EARTH FAULTS IN ITS VARIOUS AVATARS
Subsequent to my earlier technical paper on Earth Fault or Earth Leakage, this paper attempts to classify the various types of earth faults that one comes across in the protection studies.
Stand-by Earthfault Relay: It would appear from the term as if the earth fault itself is stand-by. Not so! It is the name given to the protection relay. In large switchboards, the incomer breaker will have an earth fault protection relay, either instantaneous or time delayed. As earth faults in electric power systems can cause danger to the life of humans & animals, it is insisted upon by the authorities, that, in addition to the main earth fault protective relay already provided in the incomer breaker, one should go for a stand-by or back-up earth fault relay. Just in case, the main earth fault relay does not operate during an earth fault, the stand-by earth fault relay will operate and isolate the faulty portion. This is something similar to having stand-by players or substitutes in sports events like Cricket, Football, etc. Normally, the main earth fault relay will be fed either from a separate neutral CT or in the residual path of the phase CTs. Whereas, the stand-by earth fault relay will essentially be fed from a neutral CT only, mounted in the path of the earthing lead from the source neutral to earth. Also, the main earth fault relay may be instantaneous or time delayed. Whereas, the stand-by earth fault relay is always time delayed. It has to be properly co-ordinated with the main Earthfault relay. The IEEE Device Function Number for the Stand-by Earth Fault Relay is 51G.
Restricted Earth Fault Relay: Again, it is the name given to the protection relay rather than the earth fault itself. Here, as the name suggests, the relay will operate, only if the earth fault is within the restricted or confined zone, which will be pre-defined, such as a transformer winding or a generator winding. The system may have an unrestricted Earthfault relay too, but, this relay may or may not sense the earth faults happening within the zone. Mostly, may not sense, due to the possibility that the fault current within the machine, may be lesser than the setting in the relay (Particularly, for close to the neutral faults, in impedance earthed systems). Even if this unrestricted earth fault relay senses the in-zone earth faults, the operating time of this relay will be too large (due to co-ordination requirements with downstream relays), that by the time this relay operates the machine winding might have damaged already. As this protection is normally suggested for very critical machinery in the plant, it is all the more important that there should be quick isolation, in the event of a fault, before damage is effected. REF relay is such a relay, which will not have any intentional time delay, so that, in the event of fault within the zone, it operates instantaneously, say, within 20 to 50 milliseconds, thereby protecting the machine from damage. It is also all the more important that this REF relay MUST NOT operate for any external (out-of-zone) earth faults. The IEEE Device Function Number for the Restricted Earth Fault Relay is 64R.
Sensitive Earth Fault Relay: This is similar to the conventional non-directional, un-restricted Earthfault relay, except that the current settings are very low. In impedance earthed systems and in un-earthed systems, in the event of an earth fault, the fault current will be so small, that it will not be sensed by conventional earth fault protection relay. This may also happen in solidly earthed systems, in the event of a high impedance earth fault. In such cases, the conventional earth fault protection relay may not operate as the current would be below the minimum possible setting in the relay. Nevertheless, the small fault current can still create an unwanted touch potential that could circulate a “shock” current thro the body of an operator coming in contact with the faulted body. In such cases, sensitive earth fault relay is relied upon to offer protection. The sensitive earth fault protection can be time-delayed or instantaneous or both. The IEEE Device Function Number for the Sensitive Earth Fault Relay is 51NS (if time delayed) or 50NS (if Instantaneous).
It is prudent to use a CBCT for feeding the Sensitive Earth Fault Relay. Because, with conventional CTs, the relay operation may be jeopardized due to:
i) Individual CT errors might add up so that the fault current may not be exactly reproduced to the relay
ii) The fault phase’s CT will feed the exciting currents of the unfaulted phase CTs, thus again the actual fault current available to the relay is not the exact replica of the primary fault current.
The generally suggested setting for sensitive earth fault relay is 2A (Primary earth fault current). This is understandable, as with a 2A primary Earthfault current and with 5 Ohm Equipment Earth Resistance (vide the Indian National Electric Code, 2011), the touch potential that can develop is about 10V. If a person comes in contact with the faulted equipment, this 10V touch potential will circulate a current of about 10mA through the operator’s body (assuming a wet skin resistance of 1000 Ohms, vide Dalziel). A current of 10mA thro human body is the “muscle contraction threshold” current and the person may not relieve all by himself/herself from the equipment. And, longer exposure to 10mA current can cause ventricular fibrillation and even tissue burning, which are fatal.
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Hope the above article is useful.
Please do not hesitate to give-in your feedback and further queries, if any.
K. Sivakumar