As simple as 1,2,3

The British and European Standard BS EN 60898-1 is referred to in the industry as the standard for miniature circuit-breakers (mcbs). Its scope states that they are designed for use by uninstructed people and for not being maintained. Many tests are therefore conducted on circuit-breakers to check and ensure the safety of the person who returns the device to service. More often than not that person is uninstructed whether the installation is in domestic or commercial premises.

To comply with BS 7671 (16th Edition of the IEE’s Wiring Regulations), it is essential that those responsible for electrical circuit design and installation are fully aware of, and can apply the standard information provided to, circuit design.

BS EN 60898-1 prescribes that certain circuit-breakers may be classified into energy limiting classes. This is where the number 3 that often appears on the labelling comes in and this number could equally be 1, 2 or 3. The number refers to the energy limiting class of the particular device on which it appears. This information helps the design engineer with cable and discrimination calculations.

A typical example of the marking and other product information shown on mcbs is illustrated in figure 1. The rated current together with the instantaneous tripping symbol, in this case C32, has to be readily visible when the circuit-breaker is installed. For small devices, other information can be put on the side or back of the circuit-breaker.

Circuit-breakers of B and C type, having rated current up to 40 A and short circuit breaking capacities of 3, 4·5, 6 and 10 kA, may be classified according to the limits within which their I2t characteristics lie.

I2 represents the electrodynamic stress generated, which can force busbars apart and take cables off their cleats. I2t is proportional to the energy let through the device of which the thermal effects increase the conductor temperature and may, if not correctly assessed, result in an upstream fuse element melting, leading to a loss of electrical supply.

I2t is measured according to the specified tests in the standard (see table 1) for examples of maximum permissible values.

The circuit-breakers are classified into these energy-limiting classes to help the design engineers and installers with selection. Frequently it is necessary to obtain discrimination with the fuse on the supply side and evaluate cable fault current protection.

Energy limiting class 3 is in fact top of the class, providing the lowest energy let-through into the circuit. Engineers should be aware that these are maximum values.

Frequently, mcbs will exceed the standard. For example, with the Hager 16 A 10 kA C-Type, I2t = 29 kA2S; this is a 65% reduction in energy let-through when compared with the product standard. This information provides the engineer with an accurate and time-saving tool for circuit design.

The following examples demonstrate the application of I2t in circuit design.

Example 1: Fault current discrimination case study

BS 7671 provides the requirements for discrimination for electrical installations:

• Regulation 314-01-01 – every installation shall be divided into circuits as necessary to avoid danger and minimise inconvenience in the event of a fault;
• Regulation 314-01-02 – due account shall be taken of the consequences of the operation of any single protective device;
• Regulation 533-01-06 – where necessary to prevent danger, the characteristics and setting of a device for overcurrent protection shall be such that any intended discrimination in its operation is achieved.

From these regulations it is clear that the continuity of electrical supply to circuits which are electrically sound could be considered essential in the event of a fault on another circuit. Only the protective device closest to the fault operates, thus achieving discrimination.

For fault currents, the discrimination of a circuit-breaker, in relation to the fuse on the supply side, exists up to the current value, where the I2t value let through by the circuit-

breaker is less than the pre-arcing I2t of the fuse. This can be shown mathematically:

Downstream mcb I2t let through < Upstream fuse pre-arcing I2t

Information from the energy-limiting class can now be applied to figure 2.

First, a quick check to verify compliance with Regulation 434-03-01 – breaking capacity shall not be less than the prospective fault current. The mcb rated short circuit breaking capacity (Icn) is 10 kA ensuring full compliance.

Referring to table 1, the I2t let through for the mcb is 84 kA2S. Now using the formula:

mcb let through < pre-arc fuse then 84 kA2S < 101 kA2S.

This proves that fault current discrimination is achieved.

Example 2: Cable protection study on final circuit conductor in figure 2

The circuit is unlikely to carry overload current due to the characteristics of the load; therefore only fault current protection is required.

This is endorsed in Regulation 473-01-04(ii) – “devices for protection against overload need not be provided for a conductor, because of the characteristics of the load they are not likely to carry overload current”.

This may result in cost savings in conductor size as the design current is used to size its current carrying capacity, but voltage drop, protection against indirect contact and protection against fault current must also be checked.

We can assume for the final circuit conductor in figure 2 that the current carrying capacity voltage drop and protection against indirect contact has been calculated, resulting in the selection of a 4 mm2 thermoplastics (pvc) insulated copper conductor.

Correction factors result in the IZ (current carrying capacity under installed conditions) being 14·4 A. This is less than the nominal rating of the mcb, but as protection against overload current need not be provided this will not deviate from the regulations.

As the mcb is provided for fault current protection only, we must ensure that it protects the conductors from thermal damage and fully complies with the fault current regulations.

Regulation 434-03-03 provides the formula that can, as an approximation, be used for assessing protection against fault current:

t = k2S2/I2

where t is the maximum duration of fault current expressed in amperes; S is the nominal cross-sectional area of the conductor in mm2; I is the value of fault current in amperes rms and k is a factor dependent upon the conductor and insulation material.

When the operating time of the protective device is less than 0·1 seconds, where asymmetry of the current is of importance, the value of k2S2 for the cable shall be greater than the I2t let through of the device as quoted by the manufacturer. It is not the fault current squared multiplied by the disconnection time.

This can be expressed mathematically as: k2S2 >I2t.

It is necessary to determine the operating time and maximum let through energy of the mcb at minimum and maximum fault current and to ensure that the conductor can withstand the highest value (table 2).

To calculate the thermal withstand of the 4 mm2 conductor we must extract the k factor from table 43A of the regulations. For a copper conductor with 70° thermoplastics (general purpose pvc) insulation this is 115. The k2S2 is therefore 1152 x 42 which equals 211 600.

We can compare this value to the highest I2t value in table 2 to quickly and easily verify full protection, since 211 600 A2S > 84 000 A2S.

Conclusion

BS EN 60898-1 has given extensive application and design information to the design engineer. From which source the design engineer extracts this information may be of commercial interest and consideration should be given to the following:

• the values derived from the energy limiting class in the standard could be applied to any manufacturer’s mcb in that class, requiring only one set of figures;
• mcbs will often surpass the maximum values laid down in the standard resulting in lower let through energy. Incorporating this lower value into the circuit design could lead to improved discrimination and cable protection providing a more economic design. Manufacturers’ data is normally easily available in tabular form.

Whichever way engineers and contractors now view the number in the little square box they can appreciate that specifying an energy-limiting class 3 mcb will considerably reduce the thermal effects and electrodynamic forces created by large fault currents.

Hager runs a free training seminar from its five regional centres to help full understanding of BS EN 60898-1. It covers mcb labelling, tripping characteristics, energy-limitation class and their application in circuit design.