Adopted by NERC Board of Trustees: November 13, 2008 Page 1 of 21 

Glossary of Terms Used in Reliability Standards 

Updated April 20, 2009 

Term Acronym Definition 

Adequacy The ability of the electric system to supply the aggregate electrical 

demand and energy requirements of the end-use customers at all times, 

taking into account scheduled and reasonably expected unscheduled 

outages of system elements. 

Adjacent Balancing Authority A Balancing Authority Area that is interconnected another Balancing 

Authority Area either directly or via a multi-party agreement or 

transmission tariff. 

Adverse Reliability Impact The impact of an event that results in frequency-related instability; 

unplanned tripping of load or generation; or uncontrolled separation or 

cascading outages that affects a widespread area of the Interconnection. 

After the Fact ATF A time classification assigned to an RFI when the submittal time is 

greater than one hour after the start time of the RFI. 

Agreement A contract or arrangement, either written or verbal and sometimes 

enforceable by law. 

Altitude Correction Factor A multiplier applied to specify distances, which adjusts the distances to 

account for the change in relative air density (RAD) due to altitude from 

the RAD used to determine the specified distance. Altitude correction 

factors apply to both minimum worker approach distances and to 

minimum vegetation clearance distances. 

Ancillary Service Those services that are necessary to support the transmission of 

capacity and energy from resources to loads while maintaining reliable 

operation of the Transmission Service Provider's transmission system in 

accordance with good utility practice. (From FERC order 888-A.) 

Anti-Aliasing Filter An analog filter installed at a metering point to remove the high 

frequency components of the signal over the AGC sample period. 

Area Control Error ACE The instantaneous difference between a Balancing Authority’s net actual 

and scheduled interchange, taking into account the effects of Frequency 

Bias and correction for meter error. 

Area Interchange 

Methodology 

The Area Interchange methodology is characterized by determination of 

incremental transfer capability via simulation, from which Total Transfer 

Capability (TTC) can be mathematically derived. Capacity Benefit 

Margin, Transmission Reliability Margin, and Existing Transmission 

Commitments are subtracted from the TTC, and Postbacks and 

counterflows are added, to derive Available Transfer Capability. Under 

the Area Interchange Methodology, TTC results are generally reported 

on an area to area basis. 

Arranged Interchange The state where the Interchange Authority has received the Interchange 

information (initial or revised). 

The newly approved terms 

are included in the shaded 

table rows below. 
Glossary of Terms Used in Reliability Standards 

Adopted by NERC Board of Trustees: November 13, 2008 Page 2 of 21 

Term Acronym Definition 

Automatic Generation Control AGC Equipment that automatically adjusts generation in a Balancing 

Authority Area from a central location to maintain the Balancing 

Authority’s interchange schedule plus Frequency Bias. AGC may also 

accommodate automatic inadvertent payback and time error correction. 

Available Flowgate Capability AFC A measure of the flow capability remaining on a Flowgate for further 

commercial activity over and above already committed uses. It is 

defined as TFC less Existing Transmission Commitments (ETC), less a 

Capacity Benefit Margin, less a Transmission Reliability Margin, plus 

Postbacks, and plus counterflows. 

Available Transfer Capability ATC A measure of the transfer capability remaining in the physical 

transmission network for further commercial activity over and above 

already committed uses. It is defined as Total Transfer Capability less 

Existing Transmission Commitments (including retail customer service), 

less a Capacity Benefit Margin, less a Transmission Reliability Margin, 

plus Postbacks, plus counterflows. 

Available Transfer Capability 

Implementation Document 

ATCID A document that describes the implementation of a methodology for 

calculating ATC or AFC, and provides information related to a 

Transmission Service Provider’s calculation of ATC or AFC. 

ATC Path Any combination of Point of Receipt and Point of Delivery for which ATC 

is calculated; and any Posted Path1. 

Balancing Authority BA The responsible entity that integrates resource plans ahead of time, 

maintains load-interchange-generation balance within a Balancing 

Authority Area, and supports Interconnection frequency in real time. 

Balancing Authority Area The collection of generation, transmission, and loads within the metered 

boundaries of the Balancing Authority. The Balancing Authority 

maintains load-resource balance within this area. 

Base Load The minimum amount of electric power delivered or required over a 

given period at a constant rate. 

Blackstart Capability Plan A documented procedure for a generating unit or station to go from a 

shutdown condition to an operating condition delivering electric power 

without assistance from the electric system. This procedure is only a 

portion of an overall system restoration plan. 

Block Dispatch A set of dispatch rules such that given a specific amount of load to 

serve, an approximate generation dispatch can be determined. To 

accomplish this, the capacity of a given generator is segmented into 

loadable “blocks,” each of which is grouped and ordered relative to other 

blocks (based on characteristics including, but not limited to, efficiency, 

run of river or fuel supply considerations, and/or “must-run” status). 

 

1

 See 18 CFR 37.6(b)(1) 
Glossary of Terms Used in Reliability Standards 

Adopted by NERC Board of Trustees: November 13, 2008 Page 3 of 21 

Term Acronym Definition 

Bulk Electric System As defined by the Regional Reliability Organization, the electrical 

generation resources, transmission lines, interconnections with 

neighboring systems, and associated equipment, generally operated at 

voltages of 100 kV or higher. Radial transmission facilities serving only 

load with one transmission source are generally not included in this 

definition. 

Burden Operation of the Bulk Electric System that violates or is expected to 

violate a System Operating Limit or Interconnection Reliability Operating 

Limit in the Interconnection, or that violates any other NERC, Regional 

Reliability Organization, or local operating reliability standards or 

criteria. 

Business Practices Those business rules contained in the Transmission Service Provider’s 

applicable tariff, rules, or procedures; associated Regional Reliability 

Organization or regional entity business practices; or NAESB Business 

Practices. 

Capacity Benefit Margin CBM The amount of firm transmission transfer capability preserved by the 

transmission provider for Load-Serving Entities (LSEs), whose loads are 

located on that Transmission Service Provider’s system, to enable access 

by the LSEs to generation from interconnected systems to meet 

generation reliability requirements. Preservation of CBM for an LSE 

allows that entity to reduce its installed generating capacity below that 

which may otherwise have been necessary without interconnections to 

meet its generation reliability requirements. The transmission transfer 

capability preserved as CBM is intended to be used by the LSE only in 

times of emergency generation deficiencies. 

Capacity Benefit Margin 

Implementation Document 

CBMID A document that describes the implementation of a Capacity Benefit 

Margin methodology. 

Capacity Emergency A capacity emergency exists when a Balancing Authority Area’s 

operating capacity, plus firm purchases from other systems, to the 

extent available or limited by transfer capability, is inadequate to meet 

its demand plus its regulating requirements. 

Cascading The uncontrolled successive loss of system elements triggered by an 

incident at any location. Cascading results in widespread electric service 

interruption that cannot be restrained from sequentially spreading 

beyond an area predetermined by studies. 
Glossary of Terms Used in Reliability Standards 

Adopted by NERC Board of Trustees: November 13, 2008 Page 4 of 21 

Term Acronym Definition 

Cascading Outages 

NOTE: On December 27, 

2007, the Federal Energy 

Regulatory Commission 

remanded the definition of 

“Cascading Outage” to 

NERC. On February 12, 

2008, the NERC Board of 

Trustees withdrew its 

November 1, 2006 

approval of that definition, 

without prejudice to the 

ongoing work of the FAC 

standards drafting team 

and the revised standards 

that are developed 

through the standards 

development process. 

Therefore, the definition is 

no longer in effect. 

The uncontrolled successive loss of Bulk Electric System Facilities 

triggered by an incident (or condition) at any location resulting in the 

interruption of electric service that cannot be restrained from spreading 

beyond a pre-determined area. 

Clock Hour The 60-minute period ending at :00. All surveys, measurements, and 

reports are based on Clock Hour periods unless specifically noted. 

Cogeneration Production of electricity from steam, heat, or other forms of energy 

produced as a by-product of another process. 

Compliance Monitor The entity that monitors, reviews, and ensures compliance of 

responsible entities with reliability standards. 

Confirmed Interchange The state where the Interchange Authority has verified the Arranged 

Interchange. 

Congestion Management 

Report 

 A report that the Interchange Distribution Calculator issues when a 

Reliability Coordinator initiates the Transmission Loading Relief 

procedure. This report identifies the transactions and native and 

network load curtailments that must be initiated to achieve the loading 

relief requested by the initiating Reliability Coordinator. 

Constrained Facility A transmission facility (line, transformer, breaker, etc.) that is 

approaching, is at, or is beyond its System Operating Limit or 

Interconnection Reliability Operating Limit. 

Contingency The unexpected failure or outage of a system component, such as a 

generator, transmission line, circuit breaker, switch or other electrical 

element. 

Contingency Reserve The provision of capacity deployed by the Balancing Authority to meet 

the Disturbance Control Standard (DCS) and other NERC and Regional 

Reliability Organization contingency requirements. 

Contract Path An agreed upon electrical path for the continuous flow of electrical power 

between the parties of an Interchange Transaction. 
Glossary of Terms Used in Reliability Standards 

Adopted by NERC Board of Trustees: November 13, 2008 Page 5 of 21 

Term Acronym Definition 

Control Performance 

Standard 

CPS The reliability standard that sets the limits of a Balancing Authority’s 

Area Control Error over a specified time period. 

Corrective Action Plan A list of actions and an associated timetable for implementation to 

remedy a specific problem. 

Cranking Path A portion of the electric system that can be isolated and then energized 

to deliver electric power from a generation source to enable the startup 

of one or more other generating units. 

Critical Assets Facilities, systems, and equipment which, if destroyed, degraded, or 

otherwise rendered unavailable, would affect the reliability or operability 

of the Bulk Electric System. 

Critical Cyber Assets Cyber Assets essential to the reliable operation of Critical Assets. 

Curtailment A reduction in the scheduled capacity or energy delivery of an 

Interchange Transaction. 

Curtailment Threshold The minimum Transfer Distribution Factor which, if exceeded, will 

subject an Interchange Transaction to curtailment to relieve a 

transmission facility constraint. 

Cyber Assets Programmable electronic devices and communication networks including 

hardware, software, and data. 

Cyber Security Incident Any malicious act or suspicious event that: 

 Compromises, or was an attempt to compromise, the Electronic 

Security Perimeter or Physical Security Perimeter of a Critical 

Cyber Asset, or, 

 Disrupts, or was an attempt to disrupt, the operation of a Critical 

Cyber Asset. 

Delayed Fault Clearing Fault clearing consistent with correct operation of a breaker failure 

protection system and its associated breakers, or of a backup protection 

system with an intentional time delay. 

Demand 1. The rate at which electric energy is delivered to or by a system or 

part of a system, generally expressed in kilowatts or megawatts, at a 

given instant or averaged over any designated interval of time. 

2. The rate at which energy is being used by the customer. 

Demand-Side Management DSM The term for all activities or programs undertaken by Load-Serving 

Entity or its customers to influence the amount or timing of electricity 

they use. 

Direct Control Load 

Management 

DCLM Demand-Side Management that is under the direct control of the system 

operator. DCLM may control the electric supply to individual appliances 

or equipment on customer premises. DCLM as defined here does not 

include Interruptible Demand. 
Glossary of Terms Used in Reliability Standards 

Adopted by NERC Board of Trustees: November 13, 2008 Page 6 of 21 

Term Acronym Definition 

Dispatch Order A set of dispatch rules such that given a specific amount of load to 

serve, an approximate generation dispatch can be determined. To 

accomplish this, each generator is ranked by priority. 

Dispersed Load by 

Substations 

 Substation load information configured to represent a system for power 

flow or system dynamics modeling purposes, or both. 

Distribution Factor DF The portion of an Interchange Transaction, typically expressed in per 

unit that flows across a transmission facility (Flowgate). 

Distribution Provider Provides and operates the “wires” between the transmission system and 

the end-use customer. For those end-use customers who are served at 

transmission voltages, the Transmission Owner also serves as the 

Distribution Provider. Thus, the Distribution Provider is not defined by a 

specific voltage, but rather as performing the Distribution function at any 

voltage. 

Disturbance 1. An unplanned event that produces an abnormal system condition. 

2. Any perturbation to the electric system. 

3. The unexpected change in ACE that is caused by the sudden failure 

of generation or interruption of load. 

Disturbance Control Standard DCS The reliability standard that sets the time limit following a Disturbance 

within which a Balancing Authority must return its Area Control Error to 

within a specified range. 

Disturbance Monitoring 

Equipment 

DME Devices capable of monitoring and recording system data pertaining to a 

Disturbance. Such devices include the following categories of 

recorders2

: 

 Sequence of event recorders which record equipment response 

to the event 

 Fault recorders, which record actual waveform data replicating 

the system primary voltages and currents. This may include 

protective relays. 

 Dynamic Disturbance Recorders (DDRs), which record incidents 

that portray power system behavior during dynamic events such 

as low-frequency (0.1 Hz – 3 Hz) oscillations and abnormal 

frequency or voltage excursions 

Dynamic Interchange 

Schedule or 

Dynamic Schedule 

 A telemetered reading or value that is updated in real time and used as 

a schedule in the AGC/ACE equation and the integrated value of which is 

treated as a schedule for interchange accounting purposes. Commonly 

used for scheduling jointly owned generation to or from another 

Balancing Authority Area. 

 

2

 Phasor Measurement Units and any other equipment that meets the functional requirements of DMEs may qualify 

as DMEs.
Glossary of Terms Used in Reliability Standards 

Adopted by NERC Board of Trustees: November 13, 2008 Page 7 of 21 

Term Acronym Definition 

Dynamic Transfer The provision of the real-time monitoring, telemetering, computer 

software, hardware, communications, engineering, energy accounting 

(including inadvertent interchange), and administration required to 

electronically move all or a portion of the real energy services associated 

with a generator or load out of one Balancing Authority Area into 

another. 

Economic Dispatch The allocation of demand to individual generating units on line to effect 

the most economical production of electricity. 

Electrical Energy The generation or use of electric power by a device over a period of 

time, expressed in kilowatthours (kWh), megawatthours (MWh), or 

gigawatthours (GWh). 

Electronic Security Perimeter The logical border surrounding a network to which Critical Cyber Assets 

are connected and for which access is controlled. 

Element Any electrical device with terminals that may be connected to other 

electrical devices such as a generator, transformer, circuit breaker, bus 

section, or transmission line. An element may be comprised of one or 

more components. 

Emergency or 

BES Emergency 

 Any abnormal system condition that requires automatic or immediate 

manual action to prevent or limit the failure of transmission facilities or 

generation supply that could adversely affect the reliability of the Bulk 

Electric System. 

Emergency Rating The rating as defined by the equipment owner that specifies the level of 

electrical loading or output, usually expressed in megawatts (MW) or 

Mvar or other appropriate units, that a system, facility, or element can 

support, produce, or withstand for a finite period. The rating assumes 

acceptable loss of equipment life or other physical or safety limitations 

for the equipment involved. 

Emergency RFI Request for Interchange to be initiated for Emergency or Energy 

Emergency conditions. 

Energy Emergency A condition when a Load-Serving Entity has exhausted all other options 

and can no longer provide its customers’ expected energy requirements. 

Equipment Rating The maximum and minimum voltage, current, frequency, real and 

reactive power flows on individual equipment under steady state, shortcircuit and transient conditions, as permitted or assigned by the 

equipment owner. 

Existing Transmission 

Commitments 

ETC Committed uses of a Transmission Service Provider’s Transmission 

system considered when determining ATC or AFC. 

Facility A set of electrical equipment that operates as a single Bulk Electric 

System Element (e.g., a line, a generator, a shunt compensator, 

transformer, etc.) 
Glossary of Terms Used in Reliability Standards 

Adopted by NERC Board of Trustees: November 13, 2008 Page 8 of 21 

Term Acronym Definition 

Facility Rating The maximum or minimum voltage, current, frequency, or real or 

reactive power flow through a facility that does not violate the applicable 

equipment rating of any equipment comprising the facility. 

Fault An event occurring on an electric system such as a short circuit, a 

broken wire, or an intermittent connection. 

Fire Risk The likelihood that a fire will ignite or spread in a particular geographic 

area. 

Firm Demand That portion of the Demand that a power supplier is obligated to provide 

except when system reliability is threatened or during emergency 

conditions. 

Firm Transmission Service The highest quality (priority) service offered to customers under a filed 

rate schedule that anticipates no planned interruption. 

Flashover An electrical discharge through air around or over the surface of 

insulation, between objects of different potential, caused by placing a 

voltage across the air space that results in the ionization of the air 

space. 

Flowgate 1.) A portion of the Transmission system through which the Interchange 

Distribution Calculator calculates the power flow from Interchange 

Transactions. 

2.) A mathematical construct, comprised of one or more monitored 

transmission Facilities and optionally one or more contingency Facilities, 

used to analyze the impact of power flows upon the Bulk Electric 

System. 

Flowgate Methodology The Flowgate methodology is characterized by identification of key 

Facilities as Flowgates. Total Flowgate Capabilities are determined 

based on Facility Ratings and voltage and stability limits. The impacts of 

Existing Transmission Commitments (ETCs) are determined by 

simulation. The impacts of ETC, Capacity Benefit Margin (CBM) and 

Transmission Reliability Margin (TRM) are subtracted from the Total 

Flowgate Capability, and Postbacks and counterflows are added, to 

determine the Available Flowgate Capability (AFC) value for that 

Flowgate. AFCs can be used to determine Available Transfer Capability 

(ATC). 

Forced Outage 1. The removal from service availability of a generating unit, 

transmission line, or other facility for emergency reasons. 

2. The condition in which the equipment is unavailable due to 

unanticipated failure. 

Frequency Bias A value, usually expressed in megawatts per 0.1 Hertz (MW/0.1 Hz), 

associated with a Balancing Authority Area that approximates the 

Balancing Authority Area’s response to Interconnection frequency error. 
Glossary of Terms Used in Reliability Standards 

Adopted by NERC Board of Trustees: November 13, 2008 Page 9 of 21 

Term Acronym Definition 

Frequency Bias Setting A value, usually expressed in MW/0.1 Hz, set into a Balancing Authority 

ACE algorithm that allows the Balancing Authority to contribute its 

frequency response to the Interconnection. 

Frequency Deviation A change in Interconnection frequency. 

Frequency Error The difference between the actual and scheduled frequency. (FA – FS) 

Frequency Regulation The ability of a Balancing Authority to help the Interconnection maintain 

Scheduled Frequency. This assistance can include both turbine governor 

response and Automatic Generation Control. 

Frequency Response (Equipment) The ability of a system or elements of the system to react 

or respond to a change in system frequency. 

(System) The sum of the change in demand, plus the change in 

generation, divided by the change in frequency, expressed in megawatts 

per 0.1 Hertz (MW/0.1 Hz). 

Generator Operator The entity that operates generating unit(s) and performs the functions of 

supplying energy and Interconnected Operations Services. 

Generator Owner Entity that owns and maintains generating units. 

Generator Shift Factor GSF A factor to be applied to a generator’s expected change in output to 

determine the amount of flow contribution that change in output will 

impose on an identified transmission facility or Flowgate. 

Generator-to-Load 

Distribution Factor 

GLDF The algebraic sum of a Generator Shift Factor and a Load Shift Factor to 

determine the total impact of an Interchange Transaction on an 

identified transmission facility or Flowgate. 

Generation Capability Import 

Requirement 

GCIR The amount of generation capability from external sources identified by 

a Load-Serving Entity (LSE) or Resource Planner (RP) to meet its 

generation reliability or resource adequacy requirements as an 

alternative to internal resources. 

Host Balancing Authority 1. A Balancing Authority that confirms and implements Interchange 

Transactions for a Purchasing Selling Entity that operates generation 

or serves customers directly within the Balancing Authority’s metered 

boundaries. 

2. The Balancing Authority within whose metered boundaries a jointly 

owned unit is physically located. 

Hourly Value Data measured on a Clock Hour basis. 

Implemented Interchange The state where the Balancing Authority enters the Confirmed 

Interchange into its Area Control Error equation. 

Inadvertent Interchange The difference between the Balancing Authority’s Net Actual Interchange 

and Net Scheduled Interchange. 

(IA – IS) 
Glossary of Terms Used in Reliability Standards 

Adopted by NERC Board of Trustees: November 13, 2008 Page 10 of 21 

Term Acronym Definition 

Independent Power Producer IPP Any entity that owns or operates an electricity generating facility that is 

not included in an electric utility’s rate base. This term includes, but is 

not limited to, cogenerators and small power producers and all other 

nonutility electricity producers, such as exempt wholesale generators, 

who sell electricity. 

Institute of Electrical and 

Electronics Engineers, Inc. 

IEEE 

Interchange Distribution 

Calculator 

IDC The mechanism used by Reliability Coordinators in the Eastern 

Interconnection to calculate the distribution of Interchange Transactions 

over specific Flowgates. It includes a database of all Interchange 

Transactions and a matrix of the Distribution Factors for the Eastern 

Interconnection. 

Interchange Energy transfers that cross Balancing Authority boundaries. 

Interchange Authority The responsible entity that authorizes implementation of valid and 

balanced Interchange Schedules between Balancing Authority Areas, and 

ensures communication of Interchange information for reliability 

assessment purposes. 

Interchange Schedule An agreed-upon Interchange Transaction size (megawatts), start and 

end time, beginning and ending ramp times and rate, and type required 

for delivery and receipt of power and energy between the Source and 

Sink Balancing Authorities involved in the transaction. 

Interchange Transaction An agreement to transfer energy from a seller to a buyer that crosses 

one or more Balancing Authority Area boundaries. 

Interchange Transaction Tag 

or 

Tag 

 The details of an Interchange Transaction required for its physical 

implementation. 

Interconnected Operations 

Service 

 A service (exclusive of basic energy and transmission services) that is 

required to support the reliable operation of interconnected Bulk Electric 

Systems. 

Interconnection When capitalized, any one of the three major electric system networks in 

North America: Eastern, Western, and ERCOT. 

Interconnection Reliability 

Operating Limit 

IROL A System Operating Limit that, if violated, could lead to instability, 

uncontrolled separation, or Cascading Outages that adversely impact the 

reliability of the Bulk Electric System. 

Interconnection Reliability 

Operating Limit Tv

IROL Tv The maximum time that an Interconnection Reliability Operating Limit 

can be violated before the risk to the interconnection or other Reliability 

Coordinator Area(s) becomes greater than acceptable. Each 

Interconnection Reliability Operating Limit’s Tv shall be less than or 

equal to 30 minutes. 
Glossary of Terms Used in Reliability Standards 

Adopted by NERC Board of Trustees: November 13, 2008 Page 11 of 21 

Term Acronym Definition 

Intermediate Balancing 

Authority 

 A Balancing Authority Area that has connecting facilities in the 

Scheduling Path between the Sending Balancing Authority Area and 

Receiving Balancing Authority Area and operating agreements that 

establish the conditions for the use of such facilities 

Interruptible Load 

or 

Interruptible Demand 

 Demand that the end-use customer makes available to its Load-Serving 

Entity via contract or agreement for curtailment. 

Joint Control Automatic Generation Control of jointly owned units by two or more 

Balancing Authorities. 

Limiting Element The element that is 1. )Either operating at its appropriate rating, or 2,) 

Would be following the limiting contingency. Thus, the Limiting Element 

establishes a system limit. 

Load An end-use device or customer that receives power from the electric 

system. 

Load Shift Factor LSF A factor to be applied to a load’s expected change in demand to 

determine the amount of flow contribution that change in demand will 

impose on an identified transmission facility or monitored Flowgate. 

Load-Serving Entity Secures energy and transmission service (and related Interconnected 

Operations Services) to serve the electrical demand and energy 

requirements of its end-use customers. 

Misoperation  Any failure of a Protection System element to operate within the 

specified time when a fault or abnormal condition occurs within a 

zone of protection. 

 Any operation for a fault not within a zone of protection (other than 

operation as backup protection for a fault in an adjacent zone that 

is not cleared within a specified time for the protection for that 

zone). 

 Any unintentional Protection System operation when no fault or 

other abnormal condition has occurred unrelated to on-site 

maintenance and testing activity. 

Native Load The end-use customers that the Load-Serving Entity is obligated to 

serve. 

Net Actual Interchange The algebraic sum of all metered interchange over all interconnections 

between two physically Adjacent Balancing Authority Areas. 

Net Energy for Load Net Balancing Authority Area generation, plus energy received from 

other Balancing Authority Areas, less energy delivered to Balancing 

Authority Areas through interchange. It includes Balancing Authority 

Area losses but excludes energy required for storage at energy storage 

facilities. 
Glossary of Terms Used in Reliability Standards 

Adopted by NERC Board of Trustees: November 13, 2008 Page 12 of 21 

Term Acronym Definition 

Net Interchange Schedule The algebraic sum of all Interchange Schedules with each Adjacent 

Balancing Authority. 

Net Scheduled Interchange The algebraic sum of all Interchange Schedules across a given path or 

between Balancing Authorities for a given period or instant in time. 

Network Integration 

Transmission Service 

 Service that allows an electric transmission customer to integrate, plan, 

economically dispatch and regulate its network reserves in a manner 

comparable to that in which the Transmission Owner serves Native Load 

customers. 

Non-Firm Transmission 

Service 

 Transmission service that is reserved on an as-available basis and is 

subject to curtailment or interruption. 

Non-Spinning Reserve 1. That generating reserve not connected to the system but capable of 

serving demand within a specified time. 

2. Interruptible load that can be removed from the system in a specified 

time. 

Normal Clearing A protection system operates as designed and the fault is cleared in the 

time normally expected with proper functioning of the installed 

protection systems. 

Normal Rating The rating as defined by the equipment owner that specifies the level of 

electrical loading, usually expressed in megawatts (MW) or other 

appropriate units that a system, facility, or element can support or 

withstand through the daily demand cycles without loss of equipment 

life. 

Nuclear Plant Generator 

Operator 

Any Generator Operator or Generator Owner that is a Nuclear Plant 

Licensee responsible for operation of a nuclear facility licensed to 

produce commercial power. 

Nuclear Plant Off-site Power 

Supply (Off-site Power) 

The electric power supply provided from the electric system to the 

nuclear power plant distribution system as required per the nuclear 

power plant license. 

Nuclear Plant Licensing 

Requirements (NPLRs) 

Requirements included in the design basis of the nuclear plant and 

statutorily mandated for the operation of the plant, including nuclear 

power plant licensing requirements for: 

1) Off-site power supply to enable safe shutdown of the plant during 

an electric system or plant event; and 

2) Avoiding preventable challenges to nuclear safety as a result of an 

electric system disturbance, transient, or condition. 

Nuclear Plant Interface 

Requirements (NPIRs) 

 The requirements based on NPLRs and Bulk Electric System 

requirements that have been mutually agreed to by the Nuclear Plant 

Generator Operator and the applicable Transmission Entities. 

Off-Peak Those hours or other periods defined by NAESB business practices, 

contract, agreements, or guides as periods of lower electrical demand. 
Glossary of Terms Used in Reliability Standards 

Adopted by NERC Board of Trustees: November 13, 2008 Page 13 of 21 

Term Acronym Definition 

On-Peak Those hours or other periods defined by NAESB business practices, 

contract, agreements, or guides as periods of higher electrical demand. 

Open Access Same Time 

Information Service 

OASIS An electronic posting system that the Transmission Service Provider 

maintains for transmission access data and that allows all transmission 

customers to view the data simultaneously. 

Open Access Transmission 

Tariff 

OATT Electronic transmission tariff accepted by the U.S. Federal Energy 

Regulatory Commission requiring the Transmission Service Provider to 

furnish to all shippers with non-discriminating service comparable to that 

provided by Transmission Owners to themselves. 

Operating Plan A document that identifies a group of activities that may be used to 

achieve some goal. An Operating Plan may contain Operating 

Procedures and Operating Processes. A company-specific system 

restoration plan that includes an Operating Procedure for black-starting 

units, Operating Processes for communicating restoration progress with 

other entities, etc., is an example of an Operating Plan. 

Operating Procedure A document that identifies specific steps or tasks that should be taken 

by one or more specific operating positions to achieve specific operating 

goal(s). The steps in an Operating Procedure should be followed in the 

order in which they are presented, and should be performed by the 

position(s) identified. A document that lists the specific steps for a 

system operator to take in removing a specific transmission line from 

service is an example of an Operating Procedure. 

Operating Process A document that identifies general steps for achieving a generic 

operating goal. An Operating Process includes steps with options that 

may be selected depending upon Real-time conditions. A guideline for 

controlling high voltage is an example of an Operating Process. 

Operating Reserve That capability above firm system demand required to provide for 

regulation, load forecasting error, equipment forced and scheduled 

outages and local area protection. It consists of spinning and nonspinning reserve. 

Operating Reserve – Spinning The portion of Operating Reserve consisting of: 

 Generation synchronized to the system and fully available to serve 

load within the Disturbance Recovery Period following the 

contingency event; or 

 Load fully removable from the system within the Disturbance 

Recovery Period following the contingency event. 

Operating Reserve – 

Supplemental 

 The portion of Operating Reserve consisting of: 

 Generation (synchronized or capable of being synchronized to the 

system) that is fully available to serve load within the Disturbance 

Recovery Period following the contingency event; or 

 Load fully removable from the system within the Disturbance 

Recovery Period following the contingency event. 
Glossary of Terms Used in Reliability Standards 

Adopted by NERC Board of Trustees: November 13, 2008 Page 14 of 21 

Term Acronym Definition 

Operating Voltage The voltage level by which an electrical system is designated and to 

which certain operating characteristics of the system are related; also, 

the effective (root-mean-square) potential difference between any two 

conductors or between a conductor and the ground. The actual voltage 

of the circuit may vary somewhat above or below this value. 

Operational Planning Analysis An analysis of the expected system conditions for the next day’s 

operation. (That analysis may be performed either a day ahead or as 

much as 12 months ahead.) Expected system conditions include things 

such as load forecast(s), generation output levels, and known system 

constraints (transmission facility outages, generator outages, equipment 

limitations, etc.). 

Outage Transfer Distribution 

Factor 

OTDF In the post-contingency configuration of a system under study, the 

electric Power Transfer Distribution Factor (PTDF) with one or more 

system Facilities removed from service (outaged). 

Overlap Regulation Service A method of providing regulation service in which the Balancing 

Authority providing the regulation service incorporates another 

Balancing Authority’s actual interchange, frequency response, and 

schedules into providing Balancing Authority’s AGC/ACE equation. 

Participation Factors A set of dispatch rules such that given a specific amount of load to 

serve, an approximate generation dispatch can be determined. To 

accomplish this, generators are assigned a percentage that they will 

contribute to serve load. 

Peak Demand 1. The highest hourly integrated Net Energy For Load within a Balancing 

Authority Area occurring within a given period (e.g., day, month, 

season, or year). 

2. The highest instantaneous demand within the Balancing Authority 

Area. 

Performance-Reset Period The time period that the entity being assessed must operate without any 

violations to reset the level of non compliance to zero. 

Physical Security Perimeter The physical, completely enclosed (“six-wall”) border surrounding 

computer rooms, telecommunications rooms, operations centers, and 

other locations in which Critical Cyber Assets are housed and for which 

access is controlled. 

Planning Authority The responsible entity that coordinates and integrates transmission 

facility and service plans, resource plans, and protection systems. 

Planning Coordinator See Planning Authority. 

Point of Delivery POD A location that the Transmission Service Provider specifies on its 

transmission system where an Interchange Transaction leaves or a 

Load-Serving Entity receives its energy. 
Glossary of Terms Used in Reliability Standards 

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Term Acronym Definition 

Point of Receipt POR A location that the Transmission Service Provider specifies on its 

transmission system where an Interchange Transaction enters or a 

Generator delivers its output. 

Point to Point Transmission 

Service 

PTP The reservation and transmission of capacity and energy on either a firm 

or non-firm basis from the Point(s) of Receipt to the Point(s) of Delivery. 

Postback Positive adjustments to ATC or AFC as defined in Business Practices. 

Such Business Practices may include processing of redirects and 

unscheduled service. 

Power Transfer Distribution 

Factor 

PTDF In the pre-contingency configuration of a system under study, a 

measure of the responsiveness or change in electrical loadings on 

transmission system Facilities due to a change in electric power transfer 

from one area to another, expressed in percent (up to 100%) of the 

change in power transfer 

Pro Forma Tariff Usually refers to the standard OATT and/or associated transmission 

rights mandated by the U.S. Federal Energy Regulatory Commission 

Order No. 888. 

Protection System Protective relays, associated communication systems, voltage and 

current sensing devices, station batteries and DC control circuitry. 

Pseudo-Tie A telemetered reading or value that is updated in real time and used as 

a “virtual” tie line flow in the AGC/ACE equation but for which no 

physical tie or energy metering actually exists. The integrated value is 

used as a metered MWh value for interchange accounting purposes. 

Purchasing-Selling Entity The entity that purchases or sells, and takes title to, energy, capacity, 

and Interconnected Operations Services. Purchasing-Selling Entities may 

be affiliated or unaffiliated merchants and may or may not own 

generating facilities. 

Ramp Rate 

or 

Ramp 

 (Schedule) The rate, expressed in megawatts per minute, at which the 

interchange schedule is attained during the ramp period. 

(Generator) The rate, expressed in megawatts per minute, that a 

generator changes its output. 

Rated Electrical Operating 

Conditions 

 The specified or reasonably anticipated conditions under which the 

electrical system or an individual electrical circuit is intend/designed to 

operate 

Rating The operational limits of a transmission system element under a set of 

specified conditions. 

Rated System Path 

Methodology 

The Rated System Path Methodology is characterized by an initial Total 

Transfer Capability (TTC), determined via simulation. Capacity Benefit 

Margin, Transmission Reliability Margin, and Existing Transmission 

Commitments are subtracted from TTC, and Postbacks and counterflows 

are added as applicable, to derive Available Transfer Capability. Under 

the Rated System Path Methodology, TTC results are generally reported 

as specific transmission path capabilities. 
Glossary of Terms Used in Reliability Standards 

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Term Acronym Definition 

Reactive Power The portion of electricity that establishes and sustains the electric and 

magnetic fields of alternating-current equipment. Reactive power must 

be supplied to most types of magnetic equipment, such as motors and 

transformers. It also must supply the reactive losses on transmission 

facilities. Reactive power is provided by generators, synchronous 

condensers, or electrostatic equipment such as capacitors and directly 

influences electric system voltage. It is usually expressed in kilovars 

(kvar) or megavars (Mvar). 

Real Power The portion of electricity that supplies energy to the load. 

Reallocation The total or partial curtailment of Transactions during TLR Level 3a or 5a 

to allow Transactions using higher priority to be implemented. 

Real-time Present time as opposed to future time. (From Interconnection Reliability 

Operating Limits standard.) 

Real-time Assessment An examination of existing and expected system conditions, conducted 

by collecting and reviewing immediately available data 

Receiving Balancing Authority The Balancing Authority importing the Interchange. 

Regional Reliability 

Organization 

 1. An entity that ensures that a defined area of the Bulk Electric System 

is reliable, adequate and secure. 

2. A member of the North American Electric Reliability Council. The 

Regional Reliability Organization can serve as the Compliance 

Monitor. 

Regional Reliability Plan The plan that specifies the Reliability Coordinators and Balancing 

Authorities within the Regional Reliability Organization, and explains how 

reliability coordination will be accomplished. 

Regulating Reserve An amount of reserve responsive to Automatic Generation Control, which 

is sufficient to provide normal regulating margin. 

Regulation Service The process whereby one Balancing Authority contracts to provide 

corrective response to all or a portion of the ACE of another Balancing 

Authority. The Balancing Authority providing the response assumes the 

obligation of meeting all applicable control criteria as specified by NERC 

for itself and the Balancing Authority for which it is providing the 

Regulation Service. 

Reliability Adjustment RFI Request to modify an Implemented Interchange Schedule for reliability 

purposes. 
Glossary of Terms Used in Reliability Standards 

Adopted by NERC Board of Trustees: November 13, 2008 Page 17 of 21 

Term Acronym Definition 

Reliability Coordinator The entity that is the highest level of authority who is responsible for the 

reliable operation of the Bulk Electric System, has the Wide Area view of 

the Bulk Electric System, and has the operating tools, processes and 

procedures, including the authority to prevent or mitigate emergency 

operating situations in both next-day analysis and real-time operations. 

The Reliability Coordinator has the purview that is broad enough to 

enable the calculation of Interconnection Reliability Operating Limits, 

which may be based on the operating parameters of transmission 

systems beyond any Transmission Operator’s vision. 

Reliability Coordinator Area The collection of generation, transmission, and loads within the 

boundaries of the Reliability Coordinator. Its boundary coincides with 

one or more Balancing Authority Areas. 

Reliability Coordinator 

Information System 

RCIS The system that Reliability Coordinators use to post messages and share 

operating information in real time. 

Remedial Action Scheme RAS See “Special Protection System” 

Reportable Disturbance Any event that causes an ACE change greater than or equal to 80% of a 

Balancing Authority’s or reserve sharing group’s most severe 

contingency. The definition of a reportable disturbance is specified by 

each Regional Reliability Organization. This definition may not be 

retroactively adjusted in response to observed performance. 

Reserve Sharing Group A group whose members consist of two or more Balancing Authorities 

that collectively maintain, allocate, and supply operating reserves 

required for each Balancing Authority’s use in recovering from 

contingencies within the group. Scheduling energy from an Adjacent 

Balancing Authority to aid recovery need not constitute reserve sharing 

provided the transaction is ramped in over a period the supplying party 

could reasonably be expected to load generation in (e.g., ten minutes). 

If the transaction is ramped in quicker (e.g., between zero and ten 

minutes) then, for the purposes of Disturbance Control Performance, the 

Areas become a Reserve Sharing Group. 

Resource Planner The entity that develops a long-term (generally one year and beyond) 

plan for the resource adequacy of specific loads (customer demand and 

energy requirements) within a Planning Authority Area. 

Response Rate The Ramp Rate that a generating unit can achieve under normal 

operating conditions expressed in megawatts per minute (MW/Min). 

Request for Interchange RFI A collection of data as defined in the NAESB RFI Datasheet, to be 

submitted to the Interchange Authority for the purpose of implementing 

bilateral Interchange between a Source and Sink Balancing Authority. 

Right-of-Way (ROW) A corridor of land on which electric lines may be located. The 

Transmission Owner may own the land in fee, own an easement, or have 

certain franchise, prescription, or license rights to construct and 

maintain lines. 

Scenario Possible event. 
Glossary of Terms Used in Reliability Standards 

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Term Acronym Definition 

Schedule (Verb) To set up a plan or arrangement for an Interchange Transaction. 

(Noun) An Interchange Schedule. 

Scheduled Frequency 60.0 Hertz, except during a time correction. 

Scheduling Entity An entity responsible for approving and implementing Interchange 

Schedules. 

Scheduling Path The Transmission Service arrangements reserved by the PurchasingSelling Entity for a Transaction. 

Sending Balancing Authority The Balancing Authority exporting the Interchange. 

Sink Balancing Authority The Balancing Authority in which the load (sink) is located for an 

Interchange Transaction. (This will also be a Receiving Balancing 

Authority for the resulting Interchange Schedule.) 

Source Balancing Authority The Balancing Authority in which the generation (source) is located for 

an Interchange Transaction. (This will also be a Sending Balancing 

Authority for the resulting Interchange Schedule.) 

Special Protection System 

(Remedial Action Scheme) 

 An automatic protection system designed to detect abnormal or 

predetermined system conditions, and take corrective actions other than 

and/or in addition to the isolation of faulted components to maintain 

system reliability. Such action may include changes in demand, 

generation (MW and Mvar), or system configuration to maintain system 

stability, acceptable voltage, or power flows. An SPS does not include 

(a) underfrequency or undervoltage load shedding or (b) fault conditions 

that must be isolated or (c) out-of-step relaying (not designed as an 

integral part of an SPS). Also called Remedial Action Scheme. 

Spinning Reserve Unloaded generation that is synchronized and ready to serve additional 

demand. 

Stability The ability of an electric system to maintain a state of equilibrium during 

normal and abnormal conditions or disturbances. 

Stability Limit The maximum power flow possible through some particular point in the 

system while maintaining stability in the entire system or the part of the 

system to which the stability limit refers. 

Supervisory Control and Data 

Acquisition 

SCADA A system of remote control and telemetry used to monitor and control 

the transmission system. 

Supplemental Regulation 

Service 

 A method of providing regulation service in which the Balancing 

Authority providing the regulation service receives a signal representing 

all or a portion of the other Balancing Authority’s ACE. 

Surge A transient variation of current, voltage, or power flow in an electric 

circuit or across an electric system. 
Glossary of Terms Used in Reliability Standards 

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Term Acronym Definition 

Sustained Outage The deenergized condition of a transmission line resulting from a fault or 

disturbance following an unsuccessful automatic reclosing sequence 

and/or unsuccessful manual reclosing procedure. 

System A combination of generation, transmission, and distribution components. 

System Operating Limit The value (such as MW, MVar, Amperes, Frequency or Volts) that 

satisfies the most limiting of the prescribed operating criteria for a 

specified system configuration to ensure operation within acceptable 

reliability criteria. System Operating Limits are based upon certain 

operating criteria. These include, but are not limited to: 

 Facility Ratings (Applicable pre- and post-Contingency equipment or 

facility ratings) 

 Transient Stability Ratings (Applicable pre- and post-Contingency 

Stability Limits) 

 Voltage Stability Ratings (Applicable pre- and post-Contingency 

Voltage Stability) 

 System Voltage Limits (Applicable pre- and post-Contingency 

Voltage Limits) 

System Operator An individual at a control center (Balancing Authority, Transmission 

Operator, Generator Operator, Reliability Coordinator) whose 

responsibility it is to monitor and control that electric system in real 

time. 

Telemetering The process by which measurable electrical quantities from substations 

and generating stations are instantaneously transmitted to the control 

center, and by which operating commands from the control center are 

transmitted to the substations and generating stations. 

Thermal Rating The maximum amount of electrical current that a transmission line or 

electrical facility can conduct over a specified time period before it 

sustains permanent damage by overheating or before it sags to the 

point that it violates public safety requirements. 

Tie Line A circuit connecting two Balancing Authority Areas. 

Tie Line Bias A mode of Automatic Generation Control that allows the Balancing 

Authority to 1.) maintain its Interchange Schedule and 2.) respond to 

Interconnection frequency error. 

Time Error The difference between the Interconnection time measured at the 

Balancing Authority(ies) and the time specified by the National Institute 

of Standards and Technology. Time error is caused by the accumulation 

of Frequency Error over a given period. 

Time Error Correction An offset to the Interconnection’s scheduled frequency to return the 

Interconnection’s Time Error to a predetermined value. 
Glossary of Terms Used in Reliability Standards 

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Term Acronym Definition 

TLR Log Report required to be filed after every TLR Level 2 or higher in a 

specified format. The NERC IDC prepares the report for review by the 

issuing Reliability Coordinator. After approval by the issuing Reliability 

Coordinator, the report is electronically filed in a public area of the NERC 

Web site. 

Total Flowgate Capability TFC The maximum flow capability on a Flowgate, is not to exceed its thermal 

rating, or in the case of a flowgate used to represent a specific operating 

constraint (such as a voltage or stability limit), is not to exceed the 

associated System Operating Limit. 

Total Transfer Capability TTC The amount of electric power that can be moved or transferred reliably 

from one area to another area of the interconnected transmission 

systems by way of all transmission lines (or paths) between those areas 

under specified system conditions. 

Transaction See Interchange Transaction. 

Transfer Capability The measure of the ability of interconnected electric systems to move or 

transfer power in a reliable manner from one area to another over all 

transmission lines (or paths) between those areas under specified 

system conditions. The units of transfer capability are in terms of 

electric power, generally expressed in megawatts (MW). The transfer 

capability from “Area A” to “Area B” is not generally equal to the transfer 

capability from “Area B” to “Area A.” 

Transfer Distribution Factor See Distribution Factor. 

Transmission An interconnected group of lines and associated equipment for the 

movement or transfer of electric energy between points of supply and 

points at which it is transformed for delivery to customers or is delivered 

to other electric systems. 

Transmission Constraint A limitation on one or more transmission elements that may be reached 

during normal or contingency system operations. 

Transmission Customer 1. Any eligible customer (or its designated agent) that can or does 

execute a transmission service agreement or can or does receive 

transmission service. 

2. Any of the following responsible entities: Generator Owner, LoadServing Entity, or Purchasing-Selling Entity. 

Transmission Line A system of structures, wires, insulators and associated hardware that 

carry electric energy from one point to another in an electric power 

system. Lines are operated at relatively high voltages varying from 69 

kV up to 765 kV, and are capable of transmitting large quantities of 

electricity over long distances. 

Transmission Operator The entity responsible for the reliability of its “local” transmission 

system, and that operates or directs the operations of the transmission 

facilities. 
Glossary of Terms Used in Reliability Standards 

Adopted by NERC Board of Trustees: November 13, 2008 Page 21 of 21 

Term Acronym Definition 

Transmission Operator Area The collection of Transmission assets over which the Transmission 

Operator is responsible for operating. 

Transmission Owner The entity that owns and maintains transmission facilities. 

Transmission Planner The entity that develops a long-term (generally one year and beyond) 

plan for the reliability (adequacy) of the interconnected bulk electric 

transmission systems within its portion of the Planning Authority Area. 

Transmission Reliability 

Margin 

TRM The amount of transmission transfer capability necessary to provide 

reasonable assurance that the interconnected transmission network will 

be secure. TRM accounts for the inherent uncertainty in system 

conditions and the need for operating flexibility to ensure reliable system 

operation as system conditions change. 

Transmission Reliability 

Margin Implementation 

Document 

TRMID A document that describes the implementation of a Transmission 

Reliability Margin methodology, and provides information related to a 

Transmission Operator’s calculation of TRM. 

Transmission Service Services provided to the Transmission Customer by the Transmission 

Service Provider to move energy from a Point of Receipt to a Point of 

Delivery. 

Transmission Service Provider The entity that administers the transmission tariff and provides 

Transmission Service to Transmission Customers under applicable 

transmission service agreements. 

Vegetation All plant material, growing or not, living or dead. 

Vegetation Inspection The systematic examination of a transmission corridor to document 

vegetation conditions. 

Wide Area The entire Reliability Coordinator Area as well as the critical flow and 

status information from adjacent Reliability Coordinator Areas as 

determined by detailed system studies to allow the calculation of 

Interconnected Reliability Operating Limits.