From Table B6 of the OSG, the maximum measured Z s value is1.75 Ω at an ambient temperature of 10 oC. IEC 60947-2 governs circuit breakers (CBs) for industrial applications. They protect electrical power distribution of up to 1000 volts a.c. and 1500 volts d.c. with an entire spectrum of rated currents from 0,5 to 6300A. Utilities and manufacturing facilities often use air circuit breakers (ACBs), molded case circuit breakers (MCCBs), and miniature circuit breakers (MCBs)

For example, the BS EN 60898-1 standard describes B, C and D operating curves with ratio to rated current. But the BS EN 60947-2 standard prescribes an instantaneous tripping release which may be provided with a plus or minus 20% tolerance and adjustable as illustrated in Fig. 1. For this reason, manufacturers provide additional curves K, Z and MA to the B, C and D curves. The maximum Z s values to achieve disconnection time vary with different types of protective devices and also between manufacturers. Appendix 3 of BS 7671 recommends that wherever possible designers use manufacturer- specific data. Table B6 of the OSG and Table A4 of GN 3 contain maximum Z s values for circuit-breakers based on the cross-sectional area (csa) of the protective conductor and at an assumed conductor temperature of 10°C. The table is based on nominal rather than actual voltage - 230V nominal is usually taken to be anything within the -6% to +10% range - i.e. between 216.2V and 253V. It's very common for DNOs to have far above 230V at their transformers to compensate for voltage drop in the distribution system - typically a 433/250V. So as long as your supply doesn't go above 253V (during times of low load say) I'd say it's acceptable to use the 230V nominal column (e.g. 0.4s for TN small final circuits). In many European countries there is no table, they only have the formula in their national standards. Note that BS7671:2018 clause 411.4.202 does state that the permissible Zs can be calculated and in my opinion it is the best method to ensure that we are not failing Zs values that are still low enough to achieve the required disconnect time. I will give you an example:Circuit-breakers to BS EN 60947-2 are intended for operation by skilled users and they must be maintained. They are suitable for Pollution Level 3, which includes unheated rooms, boiler rooms, industrial and farming areas.

Alternatively, the maximum Z s value can calculated using the time- current characteristic of the MCCB being used, as shown in Fig. 2 for a 32 A MCCB. These certified CBs are intended for indoor use, and under pollution and humidity-free conditions: households or similar installations where overcurrent protection will not likely be maintained by unskilled users. In other words – in the final distribution electrical switchboards of buildings in which the nominal current does not exceed 125A. Usually, these circuit breakers are sold by electrical retailers: they are simple to install, safe, and easy to use even after many years without maintenance. Example of Acti9 circuit breaker used for buildings and industry applications The values of earth loop impedance shown in these tables must compensate for conductor temperature rise, if the measurement of loop impedance is taken at ambient temperature. A useful rule of thumb is to allow for a temperature rise from 20 degrees to 70 degrees by multiplying the listed value by 0.8. The measured value can then be compared to the compensated value. The most suitable solution is for MCBs that are certified by both IEC standards since their performance meets requirements for use in residential installations and is high enough for use in industry and infrastructure applications. Due to the high level of protection performance of these CBs, they should be used at least for the incoming electrical switchboards in buildings applications. Appliance for MCB’s to standards in different applications of useHow comes BS 88 and bs1361 fuses have two tables for 0.4s and 5S but 60898 fuses have one which covers both. Although cbs from different manufacturers may appear similar, the technical performance, dimensions and terminations are not always compatible. Or take the tripping characteristics: IEC 60898-1 clearly describes B, C, and D curves with ratio to rated current, while in IEC 60947-2 the instantaneous tripping release may be adjustable according to the need of the user, or pre-defined by the manufacturer, with ±20% tolerance. This is the reason why manufacturers additionally provide a wide scope of different curves: K, Z, MA. Yes. The rated voltage currently required in industrial use CBs is 400 V, 440 V, 690 volts, or higher values up to 1000 V. Compare those numbers to the usual value 230V/400 V for residential MCBs. Reference ambient temperature is 30°C for households. The same goes for impulse withstand voltage (Uimp), IEC 60898-1 requires 4kV, in line with the use for final circuits. Whereas for industrial circuit breakers, the usual values of Uimp are 6 or 8kV, in line with the position of the circuit breaker at the origin of the installation. I can say is that mix-ups of standards happen. The ideal way to avoid them is with tightly worded and highly enforced national regulations. And, for prescribers to specify what CBs will be used for and then confirm that the standard governs that usage.

Where manufacturers’ values of Z s for MCBs are used, they will often recommend multiplying the measured value of Z s by a correction factor, to correct for ambient temperature. For example, Schneider, recommend using a factor of 0.8, However when I look at table 41.1 it is stating that if Uo is 240Vac the permissible disconnection time is only 0.2 seconds. Notes 2 and 3 at the end of Tables 41.2, 41.3 and 41.4 in BS 7671 indicate that the maximum Z s values in those tables should not be exceeded when the line conductors are at the appropriate maximum permitted temperature (70 o C for thermoplastic). The figures in these tables can be used for design purposes.Typically IEC 60898-1 certified circuit breakers meet required performance to prove proper protection of household installations: Pollution degree 2, impulse voltage 4kV, isolation voltage is the same as nominal voltage 400V. Also, the requirements of this standard are adapted for non-technical users which is why limited technical information is printed in detail on CBs. Circuit-breakers to BS EN 60898-1 can also comply with BS EN 60947-2 but the short-circuit breaking capacity of each may be different. Some manufacturers state that their cbs with a short-circuit capacity of say, 10 kA comply with BS EN 60898-1 and the same cb with a short-circuit capacity of 15 kA complies with BS EN 60947-2. Maximum Z s values for MCCBs to BS EN 60947-2 are not included in BS 7671. Therefore, manufacturers’ tables should be consulted for the rating of the MCCB to be used.

In response to the question “What’s the difference between standards IEC 60898-1 and IEC 60947-2?”, I’m tending to answer with another question: What do they have in common? They are both standards that specify requirements for low-voltage circuit breakers. Is there more in common than different? Let’s look into it. Which one is for homes, which one is for industry? I am being told by an inspector that we have to use 230Vac when performing permissible earth loop impedance calculations as per BS7671:2018 clause 411.4.4 Zs × Ia ≤ U0 × Cmin. It is convenient when the performance of a cb meets the requirements of both standards and is therefore suitable for residential, industrial and commercial installations. That's quite a large difference and hence the reason why the national standards in other European countries do not include Zs tables, only the formula is provided.An MCCB with a rated current value (I n) of 32 A is connected to a circuit forming part of a TN supply system which has a U 0 value of 230 V. The MCCB is required to disconnect in 0.4 seconds. Calculate the maximum Z s value. BS 3871 circuit breakers were also manufactured as Type 4. Generic data cannot be used for Type 4 circuit breakers and the manufacturer’s data must be used to calculate maximum values of earth loop impedance. The more common forms of Type 4 are listed in the following tables, together with their maximum values of Z s, for 0.4s and 5s disconnection times. FEDERAL Where a more frequent switching is required, such as the switching on and off of banks of luminaires, the manufacturer’s instruction should be followed and preferably an alternative device should be selected. However if we calculate it we get a higher permissible Zs while still being in compliance with BS7671 411.4.202.