The IT earthing system (unearthed neutral) in LV

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1.5 Equivalent system for a network with unearthed or impedance-earthed neutralA few definitions and assumptions are givenbelow in order to define the equivalent systemfor this network (see fig. 4 ):c the neutral point is unearthed or earthed by animpedance (Z N ) of high value (normally 1 kΩ to2kΩ) whose earth connection is equivalent to aresistance (R B );c the load frames are interconnected either fullyor by group. For EMC reasons (see“Cahier Technique” no. 187), it is advisable tointerconnect all the application frames of thesame installation and to connect them to thesame earth connection (resistance R A );c the earth connections (R A and R B ) areinterconnected (in most cases), or separate.NB: Two earth connections are considered to beseparate if they are more than 8 m apart;c each live conductor has, with respect to earth,an impedance made up of a resistance and acapacity.N321NPEZ NRN R1 R2 R3CN C1 C2 C3R BR AFig. 4 : equivalent system of a network with unearthed or impedance-earthed neutral.Cahier Technique Schneider Electric no. 178 / p.8
2 The 1 st insulation fault with the IT earthing systemIn normal operating conditions, safety of personsis guaranteed when contact voltage is less than50 V as per standard IEC 60364 (NF C 15-100).When this contact voltage is exceeded, thesestandards require automatic opening of thecircuit. The following section shows how use ofthe IT earthing system for network operationprevents tripping on the first insulation fault.2.1 Calculating fault currents and contact voltage on the first faultGeneral case (resistive fault)Should a fault with a resistive value Rd occurbetween phase 3 and the earth, a fault current I dflows in the neutral impedance and in thecapacities C1, C2 and C3 (see fig. 3a).Assuming that the phase-to-earth capacities arebalanced (C1 = C2 = C3 = C), the fault currenthas the following value:1+ 3j CωZI d =UN0Rd+ ZN + 3jCωZNRdThe capacitive current is written as:3j CωZI c =UN0Rd+ ZN + 3jCωZNRdand the current in the impedance Z N :UI N =0Rd+ ZN + 3jCωZNRdThe contact voltage U C (contact voltage betweenthe frame of a faulty device and another frame orthe earth) is calculated from the fault current I dflowing in the earth connection resistance R A ofthe application frames if they are notinterconnected, else R B (only network earthconnection):U C = R A I d .Case of the full faultThis paragraph calculates the configurationgenerating the highest contact voltage (U C ):thus for a fault occuring on a frame with earthconnection separate from that of Z N .By application of the above formulae, whereR d = 0, we obtain:UI d = 0ZN+ 3j CωUUc = R 0AZ N + 3j CωThe capacitive current is equal to:I C = +3j Cω U 0and the current in impedance Z N :UI N = 0ZNIn the various examples below, studied forZ N = ∞ (unearthed neutral) and Z N = 1 kΩ(impedance-earthed neutral), the calculationsare made for a network in the IT system,400 VAC (U 0 = 230 V), where:R A , earth connection resistance = 10 ΩR d , insulation fault value = 0 to 10 kΩ.c Case 1:Low capacity network (e.g. limited to anoperating theatre)C1 = C2 = C3 = C = 0.3 µF per phase.c Case 2:Power network, whereC1 = C2 = C3 = C = 1.6 µF per phase.c Case 3:Very long power network, whereC1 = C2 = C3 = C = 10 µF per phase, i.e.roughly 40 km of cables!The results of all these calculations, grouped inthe table in figure 5 , confirm the low faultvoltage (≈ 20 V in the most unfavourable cases),ensuring continuity of operation, without risk forpersons, of a network designed using the ITsystem. They prove that addition of animpedance between the neutral and the earthhas very little effect on contact voltage.R d (kΩ) 0 0.5 1 10Case 1 Z N = ∞ U C (V) 0.72 0.71 0.69 0.22C R =1µF I d (A) 0.07 0.07 0.07 0.02Z N = 1 kΩ U C (V) 2.41 1.6 1.19 0.21I d (A) 0.24 0.16 0.12 0.02Case 2 Z N = ∞ U C (V) 3.61 2.84 1.94 0.23C R =5µF I d (A) 0.36 0.28 0.19 0.02Z N = 1 kΩ U C (V) 4.28 2.53 1.68 0.22I d (A) 0.43 0.25 0.17 0.02Case 3 Z N = ∞ U C (V) 21.7 4.5 2.29 0.23C R =30µF I d (A) 2.17 0.45 0.23 0.02Z N = 1 kΩ U C (V) 21.8 4.41 2.26 0.23I d (A) 2.18 0.44 0.23 0.02Fig. 5 : comparison of fault currents and contactvoltages on a first fault.Cahier Technique Schneider Electric no. 178 / p.9
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