TECHNICALTOPIC Supplying adetac - napitonline.com

1 8 NAPI T087 044 41392 ww concernsra ised onthe NAPITH elplineThe supplyof electricityfro m a main buildingtodetac hed outb uildin gs, such as garages or she ds,con tinu es to co nce rn so me callersto the NAPITtec hnic al helplin e. Ther e are severalscenarios: th e supplyin th e main bu ilding is TN-S or TN-C-San d th ere are no extran eou s-c ondu ctive-partsinth e detach ed out bu ilding th e supplyin th e main bu ilding is TN-S and thereare ex tran eo us- co ndu ct ive-par ts in th e det achedoutbu ilding th e supplyin th e main bu ilding is TN-C-Sandth er e are extran eou s- con ductive-partsin thedet ac hed ou tb th is art icle.

2 I ll con sider the first sit uation andassu me that the exist ing buildin g are domesticprem ises with a TN- S or a TN-C -S ll furtherassu me that the garagehas no extran eous-con duc tive -par ts ( no wate r su pply) . In thenext issue , we ll co ve r th e oth er two sce Dpro tec ted socke t-out letsWh atev er th e detach ed building is used fo r, anyso cket- ou tlet s within it must be co nsideredasreaso nab ly like ly to be used to su pply electricalequ ipm ent for use outdo ors. Th er efore , inac cordanc e with Regu lation 47 1-16-01,such socke t-outlets must be pro tected by a 30 mA residualcurrent dev ice (R CD).

3 Th e qu estion is some timesaske d ab out th e locatio n of th e RCD. There arevario us ways of ac hie vin g this. Th e RCD pr ot ectioncould be pro vided with in th e house by : taking the garage supplyfr om th e RCD-protectedside of a split- board in th e house using an RCB O in th e co nsu me r unit in the houseto pro tec t th e su pply to th e rnativ ely, the RCD pro tectio n co uld be providedwith in th e garag e by: selec ting a consu mer unit which has RCDpro te ctio n inc orpo rated in the main swit ch using an RCB O in th e co nsu me r unit in thegarag e to pro tec t th e so cket-o utlet(s) wit hinth e garag e installingso cket-o utlet(s) in th e garagewithRCD pro tec tio n inco rpo rated (calledSRCDs).

4 The supplycableCo nsiderable car e must be giv en to the type of cableused to su pply th e outbu ilding and how it is to beLightLightswitchSupplytotakenfromspare waySocket-outlet(protected by30mARCD)TN-SorTN-C-SsupplyEarthenwarep ipeDetatched Garage2 wayconsumerunit20 AMCBE xistingconsumerunitFi g 1 Supplyingade tacTEC HNI CA L TOPI CNAP IT08704441392 ww 9run. Typ ically , a swa (st eel wire armo ured) cableor ano ther type of compositepvc insulatedcablewith su itab le me ch anical prot ectio n would beselec ted. For example,a flat tw in an d earthcab le co uld be inst alled in an ear the nware pipe.

5 Ot herw ise, markin g tape of cable tiles must beused. Whi le BS 76 71 cont ains no re quire men tfor th e mini mum depth of unde rgro un d cables,they mu st be burie d deep eno ugh to avoid beingdam aged by an y gr ou ndw ork wh ich is lying agar ag eFig. 1 sh ows a compo site (e .g. tw in an d earth)cab le run fro m an existingconsu mer unit in thehou se to th e de tac hed garage. A spare way in theexistingco nsu me r unit has be en ut ilised and a 20 Aminiatu re circu it br eaker (m cb) pro vides overcurrentpro tectio n.

6 A two- way con su mer un it has beenins talled in the gar ag e to su pply th e lightingcircuitand th e RCD- pr ot ected socket-out let. Th e supplyto the house could be eit her TN- S or TN-C-Sand inthis case it wo uld not matte r in ter ms of design ingthe elec trical supplyfor th e gar alterna tive des ignIf a spare way was no t availablein th e existingcon sum er unit, th e su pply co uld be taken froma sw itc hed fused connection unit co nnectedtoa rin g fin al circ uit within th e hou se. An alt ernativeto th e use of the two-w ay co nsu mer un it in thegarag e wo uld be to term inate th e su pply cablein a dou ble- po le sw itch in th e garagefor localiso lation.

7 Fro m th e do uble-po le sw itch, an SRCDso ck et- ou tlet could be su pplied and the lightingcou ld be su pplied via a sw itch ed fu sed connectionunit. Fig. 2 illu stratesth is lusionIt is hoped that this article,to geth er with thediag ram s, has been helpfu l in pro viding clearguidan ce for th e su pply of electricityto a typicaldetac hed gar ag e. Ne xt tim e we ll co nsider howto su pply a det ac he d wo rksh op which hasextr aneo us-c ondu ll look firstat using a TN-S su pply th en a TN -C -S doutbuildingSocket-outletFusedconnection unitDoublepoleswitchSocket-outlet(protec ted by30mA RCD)TN-SorTN-C-SsupplyFusedconnectionuni tEarthenwarepipeDetatched GarageLightswitchLightFig 2