CZAZ-SZ is a freely programmably device. Three independent sets of protection algorithms (bus-system 1, bus-system 2, sum) constantly monitor balance of incoming and otugoing currents in substation. Each of algorithm consist current and phase-comparision element. Operate signal is generated by AND „two of two” logic. That means, that both of algoritms („bus” and „sum”) must be simultanously activated in order to generate the trip signal. In some cases when the bus coupler bay is unequipped with CT, the sequential operation mode could be applied. I.e. when „sum” algoritm operates, the bus coupler CB open first. Next CB which has to be opened is selected by bus algorythms of section 1 and 2. In case of detection 5% deviation in current inputs, the device operation is blocked after 5 sec delay. Currents unbalanced condition when current values are not significantly higher than the rated values should be interpreted as improper connection of the device with substation secondary circuits. The device shows occurance of failure by set of RGD LED diodes and 7” LCD screen. All signals can be grouped to one ALARM or Failure signal. 7” LCD touchscreen allows to monitor switchgear breakers, settings change, preview of inputs and outputs statuses, language change, preview of time, event recorder or measured values.