Maximum Zs Values and fuse standards to BS7671

13 October 2012

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 (Z_s) 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(However, fuses complying with these withdrawn standards will be found in existing electrical installations for some years to come.)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 Z_s values in Tables 41.2, 41.3 and 41.4 are obtained.

This article will look at the maximum Z_s values for BS88-3 fuses in Table 41.4(b) as an example of how these maximum Z_s values are obtained.

The formula

The maximum earth loop impedance, Zs values are arrived at using the formula in Regulation 411.4.5:

Z_s x I_a = U_o

where:

Z_s = the earth fault loop impedance in ohms

I_a = 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 411.3.2.3 for TN systems).

Where an RCD is used, the current I_a 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 411.3.2.3 for TN systems).

U₀ = the nominal voltage to Earth in volts (ie. 230 V for standard a.c. single-phase and three-phase circuits)

Appendix 3

The value of I_a 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 Z_s 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:

U_o

Z_s = I_a

U₀ = 230V.

To obtain I_a, 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:

U_o

Z_s = I_a

230

Z_s = 113

Z_s = 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 Z_s 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 – rem

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