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Uef, Sef, Ref, Lv Systems, Uk Standards & Practices


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Have run into these three terms: Unrestricted Earth Fault (UEF), Standby Earth Fault (SEF) and Restricted Earth Fault (REF) within the context of British Standards and UK LV wiring practices.Explained from a USA perspective UEF seems like regular equipment ground fault, SEF seems like equipment ground fault protection but using a ground return CT in the proximity of the transformer's bonding, sensing "all" ground current returning to the transformer. SEF is usually set as discriminating (selective) back up to UEF. Correct me if my understanding is incorrect please.


REF is supposed to be some sort of sensitive and fast differential GF protection sensing for a GF in the zone between the service entrance equipment and the transformer, including transformer secondary windings. REF is usually set sensitive and fast as it is a differential protection mode and does not need to be selective. Correct me if my understanding is incorrect please.


However, it seems (or some so alledge) that REF can be provided in LV equipment without resorting to a differential protection mode. Can somebody explain how the system is wired to achieve that? How can one use fast and sensitive GF protection in such a manner as to not cause excessive tripping for downstream faults, i.e. system instability? It seems to me that REF "must" be differential if it is to function as intended. Any clarification on this point would be appreciated. If somebody can point me to standards documentation or manufacturer's application literature I would appreciate it.


Secondly, UEF is used to trip the LV incoming main, SEF can be used to trip the LV incoming main and (optionaly) a MV feeder to the transformer, REF is always used to trip a feeder to the transformer. If the transformer is fed from a fused switch there is no point in having REF. Is this explanation correct?


In the above I am assuming LV power coming from a transformer, however, all of the above would also apply if fed from a generator. Correct?


Thank you in advance for your assistance.



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