Motor protection for low-power current transformer

Motor protection for low-power current transformer – SIPROTEC 7SK81


The SIPROTEC 7SK81 provides 4 low-power current transformer inputs and optionally 3 low-power voltage transformer inputs. With the same low-power current transformer (LPCT) a wide range of primary rated line currents can be covered. Objects with rated currents in the range of 20 A to 2500 A can be protected when using low-power current transformers.

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Features & functions

Motor protection for low-power current transformer SIPROTEC 7SK81


The following low-power current transformer ratios are suitable for the following primary current operating ranges: 

300 A /225 mV for a primary operating current range of 60 A to 630 A 

600 A /225 mV for a primary operating current range of 120 A to 1250 A 

1250 A /225 mV for a primary operating current range of 250 A to 2500 A 

100 A /225 mV for a primary operating current range of 20 A to 200 A. 


The SIPROTEC 7SK81 is a multi-functional motor protection relay. It is designed for protection of asynchronous motors of all sizes. The relays have all the required functions to be applied as a backup relay to a transformer differential relay. 

The relay provides numerous functions to respond flexibly to the system requirements and to deploy the invested capital economically. Examples for this are: exchangeable interfaces, flexible protection functions and the integrated automation level (CFC). Freely assignable LEDs and a six-line display ensure a unique and clear display of the process states. In combination with up to 9 function keys, the operating personnel can react quickly and safely in any situation. This guarantees a high operational reliability.



  • 4 current inputs
  • 0/3 voltage inputs
  • 3/7 binary inputs (thresholds configurable using software)
  • 5/8 binary outputs (2 changeover)
  • 0/5 RTD inputs
  • 1 life contact
  • Pluggable voltage terminals.


  • Instantaneous and definite time-overcurrent protection (phase/neutral) (50, 50N, 51, 51N)
  • Directional time-overcurrent protection, ground (67N)
  • Directional overcurrent protection, ground (definite/inverse) (67Ns, 50Ns))
  • Displacement voltage, zero-sequence voltage (59N)
  • Trip-circuit supervision (38)
  • Undercurrent monitoring (37)
  • Temperature monitoring (38)
  • Thermal overload protection (49)
  • Load jam protection (51M)
  • Locked rotor protection (14)
  • Restart inhibit (66/86)
  • Undervoltage/overvoltage protection (27/59)
  • Forward-power, reverse-power protection (32)
  • Power factor (55)
  • Overfrequency/underfrequency protection(81O/U)
  • Breaker failure protection (50B/F)
  • Phase-balance current protection (negative-sequence protection) (46)
  • Unbalance-voltage protection and/or phase-sequence monitor-ing (47)
  • Start-time supervision (48)
  • Lockout (86)
  • Rate-of-frequency-change protection (81R)


Motor protection 

The SIPROTEC 7SK81 device is specifically designed to protect induction-type asynchronous motors.


  • Inputs for low power VTs and CTs according IEC 61869-6 (formerly IEC 60044-7 and IEC 60044-8)
  • Removable terminal blocks
  • Binary input thresholds settable using DIGSI (3 stages)
  • 9 programmable function keys
  • 6-line display
  • Buffer battery exchangeable from the front
  • USB front port
  • 2 additional communication ports
  • Integrated switch for low-cost and redundant optical Ethernet rings
  • Ethernet redundancy protocol RSTP for highest availability
  • Relay-to-relay communication through Ethernet with IEC 61850 GOOSE
  • Millisecond-accurate time synchronization through Ethernet with SNTP.