What is “Self-Power”?

The concept “Self-Power” defines the supplying mode of electronic protection relays for Medium Voltage.
It means that there is no need for auxiliary voltage to power the relay and that the energy is obtained directly from the line that we are protecting.

This supplying mode means that the maintenance of transformer and distribution substations is massively reducing. All batteries, chargers, and other external power elements are made redundant.

How do we obtain power from the line?

The energy is obtained through current transformers installed in the line we want to protect.

Two options are respecting to the transformers:

  • Standard transformers /1 or /5
  • Specific transformers: in this case, the secondary current is not standard, and they provide some particular feature respect the standard Cts like a higher saturation level, or a more comprehensive measurement range.

SF6 Circuit breakers and self-powered relays are substituting in RMU’s based on switches with three fuses, but… Why?

Advantages of Self-powered Relays vs. Fuses

  • Low current faults: Fuses only can break high current faults. Circuit breakers with self-powered relays can detect and clear low current faults avoiding the destruction of the MV/LV transformers.
  • Protection Selectivity: The discrimination with fuse curves becomes difficult. It is possible to choose the right curve in self-powered relays to ensure coordination between MV and LV protection.
  • Inrush current: Transformer energizing produces high transient inrush current. This is a natural phenomenon, and the protection should not operate. Fuses are not able to distinguish this current, and the trip cannot be blocked. A circuit breaker with a self-powered relay allows greater flexibility to avoid the trip.
  • Temperature: Fuses are weak into a harsh environment, and the different temperatures can provoke two situation:
    • High temperature: Undesired trips.
    • Low temperature: The trip does not happen; instead, the fault current is high enough.
  • Saving Costs: Circuit-breaker with self-power reduces costs for two reasons:
    • Self-powered relays are maintenance-free.
    • There is no need to keep large stocks of fuses.

Which opening mechanism will be used with self powered relays?

Self powered Relays do power themselves from the primary current and the energy is scarce, so an efficient management of this energy is indispensable.

Due to this situation using coils is not viable because they have a very high electrical consumption. The solution is to use a striker.

But what is a striker?

It’s a bistable device with a simple action. It consists on an electromagnet that is charged when the breaker is closed (To trip, it required low energy, 100 millijoules approximately) The striker shaft is moved by a spring. And activated by a polarized low power electrical signal, supplied by the relay if a fault occurs.

Could a Self-powered Relay be used if the opening mechanism is a coil instead of a striker?

Yes, it is possible to use a Self-powered Relays if the installed opening mechanism is a coil, but it is necessary to install an external device with the relay.
Fanox offers amongst its products catalogue the TCM adapter. This device is connected to the trip contact of the relay and provides the required energy to trip a coil.

How does TCM work?

TCM is an energy accumulator that can be charged, through its auxiliary supply terminals, before the installation or once the device is installed, if in the installation auxiliary supply is available. The charging time is 10 seconds approximately and once the power supply is lost the TCM has three days of autonomy.

Main advantages over other brands:

  • The relays are self-powered by the current measured by the CTs fitted on the lines. The MAIN ADVANTAGE comparing with other self-powered relays in the market is that Fanox relays do not require internal batteries. This supplying mode implies that the maintenance of transformer and distribution substations is massively reducing
  • High electromagnetic compatibility makes FANOX relays the safest in the market. KEMA certification proves it.
  • Five years warranty al least.
  • If the installation use Standard CTs /1A or /5A can save money due to specific CTs are not required.
  • Fanox self-powered relays can trip all the strikers in the market. Thanks to a setting that allows the user to select the required voltage by the striker.
  • LOCAL AND REMOTE communication.
  • Very intuitive menu, straightforward to set.
  • Our flexible design offers solutions for all the applications worldwide: coils, strikers, dual-powered installations, etc.

Save money using ONE SELF-POWERED Relay instead of TWO separate devices

RMU’s protected with the most innovative technology

One single relay with different settings for operation and maintenance tasks, is becoming a mandatory requirement nowadays. Not only does It speed up the process, but it also complies with severer safety procedures and to optimizes the size of the RMU’s.

Our Self powered Range will allow the user to manage different applications with one device. Provided with four settings groups, it provides the most reliable protection solution, to facilitate the settings process and to offer the best protection even when auxiliary supply is not available.

No one in the market gives more quality and specifications with such expertise.

Besides, all models can be powered from an external battery to facilitate the commissioning of the centers (the settings and configuration procedure can be carried out without installing the relay) to manage the incidents that may occur and also to maintain the devices in adverse conditions.

Related Products


The SIA-B Self-powered Protection Relay is a Secondary Distribution Protection prepared to work with RMUs, MRMUs, and all SF6 insulated Switchgears in the market for MV applications. It is impeccably suited for distributed renewable generation operators such as solar parks and wind turbines.

SIA-B Protection Relay with specific CT’s, provides a comprehensive range of protection functions in amazingly compact and lightweight housing, preventative maintenance, and remarkable cost savings.

Remote communications RS485 Modbus RTU

  • Self-Powered & Dual-Powered (Self+Vaux)
  • Low consumption
  • Wide protection range through the Sensors
  • Load data profiling
  • Disturbance fault recording
  • With internal commissioning battery as optional

Ansi Code Functions
50, 51, 50N, 51N, SHB, 49T, 49, TB, PGC

Additional Functions



SIA-B is the most reliable dual powered (Self-Powered + Universal Vaux) protection relay offering a high performance previously available only in relays with Vaux:

Remote communications for SCADA (Modbus or DNP3.0 protocols)

  • Up to 10 COMTRADE records
  • Advanced protection functions
  • Completely configurable I/O thanks to advanced logic

Ultra-fast SOTF
Very low start-up levels and minimum consumption <2 VA
Compact design

Ansi Code Functions
50, 51, 50G, 51G, 49, 46, SHB, 49T, TB, 52, 50BF, CLP, 68, PGC

Additional Functions



Counting with extensive experience of successfully operating devices in the field worldwide and thanks to these unique advantages, SIA-C enjoys top recognition among users all around the planet.

Above all, the different mechanics available by model list make our SIA-C the best choice for our customers, covering all the needs of the market

Remote communications RS485 Modbus RTU or IEC 60870-5-103

  • Self-Powered & Dual-Powered (Self+Vaux)
  • Standard CTs /1 or /5
  • Low start-up levels
  • Load data profiling
  • Self-diagnosis (watchdog)
  • Non-volatile RAM memory (up to 1.024 events and 20 fault report), maintaining date & time even in SP mode, thanks to its internal RTC
  • With internal commissioning battery as optional
  • Withdrawable model as optional

Ansi Code Functions
50, 51, 49T, 51G, 50G, 46, 46BC, 49, SHB, CLP, 79, 52, 50BF, 74TCS, 60CTS, 68, PGC

Additional Functions