Max Earth Loop Zs for MCBs Tables- Download

by Editor on March 30, 2016

Online reference for maximum permissible earth loops for MCBs & RCBOs. The file can be viewed or downloaded. Here’s the link:

Max Zs Values for MCBs & RCBOs

About Max Zs

The values given in this table are for maximum permissible measured earth fault loop impedances or Zs for each circuit as per Table B6 from the Onsite Guide.
The earth fault loop impedances correspond to the appropriate disconnection time for MCBs BS3871 and BS-EN 60898 and RCBOs BS-EN 61009 . These were taken from a comparison of the time/current characteristics of each device and the adiabatic equation given in Regulation 543.1.3 .

The maximum earth loop impedance values which you’ll find in both the On-Site Guide and GN3 charts are lower than those in Tables 41.2, 41.3 and 41.4 of the BS7671 Wiring Regulations. They have been calculated at an assumed conductor temperature of 10°C whilst the design figures given in the Regs are at the conductor’s maximum permitted operating temperature. The correction factor used is 1.25.

The Onsite-Guide also states that for smaller section conductors the impedance may also be limited by the adiabatic equation of Regulation 543.1.3.
A value of k of 115 from Table 54.3 of BS 7671 is used. The k value is based on both the thermoplastic (PVC) and LSHF cables operating at a maximum temperature of 70 °C.
This is suitable for PVC insulated and sheathed cables to Table 4, 7 or 8 of BS 6004 and for thermosetting (LSHF) insulated and sheathed cables to Table 3, 5, 6 or 7 of BS 7211.


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