Restricted Earth Fault Protection 

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The windings of many medium and small sized transformers are protected by restricted earth fault (REF) systems. The illustration shows the principal of REF protection.

Basic Operation

Restricted Earth Fault Operation
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'.

REF protection is fast and can isolate winding faults extremely quickly, thereby limiting damage and consequent repair costs. If CTs are located on the transformer terminals only the winding is protected. However, quite often the secondary CT is placed in the distribution switchboard, thereby extending the protection zone to include the main cable.

Without REF, faults in the transformer star secondary winding need to be detected on the primary of the transformer by the reflected current. 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). As the magnitude of the currents remain relatively large on the secondary (particularly if solidly earthed), nearly the entire winding can be protected using REF.

It should be remembered that the protection as illustrated covers only the secondary of the transformer. Sometimes REF protection is added to the primary as well (although if primary protection is required it may be preferable to consider full differential protection).

As it is essential that the current in the CTs be balanced during normal conditions (and through faults), historically REF has been implemented using High Impedance Relays. CT's have also been specified as matched pairs and the impedance of leads/wires and interconnecting cables has had a large influence on the functioning of the relay.

Measurement errors associated with mismatched CTs, saturation and varying lead resistance have been responsible for nuisance tripping and the system can be difficult to commission.  This may be the reason some people avoid the use of REF. Recent advances in numerical relay technology have all but eliminated these issues, making the implementation of REF relatively easy, ensuring no nuisance tripping and simplifying commissioning.

In the diagram 87N is the ANSI Device Number for restricted earth fault.

See Also

ANSI (IEEE) Protective Device Numbering

Steven McFadyen's avatar Steven McFadyen

Steven has over twenty five years experience working on some of the largest construction projects. He has a deep technical understanding of electrical engineering and is keen to share this knowledge. About the author

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  1. Raman's avatar Raman says:
    7/27/2012 5:14 PM

    Thank you. You have explained it in simple terms. Can you also Please clarify the following :

    For a radial transformer feeder provided with REF protection on LV side, does the operation of REF relay be wired to trip the primary HV side or secondary LV side Breaker?

    For parallel operation system, does operation of REF be connected to trip both primary and secondary Breakers.?

    It would be great if the breaker linkage is shown as part of the schematic.

    Can you please clarify?


    • Steven's avatar Steven says:
      7/27/2012 7:52 PM

      REF is a fault in the winding and you need to isolate the transformer - trip the MV/HV side.

      The REF is unit protection and restricted to one transformer. If you have two in parallel you could isolate only the faulty transformer. Just make sure the other transformer can support the full load.

      I don't have access to my normal computer at the moment, but as soon as I do I will look at updating the schematic.

    • Steven's avatar Steven says:
      1/26/2013 10:28 AM

      Image now updated.

  2. LYH's avatar LYH says:
    8/21/2012 7:10 AM

    Further question: Is REF protection should be implemented on a small 6/0.4kV, 500kVA oil type transformer?

    • Steven's avatar Steven says:
      8/24/2012 12:25 PM

      There is a cost to REF and at some stage it becomes expensive in relation to the transformer size. At 500 kVA, I would tend to think the cost may not be justified (unless the transformer were particularly important).

      It would be interesting if other site users have opinion on this and can add to the discussion.

  3. waji's avatar waji says:
    9/26/2012 11:42 AM

    Can you please explain how a transformer can detect an earth fault by using using phase CTs?

  4. rajadurai's avatar rajadurai says:
    10/19/2012 4:33 PM

    What class of cts used for REF and SEF protection of transformers

    • Amit's avatar Amit says:
      10/23/2012 9:52 AM

      For REF protection, PS class CT are used for high accurecy and for SEF 5P10 type CTs are used.

  5. vishal's avatar vishal says:
    12/22/2012 11:25 AM

    I want to just ask that how we can restrict the earth fault detection of a Particular Critical System let us consider an alternator and what is the Logic Behind that?what are the things to keep in mind and what is the difference between Overcurrent Earth Fault and REF ?How the CTs are arranged in Differential Protection what is the logic behind the arrangement .

  6. Daniel's avatar Daniel says:
    1/5/2013 7:29 AM


    Is it a MUST that the knee point voltage, Vkp and excitation current, Imag of the Neutral CT and phase CT to be the same?

    if Vkp and Imag of both CT not the same, what will be the consequences on the Bias Differential and REF protection?

    Anybody can advise me on these? Thanks

    • Steven's avatar Steven says:
      9/17/2013 12:30 PM

      On a through fault (if the CTs saturate) the currents will be different and the relay will see a zone fault where one does not exist. This could result in a nuisance trip.

      Some modern relays are able to compensate for differences in CT performance. This would need to be investigated on a case by case basis.

  7. black mamba's avatar black mamba says:
    4/4/2013 7:45 PM

    what's the difference between ANSI 87N and ANSI 64REF?
    Is the polarity of connection samefor both protection schemes?

    • Steven's avatar Steven says:
      4/7/2013 5:27 AM

      I think the intent is that both mean the same thing. Which is the correct or better to use may be open to interpretation.

      To me, 87N is more appropriate as REF is a differential type protection (ANSI 87 – differential protection relay). I think 64 is strictly more relevant to detecting earth/ground faults due to breakdown of insulation.

  8. Skara's avatar Skara says:
    5/21/2013 9:03 PM

    goodday gents,

    kindly please assaist with what is the difference between high impendance REF and low impedance REF

    • Steven's avatar Steven says:
      5/22/2013 10:37 AM

      The primary difference is in the type of relay used. High impedance relays have a high input impedance and draw very little current under normal conditions. Low impedance relays are the opposite.

      There is a lot of debate on which to use and some theory. My view is that low impedance is preferred. It's simpler, there is more flexibility in the CTs and inputs and coupled with all the functions of a modern numerical relay it eliminates a lot of problems that exist in high impedance systems.

      There is also a technical comparison on the two types here which should help you:

  9. Freddie's avatar Freddie says:
    5/30/2013 1:57 PM

    What can cause 64REF to trip off a transformer during disturbances from the grid

    • Steven's avatar Steven says:
      6/2/2013 9:37 AM

      It could be many things.

      A common reason could be unbalanced/incorrectly selected CTs causing false residual current and nuisance tripping. Similarly, if your CTs are becoming saturated they can become unbalanced and give problems.

      You should also make sure that the CT ratios, phase shift, logic, etc. are programmed correctly in to the relay and that your polarities/wiring are fine.

      It is difficult to remotely diagnose this type of issue. You need to work through the circuits/issues logically until you can narrow it down. If you are also able to get a recording of the voltages and currents during an incident this will help you a lot.

  10. SYAM's avatar SYAM says:
    9/3/2013 5:23 PM

    Hi Steven,

    "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)".

    Can you please explain why..?

    I have a few queries also (it may sound stupid).

    Query 1. I am bit confused between the operation of REF and Standby earth fault relay. Can you help me in understanding with some diagrams or any links to refer.?

    Query 2. In our area we use separate CTs for REF and SEF+ OC protection. i.e 4 or 5 nos PS class CTs (3 in Phases 2 in Neutral) along with 4 no 5P10 class CT ( 3 in Phases 1 in Neutral for SEF+OC. We use separate relays for REF, SEF, & OC For SEF we use Induction (disc type) relay. My query is that is it possible to use only one type of (PS Class) CT for all the protections..? Is it possible to use a single relay(if the relay is capable of) to incorporate all the protections.?

    Query 3.Can I use Class PS CT for OC or SEF protection or for metering purpose.?

    Query 4. When a fault occurs between one phase and the ground will the fault current travel back from the ground through the neutral to the fault point..? (using the concept that current flows only in a closed circuit)
    For faults outside the Zone SEF will operate, REF will not operate.
    For Faults inside the Zone the REF will operate When an earth fault happens inside the zone will the CT provided for SEF sense the fault current. (i mean when a current flows in the primary (neutral) a proportional current in the secondary)?

    Please clarify my doubts.It may sound silly or stupid but I would like to know why and how.

    Thanks in advance



    • Steven's avatar Steven says:
      9/17/2013 12:25 PM

      In answer:

      1 - there is an REF diagram in the note. I'll try to elaborate on this when I get more time.

      2. - yes it is possible. I would also look at more modern numerical relays for your application.

      3. - yes, but you need to check the CT calculations. I would prefer separate CT for the meter.

      4. - SEF should still sense the fault and act as backup protection.

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