Operational security is the transmission system's capability to retain a normal state or to return to a normal state as soon as possible, and which is characterised by operational security limits (Article 3(1) of the Network Code on System Operation). 

Where the level of reactive power ancillary services is not sufficient for maintaining operational security, Transmission System Operators (TSOs) prepare and activate remedial actions (Article 109(3) of the Network Code on System Operation). 

In such a case the TSO informs the neighbouring TSOs.

According to the Network Code on Network Operation each TSO is required to publish the levels of reserve capacity necessary to maintain operational security.

For each operational planning timeframe, each TSO is also required to assess, against their forecasts, whether its available reactive power ancillary services are sufficient to maintain the operational security of the transmission system.

Network Code on System Operation, Articles 3(1), 108, 109

ACER Public Consultation on the proposals of the Electricity Transmission System Operators for the methodologies for Coordinating Operational Security Analysis and Relevant Asset Outage Coordination as mandated by the EU Regulation establishing a guideline on electricity transmission system operation, 25 January 2019