Calibrating current relay circuit using portable light transformer

Mobile light transformers are generally 220/12, 24, 36V, with a capacity of 0.5kVA, and are mainly used for temporary lighting (using low-voltage bulbs). As shown in the figure is the calibration current relay circuit. According to relevant regulations, the quick-break protection in the power system and the current relay used in over-current protection should be calibrated regularly every year. After connecting the wires according to the above circuit, close the knife switch QS and slowly adjust the voltage regulator TA so that the reading of the ammeter AP gradually increases to the value on the dial of the current relay KA. When KA acts, its contact KA connects the indicator light H circuit, and H lights up. At this time, the current value indicated by AP is the "action current value" of KA; then slowly adjust TA to reduce the reading of ammeter AP. When H goes out, the current value of AP is the "return current value". Dividing the "return current value" by the "action current value" is the "return coefficient" of the current relay KA. A value between 0.85 and 0.9 is considered qualified. T is the portable light transformer.