May 04, 2010
Event Categories and Levels of Analysis
Each event is categorized to help NERC, regional entity Event Analysis (EA) staff, and
registered entities to determine an appropriate level of analysis or review.
Events initiated by natural phenomena such as earthquakes, tornadoes, hurricanes, ice storms,
etc., will not be specifically analyzed although the resulting outages are encompassed by the EA
categories. These natural phenomena events will be reviewed to determine if there were any
unexpected system behaviors that may warrant a specific EA.
Event Analysis Levels (Refer to the EAWG processes, procedures and
templates for details)
NERC staffs and the affected regional entity collaboratively determine the appropriate level of
any Event Analysis that should be conducted. The following provides a summary of the various
reports that are required per the EAWG process:

Basic Report – prepared by impacted entities, sent to NERC and the affected
regional entity. The report may come from a variety of sources such as an RCIS
message, EOP-004, OE-417, and/or ES-ISAC report.

Summary Report – prepared by impacted entities, sent to the regional entity and
NERC

Detailed Report – prepared by an EA Team including membership from the
impacted entities, affected regional entities and NERC staff. Most single-region
analyses are facilitated and closed by the regions. Multi-regional category four
events may be facilitated and closed by a region or NERC.
Typically, a Basic report will be prepared for all categories by the impacted entity. In addition,
the following reports will be developed based on the event category levels. Typically, the
following analysis levels are performed:

Category 1 - Basic Report (normally there is no follow-up anticipated for category 1
reports)

Category 2 - Summary Report

Category 3 - Summary Report and follow-up as directed by the regional entity

Category 4 – Detailed Report developed by a regional entity or NERC led event
analysis team

Category 5 – Detailed Report developed by NERC led event analysis team
May 04, 2010
Operating Reliability Events 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 5 categories that
account for their different system impact and help to determine the level of analysis that is
warranted1.
Category 1
a.
b.
c.
d.
e.
f.
g.
h.
i.
j.
k.
An event resulting in one or more of the following:
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)
Loss of multiple BES transmission components. (Simultaneous or common mode
event)
Frequency below the Low Frequency Trigger Limit (FTL) more than 5 minutes.
Frequency above the High FTL more than 5 minutes.
Intended and controlled system separation by proper Special Protection System
Scheme (SPS) / Remedial Action Scheme (RAS) of action of Alberta from the
Western Interconnection, New Brunswick or Florida from the Eastern
Interconnection.
Proper SPS/RAS actuation resulting in load loss of 100 MW or more.
SPS/RAS misoperation.
Unintended loss of an entire dc converter station
System wide voltage reduction of 3% or more
Public appeal to reduce the use of electricity for purposes of maintaining the
continuity of the BES
Fuel supply emergencies that could impact BES adequacy or reliability
Category 2: An event resulting in one or more of the following:
a.
b.
c.
d.
e.
f.
g.
1
2
Loss of BES control center voice or data functionality for 30 minutes or more.
Unplanned evacuation from BES control center facility
Voltage excursions equal to or greater than ±10% lasting more than 5 minutes
The loss of an entire switching station (4 or more line terminals, 100 kV or above).
The loss of an entire generation station of 5 or more generators (aggregate generation
of 500 MW or higher).
Loss of off-site power (LOOP) to a nuclear generating station.
UFLS, UVLS, or manual load shedding resulting in a loss of load of 100 MW or
greater.
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.
May 04, 2010
h.
i.
j.
k.
The planned automatic rejection of generation through SPS/RAS of 500 MW to
3,000 MW in the Eastern Interconnection or Western Interconnection, or 500 MW to
1,500 MW in Texas, and Québec Interconnections.
Unintended system separation resulting in an island of a combination of load and
generation of 100 MW to 4,999 MW
Equipment failures or SPS/RAS actions which result in the loss of firm system
demands for more than 15 minutes, as described below:
a. 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.
b. All other entities are required to report all such losses of firm demands totaling
more than 200 MW or 50% of the total customers being supplied immediately
prior to the incident, whichever is less.
An Interconnection Reliability Operating Limit (IROL) violation as defined in
reliability standard TOP-007.
Category 3: An event resulting in one or more of the following:
a.
b.
c.
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 Texas or Québec Interconnections.
Unintended system separation resulting in an island of a combination of load and
generation of 5,000 MW to 10,000 MW.
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.
b.
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).
Unintended system separation resulting in an island of a combination of load and
generation 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.
c.
The loss of load of 10,000 MW or more.
The loss of generation of 10,000 MW or more.