BackgroundInside or nearby your panel (fuse box) will be your main earthing terminal where all the earth conductors from your final subcircuits and service bonding are joined. This is then connected via the 'earthing conductor' to a real earth somehow. Mains electricity systems are categorised according to how the earthing is implemented. The earthing conventions used in Europe are described below.Note that the phase or 'hot' conductors are formally known as 'line' and that 'live' includes both line and neutral conductors. In these descriptions, 'system' includes both the supply and the installation. Description of lettersFirst letter:T The live parts in the system have one or more direct (low impedance) connections to earth. I The live parts in the system have no connection to earth, or are connected only indirectly through a high impedance. Second letter: T The protective earth conductors are connected to a local earth electrode only N The protective earth conductors are connected to the neutral at some point Remaining letter(s): C Combined neutral and protective earth functions. A single conductor serves both functions. S Separate neutral and protective earth functions. Separate conductors for earth and neutral are used. ITSupply has no direct earth connection. However, as no material is a perfect insulator, there is always a high impedance path to the surroundings via cables and equipment. This means that any large system will have a potential to earth. The protective earth conductors mainly serve as bonding between equipment and exposed metal. The installation is often provided with an earth rod, but the current from one live conductor to an earth rod will be very small.This means that the first live to earth fault that occurs goes undetected, but if a second fault occurs on a different live conductor (phase or on neutral) there will be a short circuit between the faults. There are both upsides and downsides to this: The upside is that you can continue to use equipment with a earth fault without risk. A device called permanent insulation monitor (PIM) warning of the fault rather than cutting power can be installed, and the problem can then be adressed when it is deemed possible to do so. In critical applications, such as an operating theatre, this can be beneficial. TTNo earth provided by supplier; installation requires own earth rod. Commonly used with overhead supply lines or in countries where such are common, as a broken neutral doesn't make the protective earth live. (Compare with TN-C and TN-C-S below) Also, if the service only has line conductors (lacks neutral) of which none is earthed, the only altenatives would be the IT or TN-S system. Despite the fact that the transformer has a low impedance connection to earth, the impedance from it to the installation earth rod is too high to allow a fault current high enough to blow a fuse or trip a breaker. Therefore some device to detect an earth fault is necessary. Today a current sensing earth leakage breaker (RCD/GFI) is typically used, although other methods exist. Trip ratings are typically in the 100-500mA range for a 230V residential service. The drawbacks include risk of malfunction in the protective device and difficulties of achieving discrimination between protective devices. An earth fault in one appliance can result in the whole building losing power if the protective devices are not properly coordinated.TN-SThe supplier provides a separate earth conductor back to the substation. Found primarily in United Kingdom. A costly but otherwise attractive arrangement that is becoming increasingly popular in other countries. An line to earth fault in appliance or device will result in a short-circuit which trips breaker or blows fuse. This is a simple and reliable way of preventing electric shock, although there is a certain risk of fire if the fault resistance is high. The use of a current sensing earth leakage breaker eliminates this danger, and is therefore mandated in some countries for fire protection.TN-CNeutral used as protective earth throughout supply and installation, without separate earth conductor. Used e.g. in old German installations and old dryer circuits in the United States. A line to earth fault is from a system point of view the same as a line to neutral fault, and it trips the breaker.Very questionable safety, as a broken neutral connection will result in appliances having energized cases. It also means that the potential of the earthing can be different at different outlets, due to the voltage drop in the neutral conductor. A voltage drop in the range of 10V from circuit breaker to socket in a 230V system is usually deemed acceptable. This difference in potential can be devastating to electronic devices which are connected to each other, such as computers in a network. TN-C-SService has combined neutral and earth which are separated out in the installation. This shows up as the earthing conductor going onto the connection block with the neutral conductor of the supplier, at the meter or the panel. This is the standard system used in North America and large parts of Europe. It is called 'protective multiple earthing' in the UK and 'main earth neutral' down under. Provided that the utility can guarantee a good earthing of the neutral it is safe, unless the neutral is broken somewhere before the split. That is very unlikely to happen with large wires and underground services, but to offer a degree of protection it does happen, all metalwork that extends throughout the building, such as water pipes, are connected to the earth in what is called "mains bonding". This is not required in all countries. The TN-C-S arrangement is far superior to TN-C and less expensive than TN-S. |