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What is the maximum earth fault loop impedance Zs for the gv7? What values are taken into consideration to calculate the Zs values?

For a TN system, section 413-02-08 of BS7671 defines the maximum earth fault loop impedance based on the following condition being met:
                                                                                  
Where:
 
Zs - is the maximum earth fault loop impedance.
U0 - is the nominal voltage to earth (for a 400V 3 phase system this is 230V).
la - is the current causing the operation of the circuit breaker in less than 0.4 seconds.
 
All GV7 units have a maximum magnetic trip level of 14 x the max current setting plus a 20% tolerance thus you can use the table below for 230V.

In practice, the thermal setting also scales the magnetic setting and therefore the above values are the maximum. If the measured earth fault loop impedance is above the values in the table, it may be possible that it meets the requirements of the lEE Regulations if the thermal setting is lowered.
 
 Important Note: Where an installation has been specified on the basis of Zs value calculated from a less than maximum short circuit trip level (as described in the previous paragraph) it is necessary to ensure that no unauthorized or inadvertently upward adjustment of the thermal setting is possible.
If it isn’t practical to do so, then the Zs values for the maximum trip current.
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