Guide to Construction of ArTu switchgear - Abb

8 Protection against the effects of short-circuit8.2 Short-circuit current and suitability of switchgear for the plantGENERALSTANDARDS ANDCHARACTERISTICSVERIFYING THEOVERTEMPERATURESPROTECTION AGAINSTSHORT-CIRCUIT ANDELECTRIC SHOCKSPRACTICAL INDICATIONSCERTIFICATION ANDCONSTRUCTIONCERTIFICATESDECLARATION OFCONFORMITYThe short-time short-circuit withstand current ofthe switchgear Icw (r.m.s. value) is knownYESIcc (prospective of the plant) < Icw(of the switchgear)YESSwitchgearsuitableThe following diagrams show the method to determine compatibility of the switchgear with the plant.ExampleExisting plant data:Vn=400 Vfn=50HzIcc=35kALet us suppose that in an existing plant there is electric switchgear with an Icw of 35kA andthat, in the installation point of the switchgear, the prospective short-circuit current is 35kA.Now let us imagine that it is decided to extend the power of the plant and that the short-circuitvalue increases to 60 kA.Plant data afterextension:NOThere is a circuit-breaker onthe supply side of theswitchgear which for theprospective Icc has an I2t < I2t(of the switchgear) and a limitedpeak current Ip < Ip (switchgear)NOSwitchgearnot suitableIt must, of course, be checked that the breaking capacities of the apparatus inside the switchgear are compatible with theshort-circuit values of the plant.Vn=400 Vfn=50HzIcc=60kASince the switchgear Icw is lower than the short-circuit current of the plant, to check that theexisting switchgear is still compatible, you must:- determine the values of I 2 t and of Ip let through by the circuit-breaker placed on thesupply side of the switchgear- check that the protection devices located inside the switchgear have adequate breakingcapacity, individually or for back-up.Icw = 35kA from which:I 2 t switchgear= 35 2 x1 =1225 MA 2 sIp switchgear= 73.5 kALet us suppose that, on the supply side of the switchgear, a Tmax T5H moulded-case circuit-breaker(Icu=70kA@415V) is installedI 2 t interruttore< 4MA 2 sIp interruttore I 2 t switchgearIp circuit-breaker> Ip switchgearThe conditioned short-circuit current of the switchgear(r.m.s. value) is knownIcc (prospective of the plant) < Icc (conditioned of theswitchgear) (with a specifiedprotection device)YESSwitchgearsuitableThe switchgear (structure and busbar system) turns out to be suitable.With regard to the circuit-breakers located inside the switchgear, let us suppose that theseare Tmax T1,T2,T3 moulded-case circuit-breakers, version N with Icu=36kA@415V. From theBack-up tables it can be seen that the circuit-breakers present in the switchgear are suitablefor the plant as their breaking capacity is increased to 65 kA by the T5H circuit-breaker placedon the supply side.NOSwitchgearnot suitable46 ABB SACEGuide to Construction of ArTu switchgear