The IT earthing system (unearthed neutral) in LV

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5 Advantages and disadvantages ofthe IT earthing system in LVThe main advantage of using the IT earthingsystem for network operation is without doubtthe continuity of supply it offers, as there is noneed to trip on the first fault (as described in thesection below). Another of this system’s strongpoints is guaranteed safety against the firehazard and for control and monitoring circuits ofmachine tools.However, to benefit fully from such advantages,the restrictions of this system must also beconsidered.5.1 Increased availabilityThis advantage can be confirmed by a simpleprobability calculation.Let us assume that the occurrence of aninsulation fault in an electrical installation is onefault every three months (90 days),1i.e. λ =90 jand the time needed to track and put right thefaulty part is one day,i.e. µ =1j.The Markof graph technique gives therepresentation shown in figure 22 and enablesus to calculate that the average time betweentwo double faults is 8,190 days!This corresponds to an average electrical poweravailability that is 91 times better in IT than in TNor TT.Consequently, preference is frequently given tothe IT earthing system for use in:c hospitals,c airport take-off runways,Nofaultλ = 1 90 jµ = 1j1faultλ = 1 90 jµ = 1j2faultsFig. 22 : a Markof graph shows that average electricalpower availability is 91 times better in IT than in TN orTT.c vessels,c plants with continuous manufacturingprocesses,c laboratories,c cold storage units,c electrical power plants.5.2 Increased safety against the fire hazardElectricity is often the cause of fire. Standardsset the threshold for this risk at 500mA on aninsulation fault (NF C 15-100, part 482.2.10). Thisvalue can be considerably exceeded, particularlywith stray currents that flow through buildingstructures when faults occur in the TN system.Also worthy of note is that the IT is the onlyearthing system that monitors insulation of theneutral conductor, compared with the TNS whichcan insidiously turn into a TNC on a neutral-PEfault with an increase in the fire hazard.It is because the current of the first fault isparticularly low that the IT earthing system hasbeen chosen for use in certain establishments atrisk from fire and explosion (see chapter 1).Furthermore the first PIMs were used infiredamp mines.Cahier Technique Schneider Electric no. 178 / p.22
5.3 Less downtime on control and monitoring circuitsThe relay diagram illustrated in figure 23 withthe TN earthing system shows three possibleinsulation faults which, when full, result inimmediate downtime, whose material andeconomic consequences are rarely negligible.These faults have the same consequences withthe TT system.In particular, faults c and d cause tripping of themaster protection device, and prevent allsubsequent operations, such as for example theorder to change direction on a transporter bridge!These same first faults which can causeoperating malfunctions or even accidents withthe TN and TT systems, have no effect with theIT system, except if they occur as the secondfault (extremely unlikely, see section 5.1).These examples show that even if safety ofpersons with respect to the electrical hazard isguaranteed by the various earthing systems, orby use of Safety by Extra Low Voltage (SELV),safety of persons with respect to mechanicalhazards may not be guaranteed in certain cases.More care must therefore be taken when wiringsuch circuits in the TT and TN than in the ITsystem, as the latter warns the operator of theincident (first insulation fault), thereby guardingagainst electrical and mechanical hazards. PIMsare increasingly used for just this purpose, tomonitor automation networks.An additional solution is often advisable,particularly with relays using electronic devicessensitive to electromagnetic disturbances. Theaim is to supply all the control and monitoringcircuits separately by means of a LV/LVtransformer with separate windings.Despite this, as stated in chapter 2, use of the ITearthing system has its limits which aredescribed in the section below.32N1NPEI dAMR a b c dBFault a cannot be detected.Fault b prevents the off function.Faults c and d cause ashort-circuit.Ma b c dFault a cannot be detected.Faults b, c and d cause ashort-circuit.AFig. 23 : monitoring circuit may be concerned by several types of insulation faults always resulting in downtimewith TT and TN system.5.4 Restrictions and precautions for using the IT earthing systemThe restrictions for using the IT system arelinked to loads and networks.Limits linked to loadsc With a high earth capacitive coupling(presence of filters).A number of devices fitted with capacitive filters(see fig. 24 ) offer the same disadvantage, dueto their number, as very long networks when theIT system is used.These capacitive leakages have a particularity,with respect to distributed capacity mainly due tonetwork cables, i.e. they can be unbalanced.DeviceMicro-computerUPSVariable speed controlersFluorescent tubes(in ramps of 10)Network/earth capacity20 nF to 40 nF40 nF70 nF20 nFFig. 24 : guideline capacitive values for HF filters builtinto various devices.Cahier Technique Schneider Electric no. 178 / p.23
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