According to Article 3(2)(37) of the Network Code on System Operation, emergency state means the system state in which one or more operational security limits are violated.

Other system states are:

- normal state, 

- blackout state, 

- alert tate,

- restoration.

The criteria to be used by the Transmission System Operator (TSO) when classifying the system state of its transmission system as emergency state are as follows:

i. there is at least one deviation from operational security limits and times defined according to the Network Code on System Operation and to the Emergency and Restoration Network Code (NC ER); or

ii. frequency is outside the frequency limits for the  ormal state and outside the frequency limits for the alert state as defined in Network Code on System Operation; or

iii. at least one measure of the system defence plan is activated; or

iv. there is a complete loss for more than 30 minutes of all tools and facilities which are required for system operation:

- facilities for monitoring the system state of the transmission system, including state estimation applications;

- means for controlling switching;

- means of communication with control centres of other TSOs;

- tools for operational security analysis. 

According to Article 40 of the NC ER each TSO, when in the emergency state, is entitled to gather the following information:

(a) from Distribution System Operators (DSOs) the necessary information about at least:

(i) the part of their network that is in island operation;

(ii) the ability to synchronise parts of their network that is in island operation; and

(iii) the capability to start island operation;

(b) from significant grid users (SGUs) and restoration service providers, information about at least the following conditions:

(i) the current status of the installation;

(ii) the operational limits;

(iii) the full activation time and the time to increase generation; and

(iv) the time critical processes.

Network Code on System Operation, Article 3(2)(37), Article 18(3)

Emergency and Restoration Network Code (NC ER), Article 40