Overcurrent protection for low-power current transformer

Overcurrent protection for low-power current transformer – SIPROTEC 7SJ81


The SIPROTEC 7SJ81 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

Overcurrent protection for low-power current transformer SIPROTEC 7SJ81


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

The SIPROTEC 7SJ81 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/Form C contacts)
  • 1 life contact
  • Pluggable voltage terminals


  • Instantaneous and definite time-overcurrent protection (phase/neutral) (50, 50N, 51, 51N)
  • Directional time-overcurrent protection (phase/ground) (67, 67N)
  • Directional sensitive ground fault detection (67Ns, 50Ns)
  • Overvoltage protection, zero-sequence system (59N)
  • Inrush restraint
  • Trip-circuit supervision (74TC)
  • Undercurrent monitoring (37)
  • Overload protection (49)
  • Undervoltage/overvoltage protection (27/59)
  • Forward-power, reverse-power protection (32)
  • Power factor (55)
  • Overfrequency/underfrequency protection(81O/U)
  • Circuit-breaker failure protection (50B/F)
  • Intermittent ground fault protection
  • Directional intermittent ground fault protection (67Ns)
  • Automatic reclosing (79)
  • Fault locator (FL)
  • Lockout (86)


Line protection

The SIPROTEC 7SJ81 units can be used for line protection of high and medium-voltage networks with grounded, low-resistance grounded, isolated or a compensated neutral point.


  • Inputs for Low power CTs and VTs 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 protocols RSTP, PRP and HSR for highest availability
  • Relay-to-relay communication through Ethernet with IEC 61850 GOOSE
  • Millisecond-accurate time synchronization through Ethernet with SNTP.