A) beyond its rated capacity. Therefore, the first step shall be to determine the maximum temperature in the enclosure of CT. Let this maximum temperature be 55°C. It will be wise decision to consider this maximum temperature as the ambient temperature for calculating the Current Transformer Thermal Rating Factor as this will provide extra safety margin. Well, for calculating the Current Transformer Thermal Rating Factor at this new ambient temperature of 55°C, we will use simple linear equation given below.

(CTTRF at New Ambient Temp. / CTTRF at 30°C Ambient)^{2 }

= (85 – New Ambient Temp.) / 55

Where CTTRF = Current Transformer Thermal Rating Factor

Thus,

(CTTRF at New Ambient Temp. / 4)^{2 }

= (85 – 55) / 55

= 30/55

So,

(CTTRF at New Ambient Temp.)^{2} = (16×30) / 55

= 8.72

Thus, the new Current Transformer Thermal Rating Factor at ambient temp. of 55°C will be 2.95.

Therefore, where the 400 ampere CT could carry (400×4.0) 1600 A primary at 30°C ambient without exceeding the manufacturer’s recommended transformer thermal rating, it can safely carry only 2.95 x 400 at 55°C.

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