SIST-TP CLC/TR 50555:2011

SLOVENSKI STANDARD
01-oktober-2011
Prekinitveni indeksi
Interruption indexes
Ta slovenski standard je istoveten z: CLC/TR 50555:2010
ICS:
27.010 Prenos energije in toplote na Energy and heat transfer
splošno engineering in general
29.240.01 2PUHåMD]DSUHQRVLQ Power transmission and
GLVWULEXFLMRHOHNWULþQHHQHUJLMH distribution networks in
QDVSORãQR general
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

TECHNICAL REPORT
CLC/TR 50555
RAPPORT TECHNIQUE
May 2010
TECHNISCHER BERICHT
ICS 27.010;29.240.01
English version
Interruption indexes
Indicateurs d'interruption Unterbrechungsindizes

This Technical Report was approved by CENELEC on 2010-05-07.

CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Croatia, Cyprus,
the Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy,
Latvia, Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia,
Spain, Sweden, Switzerland and the United Kingdom.

CENELEC
European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung

Management Centre: Avenue Marnix 17, B - 1000 Brussels

© 2010 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.
Ref. No. CLC/TR 50555:2010 E
Foreword
This Technical Report was prepared by Task Force 4, (Interruption definitions and continuity indices)
of Working Group 1 (Physical characteristics of electrical energy), of Technical Committee CENELEC
TC 8X, System aspects of electrical energy supply.
It was circulated for voting in accordance with the Internal Regulations, Part 2, Subclause 11.4.3.3
(simple majority).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN and CENELEC shall not be held responsible for identifying any or all such patent
rights.
__________
- 3 - CLC/TR 50555:2010
Contents
1 Introduction and scope .5
1.1 Introduction .5
1.2 Scope .6
1.3 Continuity indices – Needs and applications .6
2 Terms and definitions .8
3 Continuity of supply – Interruptions . 10
3.1 Background information on interruptions . 10
3.2 Interruptions as defined by EN 50160. 10
3.3 Interruption sources . 11
3.4 Classification of the interruptions . 12
4 Explanation of continuity . 14
4.1 Long interruption continuity indices. 15
4.2 Short interruption continuity indices . 18
4.3 Worst Served Customers . 19
4.4 Practices for recording short interruptions . 19
4.5 Rules for recording long interruptions . 20
4.6 Interruption indices in use across Europe . 24
5 Recommended continuity indices . 29
5.1 Philosophy and criteria for recommending indices . 29
5.2 Recommended continuity indices . 29
6 Recommended data collection and aggregation . 30
6.1 General . 30
6.2 Parameter uncertainty impact on continuity indices . 31
6.3 Classification of reported indices . 32
6.4 Example of continuity indices estimation . 32
7 Information helpful when comparing continuity indices . 34
7.1 Important differences between countries and a single recommendation . 34
7.2 Observation period for continuity indices . 35
8 Trends, new and future developments . 36
8.1 Development / improvement of new tools (technology) . 36
8.2 Recording data considerations . 36
8.3 Development in telecommunication system s . 36
8.4 Smart grids and metering aspects . 37
8.5 Simulation techniques . 37
8.6 Standardization considerations . 37
8.7 Particular incidence in LV networks . 37
8.8 Cause and effect considerations . 37
Bibliography . 38

Figures
Figure 1 – Network operator organizational levels and stakeholders .7
Figure 2 – Short interruption within a long interruption . 22
Figure 3 – Short interruption after a long interruption . 23
Figure 4 – Two consecutive short interruptions. 24
Figure 5 – Interruption scenarios . 33
Figure 6 – Risk performance versus risk exposure . 35

Tables
Table 1 – Indices for quantifying long interruptions used in European countries . 25
Table 2 – Monitoring and indices for short and transient interruptions in European countries . 28
Table 3 – System characteristics recommended . 35

- 5 - CLC/TR 50555:2010
1 Introduction and scope
1.1 Introduction
As a result of the liberalization of electricity markets, System Operators are being increasingly
encouraged to report the performance of their electric power systems to other parties, in particular the
network users and the national regulation authorities (NRA). While in the past, quality of supply was
generally considered as an implicit duty on System Operators, today quality objectives have become
more and more definite objectives agreed with the Regulator and/or part of the contracts negotiated
with the Network Users. Indeed a number of European Regulators have already defined, or planned to
1)
define, quality of supply objectives (addressing continuity of supply and/or voltage quality) to be met
by electric distribution systems. In some countries, quality of supply objectives form part of the
incentive-based regulation.
Quality of supply limits can be seen as the outer envelope of performance for each quality of supply
parameter. Specific continuity of supply Indices are established by particular Regulators in order to
facilitate benchmarking the performance of the System Operators under their jurisdiction. The indices
allow System Operators to meet their obligation to routinely report continuity of supply performance. It
is important that the objectives are seen not only as achievable but also as being cost effective
considering the needs of all the network users.
As customers expect a high continuity of supply for a reasonable price, one of the roles of a System
Operator is to optimise the continuity performance of the electric system in a cost effective manner;
the role of the Regulator being to ensure that this is carried out in a correct way taking into account the
customers’ expectations and their willingness to pay. It needs to be recognized that historically the
electrical systems in different countries have been designed in different ways based on different
technological choices, commercial approaches or climatic conditions.
There is a great variety of reliability indices used within the different European countries. Each country
has its own indices, some are system orientated and others are customer oriented. Some countries
measure separately the frequency and the duration of interruptions, others combine them into a single
value. In addition, not all the countries use the same definitions for interruptions and their
classification. For all of these reasons it is currently very difficult to compare the continuity of supply
indices between countries.
1)
Quality of electricity supply is a collective effect of all aspects of performance in the electricity supply. The quality of the
electricity supply includes as a prerequisite reliability of the electric power system, power quality and customer
relationships. For the purpose of this Technical Report the term continuity of supply is used for the availability of the
electricity.
1.2 Scope
This Technical Report provides guidance on how to calculate continuity of supply indices. These
recommended indices are more particularly given for European benchmarking of distribution network
2)
performance. For transmission network performance, more representative indices may be used.
It presents
– an overview of practices in Europe on long and short interruptions,
– definition of physical interruptions in a harmonized way,
– philosophy and criteria for recommending indices,
– a suggested common approach to continuity indices.
The fact that the networks in different parts of any particular country will be subject to different
conditions (e.g. weather and customer density) mean that it is not viable to apply common
performance standards to all networks within any one country or any group of countries without
making these targets so weak that there is a good prospect of them being achieved in all areas. The
present situation where national regulators set performance targets within their own countries is widely
regarded as being the most effective mechanism for achieving optimal socio-economic performance.
For these reasons this Technical Report does not provide common targets for the number and
duration of interruptions that should not be exceeded.
This Technical Report is designed to be a first step towards benchmarking the interruption
performance of European countries. Rules on the aggregation of interruptions, in particular short
interruptions, have not been considered in this Technical Report, however it is recognised that it might
be necessary to describe aggregation rules in a second version of the Technical Report.
1.3 Continuity indices – Needs and applications
Performance indices in general are important tools in decision making for transmission and distribution
system asset management. Such indices can be used to translate issues, which might be rather
vaguely expressed, into formalized parameters to be used in decision-making. As the reliability of the
power system is a key element in power system management, continuity indices are useful to
translate objectives such as
• to maximize power system reliability and
• to provide our customers with a supply that has the minimum number of interruptions.
into more formalized objectives and targets aimed to support asset management and stakeholder
communication.
In power system asset management, decisions must be taken at different organizational levels within
companies. Figure 1 illustrates the main decision levels as well as the most important stakeholders
that may influence decisions at different levels.

2)
For example, "Average Interruption Time" is commonly used by TSO (AIT = T x ENS / E ).
T
- 7 - CLC/TR 50555:2010
Figure 1 – Network operator organizational levels and stakeholders
The stakeholders set the general requirements, for the business, that are further translated into
internal business values and criteria. For example, compliance with the rules and regulations
concerning continuity of supply are strategic criteria important for the utilities, authorities and
regulators. Obtaining a sufficient profit or return on the assets is an important criterion for company
owners while low cost and high reliability of supply are requirements of the customers.
In order to be able to take into consideration all continuity aspects from different stakeholder
perspectives, continuity indices are needed.
Continuity indices have several applications:
a) they might be used to measure overall developments and trends;
b) they might be used in benchmarking to identify best practices and learn from others;
c) they might be included in planning objectives and/or planning restrictions;
d) they might be used in contractual arrangements;
e) they might be used by regulatory authorities;
f) they might be used in stakeholder communication.
The list above is rather general and covers company levels, national issues and international issues.
As this Technical Report focuses on the international level (European issues), the main applications
and criteria for recommending indices are given in Clause 5.

2 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
2.1
high voltage (HV)
voltage whose nominal r.m.s. value is 36 kV < U ≤ 150 kV
n
[EN 50160:2010]
NOTE Because of existing network structures, in some countries the boundary between MV and HV can be different.
2.2
interruption threshold
voltage magnitude specified for the purpose of detecting the start and the end of a voltage interruption
[EN 61000-4-30]
2.3
low voltage (LV)
voltage whose nominal r.m.s. value is U ≤ 1 kV
n
[EN 50160:2010]
2.4
medium voltage (MV)
voltage whose nominal r.m.s. value is 1 kV < U ≤ 36 kV
n
NOTE Because of existing network structures, in some countries the boundary between MV and HV can be different.
[EN 50160:2010]
2.5
network operator
party responsible for operating, ensuring the maintenance of, and if necessary developing, the supply
network in a given area and responsible for ensuring the long term ability of the network to meet
reasonable demands for electricity supply
[EN 50160:2010]
2.6
network user
party being supplied by or supplying to an electricity supply network
NOTE 1 In several countries, the term network user includes network operators connected to a supply network with the same
or higher voltage level.
[EN 50160:2010]
NOTE 2 For the purposes of this Technical Report, the terms Network User and Customer are considered to have the same
meaning and are therefore interchangeable.
2.7
normal operating condition
operating condition for an electricity network, where load and generation demands are met, system-
switching operations are made and faults are cleared by automatic protection systems, and in the
absence of exceptional circumstances, i.e.:
• temporary supply arrangement;
• in the case of non-compliance of a network user's installation or equipment with the relevant
standards or with the technical requirements for connection;
• exceptional situations such as
– exceptional weather conditions and other natural disasters,
– third party interference,
– acts by public authorities,
– industrial actions (subject to legal requirements),
– force majeure,
– power shortages resulting from external events
[EN 50160:2010]
- 9 - CLC/TR 50555:2010
2.8
planned outage
outage scheduled in advance, for maintenance or other purposes
[IEV 191-24-01]
2.9
reference voltage (for interruptions, voltage dips and voltage swells evaluation)
value specified as the base on which residual voltage, thresholds and other values are expressed in
per unit or percentage terms
NOTE For the purpose of this document, the reference voltage is the nominal or declared voltage of the supply system.
[EN 50160:2010]
2.10
residual voltage (U )
res
minimum value of U or U recorded during a voltage dip or interruption
rms(1/2) rms(1)
NOTE 1 The residual voltage is expressed as a value in volts, or as percentage or pu value of U . U is used for Class A.
din rms(1/2)
Either Urms(1/2) or Urms(1) may be used for Class S. See EN 61000-4-30:2009, 5.4.1.
[EN 61000-4-30]
NOTE 2 The terms U and referent voltage are considered to have the same meaning.
din
2.11
r.m.s. value
square root of the mean of the squares of the instantaneous values of a quantity taken over a
specified time interval
[EN 61000-4-30]
2.12
SCADA
Supervisory Control and Data Acquisition
2.13
supply interruption
condition in which the voltage at the supply terminals is lower than 5 % of the reference voltage. A
supply interruption can be classified as
• notified, when network users are informed in advance, or
• non-notified, caused by permanent or transient faults, mostly related to external events,
equipment failures or interference. An accidental interruption is classified as:
– a long interruption (longer than 3 min);
– a short interruption (up to and including 3 min)
NOTE 1 Normally, interruptions are caused by the operation of switches or protective devices.
NOTE 2 The effect of a notified interruption can be minimized by network users by taking appropriate measures.
NOTE 3 Notified interruptions are typically due to the execution of scheduled works on the electricity network.
NOTE 4 Non-notified supply interruptions are unpredictable, largely random events.
NOTE 5 For polyphase systems, an interruption occurs when the voltage falls below 5 % of the reference voltage on all
phases (otherwise, it is considered to be a dip).
NOTE 6 In some countries, the term Very Short Interruptions (VSI) or transitory interruptions are used to classify interruptions
with duration shorter than 1 s to 5 s. Such interruptions are related to automatic reclosing device operation.
[EN 50160:2010]
2.14
unplanned outage
outage that is not a planned outage
[IEV 191-24-02]
3 Continuity of supply – Interruptions
An interruption occurs when the electricity supply is not available to one or more customers.
An interruption can be the result of system incidents caused by a wide variety of possible events, such
as humans, animals, weather, insulator degradation and other natural phenomena or can be caused
by the operation of switches to allow the execution of works on the distribution network.
More commonly interruptions are caused by the operation of switches or protection devices (circuit
breakers, fuses etc). In the absence of local generation that can provide an alternative supply to a
customer’s installation, the voltage at the customer’s supply terminals will rapidly drop to zero. From a
customer perspective, the main indication of a power interruption is that the lights go out and that
other electrical equipment stops functioning. From a network operator perspective, an interruption is
indicated by alarm signals from control centres (circuit breaker opening, low voltage alarms etc), by
the knowledge of planned disconnection operations or by notification from customers that are off
supply.
3)
3.1 Background information on interruptions
When considering the continuity of supply of an electric power system, it is very important to consider
the difference between “component outages” (outages) and “supply interruptions” (interruptions). An
interruption is a situation when a customer is without electricity. An outage is a situation when a
component in the power network (e.g. a cable or a transformer) is disconnected from the rest of the
network, either manually or as a result of a protection operation.
Supply interruptions are very often caused by component outages. However not all component
outages result in supply interruptions. The start of an interruption is typically due to the start of an
outage (a “component failure”). The end of an interruption may be due to a switching operation or due
to the end of a component outage (component restoration, repair or replacement).
An outage that results in an interruption for one or more customers can have its origin at various
locations in the network. The majority of customers are connected to the low-voltage network, but a
substantial part of the interruptions experienced by low-voltage customers is due to outages that occur
at higher voltage levels. For most low-voltage and medium-voltage customers, the majority of
interruptions are due to outages that occur at the medium-voltage level.
In radial networks (typically at low or medium voltage at remote locations) there is only one supply
path to the customers. The outage of a component will immediately result in an interruption and the
interruption will only end when the component is restored, or when an alternative supply is provided,
e.g. from another part of the network or from generation. In the first case the interruption exactly
corresponds to the outage. The duration of the interruption is equal to the restore, repair or
replacement time of the outaged component. In the case of an alternative supply, the duration is equal
to the time for first notification to the time when the alternative supply was provided. Those low-voltage
and medium-voltage networks where redundancy exists are sometimes referred to as “radically
operated meshed networks”. The start of an interruption corresponds with the start of an outage, but
the interruption can be ended (electricity restored) through a switching action (“back feeding”). This is
referred to as “redundancy through switching”.
In transmission networks and in important medium-voltage networks, the alternative path not only
exists but is also used during routine operation, i.e. electricity is supplied via parallel circuits (paths). In
the event of an outage one path is disconnected, but there is no interruption because supply is
maintained via the remaining healthy path. The customers will not experience any interruption. This is
referred to as “redundancy through parallel operation”.
3.2 Interruptions as defined by EN 50160
EN 50160 takes into account the characteristics of protection and automatic reclosing systems in use
in supply networks.
th
3)
Source: 4 CEER Benchmarking Report on Quality of Electricity Supply [7].

- 11 - CLC/TR 50555:2010
On medium voltage overhead networks it is common practice to perform an automatic reclosure after
the initial tripping of a line circuit breaker on a fault. The time delay between tripping and reclosing
depends on local conditions. The time delay between the circuit breaker tripping and reclosing is often
referred to as the “dead-time”, dead-times can vary between a minimum of 0,5 s up to about 180 s.
For lines controlled by circuit breakers set with multiple reclosures, the customers supplied by the line
will experience another short interruption followed by restoration of supply or a final tripping of the
circuit breaker, depending on whether the fault has cleared spontaneously or persists. Where multiple
reclosures are not used the circuit breaker will trip and “lock-out” i.e. not reclose, under these
conditions the customers supplied by the line will suffer a long interruption until the fault is cleared and
the circuit breaker is reclosed.
In this latter case, the supply will be interrupted until the fault is located and the faulted section of line
is isolated for inspection and repair. If an alternative source of supply is provided, this can be brought
into service either by manual or automatic switching. For networks with automatic switching, the delay
is commonly ranging between 30 s and 3 min.
3.3 Interruption sources
At present in all countries, most electrical power is generated at large power plants and stepped up to
high voltage for long distance transmission. At substations, power is stepped down to medium voltage
for shorter distance distribution.
For most consumers of electricity, whether industrial, commercial or residential, the power is
converted to its final, usable voltage by transformers at or very near to the consumer's location.
Taken together, the entire system of generating plants, transmission and distribution networks can be
seen as a “grid” that interconnects the electrical supplies.
As a whole, the “grid” is very reliable and resilient and can continue to operate in the presence of
individual component failures, even where these are very large generating plants or substations.
However, the networks are still susceptible to events that could lead to interruptions, for example:
• components’ failures caused by
– lightning strikes,
– animals coming into contact with power lines,
– wind or ice can force tree limbs onto power lines and occasionally blow down power lines,
– equipment malfunction,
– equipment aging degradation,
– overload
– bushfires,
– insulator damage due to acts of vandalism
– damage to underground power lines by contractors,
– substation transformer fires or explosions,
– result of outages on customer owned facilities;
• loss of supply from another network operator;
• operation of switches to allow the execution of works on the distribution network or the result of
some request of a third party or a public authority.
In addition to these very common and local causes, there can be other wide area power failures
caused by significant disturbances to the Transmission Network where lots of people lose supply for
long periods of time. These long interruptions are sometimes referred to as “blackouts”. Although,
these events are low probability, they have been experienced in a number of developed countries.
Storms can exaggerate local vulnerabilities by disabling numerous electricity lines at the same time
and reduce repair capacity.
Because of the complexity of the electricity system the network operator needs the best available
information to identify “what-where-when and why an interruption has happened“, in order to have the
best description of the state of the distribution network and its performance.
3.4 Classification of the interruptions
Interruptions can have different duration, sources, causes and ultimately have different effects on the
customers. These differences can mean that network operators, customers and regulators look at the
power interruptions in different ways; a good classification of an interruption should take into account
all the different needs and expectations. Therefore, a good classification is the basis for an effective
means of monitoring continuity of supply.
To find some real meaning in comparisons of one network operator's or country’s reliability experience
to that of other network operators or countries it is necessary to pay close attention to the definitions
used by each country/network operator and their recording methods. What appear to be identical
indices can vary because of differences in how interruptions are defined and recorded. Thus, anyone
comparing statistics between two or more network operators or countries can expect to see noticeable
differences in their statistics due only to definitions and recording differences. Equally, to adjust the
data to account for these differences so that the data can be compared on a valid basis, requires a
great deal of additional work.
In the next sub-clauses, the main classifications will be introduced.
3.4.1 Classification based on the notice to the customers
Because customers, especially the non-domestic, can be very sensitive to interruptions, pre-
notification of a planned interruption can allow the customer to take actions to limit the consequences.
For this reason, interruptions can be divided in two classes:
• interruptions notified in advance to network users (notified interruptions);
• interruptions not-notified in advance to network users (non-notified interruptions).
The method of notification can vary from country to country, according to different local cultures,
procedures and any regulatory requirements.
Usually all interruptions due to work on the distribution network are notified interruptions with adequate
notice given to the customers. However, there can be situations where the network operator has
insufficient time to provide the customers with advance warning of an interruption, for example
operational emergencies. For this reason, non-notified interruptions can be divided into two different
classes:
• interruptions planned without notice in advance to the users;
• unplanned Interruptions.
This second classification is more oriented to the distribution operator view.
3.4.2 Classification based on the duration
A distinction is often made based on the duration of an interruption. In most European countries a long
interruption is an interruption that lasts more than three minutes.
According to EN 50160, an interruption is referred to as a “short interruption” if it lasts three minutes or
less and “Long” if it lasts more the three minutes.
Related to the definition of the duration of interruptions are the methods for its measure.
The reporting of interruption duration is the single widest variable in reliability reporting practises:
nearly every network operator has a good record of when service is restored to each interrupted
customer. Network operators usually need to estimate the beginning time of an interruption caused by
a fault in LV network.
- 13 - CLC/TR 50555:2010
Circuit breaker operations that disconnect a circuit or a whole substation are usually recorded by
SCADA or on-site equipment, so the timing is known precisely. But there is no similar recording
mechanism to identify the precise time when a line fuse operated or for example when a line is
brought down in a storm. Therefore, to compute the duration of the outage and the interruptions it has
caused, it is necessary to estimate the time of occurrence of the outage.
In some cases the definition of the "start" of each interruption is the time when it is first reported, even
though it might have occurred long before the first customer called in to report an interruption. It is
possible to account for that delay by adding a “extra duration” to the time when the call was received.
In particular, to have good measures of duration of interruptions, the following requirements would
need to be satisfied:
• for all remote controlled circuit breakers it should be possible to accurately record the exact “start”
and “end” time of an interruption that is caused by the opening of that circuit breaker;
• in all other cases:
– the “start” of the interruption is the time when it is first reported (usually by the customer);
– the “end” of the interruption is the time when it is reported, usually by the operative who has
repaired the fault or provided an alternative source of supply.
In some countries, interruptions that last less then 1 s are classified as “Transient”. This type of
interruption is of particular interest to HV and MV customers.
3.4.3 Classification based on the documented causes
To have a good representation of the performance of each network operator or in the case of
benchmarking across two or more such companies, it is very useful to have a consistent view on
defining the cause of interruptions. A simple classification by “cause” could be to consider the three
broad categories listed below:
• “Force Majeure”: e.g. orders of public authorities, meteorological conditions beyond the design
prescriptions or exceptional incidents on the electric networks, thefts;
• “External”: e.g. third party’ damages, loss of supply from another network operator, outages on
customer owned facilities;
• “Other Causes”: all other causes not covered in the first two examples.
3.4.4 Classification on the voltage level
The electricity system is composed of different interconnected networks with different voltage levels. In
general, the reliability of the system declines with each decrease in voltage level. This is because, in
general, lower voltage level networks are affected by both local low voltage interruptions and by
interruptions on the higher voltage networks.
For these reasons it is usually the case that the continuity indices improve for each increase in voltage
level. Therefore, it is necessary for a network operator to know the performance of the network at each
voltage level. Furthermore, in case of benchmarking, because not all the network companies own the
network at all voltage levels, and not with the same proportion, it is necessary to distinguish the
continuity indices with respect to the origin of the interruptions.
A good but still simple classification by voltage level is:
• Extra High Voltage (EHV) Transmission Network;
• High Voltage (HV) Transmission or Distribution Network (i.e. HV side of Primary Substations and
HV circuits);
• Medium Voltage (MV) Distribution Network (i.e. MV side of Primary Substations, MV circuits and
MV side of Secondary Substations);
• Low Voltage (LV) Distribution Network (i.e. LV side of Secondary Substations, LV circuits;
• Interconnected Distribution Network (i.e. Other companies networks).

3.4.5 Classification on the Distribution Network
The electricity system is composed of different types of assets, in terms of construction and operation,
for example: insulated overhead lines, bare overhead lines, underground cables.
In general, because underground cables are less affected by meteorological conditions, the higher the
percentage of network that is cable compared to overhead lines, the higher the reliability of the
electrical system. On the other hand, a cable is likely to be more expensive than a overhead line.
Hence it is necessary to consider the costs as well as the benefits of a network with higher reliability.
Different countries have distribution networks that are composed of different percentages of cables
and overhead lines. A simple classification by network construction is to disaggregate the cause of
interruption in accordance with the three broad categories listed below:
• interruptions due to faults on underground cable;
• interruptions due to faults on bare overhead lines;
• interruptions due to faults on insulated overhead lines.
3.4.6 Exceptional events excluded from statistics
Even though not in use in all European countries, the concept of exceptional events is still very
common in Europe with diverse regulatory applications and various classifications being used.
Exceptional events are usually classified according to the following categories [7]:
• extraordinary situations;
• force majeure;
• emergency situations;
• multiple incident situations;
• security situations;
• highly critical power situations.
The concept of exceptional events is usually used to better determine the performance of a network
operator by excluding those events from the reliability statistics; the exclusion has also been used for
settlement of disputes in civil law. The definition of exceptional events in European countries is done
4)
by various bodies; e.g., national regulation authorities, distribution network operator (DSO) ,
transmission system operator (TSO), Government, local authorities, etc.
In some European countries, some statistical methods are used to identify the “Force Majeure”
causes. The main difficulty of these methods is related to the low frequency of these events when a
small area is being considered. For a whole country, experiences with application of a statistical
method are good. In Ireland, the IEEE 1366 method of the Major Event Days is in use. In Austria,
since two years, VEÖ has made good experiences with application of this method, after verifying the
results as being plausible.
The Italian Regulator has defined a statistical method starting from a completely different point of view:
the performance of each DNO is measured over periods of 6 h. It is then possible to use statistical
methods to identify the “Normal” or “Exceptional” periods, by consider the 1 460 x 6 hour-periods in
any one year. The interruptions started in the “Exceptional periods” are considered as “Statistical
Force Majeure”.
4 Explanation of continuity
Interruptions are often classified depending on their duration. In most European countries, an
interruption is generally classified as a long interruption if it lasts longer than three minutes. This
definition is also in accordance with EN 50160. Across the various European countries there are
currently differences in the way that continuity indices are determined for long and short interruptions.
The more common indices in use are presented in the following sub-clauses of this Technical Report.

4)
Some countries use the term Distribution Network Operator (DNO) rather than DSO.

- 15 - CLC/TR 50555:2010
4.1 Long interruption continuity indices
This sub-clause provides a brief explanation of some of the more common continuity indices employed
in different European countries for calculating long interruptions.
System Average Interruption Duration Index (SAIDI)
SAIDI gives the average amount of time per year that the supply to a customer is interrupted (long
interruption). It is expressed in minutes per customers per year and calculated by using the following
expression:
N ×r
∑
i i
i
SAIDI =
N
T
5)
Where the summation is taken over all incidents , either at all voltage levels or only at selected
voltage levels; r gives the restoration time for each incident; N gives the number of customers
i i
interrupted by each incident; N gives the total number of customers in the system for which the index
T
is calculated.
NOTE the restoration time is different for different groups of customers involved in the same incident; therefore, the sum must
be extended to each group of customers experiencing the same restoration time.
System Average Interruption Frequency Index (SAIFI)
SAIFI gives the average number of times per year that the supply to a customer is interrupted (long
interruption). It is expressed in interruptions per customer per year and calculated using the following
expression:
∑N
i
i
SAIFI =
N
T
Customer Average Interruption Duration Index (CAIDI)
CAIDI gives the average duration of an interruption. It is expressed in minutes per interruption and
calculated using the following expression:
N ×r
∑
i i
i
CAIDI =
N
∑
i
i
It can also be obtained as the ratio of SAIDI and SAIFI.
These three indices (SAIDI, SAIFI and CAIDI) are the main indices used in the majority of countries.
These indices are defined in IEEE 1366-2003 [5] (and in other documents), where weighting based on
number of customers is used and the interruptions counted in the calculation of the indices are
interruptions lasting longer than 5 min (sustained interruptions). This is different to the practice in
European countries where the common practice is to consider long interruptions to be interruptions
lasting longer than 3 min. With both SAIFI and SAIDI a reduction in value indicates an improvement in
the continuity of supply. With CAIDI this is not the case: a reduction in both SAIDI and SAIFI could still
result in an increase in CAIDI. Whereas CAIDI remains a useful index, it is not suitable for
comparisons or for trend analysis.
Customer Minutes Lost (CML)
CML is used in United Kingdom as synonym for SAIDI. CML is the duration of interruptions to supply
per year (CML) – average customer minutes lost per customer per year, where an interruption of
supply to customer(s) lasts for three minutes or longer, calculated as:
The sum of the customer minutes lost for all restoration stages for all incidents
The total number of customers
5)
For the purposes of this Technical Report the term “incident” refers to an event of situation that will result in the
interruption of electricity to one or more customers, for example an incident might be a fault on the network.

Customer Interruptions (CI)
6)
CI is used in United Kingdom instead of SAIFI . CI is the number of customers interrupted per year
(CI) – the number of customers whose supplies have been interrupted per 100 customers per year
over all incidents, where an interruption of supply lasts for three minutes or longer, excluding re-
interruptions to the supply of customers previously interrupted during the same incident. CI is
calculated as:
The sum of the number of customers interrupted for all incidents × 100
The total number of customers
Transformer SAIDI (T-SAIDI)
T-SAIDI is used in Finland for mean duration of interruptions in se
...