Selection criteria for the TT, TN and IT systems
From Electrical Installation Guide
Selection does not depend on safety criteria.
The three systems are equivalent in terms of protection of persons if all installation and operating rules are correctly followed.
The selection criteria for the best system(s) depend on the regulatory requirements, the required continuity of service, operating conditions and the types of network and loads
In terms of the protection of persons, the three system earthing arrangements (SEA) are equivalent if all installation and operating rules are correctly followed. Consequently, selection does not depend on safety criteria.
It is by combining all requirements in terms of regulations, continuity of service, operating conditions and the types of network and loads that it is possible to determine the best system(s) (see Fig. E16).
Selection is determined by the following factors:
- Above all, the applicable regulations which in some cases impose certain types of SEA
- Secondly, the decision of the owner if supply is via a private MV/LV transformer (MV subscription) or the owner has a private energy source (or a separate-winding transformer)
If the owner effectively has a choice, the decision on the SEA is taken following discussions with the network designer (design office, contractor).
The discussions must cover:
- First of all, the operating requirements (the required level of continuity of service) and the operating conditions (maintenance ensured by electrical personnel or not, in-house personnel or outsourced, etc.)
- Secondly, the particular characteristics of the network and the loads (see Fig. E17 )
|Fault current||-||- -||- -||+||- -||Only the IT system offers virtually negligible first-fault currents|
|Fault voltage||-||-||-||+||-||In the IT system, the touch voltage is very low for the first fault, but is considerable for the second|
|Touch voltage||+/- -||-||-||+||-||In the TT system, the touch voltage is very low if system is equipotential, otherwise it is high|
|Protection of persons against indirect contact||+||+||+||+||+||All SEAs (system earthing arrangement) are equivalent, if the rules are followed|
|Protection of persons with emergency generating sets||+||-||-||+||-||Systems where protection is ensured by RCDs are not sensitive to a change in the internal impedance of the source|
|Protection against fire (with an RCD)||+||+||Not allowed||+||+|| All SEAs in which RCDs can be used are equivalent.
The TN-C system is forbidden on premises where there is a risk of fire
|Continuous overvoltage||+||+||+||-||+||A phase-to-earth overvoltage is continuous in the IT system if there is a first insulation fault|
|Transient overvoltage||+||-||-||+||-||Systems with high fault currents may cause transient overvoltages|
|Overvoltage if transformer breakdown(primary/secondary)||-||+||+||+||+||In the TT system, there is a voltage imbalance between the different earth electrodes. The other systems are interconnected to a single earth electrode|
|Immunity to nearby lightning strikes||-||+||+||+||+||In the TT system, there may be voltage imbalances between the earth electrodes. In the TT system, there is a significant current loop between the two separate earth electrodes|
|Immunity to lightning strikes on MV lines||-||-||-||-||-||All SEAs are equivalent when a MV line takes a direct lightning strike|
|Continuous emission of an electromagnetic field||+||+||-||+||+||Connection of the PEN to the metal structures of the building is conducive to the continuous generation of electromagnetic fields|
|Transient non-equipotentiality of the PE||+||-||-||+||-||The PE is no longer equipotential if there is a high fault current|
|Continuity of service|
|Interruption for first fault||-||-||-||+||+||Only the IT system avoids tripping for the first insulation fault|
|Voltage dip during insulation fault||+||-||-||+||-||The TN-S, TNC and IT (2nd fault) systems generate high fault currents which may cause phase voltage dips|
|Special devices||-||+||+||-||-||The TT system requires the use of RCDs. The IT system requires the use of IMDs|
|Number of earth electrodes||-||+||+||-/+||-/+||The TT system requires two distinct earth electrodes. The IT system offers a choice between one or two earth electrodes|
|Number of cables||-||-||+||-||-||Only the TN-C system offers, in certain cases, a reduction in the number of cables|
|Cost of repairs||-||- -||- -||-||- -||The cost of repairs depends on the damage caused by the amplitude of the fault currents|
|Installation damage||+||-||-||++||-||Systems causing high fault currents require a check on the installation after clearing the fault|
[a] IT-net when a first fault occurs.
[b] IT-net when a second fault occurs.
Fig. E16: Comparison of system earthing arrangements
|Type of network||Advised||Possible||Not advised|
|Very large network with high-quality earth electrodes for exposed conductive parts (10 Ω max.)||TT, TN, IT [a] or mixed|
|Very large network with low-quality earth electrodes for exposed conductive parts (> 30 Ω)||TN||TN-S|| IT [a] |
|Disturbed area (storms) (e.g. television or radio transmitter)||TN||TT||IT [b]|
|Network with high leakage currents (> 500 mA)||TN[d]|| IT[d]
|Network with outdoor overhead lines||TT[e]||TN[e][f]||IT[f]|
|Emergency standby generator set||IT||TT||TN[g]|
|Type of loads|
|Loads sensitive to high fault currents (motors, etc.)||IT||TT||TN[h]|
|Loads with a low insulation level (electric furnaces,welding machines, heating elements, immersion heaters, equipment in large kitchens)||TN[i]||TT[i]||IT|
|Numerous phase-neutral single-phase loads (mobile, semi-fixed, portable)|| TT[j]
|Loads with sizeable risks (hoists, conveyers, etc.)||TN[k]||TT[k]||IT[k]|
|Numerous auxiliaries (machine tools)||TN-S|| TN-C
|Supply via star-star connected power transformer[n]||TT|| IT
|Premises with risk of fire||IT[o]|| TN-S[o]
|Increase in power level of LV utility subscription, requiring a private substation||TT[p]|
|Installation with frequent modifications||TT[q]|| TN[r]|
|Installation where the continuity of earth circuits is uncertain (work sites, old installations)||TT[s]||TN-S|| TN-C|
|Electronic equipment (computers, PLCs)||TN-S||TT||TN-C|
|Machine control-monitoring network, PLC sensors and actuators||IT[t]||TN-S, TT|
[a] When the SEA is not imposed by regulations, it is selected according to the level of operating characteristics (continuity of service that is mandatory for safety reasons or desired to enhance productivity, etc.). Whatever the SEA, the probability of an insulation failure increases with the length of the network. It may be a good idea to break up the network, which facilitates fault location and makes it possible to implement the system advised above for each type of application.
[b] The risk of flashover on the surge limiter turns the isolated neutral into an earthed neutral. These risks are high for regions with frequent thunder storms or installations supplied by overhead lines. If the IT system is selected to ensure a higher level of continuity of service, the system designer must precisely calculate the tripping conditions for a second fault.
[c] Risk of RCD nuisance tripping.
[d] Whatever the SEA, the ideal solution is to isolate the disturbing section if it can be easily identified.
[e] Risks of phase-to-earth faults affecting equipotentiality.
[f] Insulation is uncertain due to humidity and conducting dust.
[g] The TN system is not advised due to the risk of damage to the generator in the case of an internal fault. What is more, when generator sets supply safety equipment, the system must not trip for the first fault.
[h] The phase-to-earth current may be several times higher than In, with the risk of damaging or accelerating the ageing of motor windings, or of destroying magnetic circuits.
[i] To combine continuity of service and safety, it is necessary and highly advised, whatever the SEA, to separate these loads from the rest of the installation (transformers with local neutral connection).
[j] When load equipment quality is not a design priority, there is a risk that the insulation resistance will fall rapidly. The TT system with RCDs is the best means to avoid problems.
[k] The mobility of this type of load causes frequent faults (sliding contact for bonding of exposed conductive parts) that must be countered. Whatever the SEA, it is advised to supply these circuits using transformers with a local neutral connection.
[l] Requires the use of transformers with a local TN system to avoid operating risks and nuisance tripping at the first fault (TT) or a double fault (IT).
[l2] With a double break in the control circuit.
[m] Excessive limitation of the phase-to-neutral current due to the high value of the zero-phase impedance (at least 4 to 5 times the direct impedance). This system must be replaced by a star-delta arrangement.
[n] The high fault currents make the TN system dangerous. The TN-C system is forbidden.
[o] Whatever the system, the RCD must be set to Δn ≤ 500 mA.
[p] An installation supplied with LV energy must use the TT system. Maintaining this SEA means the least amount of modifications on the existing network (no cables to be run, no protection devices to be modified).
[q] Possible without highly competent maintenance personnel.
[r] This type of installation requires particular attention in maintaining safety. The absence of preventive measures in the TN system means highly qualified personnel are required to ensure safety over time.
[s] The risks of breaks in conductors (supply, protection) may cause the loss of equipotentiality for exposed conductive parts. A TT system or a TN-S system with 30 mA RCDs is advised and is often mandatory. The IT system may be used in very specific cases.
[t] This solution avoids nuisance tripping for unexpected earth leakage.
Fig. E17: Influence of networks and loads on the selection of system earthing arrangements