power distribution - Siemens

18TECHNICALTechnicalGround Fault ProtectionThe term “low magnitude” arcing groundfault is a deceptive description of thistype fault. What is meant by this is thatthe fault current magnitude is lowcompared to that of a bolted fault. Evenso, the arc energy released at the pointof the fault can cause much damage andmay result in a fire. A ground fault is aninsulation failure between an energizedconductor and ground. A phase-to-groundarcing fault, unlike a phase-to-phasebolted fault, is a high-impedance typefault. The factors that contribute to thishigh impedance are the resistance of thearc and the impedance of the return path.This return path is usually metal conduit,raceway, busway housing or switchboardframes. Another contributing factor is thespasmodic nature of the arc. The circuitbreaker or fuse protecting the circuitdetects the fault current, but the actualground fault current magnitude is everchanging due to arc elongating blowouteffects, self-clearing attempts and arcreignition.These current limiting effects make thecircuit breaker or fuse incapable ofdetecting the actual damage that isoccurring. This is not to imply that thesedevices are inadequate. The problem isone of system protection because thecircuit breaker must be adjusted (or fusesize selected) so as to hold withouttripping under momentary overloadconditions, such as motor starting currentor transformer inrush current. Therefore,the circuit breaker or fuse cannot openquickly enough under relatively lowmagnitude faults to limit the arcingdamage.Figure 10 illustrates the basic problem.Shown is a typical distribution systemwith a 1600 ampere main serviceentrance unit with a circuit breaker(single line “a”) or fused serviceprotector (single line “b”). A ground faultof 1500 amperes on the bus would affectbut would not open either device. A 4000ampere ground fault would be cleared inapproximately 35 seconds by the circuitbreaker and in 230 seconds by the fuse.To allow a fault of this magnitude topersist for this length of time wouldcreate more than 92,000 kW seconds ofarc energy. As a result of tests made, ithas been determined that an arc with avalue of 1050 kW seconds of energywould vaporize about 1.0 cubic in. ofcopper or 2.5 cubic in. of aluminum.Obviously a fault of the magnitudeshown in Figure 10 could cause aconsiderable amount of damage.The nature of low-level arcing groundfaults makes impractical their detectionFigure 10. Ground Fault Protectionby a traditional overcurrent devices. Tocomplete total protection of the systemagainst all possible types of faults, othermeans are utilized to detect ground faultcurrents, including:▪ Zero sequence method▪ Source ground current (or groundreturn) method▪ Residual connection methodZero Sequence MethodThis is commonly used when groundfault protection is provided for equipmentemploying electromechanical tripdevices. The scheme uses a corebalance type current transformer (groundsensor) which encircles all phaseconductors (and neutral on four wiresystem) to detect ground faults.The operation of this system is such thatunder normal operating conditions (eg.,no ground fault on the system) there isSelectionno output from the ground sensor to thetripping relay because the vector sum ofall the currents through the sensorwindow is zero.(I a + l b + I c + I n = 0)If a ground fault occurs on the system,there is now an additional current (I g )seen by ground sensor which returns tothe source by a path other than throughthe sensor window. The sensor nowsees an unbalance caused by Ig andoperates the ground relay which trips thecircuit protector.(I a + l b + l c + I n = I g )The ground sensor is locateddownstream from the point at which thesystem is grounded and can be mountedeither on the line side or load side of themain disconnect device. This method canbe used on incoming main disconnect oron feeders.18-6Siemens Power Distribution & Control, SPEEDFAX 2007-2008 Product Catalog