Windings of many smaller transformers are protected by restricted earth fault (REF) systems. Having said that, I have come across a lot of installations which would have benefited from this type of protection, but for whatever reason it has not bee installed.
The illustration shows the principal of REF protection. Under normal conditions and by application of Kirchhoff’s laws the sum of currents in both current transformers (CTs) equals zero. If there is an earth fault between the CTs then some current will bypass the CT's and the sum of currents will not be zero. By measuring this current imbalance faults between the CTs can be easily identified and quickly cleared. Fault detection is confined to the zone between the two CTs hence the name 'Restricted Earth Fault'.
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