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The calculation procedure is detailed in document 'Technical Guidance Note (TGN) High Impedance
Restricted Earth Fault Protection'. Note that the required stability level shall be taken as 1.3 x motor start
current.
•
Connect an auxiliary contact from the CB or contactor controlling the motor to a relay binary input (BI).
This BI can be used to apply in an alternative Setting Group (e.g. Group 2) for the set drop off delay of
the BI. Setting Group 2 can include increased earth fault pickup and/or time delay settings.
•
Add a time delay to the earth fault operation.
2.6
High Impedance Restricted Earth Fault Protection(87REF)
Restricted Earth Fault (REF) protection is applied to motors to detect low level earth faults in the stator windings.
Current transformers are located as shown in Figure 2.6-1. During normal operation no current will flow in the
relay element. When an internal earth fault occurs, the currents in the CTs will not balance and the resulting
unbalance flows through the relay.
The REF function is configured to provide an instantaneous trip output to minimise damage from developing
winding faults.
The calculation for the value of the Stabilising Resistor (Rstab) is based on the worst case where for the
maximum current one CT fully saturates and the other balancing CT does not saturate at all.
The required fault setting (primary operate current) of the protection is chosen; typically, this is less than 10% of
the motor rated current.
Additional external Non-Linear Resistor and stabilising resistor components are required.
See separate publication 'Technical Guidance Note (TGN) High Impedance Restricted Earth Fault Protection'.
2.7
Voltage Protection (27/59)
2.7.1
Under Voltage
Power system under-voltages may last for only a few cycles or continue on a steady-state basis, they can occur
due to system faults, an increase in system loading or loss of supply e.g. loss of an incoming transformer.
The motor does not need to be disconnected from the supply for short voltage dips, generally motor running will
recover when the voltage is restored.
The motor may stall when subjected to prolonged undervoltage conditions. The motor should be disconnected
from the supply and planned staged restoration of motors can be implemented when the supply is re-established.
The undervoltage trip is initiated by a voltage element operating after a definite time delay. The voltage and time
delay are set to coordinate with the system and motor undervoltage withstand. The applied settings must take into
account:
Prior to motor starting the relay may not detect voltage, depending on the VT connection. Under voltage
protection can be inhibited using the Gn 27/59 under voltage guard feature.
In service voltage dips are likely to occur during transient faults and starting of motors. The time delay
will consider the voltage dip duration for which re-acceleration is possible.
Motor starting can result in voltage depression to 80% of nominal, the voltage setting is likely to be below
this value.
©2018 Siemens Protection Devices
Motor
A
B
C
NLR
Figure 2.6-1 Restricted Earth-fault protection
7SR105 Rho and 7SR17 Rho Applications Guide
Rstab
7SR17
Chapter 7 Page 19 of 31
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