7SR210 & 7SR220 Description of OperationUnrestricted ©2018 Siemens Protection Devices Limited Page 71 of 94Phase Difference &Phase Decreasing ?SLIPLine U/ VBlockBus U/ VBLockV BlockD&COZ Slip Frequency SettingBlockBlockBlock&CB Close TimeF AngleSlip Within Range ?COZClose&(> Split Slip & < COZ slip)Split Slip SettingLive LineLive BusFigure 4.9.8-1 Close On Zero FunctionClose on Zero will not be accurate if slow CB times are applied in conjunction with fast slip rates during testing.Practical application limits are shown below: -00.20.40.60.811.21.41.61.820 200 400 600 800CB Close Time (ms)Maximum Slip Frequency (Hz)Figure 4.9.8-2 Close On Zero Timing4.10 Live/Dead IndicationOutputs are provided to identify each voltage input as either Live or Dead using voltage user settings. Theseoutputs are controlled independently of the 25 Check Synchronising Voltage monitoring elements but operate inthe same way.Voltage detectors are provided for each phase input A, B, C & X and inputs Y & Z when fitted. If the voltage onthe respective input is below a set threshold level they can be considered to be ‘dead’. If the voltages are above avoltage setting they are classed as ‘live’. Independent voltage detectors are provided for both line and bus.If a voltage is in the dead band range then it will be classed as dead until it has reached the live band area.Similarly, if a voltage is live, it continues to be live until it has reached the dead band area. This effectively allowsfor variable amounts of hysteresis to be set. Figure 4.10-1 illustrates the voltage detector operation.Note: the area between the dead and live zones is not indeterminate. When any voltage is applied to the relay itwill ramp up the software RMS algorithm and always pass through the dead zone first.A wide range is provided for live and dead voltage detector levels but the live and dead zones must not overlap.
