What is “inrush current”?
Inrush Current or input surge current is the maximum instantaneous current drawn by electrical devices (transformers, motors etc) when first turned on. This inrush current only occurs during the first few cycles after turning on, and will have greatly reduced after about 80mS (four cycles). An inrush current as high as 16 x full load primary full-load current would be a peak value that would only occur for about half of one cycle at switch on. A multiple of 10 x full load primary current would be a realistic maximum RMS current over the first few cycles. An example would be a three phase 50KVA transformer with a 415v primary, which has a full-load primary current of 70A. Inrush current = 10 x 70A = 700A over the first few cycles. Cables, circuit breakers and associated devices must be capable of withstanding these high inrush currents.
Inrush is a transient characteristic of a transformer’s magnetizing current. The steel core allows the flux to support the applied voltage with a relatively low “exciting current”. Depending on the magnetic state of the core and where on the voltage waveform the transformer is switched in at, super-saturation can be achieved. Peak currents as high as 15 times rated can be experienced. The current is composed of the normal exciting and a large exponentially decaying D.C. component. Normal conditions are normally reached in less then 30 cycles.
