Effect of connecting transformer neutral and body earthing

OK- on a site where a low earth impedance is not easy (electrode impedance to terra-firma >1 ohm, say) then during a fault from transformer primary to core or case (- you call this the 'body' I think) then the transformer and it's metalwork rise to a dangerous potential. In that case that is earthed to a separated ' HV earth' or 'Primary earth' while the LV side neutral then has its own electrodes, some metres outside the near field of the HV earth.
If however the earth electrode impedance is very good (say less than one ohm) and/or the HV supply impedance is very high, so this rise of voltage during a primary fault would be small (only a few hundred volts), then the HV and and LV sides may share one set of linked electrodes.

To confuse matters a bit, sometimes in UK supplier slang the single shared low resistance earth sites are referred to as having 'cold' earths and those with the risk of a dangerous voltage rise are called 'hot' earths or are ' hot sites' .

In the UK most agricultural/ village wooden pole mounted transformers are 'hot' and so the HV primary earth wire comes down the pole with the transformer on it, while the LV/ neutral earth comes down the side next pole along the line 10-20m away.

Very large substations at ground level in towns and cities have far more metal in the ground, and are more often than not 'cold'. But even so, not always - sandy soil and high potential for fault currents require electrode separation in some places.

Hope this helps a bit.

Mike

OK- on a site where a low earth impedance is not easy (electrode impedance to terra-firma >1 ohm, say) then during a fault from transformer primary to core or case (- you call this the 'body' I think) then the transformer and it's metalwork rise to a dangerous potential. In that case that is earthed to a separated ' HV earth' or 'Primary earth' while the LV side neutral then has its own electrodes, some metres outside the near field of the HV earth.
If however the earth electrode impedance is very good (say less than one ohm) and/or the HV supply impedance is very high, so this rise of voltage during a primary fault would be small (only a few hundred volts), then the HV and and LV sides may share one set of linked electrodes.

To confuse matters a bit, sometimes in UK supplier slang the single shared low resistance earth sites are referred to as having 'cold' earths and those with the risk of a dangerous voltage rise are called 'hot' earths or are ' hot sites' .

In the UK most agricultural/ village wooden pole mounted transformers are 'hot' and so the HV primary earth wire comes down the pole with the transformer on it, while the LV/ neutral earth comes down the side next pole along the line 10-20m away.

Very large substations at ground level in towns and cities have far more metal in the ground, and are more often than not 'cold'. But even so, not always - sandy soil and high potential for fault currents require electrode separation in some places.

Hope this helps a bit.

Mike