Standard requirements

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).

Selectivity between circuit-breakers

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.

1. The characteristics are subject to tolerances, which must be taken into account
2. 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.
3. 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.

1. 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.

1. 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 noe the IEC/TR 61912-2 Edition 1.0 recognized method of substitution for devices of different manufacture. 
2. Proprietary software systems for determination of selectivity are available, working from time/current data supplied by individual manufacturers.