Rail measuring transformers, convert high mains currents to standardized measuring currents, current transformers avoid high costs when connecting and laying busbars, recommended from 40 A, accuracy classes: Class 0.5: for precise measurements and calibrated kWh meters, Class 1: for general measurements and uncalibrated kWh meters, Class 3: for rough measurements, relays and for protection, if the primary cable is wrapped several times around the current transformer, you get half the primary current with each wrap, whereby the power and class remain unchanged