In electricity supply a TT earthing system is one where there is no metallic connection at all between the customer's earth and the neutral terminal of the transformer in the supplier's network. Transmission lines in Lund, Sweden Electric power, often known as power or electricity, involves the production and delivery of electrical energy in sufficient quantities to operate domestic appliances, office equipment, industrial machinery and provide sufficient energy for both domestic and commercial lighting, heating, cooking and industrial processes. ... In electricity supply systems, an earthing system defines the electrical potential of the conductors relative to that of the Earths conductive surface. ... Transformers - Typical electrical configurations. ... Electricity distribution is the penultimate process in the delivery of electric power, the part between transmission and user purchase from an electricity retailer. ...

The main earth terminal of the customer's installation is connected to earth through an electrode (usually a copper rod or mesh) buried in the ground. Earth fault current must flow via this earth electrode through the soil to the earth electrode at the substation, which is in turn connected to the neutral terminal of the transformer. An electrode is a conductor used to make contact with a nonmetallic part of a circuit (e. ... General Name, Symbol, Number copper, Cu, 29 Chemical series transition metals Group, Period, Block 11, 4, d Appearance copper, metallic Atomic mass 63. ... An aerial substation A substation is the part of an electricity transmission and distribution system where voltage is transformed from low to high and vice versa using transformers. ...

Pros

You have complete control. The actions of other customers cannot put dangerous voltages on the earth system.

Cons

Earth loop impedance is very high so very little current can flow in a fault to earth. bb In electrical engineering, impedance is a measure for the manner and degree a component resists the flow of electrical current if a given voltage is applied. ...

Mitigation of specific risks

In most cases the high earth loop impedance means that a RCD must be used on the whole installation. In many cases this is a time delayed type with a trip point of at least 100mA to provide discrimination with more sensitive RCCBs installed downstream on higher risk circuits. In electrical engineering, impedance is a measure for the manner and degree a component resists the flow of electrical current if a given voltage is applied. ... In electrical installations, residual current devices (RCD) or residual current circuit breakers (RCCB) are circuit breakers that operate to disconnect their circuit whenever they detect that current leaking out of the circuit (such as current leaking to earth through a ground fault) exceeds safety limits. ...

Due to the high earth loop impedance there will be a significant voltage on the earthing system until the RCCB disconnects the fault. Bonding of pipework to the earthing system is important to eliminate a potential difference between the earthing system and the pipework. However due to the fact that high earth impedence leads to low currents this bonding does not need to be very big (6mm² in the UK).

Other earthing systems

TN-C-S earth and neutral combined in the supply network but separated in customer installation.
TN-S earth and neutral run back separately to the transformer neutral terminal.
TN-C earth and neutral are combined right throughout the installation.
A TN-C-S earthing system is one where where earth and neutral are combined in the supply wiring but are separate in the installation. ... A TN-S earthing system is one where where earth and neutral run separately right back to the supply. ... A TN-C earthing system is one where where earth and neutral are combined in the supply wiring and right through the install. ...