Selectivity with fuses

How to make selectivity with fuses

Here you can find the main rules to design selectivity between fuses or fuses and breakers. Three combinations are possible:

Upstream fuse and downstream fuse
How to design selectivity in the overload and in the short-circuit zone
Upstream circuit-breaker and downstream fuse
How to design selectivity in the overload and in the short-circuit zone
Upstream fuse and downstream circuit-breaker

Fuse / Fuse

Breaker / Fuse

Fuse / Breaker

Upstream fuse and downstream fuse

Selectivity in the overload zone

Selectivity in the short-circuit zone

Fuses with the same utilization category and different current rating have parallel characteristics; selectivity is obtained by comparison of time-current characteristics.

Time-current curves must be checked for fuses with different utilization category (e.g aM and gG), no overlapping shall be present.

This verification of selectivity between fuses is valid for operating times t ≥ 0.1 s.

Selectivity is determined from the let through energy I²t characteristics.

The selectivity limit current is the value at which the total operating I²t of the downstream fuse exceeds the pre-arcing I²t of the upstream fuse.

In the short-circuit zone the let through energy of a fuse is constant and therefore the comparison may be made from the tabulated figures provided by the manufacturer. Pre-arcing I²t is independent of voltage and current in this zone. However, arcing I²t is dependent on system voltage, thus affecting the total operating value.

In case of fuses of the same type (e.g.: gG – gG) and compliant with IEC 60269-2, total selectivity is verified when the rated current of upstream fuse is 1.6 times the rated current of the downstream fuse.

Upstream circuit-breaker and downstream fuse

Selectivity in the overload zone

Selectivity in the short-circuit zone

Selectivity is obtained by comparing time-current characteristics

It is advisable to maintain a safety margin between the maximum tripping time of the fuse characteristic and the minimum tripping time of the circuit-breaker characteristic.

Generally, a delta time ≥ 1s is enough for thermal-magnetic breakers and 100 ms in case of electronic trip units.

Selectivity in the short-circuit zone may be determined by the comparison of time-current characteristics in case of circuit-breakers with time-delay short-circuit protection release (S protection, ANSI Code 50TD).

In the zone of instantaneous tripping and for a circuit-breaker without short-circuit time-delay release, the selectivity limit current must be determined from the test data provided by the manufacturer.

In the absence of specific test data, selectivity is assured up to the fault current level at which the let-through peak current of the downstream fuse is less than the peak value corresponding to the instantaneous tripping level of the upstream device.

Ipk (fuse) ≤ I3 (CB)

Upstream fuse and downstream circuit-breaker

Selectivity in the overload zone

Selectivity in the short-circuit zone

This occurs mainly when an upstream fuse is used for the back-up protection of circuit- breakers in case of exceptionally high prospective fault levels.

In the overload zone selectivity is obtained by comparison of time-current characteristics.

It is related to the let through energy I²t characteristics.

The selectivity limit current is the value at which the let-through energy I²t of the circuit-breaker exceeds the pre-arcing I²t of the fuse (curve or value provided by the manufacturer).

Advice:
To ensure full selectivity between the upstream fuse and the downstream circuit-breaker, we recommend replacing the upstream fuse after clearance of short-circuit by the downstream circuit-breaker.

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.

4.1 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.

4.2 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.

4.2.1 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.

4.2.2 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.

For more information see the IEC/TR 61912-2 Edition 1.0

Zone selectivity

Introduction

Operatively speaking

This type of coordination is an evolution of time coordination.
In general, zone selectivity is realized by means of dialogue between the current measuring de-vices that, once the setting threshold has been detected as having been exceeded, allows the fault zone to be identified correctly and the power supply to the fault to be cut off.

It can be realized in two ways:

The measuring devices send the information on the current setting threshold having been exceeded to a supervision system and the latter identifies which protection has to intervene;
When there are current values exceeding their setting, each protection sends a lock signal by means of a direct connection or a bus to the hierarchically higher level protection (on the supply side in relation to the power flow direction) and, before intervening, checks that a similar lock signal has not arrived from the load-side protection. Thus, only the protection immediately on the supply side of the fault trips.

This second way guarantees without doubt shorter trip times. Compared with a coordination of the time type, the need to increase the intentional delay as one moves towards the power supply source is no longer necessary. The delay can be decreased to the time needed to exclude the presence of a possible lock signal coming from the load-side protection.
This type of selectivity is suitable for radial networks and, when associated with directional protec-tion, is suitable for meshed networks too.

Compared with the coordination of time type, zone selectivity permits:

a reduction in the trip times (these can be lower than hundred milliseconds);
a reduction both in the damage caused by the fault and in the interferences in the power supply system;
a reduction in the thermal and dynamic stresses on the components of the installation;
a very high number of selectivity levels.

However:

it is more burdensome both in terms of cost and of complexity of the installation;
it requires an auxiliary supply.

As a result, this solution is mainly used in systems with high rated current and short-circuit current values and with binding requirements for safety and service continuity. In particular, examples of logical selectivity are often found in the primary distribution switchgear placed closely on the load side of transformers and generators.

This type of selectivity can be realized:

between Emax 2 air circuit-breakers equipped with Ekip Touch and Ekip Hi-Touch trip units.
The ultimate selectivity limit which can be obtained is equal to the Icw
Is = Icw

between Tmax T4L,T5L and T6L moulded-case circuit-breakers equipped with PR223 EF trip units.
The ultimate selectivity limit which can be obtained is 100kA
Is = 100kA

Then, by means of S51/P1 contact module, it is possible to make a chain of zone selectivity between Tmax and Emax 2. It is also possible to realize a selectivity chain including ABB MV protections.

The operating principle of zone selectivity between ABB circuit-breakers is as follows:
In the presence of current values higher than their setting, each protection sends a lock signal (by means of a direct connection or a bus) to the hierarchically higher level protection (on the supply side in relation to the power flow direction) and, be-fore tripping, checks that an analogous lock signal has not arrived from the load-side protection. In this way, only the protection immediately to the supply side of the fault trips.

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