Other characteristics of a circuit-breaker

Familiarity with the following characteristics of LV circuit-brneakers is often necessary when making a final choice.

Rated insulation voltage (Ui)

This is the value of voltage to which the dielectric tests voltage (generally greater than 2 Ui) and creepage distances are referred to.

The maximum value of rated operational voltage must never exceed that of the rated insulation voltage, i.e. Ue ≤ Ui.

Rated impulse-withstand voltage (Uimp)

This characteristic expresses, in kV peak (of a prescribed form and polarity) the value of voltage which the equipment is capable of withstanding without failure, under test conditions.

Generally, for industrial circuit-breakers, Uimp = 8 kV and for domestic types, Uimp = 6 kV.

Category (A or B) and rated short-time withstand current (Icw)

As already briefly mentioned there are two categories of LV industrial switchgear, A and B, according to IEC 60947-2:

• Those of category A, for which there is no deliberate delay in the operation of the “instantaneous” short-circuit magnetic tripping device (see Fig. H35), are generally moulded-case type circuit-breakers, and

Fig. H35 – Category A circuit-breaker

• Those of category B for which, in order to discriminate with other circuit-breakers on a time basis, it is possible to delay the tripping of the CB, where the fault-current level is lower than that of the short-time withstand current rating (Icw) of the CB (see Fig. H36). This is generally applied to large open-type circuit-breakers and to certain heavy-duty moulded-case types. Icw is the maximum current that the B category CB can withstand, thermally and electrodynamically, without sustaining damage, for a period of time given by the manufacturer.

Fig. H36 – Category B circuit-breaker

Rated making capacity (Icm)

Icm is the highest instantaneous value of current that the circuit-breaker can establish at rated voltage in specified conditions. In AC systems this instantaneous peak value is related to Icu (i.e. to the rated breaking current) by the factor k, which depends on the power factor (cos φ) of the short-circuit current loop (as shown in Figure H37 ).

Icu	cosφ	Icm = kIcu
6 kA < Icu ≤ 10 kA	0.5	1.7 x Icu
10 kA < Icu ≤ 20 kA	0.3	2 x Icu
20 kA < Icu ≤ 50 kA	0.25	2.1 x Icu
50 kA ≤ Icu	0.2	2.2 x Icu

Fig. H37 – Relation between rated breaking capacity Icu and rated making capacity Icm at different power-factor values of short-circuit current, as standardized in IEC 60947-2

Example: A Masterpact NW08H2 circuit-breaker has a rated breaking capacity Icu of 100 kA. The peak value of its rated making capacity Icm will be 100 x 2.2 = 220 kA.

Rated service short-circuit breaking capacity (Ics)

In a correctly designed installation, a circuit-breaker is never required to operate at its maximum breaking current Icu. For this reason a new characteristic Ics has been introduced.
It is expressed in IEC 60947-2 as a percentage of Icu (25, 50, 75, 100%)

The rated breaking capacity (Icu) or (Icn) is the maximum fault-current a circuit-breaker can successfully interrupt without being damaged. The probability of such a current occurring is extremely low, and in normal circumstances the fault-currents are considerably less than the rated breaking capacity (Icu) of the CB. On the other hand it is important that high currents (of low probability) be interrupted under good conditions, so that the CB is immediately available for reclosure, after the faulty circuit has been repaired. It is for these reasons that a new characteristic (Ics) has been created, expressed as a percentage of Icu, viz: 25, 50, 75, 100% for industrial circuit-breakers. The standard test sequence is as follows:

• O - CO - CO (at Ics)
• Tests carried out following this sequence are intended to verify that the CB is in a good state and available for normal service

For domestic CBs, Ics = k Icn. The factor k values are given in IEC 60898 table XIV.

In Europe it is the industrial practice to use a k factor of 100% so that Ics = Icu.

Fault-current limitation

Many designs of LV circuit-breakers feature a short-circuit current limitation capability, whereby the current is reduced and prevented from reaching its (otherwise) maximum peak value (see Figure H38). The current-limitation performance of these CBs is presented in the form of graphs, typified by that shown in Figure H39, diagram (a)

The fault-current limitation capacity of a CB concerns its ability, more or less effective, in preventing the passage of the maximum prospective fault-current, permitting only a limited amount of current to flow, as shown in Figure H38.

Fig. H38 – Prospective and actual currents

The current-limitation performance is given by the CB manufacturer in the form of curves (see Fig. H39).

• Diagram (a) shows the limited peak value of current plotted against the rms value of the AC component of the prospective fault current (“prospective” fault-current refers to the fault-current which would flow if the CB had no current-limiting capability)
• Limitation of the current greatly reduces the thermal stresses (proportional I²t) and this is shown by the curve of diagram (b) of Figure H39, again, versus the rms value of the AC component of the prospective fault current.

LV circuit-breakers for domestic and similar installations are classified in certain standards (notably European Standard EN 60 898). CBs belonging to one class (of current limiters) have standardized limiting I²t let-through characteristics defined by that class.

In these cases, manufacturers do not normally provide characteristic performance curves.

Fig. H39 – Performance curves of a typical LV current-limiting circuit-breaker

• 

  [a]

• 

  [b]

The advantages of current limitation

Current limitation reduces both thermal and electrodynamic stresses on all circuit elements through which the current passes, thereby prolonging the useful life of these elements. Furthermore, the limitation feature allows “cascading” techniques to be used (see Coordination between circuit-breakers) thereby significantly reducing design and installation costs

The use of current-limiting CBs affords numerous advantages:

• Better conservation of installation networks: current-limiting CBs strongly attenuate all harmful effects associated with short-circuit currents
• Reduction of thermal effects: Conductors (and therefore insulation) heating is significantly reduced, so that the life of cables is correspondingly increased
• Reduction of mechanical effects: forces due to electromagnetic repulsion are lower, with less risk of deformation and possible rupture, excessive burning of contacts, etc.
• Reduction of electromagnetic-interference effects:
  • Less influence on measuring instruments and associated circuits, telecommunication systems, etc.

These circuit-breakers therefore contribute towards an improved exploitation of:

• Cables and wiring
• Prefabricated cable-trunking systems
• Switchgear, thereby reducing the ageing of the installation

Example

On a system having a prospective shortcircuit current of 150 kA rms, a Compact L circuit-breaker limits the peak current to less than 10% of the calculated prospective peak value, and the thermal effects to less than 1% of those calculated.

Cascading of the several levels of distribution in an installation, downstream of a limiting CB, will also result in important savings.

The technique of cascading allows, in fact, substantial savings on switchgear (lower performance permissible downstream of the limiting CB(s)) enclosures, and design studies, of up to 20% (overall).

Discriminative protection schemes and cascading are compatible, in the Compact NSX range, up to the full short-circuit breaking capacity of the switchgear.