Installation and demountling of the OFI is carried out on an overhead power line without removing the voltage. The indicator is attached to the mounting cup, which is screwed onto the operational insulating rod using a threaded tip.

Indicators are installed at the fork of the power line, before and after hard-to-reach sections, at the border of the balance sheet, as well as after a certain distance along long sections.

OFI distinguishes between the type of damage: short circuit or single-phase earth fault. In the event of a short circuit in the controlled section, the fault indicator responds to a current surge of more than 120 A, followed by a drop in current and voltage to zero. The operation of the connection switch is monitored by the current drop to zero. According to its principle of operation, the OFI does not work during load current surges, magnetizing current inrush, external short circuits. In the event of an external short circuit, the damaged section is switched off by an adjacent switch, the load current will continue to flow along the line, and the OFI will not operate. When installing OFI on a lightly loaded line (the load current of which does not exceed 3 A), a phase voltage measuring element is used to highlight the pre-emergency mode.

In addition to the differential, the device has a threshold current element. The operating range of the threshold current element is from 100 A to 650 A in steps of 10 A.

OFI distinguishes between permanent and intermitent fault. If the line is restored before the expiration of the time T_stab, OFI signals an intermitent fault, otherwise it indicates a permanent fault.

OFI is capable of detecting single-phase earth fault when a short circuit shunting device is installed at the supply substation. In the event of a single-phase earth fault, the short circuit shunting device briefly increases the current in the damaged phase by 30-50 A, which leads to the operation of the OFI.

OFI is powered by batteries with a long service life (batteries ensure the operation of the device in standby mode for up to 8 years). Each indicator is equipped with an internal battery charge diagnostic system. In case of battery discharge, the yellow LED alarm is activated. Replaceable indicator batteries are provided.

OFI includes a radio transmitter for transmitting information about the fact of operation and the type of damage to the control room. In this case, a transmitter is mounted on a support near the OFI installation, which, having received information from the OFI, retransmits it via the GSM communication channel. To save battery life of the OFI, the data transfer option can only be activated if transmitters are used.