Which Maximum Earth Fault Loop Impedance Values Should You Use?

Which Maximum Earth Fault Loop Impedance Values Should You Use?

NAPIT’s Don Holmes provides us with more essential technical information.                                

The maximum earth fault loop impedance values (Zs) in BS 7671 – or the equivalent tables in the IET On-Site-Guide (OSG) and IET Guidance Note 3 Inspection and Testing (GN 3) – are essential references when carrying out Zs tests.

But what if the manufacturer of a particular overcurrent device provides a maximum Zs value for the device which differs from the corresponding value in BS 7671?

Or what if the maximum tabulated Zs value in BS 7671, or the OSG or GN 3 needs to be corrected due to ambient temperature?

This article will consider which maximum values of Zs to use in such circumstances.

Earth fault loop impedance (Zs)                                                                    

The earth fault loop path from which the earth fault loop impedance (Zs) is derived in shown in Fig. 1. The requirements of BS 7671 for automatic disconnection are satisfied when:

,-. ,-. ,-0. ,-.


Zs = earth fault loop impedance.

Ia = current causing the automatic operation of the protective device within the time given in Table 41.1.

U0 = nominal voltage to Earth.

Cmin = the voltage variation factor (usually 0.95).

Calculating Maximum Zs values for moulded case circuit- breakers (MCCBs)                                                                                                  

Maximum Zs 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.

Alternatively, the maximum Zs value can calculated using the time- current characteristic of the MCCB being used, as shown in Fig. 2 for a 32 A MCCB.

Worked example 1                                                                                                 

 An MCCB with a rated current value (In) of 32 A is connected to a circuit forming part of a TN supply system which has a U0 value of 230 V. The MCCB is required to disconnect in 0.4 seconds. Calculate the maximum Zs value.


From Appendix 3 of BS 7671,

,-.=,,U-0. x ,C-.-,I-..              

From Fig 2, the MCCB trips instantaneously at 20 times In (32 A) which equals 640 A.

,-.=,230 V x 0.95-640 A.

,-.=0.34 Ω

Testing circuits for Zs values according to temperature                                  

Notes 2 and 3 at the end of Tables 41.2, 41.3 and 41.4 in BS 7671 indicate that the maximum Zs values in those tables should not be exceeded when the line conductors are at the appropriate maximum permitted temperature (70o C for thermoplastic). The figures in these tables can be used for design purposes.

However, as the resistance of conductors tends to increase with temperature, the ambient temperature at which testing is carried out is a significant factor in determining the temperature (and therefore the resistance) of conductors.

Adjustment for ambient temperature                                                    

When Zs tests are carried out at an ambient temperature of 10°C, the conductors being tested may be assumed to be also at 10°C. This means that the tabulated maximum Zs values of Tables 41.2, 41.3 and 41.4 need to be adjusted accordingly. For a cable having thermoplastic insulation with a maximum permitted temperature of 70°C, this adjustment can be made by simply multiplying the maximum Zs values in the appropriate table by 0.8 (Appendix A2 of GN 3).

Table B6 of the OSG and Table A4 of GN 3 contain maximum Zs 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 note under these tables states that Regulation 434.5.2 of BS 7671 requires that the protective conductor csa meets the requirements of BS EN 60891-1, -2 or BS EN 61009-1, or the minimum quoted by the manufacturer. Table B7 of the OSG and Table A5 of GN 3 list minimum protective conductor sizes for class 3 type B and C devices with current ratings up to 40 A. This covers typical domestic installation circuits wired in twin with cpc thermoplastic cables where the cpc is smaller than the live conductors.

When testing is carried out at an ambient temperature which is less or greater than 10oC, the correction factors listed in Table B8 of the OSG or Table A6 of GN 3 should be applied to the maximum Zs values listed.

Worked example 2                                                                                                      

A circuit is protected by a 20 A type B circuit-breaker to BS EN 60898. The ambient temperature is 20oC and the circuit is wired in 2.5 mm2 twin with cpc thermoplastic cable. Calculate the maximum measured Zs.


From Table B6 of the OSG, the maximum measured Zs value is1.75 Ω at an ambient temperature of 10oC.

From Table B8 of the OSG, the correction factor is 1.06.

Therefore the maximum measured

= 1.75 Ω 1.04

= 1.82 Ω.

Using manufacturers’ maximum Zs values                                        

Manufacturers may also provide maximum Zs values for their MCBs and RCBOS which are for conductor operating temperatures of 70o C. These values of Zs are higher than in Table 41.3 of BS 7671, as they are based on the actual operating characteristics of their own MCBs. In contrast, Table 41.3 of BS 7671 gives values suitable for all BS EN 60898 MCBs and BS EN 61009-1 RCCBOs.

The maximum Zs 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.

Where manufacturers’ values of Zs for MCBs are used, they will often recommend multiplying the measured value of Zs by a correction factor, to correct for ambient temperature. For example, Schneider, recommend using a factor of 0.8,

For other ambient temperatures, the appropriate correction factor should be used.

Relevant circuit design criteria                                                                      

There will be circumstances where a designer has specified requirements which are particular to an electrical installation.

The intended impedances may differ from BS 7671. In this case, the inspector of the installation should ask for the design criteria or arrange to forward the test results to the designer for verification. In the absence of such data the inspector should apply the requirements set out in BS 7671.

Related posts