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7SR157 Technical Manual
Section 3: Voltage Monitoring Functions
3.1 Voltage Levels (Live and Dead status)
The single phase voltage inputs can be connected to any phase to phase or phase to earth voltage transformer
winding connection. The voltage is compared to the corresponding voltage from the other side of the circuit
breaker. The live and dead voltage settings are set as a percentage of the nominal voltage setting (Line Nom
Voltage or Bus Nom Voltage).
The relay provides two options for determining the live or dead status of the Line and Bus voltage inputs. The
selection is made by changing the VT Level Mode in the VOLTAGE SETTING MENU. The two options are as follows:
3.1.1
VT Level Mode: 2 State
Voltage detectors determine the status of the line or bus. If the voltages on either the line or bus are below a set
threshold level (25 Dead Bus or 25 Dead Line) they can be considered to be 'dead'. If the voltages are within a
range, usually around the nominal voltage, determined by (25 Live Bus or 25 Live Line) settings, then they are
classed as 'live'. Independent voltage detectors are provided for both line and bus.
In this mode, if a voltage is initially at or below the dead setting then it will be classed as dead until it has
increased and reached the live setting. Similarly, if a voltage is live, it continues to be live until it has decreased
and reached the dead setting. This effectively allows for variable amounts of hysteresis to be set. Figure 3-1
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 it
will ramp up the software RMS algorithm and always pass through the dead zone first.
Although a wide range is provided for live and dead voltage detector elements, the user must ensure that the live
and dead zones do not overlap.
Figure 3-1 VT Level Mode: 2 State
Chapter 1 - Page 20 of 38
© 2022 Siemens Protection Devices Limited
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