What types of electrode can be used for TT earthing systems?Īny of the electrodes identified below, taken from Regulation 542.2.2 of BS 7671:2018+A1:2020, can be used for a TT earthing system. The requirements of this regulation are met if the earth fault loop impedance of the circuit protected by the RCD meets the requirements of Table 41.5. In order to fulfil the requirements of Regulation 411.5.3 of BS 7671:2018+A1:2020, the disconnection time shall be that required by Regulation 411.3.2.2 or 411.3.2.4 and R A × IΔ n ≤ 50 V. Higher external earth fault loop impedances are usually associated with TT earthing systems, which don’t provide sufficient fault current to operate fuses or circuit breakers within the required time and an RCD must be used for electric shock protection. ADS can be used for TT systems providing a suitably low external loop impedance can be provided, but it is not usually the case. The principle being that the low resistance earth loop and associated higher fault currents cause automatic disconnection within the required time. What is the method of protection against electric shock for TT systems?įor a TN system, automatic disconnection of supply (ADS) is the usual method of protection. The first ‘T’ of a TT earthing system relates to the earthing of the distributors transformer, the second ‘T’ is the earth electrode installed at the consumers installation as shown in Figure 1. ‘T’ is French for Terra, which translates to earth. TT earthing facilities are installed when the distributor does not provide a TN earthing system or when circumstances dictate that a TN earthing system cannot be used.
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