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What is the maximum earth fault loop impedance Zs for the Tesys U? 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:
A revision of the British Standard BS 7671:2008+A3:2015 introduces an additional factor in to the calculation of Earth Loop Impedance (Zs). This factor takes account of the ‘minimum voltage factor’
For a TN system, section 411.4.5 of BS7671:2008+A3:2015 defines the maximum earth fault loop impedance based on the following condition being met:

Calculation for Zs Earth Loop Impedance
 
 Where:
 
Zs - is the maximum earth fault loop impedance.
Uo is the minimum voltage factor to take account of voltage variations depending on time and place, changing of transformer taps and other considerations. e.g. For a 400V 3 phase system this is 230V).
la - Ia is the current causing operation of the protective device within the specified time. E.g. For motor starters this is 0.4 seconds.
Cmin is the minimum voltage factor to take account of voltage variations depending on time and place, changing of transformer taps and other considerations.
NOTE : For a low voltage supply given in accordance with ESQCR, Cmin is given the value 0.95.

Tesys U Integrated Morot starter

All model U control units have a maximum magnetic trip level of 14.2 x the max current setting plus a 20% tolerance thus you can use the table below for 230V.

Values for Earth Loop Impedance (Zs)
 

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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