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The New Fuse Standards and the Maximum Zs Tables
Amendment 1 of BS 7671:2008 contains some important changes concerning new fuse standards which took effect in March 2010. These changes can be seen in the maximum earth fault loop impedance (Zs) values in Tables 41.2 and 41.4. (Table 41.3 concerns circuit-breakers) The new fuse standards are:
BS 88-2 which has replaced BS 88-2.2 and BS 88-6.
BS 88-3 which has replaced BS 1361.
(However, fuses complying with these withdrawn standards will be found in existing electrical installations for some years to come.)
Having noted these changes, we are provided with a convenient opportunity to consider how these maximum Zs values in Tables 41.2, 41.3 and 41.4 are obtained.
This article will look at the maximum Zs values for BS88-3 fuses in Table 41.4(b) as an example of how these maximum Zs values are obtained.
The maximum earth loop impedance, Zs values are arrived at using the formula in Regulation 411.4.5:
Zs x Ia ≤ Uo
Zs = the earth fault loop impedance in ohms
Ia = the current in amps which causes the protective device to operate within the time specified in Table 41.1 (or, if appropriate, within the 5 second disconnection time specified in Regulation 4126.96.36.199 for TN systems).
Where an RCD is used, the current Ia is the rated residual operating current which provides disconnection within the time specified in Table 41.1 (or, if appropriate, within the 5 second disconnection time specified in Regulation 4188.8.131.52 for TN systems).
U0 = the nominal voltage to Earth in volts (ie. 230 V for standard a.c. single-phase and three-phase circuits)
The value of Ia can be obtained by reference to the appropriate time/current characteristic of protective devices and RCDs in Appendix 3 of BS 7671.
Example of 20A BS88-3 fuses
Let us assume we have a 230V, TN, a.c. system with overcurrent protection provided by 20A BS 88-3 fuses for a final circuit not exceeding 32 A.
Table 41.1 requires that disconnection of the circuit takes place within a maximum time of 0.4 seconds.
To calculate the maximum earth fault loop impedance Zs which will ensure that 20A BS 88-3 fuses will disconnect the circuit within 0.4 seconds, we must use the formula in Regulation 411.4.5 shown above, but transpose it to make
U0 the subject. This gives:
Zs ≤ Ia
U0 = 230V.
To obtain Ia, we turn to Fig. 3A1 in Appendix 3, which gives the time-current characteristics for BS 88-3 fuses.
The line representing the 20A fuse gives us the value of current but it’s much easier to use the table at the top right of the page. (If you need the practice, you could use the 20A line to confirm the value obtained in the table) From this table we can see that the value of current which causes a 20A BS 88-3 fuse to operate within 0.4 seconds is 113 A. Turning again to the formula:
Zs ≤ Ia
Zs ≤ 113
Zs ≤ 2.04 ohms.
As the required disconnection time is 0.4 seconds, we turn to Table 41.2(b). Here we find that the maximum Zs value for 20 A BS 88-3 fuses is 2.04 ohms.
BS 3036 and BS 1362
The other fuse standards in Tables 41.2 and 41.4 – BS 3036 and BS 1362 – remain at the same values as in BS7671:2008 , so the corresponding values of maximum Zs are also unchanged.
The same process can be followed for any of the protective devices listed in Tables 41.2, 41.3 and 41.4 to obtain the Ia value from Appendix 3 to determine the calculated values Zs.
Reference to the appropriate time/current characteristic of protective devices and RCDs in Appendix 3 of BS 7671 can also be utilised for the discrimination between protective devices.
From the maximum Zs tables and Appendix 3, we can see the relationship between the current required to operate the protective device and the earth loop impedance values.
The value of the earth fault loop impedance path Zs determines the current that will flow in the event of a fault to Earth. This value of current in turn determines the time that a particular protective device will take to operate (ie. automatic disconnection). Therefore the Zs value of a circuit is an essential characteristic in the protective measure known as automatic disconnection of supply (ADS).