Appendix B — Event Categories and Levels of Analysis
The purpose of this document is to identify events that may require further analysis. Each event
is categorized to help the Regional Entities (REs) and registered entity(s) impacted determine the
appropriate level of analysis or review.
Event Analysis Levels
Impacted registered entity(ies), the associated RE and when appropriate, NERC, will
collaboratively determine the appropriate level of any event analysis that should be conducted.
The following provides a summary of the various reports that are expected per the EAWG
Event Report — prepared by impacted entities, sent to NERC and the affected RE. The
event report is a generic term and the actual notification may come from a variety of
sources such as, but not limited to EOP-004, OE-417, and/or ES-ISAC report.
Event Analysis Report — A report detailing what happened and why. Prepared by
either the impacted entity or the impacted entity in coordination with the event analysis
team as defined in the EA process.
The following will be used to determine the level of analysis to be conducted:
Category 1 — event report. (Normally there is no follow-up anticipated for category 1
reports unless requested by the RE).
Category 2 and 3 — An event report and an event analysis report prepared by the
registered entity and follow-up as directed by the RE.
Category 4 and 5 — An event report and an event analysis report developed by an event
analysis team led by the RE or NERC.
Operating Reliability Event Categories
Operating reliability events are those events that are deemed to have significantly impacted the
reliable operation of interconnected system. These events are divided into five (5) categories
that account for their different system impact and help determine the level of analysis that is
warranted1. The lists below are intended to provide examples as to the types of events that fall
into each category. For events not covered below, the impacted entity, in conjunction with the
RE, will determine the categorization.
Category 1: An event resulting in one or more of the following:
a. The loss of a bulk power transmission component(s) in response to a single-phase line-to-
ground fault with delayed clearing2 (stuck breaker or protection system failure). (Refer to
Table I. Transmission System Standards — Normal and Emergency Conditions of the
TPL standards categories C6 through C9).
The highest category that characterizes an event shall be used.
Delayed clearing of a fault is due to failure of a circuit breaker or of any protection system component such as a relay or current
transformer, and not because of an intentional design delay.
b. Loss of three or more bulk power system (BPS) elements (i.e. generators, transmission
lines, and buses).
c. Frequency below the Low Frequency Trigger Limit (FTL) or above the High FTL for
more than 15 minutes.3
d. Intended and controlled system separation by the proper operation of a Special Protection
System Scheme (SPS) / Remedial Action Scheme (RAS) in Alberta from the Western
Interconnection, New Brunswick or Florida from the Eastern Interconnection.
e. Failure, degradation or misoperation of SPS/RAS.
f. Unintended loss of an entire dc converter station
g. System wide voltage reduction of 3 percent or more
h. Public appeal to reduce the use of electricity for purposes of maintaining the continuity of
i. Fuel supply emergencies that could impact BPS adequacy or reliability
j. Unintended system separation resulting in an island of up to 1,000 MW.
Category 2: An event resulting in one or more of the following:
a. Loss of all BPS control center voice or data functionality for 30 minutes or more.
b. Unplanned evacuation from BPS control center facility
c. Voltage excursions equal to or greater than ±10 percent lasting more than five (5)
d. The loss of an entire generation station of five (5) or more generators (aggregate
generation of 500 MW or higher).
e. Loss of off-site power (LOOP) to a nuclear generating station.
f. Load shedding resulting in a loss of load of 100 MW or greater as a result of:
1. Manual load shedding;
2. Underfrequency load shedding; or
3. Undervoltage load shedding.
g. Unintended system separation resulting in an island of 1,001 MW to 4,999 MW.
h. Equipment failures or SPS/RAS actions which result in the loss of firm system demands
for more than 15 minutes, as described below:
1. Entities with a previous year recorded peak demand of more than 3,000 MW are
required to report all such losses of firm demands totaling more than 300 MW.
2. All other entities are required to report all such losses of firm demands totaling more
than 200 MW or 50 percent of the total customers being supplied immediately prior
to the incident, whichever is less.
i. An Interconnection Reliability Operating Limit (IROL) violation as defined in reliability
In case of an FTL exceedence, the reporting of the event occurs automatically through the NERC ACE and
Frequency Monitoring System. Analysis of these events will normally be conducted by the NERC Operating
Committee’s Resources Subcommittee.
Category 3: An event resulting in one or more of the following:
a. The loss of load or generation (excluding SPS/RAS, UFLS, or UVLS actuation) of 2,000
MW or more in the Eastern Interconnection or Western Interconnection, or 1,000 MW or
more in the ERCOT or Québec Interconnections.
b. Unintended system separation resulting in an island of 5,000 MW to 10,000 MW.
c. Unintended system separation resulting in an island of Alberta from the Western
Interconnection, New Brunswick, or Florida from the Eastern Interconnection.
Category 4: An event resulting in one or more of the following:
a. The loss of load or generation from 5,001 MW to 9,999 MW (excluding SPS/RAS as
noted in Category 2, UFLS, or UVLS actuation).
b. Unintended system separation resulting in an island of more than 10,000 MW (with the
exception of Alberta, New Brunswick, and Florida as described in category 3C above).
Category 5: An event resulting in one or more of the following:
a. The loss of load of 10,000 MW or more.
b. The loss of generation of 10, 000 MW or more.