The monitoring of electric power consumption has become a key element for managing electrical installations in industrial and commercial sectors, such as manufacturing facilities, data centers, food processing industries, retail outlets, hospitals or educational establishments. Three years ago LEM introduced a system called Wi-LEM onto the market which is based on wireless sub-metering components, the EMN, enabling measurement of electricity segmented by activity (lighting, HVAC, motors, heating, etc.). Initially, the range seemed to be sufficiently comprehensive, with a metering capacity of up to 100 A. However, it was soon found to be too limited for the industrial or for the heavy-duty service domains and this was without taking into account the fact that monitoring often begins by measuring global consumption at the point of energy input — requiring a capability to measure up to 2000 A. LEM developed the RT current sensor adapted to these EMN devices, which provides the same flexibility of installation as split-core current transformers from the lower range, but with the same class 1 precision required for the sub-metering field.

The Rogowski coil, which has long been noted for its ease of installation, offered the right solution provided that its major drawback could be overcome — that of inaccuracy caused by the sensitivity to the position of the conductor inside the loop. From theory to practice a simple explanation of the Rogowski coil theory (“Die Messung der magnetischen Spannung”, Archiv für Elektrotechnik, 1912), is that it is a coil-winding that closes back on itself, wrapping the conductor to be measured like any toroidal-type current intensity transformer, the only — but major— difference being that there is no magnetic core.

While Ampère’s theorem still applies, the equations are slightly different because at the sensor output we find that the voltage is in proportion, not to the primary current, but rather to its derivative: U = M*di/dt. M is the mutual inductance between the primary conductor and the coil, which to some extent represents the coupling between the primary and secondary circuits. All the difficulty in obtaining good accuracy from this principle derives from the fact that the simplified analytical expression of this equation implicitly supposes perfect symmetry of the coil (M must be constant). However, this is never the case in practice, and we shall illustrate this by looking at the three critical points that cause M to be variable…

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