Ground Fault Protection - EandM
Ground Fault ProtectionIn addition to ensuring equipment is properly grounded, groundfault protection for people and equipment is also a concern.NEC Article 230.95 states that ground-fault protection ofequipment shall be provided for solidly grounded wye electricalservices of more than 150 volts to ground, but not exceeding600 volts phase-to-phase for each service disconnecting meansrated 1000 amperes or more.Although ground-fault protectors are not required on servicedisconnects that are less than 1000 amperes, depending on theinstallation, they still may be desirable. Ground fault interruptersdesigned to provide life protection must open a circuit at 5milliamps ( 1 milliamp). Ground fault protection for equipmentmust open a circuit when ground fault current reaches 30milliamps. Health care facilities, such as hospitals, requireadditional ground fault protection. This is outlined in NEC Article 517.17.Direct Method50One way a ground fault protector works is to install a sensoraround one conductor, normally the neutral-to-ground strap. Thisis referred to as the direct method. When an unbalanced currentfrom a line-to-ground fault occurs current will flow from groundto neutral. When the current reaches the setting of the groundfault sensor the shunt trip opens the circuit breaker, removingthe load from the line.
Zero sequencing MethodAnother way a ground fault protector works is with a sensorinstalled around all the circuit conductors, including the neutralon 4-wire systems. This is referred to as zero sequencing.During normal current flow the sum of all the currents detectedby the sensor is zero. However, a ground fault will cause anunbalance of the currents flowing in the individual conductors.When this current reaches the setting of the ground-faultsensor the shunt trip opens the circuit breaker.51
Residual Method52Separate sensors monitor current on all three phases (and theneutral on a 4-wire system. If the vectorial sum of the currentson the secondary of the sensors does not equal zero thebreaker will be tripped.
Ground FaultProtection DevicesGround fault protection is generally incorporated into aspecial type of protective device such as a molded case circuitbreaker. Ground fault protection is also available in Siemensinsulated case circuit breakers.Ground fault protection can also be supplied on variousdisconnect switches, such as the bolted pressure switch.Note: All main protective devices, except Vacu-Break fusibleswitches, can be equipped with ground fault relays tocomply with NEC requirements.53
Switchboard RatingsWhen selecting switchboards and overcurrent protectiondevices it is extremely important to know both the maximumcontinuous amperes and available fault current along withseveral other rating terms.Interrupting RatingInterrupting rating refers to the current rating a protectivedevice, such as a fuse or circuit breaker, can safely interrupt.Interrupting rating is also referred to as ampere interruptingcapacity (AIC). NEC article 110.9 states:Equipment intended to interrupt current at fault levels shall havean interrupting rating sufficient for the nominal circuit voltageand the current which is available at the line terminals of theequipment.Equipment intended to interrupt current at other than faultlevels shall have an interrupting rating at nominal circuit voltagesufficient for the current that must be interrupted.Full RatingThere are two ways to meet this requirement. The full ratingmethod is to select circuit protection devices with individualratings equal to or greater than the available fault current. Thismeans that, in the case of a building with 65,000 amperesof fault current available at the service entrance, every circuitprotection device must be rated at 65,000 amperes interruptingcapacity (AIC). Switchboards are available with short circuitwithstand ratings up to 200,000 amps. However, a full-ratedswitchboard over 100,000 AIC can be expensive because of thenecessary bus bracing.NEC and National Electrical Code are registered trademarks of theNational Fire Protection Association. Reprinted with permission from NFPA70-2002, the National Electrical Code , Copyright 2001, National FireProtection Association, Quincy, MA 02269.54
Series-RatedA full-rated switchboard is not always required. Seriesrated switchboards are UL listed and are adequate for manyapplications at a lower cost. The series-rated concept is thatthe main upstream circuit protection device must have aninterrupting rating equal to or greater than the available faultcurrent of the system, but subsequent downstream circuitprotection devices connected in series can be rated at lowervalues. This is permitted as long as the series combinationsshown have been tested and certified by UL. For example, abuilding with 42,000 amperes of available fault current mighthave the breaker at the service entrance rated at 42,000 AICand additional downstream breakers rated at 18,000 AIC.Series-rated breaker combinations must be tested in seriesin order to be UL listed. Siemens series-rated breakers arelisted in the UL “Recognized Components Directory” (yellowbooks) Volume 1. Selected series-rated breakers are listed inthe Speedfax catalog. Your Siemens sales engineer can providemore information on Siemens series-rated circuit breakers.Keep in mind that it is the protection device mounted in theswitchboard that interrupts current. Therefore, the interruptrating applies to the protective devices.Withstand RatingShort circuit withstand rating refers to the level of fault currenta piece of equipment can withstand without sustainingdamage. The standards for short circuit withstandability areset by Underwriters Laboratories (UL Standard 891). Busstructures and bracing are designed to withstand a specifiedamount of current for a specified amount of time. The shortcircuit withstand rating of a switchboard is determined bythe combined withstand, interrupting, and current limitingcapabilities of the bus, overcurrent protective devices in theswitchboard, and any overcurrent protective devices within orahead of the switchboard that may supply and protect it.55
Ampere RatingThe ampere rating refers to the current a switchboard orprotective device will carry continuously without deteriorationand without exceeding temperature rise limits.Voltage RatingThe voltage rating of a switchboard must be at least equal tothe system voltage. The voltage rating of a switchboard can behigher than the system voltage, but never less. For example, a480 VAC switchboard could be used on a 240 VAC system. A240 VAC switchboard could not be used on a 480 VAC system.Review 5561.Ground fault protection is required for grounded wyeelectrical services of more than 150 volts to ground, butnot exceeding 600 volts phase-to-phase when servicedisconnecting devices are rated at ampsor more.2.All main protective devices except -can be equippedwith ground fault relays.3.Ground fault protection is discussed in NEC Article.4.rating refers to the level of fault current apiece of equipment can withstand without sustainingdamage.5.rating refers to the maximum current aprotective device such as a fuse or circuit breaker cansafely interrupt.6.A switchboard is said to be -when the main upstream circuit protection device isequal to or greater than the available fault current, butsubsequent downstream circuit protection devicesconnected in series are rated at a lower AIC.7.refers to the current aswitchboard or protective device will carry continuouslywithout deterioration and without exceedingtemperature rise limits.
SB1, SB2, and SB3 SwitchboardsSiemens manufactures a variety of switchboards. The type ofswitchboard selected is determined by a variety of factors suchas space, load, and environment. In addition to meeting presentloads, the switchboard should be sized to accommodatereasonable future load additions. The continuous rating andthrough-bus can be sized on the basis of anticipated future loaddemand. Trip units or fuses of lower ratings can be installed tomeet present load demands and changed in the future as loadincreases. Siemens switchboards are available in Type 1 (indoor)or Type 3R (outdoor) enclosures.SB1, SB2, and SB3 Sentron switchboards can be found in avariety of industrial plants, hospitals, and commercial buildings.57
SB1 SwitchboardsSB1 switchboards are designed to be used in an applicationwhere space is a consideration. SB1 switchboards are rearaligned. The service section can be deeper than the distributionsections. By aligning the rear the switchboard can be installedagainst a wall.SB1 Ratings and DevicesThe SB1 switchboard contains front-connected main protectivedevices and through-bus ratings up to 2000 amps at 480 VAC.SB1 switchboards are front accessible with front connecteddevices. Main devices, used in the service section, are availablefrom 400 - 2000 amps. Branch devices, used in the distributionsection, are available from 15 - 1200 amps.58
SB2 SwitchboardsThe rear of SB2 switchboards align as standard. Front and rearalignment is available as an option. SB2 switchboards are frontaccessible and front connected. The following switchboardpictorial illustrates an SB2 that is front and rear aligned. Inthis example a pull section has been added to allow room topull cable up from the bottom to connections in the top ofthe service section. Bottom feed without a pull section is alsoavailable. SB2 switchboards may be mounted against a wall.SB2 Ratings and DevicesThe SB2 contains through-bus ratings up to 4000 amps at 480VAC. Main devices are available from 400 - 4000 amps. Branchdevices are available from 15 - 1600 amps.59
SB3 Switchboards60SB3 switchboards are front and rear aligned. SB3 switchboardsare designed for special configurations, such as incomingand outgoing busway connections, and automatic transferschemes. Through-bus ratings are available up to 6000 amps.Branch devices are available from 15 - 2000 amps (customconfigurations with higher ratings are available).
RCIII SwitchboardsThe branch and feeder devices in the Siemens type RCIIIswitchboards are individually mounted. This mounting methodrequires access to outgoing cable terminations from the rear.Type RCIII switchboards are rear connected and require rearaccess. Bus bar extensions from the feeder devices are runback to the rear of the switchboard for easy access. RCIIIswitchboards are front and rear aligned. The following drawingillustrates a type RCIII switchboard with Siemens insulated casecircuit breakers (ICCB) in the service and distribution sections.61
Drawout or Fixed Mounting62Depending on the protective device, it may be either drawoutor fixed mounted. Insulated case circuit breakers (ICCB), forexample, may be drawout or fixed mounted. Vacu-break fusible switches are fixed mounted.
Ratings63
Super Blue Pennant SwitchboardsThe Super Blue Pennant switchboard is designed as aservice entrance switchboard. The main service disconnect anddistribution devices are contained in a single unit. The meteringprovisions meet EUSERC (an electrical standardization coalition)specifications. Super Blue Pennant switchboards are rated for400, 600, or 800 amps with a circuit breaker main and 400 or600 amps with a fusible Vacu-Break switch main.64
Metering CompartmentThe metering compartment has provisions for mounting a utilitymeter on the door. Super Blue Pennant uses hot sequencemetering. Incoming power is connected to the main lugs.Service DisconnectThe service disconnect can be a fusible Vacu-Break switchthrough 200,000 AIC, or a circuit breaker with a maximum ratingof 65,000 AIC at 240 volts and 35,000 AIC at 480 volts.Distribution PanelDistribution kits are optional and field adaptable with ratings of400 - 800 amps. Up to 40 branch circuit provisions are availablewith an 18 branch circuit minimum.65
Commercial Metering SwitchboardsCommercial metering switchboards are designed forcommercial applications where multi-metering is required.These applications include shopping centers, office buildings,and other commercial buildings with multiple tenants.Type SMM Switchboards66Type SMM switchboards are designed to meet west coastutility and EUSERC specifications. The switchboard mainservice is rated up to 4000 amps at 480 volts. Service mainscan be circuit breakers (up to 2000 amps), insulated case circuitbreakers (up to 3000 amps), bolted pressure switches (up to4000 amps), or Vacu-Break , and HCP fusible switches (upto 1200 amps). Tenant mains, rated at 100 and 200 amps, areinterchangeable. Tenant mains can be circuit breakers, fusibleswitches, or T-fuse pullouts. The bus is braced for 65,000 amps.Higher bracing is available as an option. Metering sockets arerated for 200 amps continuous duty. The SMM switchboardsincorporate a ring type meter cover design. The meter ring mustbe removed to disengage the meter from the socket. The metercover does not have to be removed. Test blocks are standardequipment. The Type SMM switchboard shown below has athru-main section.
Type MMS SwitchboardsThe MMS switchboard is similar to the SMM, however, it isnot designed to meet west coast specifications. The mainservice is rated up to 4000 amps. Service mains can be circuitbreakers (up to 2000 A), insulated case circuit breakers (up to4000 A), bolted pressure switches (up to 4000 A), or Vacu-Breakfusible switches (up to 1200 A). Tenant mains, rated at 100 and200 amps, are interchangeable. Tenant mains can be circuitbreakers, fusible switches, or T-fuse pullouts. All meter socketsare rated at 200 amps. Wiring is for 100 amps or 200 amps,depending on the tenant main device. Depending on the tenantmain device, MMS switchboards are available with 2, 3, 4, or6 sockets. The bus is braced for 50,000 amps. Higher bracingis available as an option. Type MMS switchboards incorporatea ringless type meter cover design. The meter cover has to beremoved before the meter can be disengaged from the metersocket. Manual bypass is standard equipment.67
Speciality Service Entrance SwitchboardsSpecialty service entrance switchboards can be used in variousapplications. A specialty service entrance switchboard may, forexample, be placed ahead of a main switchboard. The specialtyswitchboard serves as the disconnect for the main switchboard.Specialty service entrance switchboards are available with asingle molded case circuit breaker, Vacu-Break fusible switch,or bolted pressure switch (not shown).68
BCT Service CubicleBCT service cubicles use molded case circuit breakers. They areavailable in current ratings from 400 - 1200 amps. BCT specialtyservice entrance switchboards use cold sequence metering asstandard and are top fed. For hot sequence metering the unitand circuit breaker can be inverted.69
SCT Service CubicleSCT service cubicles use Vacu-Break fusible switches. Theyare available with the following current ratings:120/240, 480 Y/277 volts208 Y/120, 240, 480, 600 volts208 Y/120, 240 volts400, 600, and 800 amps600 and 800 amps800 and 1200 ampsSCT service cubicles use cold sequence metering. Hotsequence metering is available.70
Enclosed BoltedPressure SwitchEnclosed bolted pressure switch specialty switchboards canbe used when metering is not required and are available withtop or bottom feed. The following drawing illustrates a top feedenclosed bolted pressure switch. Current ratings are availablefrom 800 - 4000 amps.71
Information Needed To Order SwitchboardsWhen ordering a switchboard several questions need to beanswered.1.What are the power system specifications (voltage,phases, number of wires)?2.What is the AIC rating (ampere interrupting capacity)?3.Will full or series rated be required?4.What is the NEMA Type enclosure desired?5.How many circuits are required?6.What types of overcurrent protective devices (MCCB,ICCB, Vacu-Break fusible switch, bolted pressure switch)are required?7.Does the switchboard need to be suitable for serviceentrance?8.What amperage is the switchboard rated at?9.Will the switchboard be top or bottom fed?10. Will the switchboard be hot or cold metering?11.What will the alignment be?12. What type of bus material is required (temperature/density)?13. What special modifications are needed (serialcommunications, pull sections, corner sections)?72
Review 61.SB1 switchboards are aligned.2.The maximum main bus rating of an SB1 switchboard isamps.3.The maximum main bus rating of an SB2 switchboard isamps.4.The maximum main bus rating of an SB3 switchboard isamps.5.Super Blue Pennant switchboards are rated up toamps with a circuit breaker andamps with a Vacu-Break fusible switch.6.Up to branch circuit provisions areavailable in the distribution panel of the Super BluePennant switchboard.7.The type of commercial metering switchboard used onthe west coast is Type .8.The type of specialty service entrance switchboardthat uses a molded case circuit breaker as a maindisconnect is a Type service cubicle.9.The type of specialty service entrance switchboard thatuses a Vacu-Break fusible switch as a main disconnectis a Type service cubicle.73
Review AnswersReview 11) 120; 2) 277; 3) 100; 4) NEMA, UL; 5) 408.Review 21) ampacity; 2) Time; 3) 200,000; 4) 90; 5) bus; 6) Splice;7) through-bus.Review 31) service, distribution; 2) pull box; 3) pull section; 4) 4300;5) Hot sequence; 6) Cold sequence; 7) f; 8) distribution;9) Rear aligned.Review 41) 277; 2) 120, 208, 120; 3) service entrance; 4) six;5) disconnect link.Review 51) 1000; 2) Vacu-Break fusible switches; 3) 230.95; 4)Withstand; 5) Interrupting; 6) series-rated; 7) Ampere rating.Review 61) rear; 2) 2000; 3) 4000; 4) 6000; 5) 800, 600; 6) 40; 7) SMM;8) BCT; 9) SCT.74
Final ExamThe final exam is intended to be a learning tool. The book maybe used during the exam. A tear-out answer sheet is provided.After grading the test, mail the answer sheet in for grading. Agrade of 70% or better is passing. Upon successful completionof the test a certificate will be issued.1.The requirements for switchboards are covered inNEC Article .a.c.2.overloads and heatoverloads and short circuitsshort circuits and heatground fault and heat10,000100,000b.d.50,000200,000The standard height of a Siemens switchboard isinches.a.c.5.318770The AIC rating of a Class R fuse is amps.a.c.4.b.d.Two causes of overcurrent are .a.b.c.d.3.2104083272b.d.3890The correct NEMA phase sequence for a vertical bus,as viewed from the front, left to right is .a.c.A-B-CC-A-Bb.d.A-C-BC-B-A75
6.Two adjoining switchboard sections are connectedtogether with .a.c.7.pull sectiondistribution sectionHot sequenceTop feedb.d.Cold sequenceBottom feedfrontfront and rearb.d.rearfront or rear138277b.d.240480On a three-phase, four-wire, B phase high leg,delta-connected transformer the high leg is.a.c.76b.d.On a three-phase, four-wire, wye-connectedtransformer with a secondary voltage of 480 voltsphase-to-phase, the phase-to-neutral voltage isvolts.a.c.11.service sectionpull boxA switchboard with a service section that is deeperthan the distribution section would bealigned.a.c.10.compression lugscross busis when power is still applied to the utilitymeter when the service main is switched off.a.c.9.b.d.A is used when cables fed from thebottom of a switchboard need to be routed to the topof the switchboard.a.c.8.vertical bus barssplice platesA-NC-Nb.d.B-NA-B
12.The maximum number of switches or circuit breakersused to disconnect and isolate the service from allother equipment on service-entrance equipment is.a.c.13.alwaysrarelyb.d.neveroftenMolded case circuit breakersVacu-Break fusible switchesInsulated case circuit breakersBolted pressure switchesThe is a removable link that isolates theneutral bus from, the grounded neutral bus
53 Ground Fault Ground fault protection is generally incorporated into a Protection Devices special type of protective device such as a molded case circuit breaker. Ground fault protection is also available in Siemens insulated case circuit
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The SE-601 is a microprocessor-based ground-fault relay for ungrounded dc systems. It provides sensitive ground-fault protection without the problems associated with nuisance tripping. Ground-fault current is sensed using an SE-GRM Series Ground-Reference Module—a resistor network that limits ground-fault current to 25 mA. The SE-601 is used
ground-fault protection for pv systems Photo 3. Four-pole, ground-fault protective device for 48-volt PV system Photo 1. One-pole, ground-fault protective device for 48-volt PV system can handle the worst case short-circuit currents and is oversized by a factor of 125 percent. It is an impressive demonstration when circuit breakers rated at 750 .
In this paper, the calculation of double line to ground fault on 66/33 kV transmission line is emphasized. Figure 3. Double-Line-to-Ground Fault 2.4 Three-Phase Fault In this case, falling tower, failure of equipment or . ground fault, fault current on phase a is I a 0. Fault voltages at phase b and c are: V b .
illustrated in Figure 3. This is a fault occurring with phase A to ground fault at 8 km measured from the sending bus as depicted in Figure 1. The fault signals generated using ATP/EMTP are interfaced to the MATLAB for the fault detection algorithm. (a) Sending end (b) Receiving end Figure 3. Example of ATP/EMTP simulated fault signals for AG fault
Fault types in dc systems: (a) line-to-ground fault and (b) line-to-line fault. International Journal of Innovative Research & Studies Volume 8, Issue II, FEB/2018 . A line-to -ground fault occurs when either the positive or negative pole is short-circuited to ground. The line-to -ground faults are the most common types of faults in
Project name: Ground fault in shipboard DC power system Task: In a Shipboard DC power system, large parts of the system will be connected without galvanic separation. If a ground fault occur in the system, the system need to handle the fault without interfering with normal operations. Ground fault can occur in different places and with different
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