Come Back to Ground! 1. Introduction

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Loraine Quinn
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1 Cme Back t Grund! 1. Intrductin Many ships, particularly small tugs and supply bats have used a neutral grunded distributin system fr 120 vlt single phase htel and dmestic lads. This system is identical t that used in mst Nrth American 120 vlt land based installatins. The land based installatins distribute pwer using a 3 wire system; ne wire is used fr the "live" cnnectin, ne fr the neutral and ne fr the grund bnd. Circuit prtectin is prvided by single ple circuit breakers installed in a standard single phase 3 wire panelbards. Marine installatins fllwed the same practise but ften used a 2 wire system t save n cable csts. When the tw wire system was used, it was essential t ensure that each fixture and 120 vlt device is separately bnded t grund at the fixture r device. On marine installatins, the fixtures themselves are ften installed directly nt metallic bulkheads that are part f the vessel's structure, this als being the medium fr the ship's "grund". IEEE clause 13.1 intrduced the requirement that "Circuit breakers in grunded neutral distributin panels shuld include a ple r switch fr the neutral". This requirement therefre remves the pssibility f using single ple circuit breakers fr the 120 vlt services. When tw ple circuit breakers are used, the standard single phase 3 wire distributin panel cannt be used withut mdificatin and three phase panels may be required cnfigured fr single phase lads. As the majr benefit f using a grunded single phase system was generally the cst saving in circuit breakers and panels, cmpliance with IEEE begs the questin "why grund?". Ungrunded installatins ffer ther benefits and perhaps the area f marine system grunding shuld nce again be reviewed. 2. Grunding, and bnding - it's all a matter f safety The majr factr which determines the grunding methd is safety - primarily safety f peple and secndly safety f equipment. On marine installatins, safety f equipment may als cntribute t safety f persnnel particularly if lss f vital equipment places

2 the ship in danger. The majrity f faults n a marine system are grund faults. A grund fault ccurs when the insulatin between live cnductrs and the "grund" whatever that is defined t be, n lnger maintains an effective resistance between the live cnductr and grund. On a "grunded" system, the fault will be remved by the autmatic pening f the circuit breaker. On an "ungrunded" system, the fault is detected but nt remved autmatically. This enables the service t be retained until such time as it may be shutdwn safely. Fr vital services this is bviusly advantageus. The "grund fault" may result frm a live cnductr tuching equipment metal cases. Persnnel safety can be in jepardy when a live cnductr cmes int cntact unintentinally with parts that an peratr may tuch. Fr marine installatins the grund is usually the ship's hull and all electrical equipment metallic enclsures are bnded t it. By bnding the case t the ship's hull, a direct path is prvided between the equipment and the ship's hull thereby ensuring they are bth at the same vltage level. A persn standing n the hull is then prtected against electrical shck. Furthermre, the path enables "fault current" t flw and thus allws any prtectin r detectin devices t perate. Safety is affected in tw ways. The methd f grunding dictates if the equipment will cntinue t perate when a grund fault ccurs. Failure t perate may be a hazard t the ship r ther equipment. The methd f grunding will affect the magnitude f any transient vervltages that culd ccur as a result f "restriking" grunds. Such transients may permanently damage equipment. Accrdingly the methd f grunding may detract frm a safe system by causing equipment t discnnect n a grund fault - r be permanently damaged. 3. What is the difference between a grunded and ungrunded system? A review f sme histrical data prvides an insight int marine system grunding and grunding methds. When a.c. installatins became ppular during the 's, the majrity f these systems perated with an ungrunded neutral. There were a few ntable exceptins.

3 MV Bergensfjrd (1950) used a resistance grunded neutral system The QE 2 (1968) used slidly grunded neutral systems fr dmestic and htel services fllwing land based practices. Many Danish Navy vessels used the "Petersn cil" t cnnect the neutral t grund. Sme small German, Yugslavian and Australian vessels used slidly grunded neutral systems. Marine system grunding is a tpic fr which the references are prlific! The mst cmprehensive dcument n the subject is a study cmpleted by YARD in 1982 in which the technical, cmmercial and peratinal and safety aspects f marine system grunding are extensively discussed. There are generally three methds f grunding applied t marine systems: Slid and lw impedance grunding High impedance grunding Insulated neutral (ungrunded) Slid and lw impedance grunded systems are smetimes referred t as being "effectively" grunded. These systems are arranged such that n the ccurrence f a grund fault, the shrtcircuit prtective devices perate as quickly as pssible, t discnnect the faulty circuit frm the system. High impedance grunded and insulated systems are smetimes referred t as being "nn-effectively" grunded. With this type f system it is pssible t perate with a grund fault n the system. It is imprtant hwever t include detectin and alarm equipment t ntify the peratrs when a grund fault ccurs, and t islate the grunded circuit as quickly as pssible. Fr marine systems it is generally preferred that the first grund fault will nt discnnect the grunded circuit, the electrical system r any part f it. Cntinuity f supply, and particularly f prpulsin, is cnsidered f paramunt imprtance and nneffectively grunded systems are generally preferred. Nn-effectively grunded systems The main advantages f the nn-effectively grunded system are: Cntinuity f service n the ccurrence f a grund fault

4 Grund fault currents can be kept lw The main disadvantages are A high level f insulatin may be necessary. The pssibility exists fr high transient ver-vltages t ccur Grunded circuit detectin is difficult Effectively grunded systems The main advantages f the effectively grunded system are: Equipment insulatin des nt require any special attentin Grund faults are detected autmatically and immediately islated Grund fault current flws fr a shrt perid f time thereby restricting damage Arcing grund ver-vltages can be avided The main disadvantages are: Immediate discnnectin and lss f the service High grund fault currents flw, which can cause extensive damage and the risk f explsin. It is generally accepted that there is n single "best" methd fr grunding a marine electrical system. Selectin must generally be made based n cst, peratin and safety issues. Generally in the marine wrld the desirability f maintaining a service althugh a grund fault, and having lw grund fault currents favurs the nn-effectively grunded systems (i.e. neutral pint insulated r grunded thrugh a high resistance). Frm the technical viewpint the mst suitable criteria n which t chse the grunding system must be the prspective grund fault current. The principal bjective is t design a system such that system ver-vltages are prevented, grund fault current is restricted t a lw value t limit fault damage but nt s lw that detectin becmes difficult and expensive. Finally having decided n the "best" grunding system fr yur particular applicatin, the system shuld be cnfigured such that the cnsequences f a grund fault minimise the dangers and d nt limit the peratinal aspects f the ship. This is dne by applying restricted grunded systems t specific services (e.g.

5 dmestic, htel, galley), and applying insulated systems t essential services and splitting systems and applying redundancy. Fr example: Lw pwer -lw vltage systems - slidly grunded The majrity f lw pwer, lw vltage marine systems use a slidly grunded neutral. This permits the use f the phase t neutral pint as a surce f lw vltage fr small pwer and lighting cnsumers. With this system a single grund fault will cause an vercurrent in the faulted circuit and this in turn will trip the circuit supply breaker t discnnect the service. Fr many services this is quite acceptable but fr services such as steering gear, lubricatin and fuel pumps, this culd be an unacceptable practice. Lw vltage systems - insulated neutral The majrity f lw vltage marine systems ( vlts) have an ungrunded cnfiguratin. The main reasn is that transient ver-vltages n a lw vltage system are generally within the equipment insulatin level (under 2 kv) but mre imprtantly, an ungrunded system can still perate even if there is a grund fault present. With ungrunded systems hwever it is imprtant t have a grund fault detectin system in place and t clear grund faults as quickly as pssible. It shuld be remembered that when the first fault ccurs n an insulated neutral system, the system immediately becmes "grunded" and cnsequently the next fault that ccurs will be limited by the much lwer zer sequence reactance f the system. This can cause fault currents in excess f the 3 phase fault current fr which mst switchgear is rated. Cnsequently, t avid the risk f prtectin equipment failure, any fault shuld be cleared as rapidly as pssible. The prudent electrician will check with the navigating staff t find a time slt when essential "grunded" equipment can safely be checked ut and brught back int full reliable service! Fr systems in hazardus areas, insulated neutral systems shuld be used. Althugh a grunded neutral system will prvide rapid fault clearance, the risk f a faulted cable causing flashver t grund with resulting explsin, is t high. Medium vltage systems - high impedance grunded The high impedance grunding system ffers several advantages: Lw grund fault current which limits damage and reduces the fire risk

6 Minimal grund fault flash hazard due t system vervltages Lw prtectin equipment csts. A variety f grunding methds are in use n medium vltage systems. On sme UK designed tankers, insulated neutral systems were used at 3.3 kv. Other systems use slidly grunded neutrals. Insulated neutral systems have certain inherent disadvantages and unless it is vital that particular equipment remains peratinal even thugh a grund fault exists, it is recmmended that insulated neutral systems be avided. Althugh the grund fault current is inherently small, the integrity f the system is at risk unless rapid fault detectin and clearance is carried ut. Furthermre all equipment must be capable f perating with ne phase at grund ptential. Fr high vltage equipment this may require additinal insulatin. By slidly grunding the neutral, the grund fault current will undubtedly trip the prtectin equipment and clear the fault. Hwever if the zer sequence impedance, which limits the grund fault current, is cnsiderably less than the psitive sequence impedance, the high grund fault current may exceed that due t a three phase fault. This shuld be checked when determining prtectin equipment shrt-circuit current ratings. The majrity f medium vltage systems use a resistance grund. The resistance is cnnected between the neutral pint and the ship's hull. The resistance limits the grund fault current t a lw value, but ne that is high enugh t ensure selective peratin f grund fault prtective devices. Practice in the US was t limit the grund fault current t 3 times the system capacitive charging current (Ic). As a guide 6 kv marine systems generally have a capacitance f abut 1.4 mf. This means that the resistance shuld be sized t limit the grund fault current t abut 5.5. amps (3 x Ic). This will then prevent any charge being built up n the system due t transient vervltages. Hwever marine systems generally have high lsses and cnsequent heavy damping; therefre such ver-vltages are generally nt a prblem. It is generally better t limit the grund fault current t a value just higher than 3 Ic t ensure the peratin f the grund current detectin and prtectin equipment. Althugh peratin f a high vltage system with a grund fault is pssible, it cannt be recmmended and steps shuld always be

7 taken t initially limit the fault current t as lw a value as is acceptable, and secndly t islate the equipment and repair the grund fault as sn as pssible. If this is nt dne there is always a danger that the fault will escalate t a phase-t-phase fault and cause fire r extensive equipment damage. 4. In summary: Lw-lw vltage systems (120, 220 etc.) Slidly grunded systems can be applied effectively and can reduce circuit breaker requirements frm 2 t single ple. (Nte: Sme regulatry bdies d nt permit the use f single ple breakers and wuld still require grund fault detectin equipment regardless. See IEEE , and Canadian Cast Guard regulatins.) Due t the rapid fault clearance inherent with this system, it is preferred fr dmestic, htel and catering type lads. Lw vltage systems (380, 440, 480, 600 vlts) Insulated neutral systems are the mst widely used. Such systems include grund fault detectin equipment t warn f grund faults. Grund fault ammeters can be added t give indicatin f the level and lcatin f the fault. System ver-vltages resulting frm capacitive charging are rare and generally result in vltages less than the equipment insulatin rating (2 kv). Medium vltage systems (3.3 kv 6kV 6.6 kv 13.8 kv) Practice has been t use high resistance grunded neutral systems where the fault current is limited t a value that will perate the prtectin equipment but is abve that required t encurage transient ver-vltages.

Field Instruction. Refer to General Safety Requirements in FI 10.6 Vegetation Management Work.

Field Instruction. Refer to General Safety Requirements in FI 10.6 Vegetation Management Work. Field Instructin 10.3 Specialised Insulated Plant fr Vegetatin Management Purpse This instructin utlines the minimum requirements and safe apprach distances (SADs) fr specialised insulated plant peratrs