Restricted Earth Fault
The Principles
As can be seen above, CT's are placed on L1, L2, L3 and a Neutral CT (Typically Class X CT's) are placed on the Neutral. Under normal conditions and by application of Kirchhoff’s laws the sum of all the line currents minus the sum of the neutral current equals zero. Therefore what goes in must come out, simply put any unbalances from any line CT to each other is returned through the neutral. This supplies the fundamental
Introduction
Restricted Earth Fault is protecting the zone from the Restricted Earth Fault Relay back upstream to your source of supply, typically a transformer or a generator. This does exactly what it says on the tin and protects against a fault to earth, therefore protecting your source of supply, however, only the star winding, this will not protect your delta winding as there is no physical protection.
principles for Restricted Earth Fault (REF) Protection
If there is an earth fault that occurs within the restricted zone, 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, it does this extremely quickly and limits the damage that could occur.
Typically it is key where these CT's are placed a, typically and most advisable the CT's should be as close as possible to the Mains Protection Circuit Breaker to maximise the protection of the REF zone and commonly protecting the cable/busbar/whatever it is linking your source to your breaker.
What happens if you don't have REF, if this was the case the only item that can notice the 'fault' is the protection unit on the primary side of the transformer (nothing there is NO physical connection to the LV winding, however as the winding fault position moves towards the neutral, the magnitude of the current seen on the primary rapidly decreases and could potentially not be detected (limiting the amount of winding which can be protected).
If the REF was activated I would advise you always trip both the upstream M.V circuit breaker (obviously) and the downstream LV Circuit Breaker, completely isolating the fault.
The Fundamentals
- Summation of all CT's must equal ZERO, if not you have an Earth Fault (Kirchhoff's Law)
- REF zone only protects from the CT location to the Secondary Winding (Star Point)
- REF does not protect beyond the Secondary Winding
- REF zone is always upstream, therefore, it is always looking backwards
- Always Trip both M.V Circuit Breaker & L.V Circuit Breaker
- Depending on the device some are self-powering and others require typically 24VDC, two examples are below
Example No.1 - MCAG14, self-powering 4 CT input relay
Example No.2 - Schneider Internal Micrologic 6.0X ACB + Neutral CT