Nowadays, technical installations in all industries are characterised by ever-increasing complexity and automation. From highly developed production lines to robot technology, the amount of equipment that requires a reliable power supply to function smoothly is steadily growing. Therefore, the foundations for reliability and availability of an installation are already laid by selecting the right power supply system. Along with personnel and fire protection, fail safety is the key factor when choosing an appropriate power supply. During the planning phase of an installation, three system types are available: the TN system, the TT system and the IT system.
In IT systems, however, all active parts are insulated from earth or connected to earth through a high impedance. The high impedance can be applied for metrological reasons, provided that electrical safety is not jeopardised. The earthing of the exposed conductive parts of the electrical installation is carried out either individually or collectively.
In TN systems, the star point of the supplying transformers is connected to earth through a low impedance and the exposed conductive parts of the electrical installation are connected to the protective earth (PE) of the system. In TT systems, the star point is also connected to earth through a low impedance but the exposed conductive parts of the electrical installation are earthed independently from the system earthing.
As input to unearthed IT systems, either a transformer or an independent power source, such as a battery or a generator are used. Since no active conductor is connected to earth with a low resistance, no high fault current flows in the event of a short circuit to exposed conductive part or an earth fault. The result is a low fault current the size of which depends on the insulation resistances and the capacitance of the conductors and system components to earth.
As required by the standards, an insulation monitoring device is mandatory in an IT system.
For example: If a person touches a live conductive enclosure in an intact unearthed AC 230 V system with sufficiently small system leakage capacitances, he/she won't suffer an electric shock. Under these conditions, only a small current that is not perceptible will flow through the person. The touch voltage is primarily determined by the voltage drop of the fault current via the protective earth conductor connected to the enclosure. Since the fault current (determined by the insulation resistance and the system leakage capacitance) is usually very small and the resistance of the protective earth conductor is also very small, no high touch voltages occur.
In contrast, an earthed system is based on the idea that, in the event of a fault, a sufficiently large fault current is generated, which leads to a fast disconnection of the power supply. In the event of an indirect contact, this means that if a live conductive enclosure is touched, a high fault current will flow immediately through the person due to the low-resistance connection to the power source. Protective devices, such as fuses and circuit breakers, are mandatory in order protect the person by shutting down the installation before the person suffers permanent harm.
The different philosophies can be explained as follows:
In an earthed system we assume that the fault occurs but take the necessary steps to avoid further damage.
In an unearthed system, a first fault does not interrupt the system power supply and therefore increases the availability of the system.
To date, the unearthed system is mainly used in safety-critical applications, such as intensive care units or railway signalling technology, where a failure of the power supply would have disastrous consequences. Outside these special areas, this system type is not very common in practice, even though the IT system offers numerous advantages not only in terms of safety but also availability.
The three following myths regarding disadvantages of this system type are still widely spread today:
The IT system is more expensive than an earthed system
Installing an unearthed system is indeed more expensive than a TN or a TT system but the additional non-cash benefits, such as preventive maintenance and reduced test efforts, cancel out the additional costs within a short period of time.
Faults can be quickly located in a TN system
By using insulation fault location systems (IFLS), faults can also be quickly located in IT systems.
Expensive equipment must be purchased for the IT system
By means of some technical measures that do not require any extra expenditure, standard components can also be used.
Each system type has its strengths and weaknesses, and selecting the best variant should depend on the application. In many practical cases, a combination of the three system types has proven to be the most optimum solution. The IT system has the best overall characteristics but it is only suitable for systems up to a certain size and complexity.
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