Restricted Earth Fault Relay

Thanks you for giving such a good reply and for sharing your valuable knowledge in this subject.

While searching in google for the answer for my doubt no 2, I got this

"If you use a PS class CT for metering, the metering will not be accurate and at time of a heavy fault, the meter will burn out.

A metering CT must be accurate between zero load and full load and must saturate above full load. Protection class CT must not saturate until the specified level."

The link is <http://in.answers.yahoo.com/question/index?qid=20081006072730AAQq6Zh>

Can it be true.?

One more thing

Should the CT secondary used for REF protection must be of 1 A even in the case of indoor substations. I have seen few drawings in which secondary rated ampere for the CT's used for REF is mentioned as 1 A. Is it wrong to use a 5A rated secondary. (usually all the CT secondary used in outdoor substation is of 1 A rating in which lengthy cabling is required, but for most of the CT's located inside the panel the secondary is rated as 5A). Is there any particular reason or any rule for that..?

Once again thank you electricalexpert65 and PWSlack for your reply.

i) Yes! If either a protection class CT like 5P10 or Class PS is used for metering, the meters must have a short time rating pertaining to the Accuracy Limiting Current else they would burn out. This is what I have mentioned in my answer too.

Yes, a metering CT must maintain its accuracy between 100% to 120% of rated primary current (and, NOT from zero load to full load, as mentioned by you. Even at 20% rated current, the ratio error of a metering CT would double and at 5% rated current, it would quadraple). Also, a metering CT need not saturate just above its rated primary current. The saturation level of a metering CT is denoted by a parameter called ISf (Instrument Security Factor) of FS (Factor of Safety), which is 2 or 5 or 10, but generally it is 2.

Also, it is agreed that a protection class CT like 5P10 must not saturate until the specified level. The specified level is more technically known as the ALF (Accuracy Limiting Factor). And, it must be remembered that the ALF does not specify that the CT would exactly saturate at the ALF current level; no, it is not! ALF just implies that the given protection class CT would maintain its limits of composite error at least upto the ALF current. It may saturate anywhere AFTER the ALF current.

And, for Class PS CTs, this ALF is more precisely calculated as the Knee Point Voltage, duly taking into consideration, the fault current magnitude (as reflected at the secondary terminals of the CT), the CT secondary winding resistance, the connected burdens (load burden plus the lead burden) and a safety or cushioning factor.

It is not necessary to go for a 1A Class PS CT for indoor mounted CT & REF too. The CT & Relay can be 5A. There is no hard & fast rule on that.

Thank you once again...

Sir, Very good answer. You have explained very clearly. Regards.

EE65 has explained clearly and there is not much to add. However I thought I will try to make the answer more complete.

Is it possible to use a single relay(if the relay is capable of) to incorporate all the protections.?

Yes, It is possible with current generation numerical relays to programme all protections in one relay. However protection philosophy mandates that all eggs should not be placed in one basket and hence redundancy / back up protection is always included. This backup protection (if sound engineering practises are followed) will have complete redundancy like separate CT's, Control schemes etc so that no single element failure ( CT, PT auxiliary supply etc) will lead to non clearance of the fault. In fact even with the latest generation relays provided as the first line of protection, the humble electro mechanical relay like CDG will be preferred as the backup redundancy protection.

For Faults inside the Zone the REF will operate the When an earth fault happens inside the zone will the CT provided for SBEF sense the fault current. (i mean when a current flows in the primary (neutral) a proportional current in the secondary)?

REF is zone (unit) protection and standby earth fault is non directional unbound protection. By protection philosophy, unit protections are tuned for sensitivity and instant operation for fast clearance of internal faults. However for backup protection scheme, time and current grading is followed to allow for clearance of fault by the upstream breaker closest to the fault. Therefore even if the standby protection is primed for operation for any faults downstream (including within the zone protection of REF), sufficient discrimination is provided in the settings so that the principal protection relay clears the fault.

Thank you for replying..

Anonymous your question was not at all silly but I would like to ask experts here a silly question.

If a fault happen outside the REF zone then still the fault current will travel from the transformer neutral, then how REF will not sense that current?

I have seen people don't do any co-ordination for a REF (as its instantaneous). For a scheme with bus section, if a REF operates that will not activate the lock out relay and allows the auto change over to take place. But if a SBEF will operate then 86 will operate and hold the auto change over. So it is very important for REF should not operate for a fault outside the zone.

On this subject I would like to add one point. For transformer REF relay , the relay is provided in the LT side Panel and the relay is made to trip the LT side breaker. But not the HT side breaker. Actually it should isolate the source of supply to the transformer ( HT side).But I notice even the leading Switch board manufacturers provide the tripping only LT Breakers. It should be corrected.

There is an economical reason in locating the REF Relay in the LT Panel. Because, in distribution transformers, the LV winding is generally star connected and as such you need three phase CTs & one neutral CT for REF protection of the LV winding. If you place the REF relay in the HV Breaker, then the LV CT leads have to be taken to the HV Breaker Board thereby increasing the lead burden and thus the rated burden of the LV CTs. This would make the LV CTs bulkier & costlier. If the REF Relay is located in the LV Switchboard, this can be economised. Yes, it is agreed that for REF protection, the HV Breaker must be made to trip.The LV Breaker may trip on under voltage (caused by the the HV Trip) or inter-trip signal from the HV Breaker or directly from the REF Relay. Alternately, the REF may trip the LV Breaker and the HV Breaker may be made to trip on an inter-trip arrangement, wherein for any fault trip of the LV Breaker, the associated HV Breaker would trip.

I come across peculiar scenario on this matter.

In many projects the LT Panel supplied by one source with 3 CTS provided on the phase side of the incoming supply say with CT of ratio 3000/1.Non numeric REF relay of 1 Amp incorporated in the LT Panel.

The Transformer is supplied by another source with neutral CT incorporated in the LV box. say with CT ratio 3200/5.

The anomalies are noticed during the commissioning. But the concerned people talk about remedial action for few days but do not take any action. Ultimately in many projects I have observed the REF relay is out of picture without any protection for the transformer.

Best solution would be change the NCT at the transformer from 3200/5 to 3000/1.

But it is not done due to some reason or other.

Under such circumstances can we use suitable interposing CT to match the phase CT secondary. The interposing CT can be located in the LT Panel.

What are the implications ?