International standards (IEC 60364) clearly define the various elements of grounding connections. The different grounding schemes (often referred to as the type of power system or system grounding arrangements) described characterize the method of grounding the installation downstream of the secondary winding of a MV/LV transformer and the means used for grounding the exposed conductive-parts of the LV installation supplied from it. The following terms are definitions of standardized grounding schemes.
TT System (Earthed Neutral)
One point at the supply source is connected directly to earth. All exposed- and extraneous-conductive-parts are connected to a separate earth electrode at the installation. This electrode may or may not be electrically independent of the source electrode. The two zones of influence overlap without affecting the operation of protective devices.
TT system is technique for the protection of persons. The exposed conductive parts are earthed and residual current devices (RCDs) are used. It is used also in the interruption for the first insulation fault.
The TT system is the simplest solution to design used in installations supplied directly by the public LV distribution network. It does not require continuous monitoring during operation (a periodic check on the RCDs may be necessary). The protection is ensured by special devices, the residual current devices (RCD), which also prevent the risk of fire when they are set to ≤ 500 mA. Each insulation fault results in an interruption in the supply of power, however the outage is limited to the faulty circuit by installing the RCDs in series (selective RCDs) or in parallel (circuit selection). Loads or parts of the installation which, during normal operation, cause high leakage currents, require special measures to avoid nuisance tripping, i.e. supply the loads with a separation transformer or use specific RCDs. (Figures 1 and 2) give the characteristics of TT system as reported by Wiki Electrical Installation and Schneider (2016) and Guide to the wiring regulations, 17th Edition IEE wiring regulations (2008).
Figure 1. Characteristics of TT system
Figure 2. Main characteristics of TT system
Automatic disconnection for TT system is achieved by RCD having a sensitivity of IΔn is less than or equal (50/RA), where RA is the resistance of the earth electrode for the installation, IΔn is the rated residual operating current of the RCD. For temporary supplies (to work sites,) and agricultural and horticultural premises, the value of 50 V is replaced by 25 V. An example of TT system is given in (Figure 3), in this figure, the resistance of the earth electrode of substation neutral Rn is 10 Ω. The resistance of the earth electrode of the installation RA is 20 Ω. The earth-fault loop current Id = 7.7 A and the fault voltage Uf = Id x RA = 154 V and therefore dangerous, but IΔn = 50/20 = 2.5 A so that a standard 300 mA RCD will operate in about 30 ms without intentional time delay and will clear the fault where a fault voltage exceeding appears on an exposed-conductive-part. The upper limit of resistance for an installation grounding electrode which must not be exceeded, for given sensitivity levels of RCDs at UL voltage limits of 50 V and 25 V. Table 1 gives the maximum resistance of the earth electrode. (Figure 3) gives an example of TT system as reported by Wiki Electrical Installation and Schneider (2016).
Table 1. Maximum resistance of the earth electrode
| IΔn | (50 V) | (25 V) |
| 3 A | 16 Ω | 8 Ω |
| 1 A | 50 Ω | 25 Ω |
| 500 mA | 100 Ω | 50 Ω |
| 300 mA | 166 Ω | 83 Ω |
| 30 mA | 1666 Ω | 833 Ω |