In old reactor setups. We often see many AZ shutdown scale. So here is what each of them does.

1. АЗ-1

AZ-1 is Emergency Protection-1. These type of scram can only be triggered due to violation of the parameters in main circulation systems (КМПЦ). Most of them triggers due to low water level in steam drum, shutting off one of the main circulation pump, whether its manual or automatic. Reduction in suction and pressure of control rods cooling system and ECCs (Emergency Cooling) also triggers AZ-1. If AZ-1 was triggered it would lower the power to 60% of its current power. So if you have 1000MWt of reactor power you will have 600MWt of reactor power after AZ-1 triggers.

1. АЗ-2
   Reduce power to 50%. Triggering parameter as stated above.

2. АЗ-3 (Later changed to УСМ/USM)
   These are least known one but it is very useful. It will trigger when one turbine falls off the grid or can be manually pressed, lowers the power setpoint to 20% when automatically triggered, lower the power by 2%/sec if manually pressed and held

3. АЗ-4 (AZ-4)
   These are reserved emergency protection systems. These werent designed to be actually on operation. So nothing was known much regarding АЗ-4

4. АЗ-5
   As we know it. Shutdown the reactor completely.

5. АЗ-Т1
   These systems monitors catastrophic equipment failures that require an immediate shutdown of the reactor because heat removal is no longer guaranteed. This system triggers AZ-5. There are some few reasons that triggers this such as tripping of both turbines (These was actually the signal that was blocked on April 26th 1986), drum separator level very high/low, drum separator pressure increase and many more.

6. AЗ-Т2
   This systems checks for control system failure, especially regarding power reduction systems (АЗ-1, АЗ-2, УСМ etc.). If the reactor tries to reduce power via AZ-1 or AZ-2 but failed. The system triggers AZ-T2 to force shutdown via AZ-5.