Standard requirements

The main definitions and requirements about selectivity from the international Standards.

Here you can find the main information and definitions about selectivity from the standards:

IEC 60947-1
“Low-voltage switchgear and controlgear – Part 1: General rules”.
IEC 60947-2
“Low-voltage switchgear and controlgear – Part 2: Circuit-breakers”.
IEC 60947-2 – Annex A
IEC/TR 61912-2
“Low-voltage switchgear and controlgear – Over-current protective devices – Part 2: Selectivity under over-current conditions”.
Technical Guide CEI 99-4
“Guidelines for the execution of medium/low voltage electrical substations of final customers/users”.

IEC 60947-1

IEC 60947-2

IEC 60947-2 – Annex A

IEC/TR 61912-2

Technical Guide CEI 99-4

In the Standard IEC 60947-1 there are the following definitions related to selectivity

Over-current protective co-ordination of over-current protective devices (2.5.22)
co-ordination of two or more over-current protective devices in series to ensure overcurrent discrimination (selectivity) and/or back-up protection

Over-current selectivity (2.5.23)
co-ordination of the operating characteristics of two or more over-current protective devices such that, on the incidence of over-currents within stated limits, the device intended to operate within these limits does so, while the other(s) does (do) not

In the Standard IEC 60947-2 are given the following definitions about selectivity

Over-current selectivity (2.17.1)
co-ordination of the operating characteristics of two or more over-current protective devices such that, on the incidence of over-currents within stated limits, the device intended to operate within these limits does so, while the other(s) does (do) not

Total selectivity (2.17.2)
over-current selectivity where, in the presence of two over-current protective devices in series, the protective device on the load side effects the protection without causing the other protective device to operate

Partial selectivity (2.17.3)
over-current selectivity where, in the presence of two over-current protective devices in series, the protective device on the load side effects the protection up to a given level of over-current, without causing the other protective device to operate

Selectivity limit current Is (2.17.4)
The selectivity limit current is a limiting value of current:

below which, in the presence of two over-current protective devices in series, the protective device on the load side completes its breaking operation in time to prevent the other protective device from starting its operation (i.e. selectivity is ensured);
above which, in the presence of two over-current protective devices in series, the protective device on the load side may not complete its breaking operation in time to prevent the other protective device from starting its operation (i.e. selectivity is not ensured).

Selectivity categories (4.4)
The selectivity category of a circuit-breaker shall be stated with reference to whether or not it is specifically intended for selectivity by means of an intentional time delay with respect to other circuit-breakers in series on the load side, under short-circuit conditions.

Selectivity category A: type of circuit-breaker without Icw (indicated for the energy selectivity)
Selectivity category B: type of circuit-breaker with Icw (indicated for the time selectivity)
Icw: rated short-time withstand current (for more information see the item 4.3.5.4 of IEC 60947-2 Edition 4.2)

For more information, see the items 4.4 and the Table 4 of the Standard IEC 60947-2 Edition 4.2

Co-ordination under short-circuit conditions between a circuit-breaker and another short-circuit protective device associated in the same circuit
This annex gives guidance on and requirements for the co-ordination of circuit-breakers with other SCPDs (short-circuit protective devices) associated in the same circuit, as regards selectivity as well as back-up protection.

The object of this annex is to state:

the general requirements for the co-ordination of a circuit-breaker with another SCPD;
the methods and the tests (if deemed necessary) intended to verify that the conditions for co-ordination have been met.

To ensure co-ordination under short-circuit conditions between a circuit-breaker (C1) and another short-circuit protective device (SCPD) associated with it in the same circuit, it is necessary to consider the characteristics of each of the two devices as well as their behavior as an association.

NOTE: An SCPD may incorporate additional protective means, for example, overload releases.

The SCPD may consist of a fuse (or a set of fuses) or of another circuit-breaker (C2) (see figure below).

C1 = Current-limiting circuit-breaker (L) (break-time characteristic)
C2 = Non-current-limiting circuit breaker (N) (tripping characteristic)

C1 = Non-current-limiting circuit-breaker (N) (break-time characteristic)
C2 = Circuit-breaker with intentional short-time delay (STD) (tripping characteristic)

Total selectivity between two circuit-breakers

The comparison of the individual operating characteristics of each of the two associated devices may not be sufficient, when reference has to be made to the behaviour of these two devices operating in series, since the impedance of the devices is not always negligible. It is recommended that this should be taken into account. For short-circuit currents it is recommended that reference be made to I²t instead of time.

The term "co-ordination" includes consideration of selectivity (see 2.5.23 of IEC 60947-1 and also 2.17.2 and 2.17.3) as well as consideration of back-up protection. Consideration of selectivity can either be carried out by desk study or by test, whereas the verification of back-up protection normally requires the use of tests.

3.1 Verification of selectivity
Selectivity can normally be considered by desk study alone, i.e. by a comparison of the operating characteristics of C1 and the associated SCPD, for example, when the associated SCPD is a circuit-breaker (C2) provided with an intentional time-delay. The manufacturers of both the C1 and the SCPD shall provide adequate data concerning the relevant operating characteristics so as to permit Is to be determined for each individual association.
In certain cases, tests at Is may be performed on the association, for example, when C1 is of the current-limiting type and C2 is not provided with an intentional time delay.
To obtain the desired selectivity when the associated SCPD is a circuit-breaker, an intentional short-time delay may be necessary for C2.
Selectivity may be partial or total up to the rated short-circuit breaking capacity Icu of C1. For total selectivity, the non-tripping characteristic of C2 shall lie above the tripping (break-time) characteristic of C1.

For more information, see the Annex A of the IEC 60947-2 Edition 4.2

This technical report, which serves as an application guide for the determination of selectivity between over-current protective devices of low-voltage switchgear and controlgear, summarises the definitions of the terminology and provides examples of application. This report does not deal with other forms of protection, such as power-reversal protection, directional protection and arc-protection systems.

Selectivity between circuit-breakers

Methods for the determination of selectivity between circuit-breakers, for the overload zone of operation, and for the fault current zone of operation, are given below.

UD = Upstream device:
in considering selectivity between two OCPDs (over-current protective devices), the OCPD connected in the circuit nearest to the source of supply.

DD = Downstream device:
in considering selectivity between two OCPDs (over-current protective devices), the OCPD connected in the circuit immediately following the upstream device, on the load side.

Overload zone (of over-current):
range of current, exceeding the rated current of the OCPD (over-current protective device), produced by the circuit loading in the absence of a fault in the circuit.

Fault current zone (of over-current):
range of current exceeding the overload current, produced by a fault in the circuit.

5.1.1.1 Circuit-breakers – Selectivity in the overload zone
Selectivity in the overload zone is verified by comparison of the time/current characteristics, see Figure 1a and Figure 1b (Figure 1b is applicable to moulded case circuit-breaker and air circuit-breaker only). Separation of the characteristics in both the time and current axes ensures selective operation of DD with respect to UD, in this zone. There will be a tolerance applicable to the characteristic, which should be taken into account. The manufacturer’s data should show a tolerance band or otherwise indicate the tolerance applicable, as required by the product standard.

Comparison of the operating characteristics of circuit-breakers in the overload zone.

Figure 1a

Comparison of thermal time/current characteristics in the overload zone

Figure 1b

Comparison of electronic time/current characteristics in the overload zone

NOTE:

The characteristics are subject to tolerances, which must be taken into account
It is necessary that the current scales are in amperes (or kA) for comparison of these curves. Manufacturer’s published characteristics may be given in either amperes or multiples of rated current.
A combination of thermal/magnetic and electronic characteristics is also commonly used.

5.1.1.2 Circuit-breakers – Selectivity in the fault current zone
Selectivity between circuit-breakers is elaborated in the product standards IEC 60947-2, Annex A and IEC 60898-1, Annex D, specifying the tests, where applicable, which show determination of selectivity in the fault-current zone. The methods applicable are mentioned hereunder.

NOTE:

Due to their basic construction MCBs to IEC 60898-1 are generally highly selective against MCCBs to IEC 60947-2.

5.1.1.2.1 Circuit-breakers – Selectivity determination in the fault current zone by comparison of characteristics
Determination from characteristics of selectivity between two circuit-breakers in the fault current zone, is limited to the case where UD has a short-circuit release time-delay function provided by an electronic release. Selectivity at fault currents producing instantaneous tripping of UD, for both electronic and electromagnetic releases, needs to be determined from test data provided by the manufacturer.

5.1.1.2.2 Circuit-breakers – Selectivity determination by test in the fault current zone at fault currents producing instantaneous tripping
For each combination of circuit-breakers, the selectivity limit currents is determined from test and the manufacturer will supply data, usually in the form of a chart. In each case selectivity may be total or partial as follows:

total selectivity: this means selectivity for all over-currents up to the value of the breaking capacity, i.e. DD alone will operate (to the TRIPPED position) on over-current up to this level.
partial selectivity: this means selectivity up to a value of over-current less than the breaking capacity of DD. The selectivity limit current is obtained by comparison of time/current characteristics or, in the case of instantaneous tripping of UD, from data obtained from tests and provided by the manufacturer.

NOTE:

The data for selectivity limit current in the fault current zone producing instantaneous tripping of the circuit-breaker is obtained from test data and is specific to each device type. There is no recognized method of substitution for devices of different manufacture.
Proprietary software systems for determination of selectivity are available, working from time/current data supplied by individual manufacturers.

Selectivity with low voltage fuses

The Technical Report includes all the requirements for the verification of coordination in the presence of fuses, in particular:

Section 5.1.2.1 prescribes verification of selectivity for combination of upstream circuit- breaker and downstream fuse in the overload zone.
Section 5.1.2.2 prescribes verification of selectivity for combination of upstream circuit-breaker and downstream fuse in the short-circuit zone.
Section 5.2.1.1 defines selectivity between upstream fuse and downstream circuit-breaker in the overload zone.
Section 5.2.1.2 defines selectivity between upstream fuse and downstream circuit-breaker in the short-circuit zone.
Section 5.2.2 explains selectivity between fuses in the overload and short-circuit zones.

The Technical Guide CEI 99-4, even if intended for the Italian market and rules, is based on a series of IEC Standards giving important prescriptions and rules for the design and construction of MV/LV transformer substations of end customers/users (passive users).
In particular, Annex F of the Guide is focused on the main aspects of the selective coordination of protection devices: it defines the different methods of selective coordination and gives some practical examples on how selectivity can be achieved for MV/LV supplies.

The methods to realize the selective coordination between protection devices are:

Coordination through variation of tripping times:
This coordination is achieved by using protective devices with dependent- or independent-time tripping characteristics, or a combination of both, suitably adjusted by increasing the tripping times as the devices get near to the power source.

Coordination through different fault currents:
This type of coordination is particularly suitable in case of protection devices positioned at the two ends of interconnections with significant impedance (transformers, current limiting reactances, low voltage lines). The selectivity between the two devices is guaranteed for all current values:

I > a • I''_k2

where:

I''_k2is the short-circuit current in case of fault downstream of the impedance
a is the coefficient taking into account current asymmetry.

For the current values which verify the above formula, the protection device on the supply side does not need intentional delays. For lower current values, selectivity can be achieved through timing.

Coordination by limiting the let-through energy:
This coordination is carried out in LV systems by using protection devices in series, coordinated with each other, so that the energy limited by the device placed downstream is less than the energy required to cause tripping of the device upstream. This type of selective coordination is achieved by current-limiting fusegears and circuit- breakers with instantaneous trip unit.

Coordination through dialogue (data exchange):
This type of selective coordination is achieved using protection devices connected in series by means of pilot wire or IEC 61850 protocol. The devices that detect a current flow higher than the tripping threshold send a wait or locking signal / message to block the device positioned immediately upstream. The device placed immediately upstream of the fault does not get the wait or locking signal / message and thus trips only after the delay required for data exchange. This makes it possible to keep the same tripping time for all the devices in the selectivity chain and makes this coordination particularly suitable when several devices are used.

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