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IT power supply systems protection

The network labelled with the letters IT ("Isolé" - isolated, "Terré" - grounding of non-living parts) is called an isolated system and all its living parts are isolated from ground or have one power supply system’s pole connected to ground via a large impedance. Non-living parts of the electric installation are grounded, either individually, in groups, or together by a single grounded protective conductor. They can be operated either with a central point (neutral conductor N) or without a middle outlet (middle conductor N). The undeniable advantage of an isolated IT system over a standard grounded TN system is that appliances connected to this system can operate continuously even in the event of the first fault of so-called ground connection (reduction of the power supply system’s insulation status below the set critical level). This increases the operational reliability of the entire system and also increases the safety of people who use it or are dependent on it. Therefore, the IT power supply system is mainly used in hospital facilities, in heavy industry such as mining, metallurgy, engineering, shipbuilding, but also in traction systems and means of transport or in photovoltaic (PV) applications.

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image/svg+xml <line id="line8901" y2="28272.83" x2="43054.359" y1="28551.93" x1="42585.961" class="fil3 str0" style="fil</BODY></html> A PHOTOVOLTAIC B INDUSTRY C HOSPITALS D TRANSPORT AND INFRASTRUCTURE E MINING INDUSTRY

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  1. PHOTOVOLTAIC
    In photovoltaic (PV) systems, we also encounter an isolated IT power supply system. It is an IT power supply system in almost all cases on the DC side. On the AC side, it is then more just in large power plants, when the ungrounded system is led out of an inverter up to the terminals of the distribution transformer. You can learn everything about surge protection devices in these applications directly in the solutions for PV systems according to the type of object.
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  2. INDUSTRY
    In the industrial sector, the ungrounded system ensures greater reliability of electricity supply, the outage of which would mean large financial, technological or material losses. Foundries, rolling mills or chemical plants can be an illustrative example of these operations, where transients and disturbances from frequency converters often occur. In practice, we most often encounter three-phase ungrounded systems with different voltage levels, the very most common electrical distribution is then 3x 400 V IT. In these applications, the beginning of the ungrounded system is from the main switchboard, where it is suitable to install a lightning and surge arrester type T1 + T2. Product HLSA12,5-440/3+1 IT is efficient for the considered voltage level. If remote monitoring of the surge protection device’s status is required, versions marked as S can be used, i.e., for example HLSA12,5-440/3+1 S IT. The potential-free contact of this signalling is then connected to the building control system (typically as control PLC input), where it constantly informs the personnel about the surge protection device’s status without the need for physical control in the switchboard.

    In following subsidiary switchboards, which are more than the effective protection distance of the surge protection away from the main switchboard, it is appropriate to install a surge arrester type T2. A suitable product for three-phase distribution is HSA-440/3+1 S IT with remote status monitoring, for single-phase distribution it is HSA-440/2+1 S IT. If some subsidiary switchboard is installed closer, then we choose the SPD according to the need to protect the appliances powered by it. But pay attention to the energy coordination of surge protection devices, that is, the distance between the individual stages, which does not have to be observed. In this case, it is necessary to insert HI series decoupling elements with an appropriate nominal current between the SPD type T1+T2 in the main switchboard and type T2 or T3 in the subsidiary switchboard. It is for example product HI32 with 32 A nominal current. This will ensure proper SPD function. The recommended installation is directly in the subsidiary switchboard before the next stage.

    If it is necessary to protect final appliance sensitive to high-frequency interference in the range of hundreds of kHz to tens of MHz, it is suitable to equip these circuits with T3 type surge protections with EMI/RFI filter. These are serial protections, therefore they are dimensioned for the nominal current of the protected appliance, resp. for its back up fuse. The most common representative of this protection is HSAF16 S IT for 230 V AC voltage with a nominal load current of 16 A, containing remote monitoring, marked with the letter S. They are installed as close as possible to the protected appliance and their efficiency is the highest up to a distance of 5 m. If the device is not sensitive to EMI/RFI interference, a standard T3 type surge arrester can be installed, e.g., socket intended for IT power supply systems with integrated surge protection. The socket contains a red LED fault signalling to control functionality status of the surge protection. Its effective protection distance is the same as for the above-mentioned type, 5 m of cable line.

    You can learn more about the protection of other parts of the distributions, such as the common TN network electrical distribution, data and communication transfers, antenna systems etc., in other solutions.
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  3. HOSPITALS
    In hospitals, ungrounded systems are only used in critical rooms such as operating theatres or intensive care units (ICU) or anesthesiology-resuscitation department. There they ensure the reliable operation of equipment important for surgeries process and for maintaining the vital functions of patients, and a power outage could cause irreversible damages to patients' health and lives. IT power supply systems are specifically called Medical IT power supply system (MIS) in a hospital environment. MIS standardly occurs as a single-phase network with 230 V / 50 Hz nominal voltage. Beginning of the IT system is behind the isolating transformer, that is located in the switchboard of the Medical IT power supply system (MIS) close to operating theatres and medical locations of group II.

    Each of these premises has its own ungrounded system. It is advisable to install a surge arrester type T2, e.g., HSA-275/2+1 IT at the beginning behind the isolating transformer. If remote monitoring of the surge protection device’s status is required, versions marked as S can be used, i.e., for example HSA-275/2+1 S IT. The potential-free contact of this signalling is then connected to the building control system (typically as control PLC input), where it constantly informs the personnel about the surge protection device’s status without the need for physical control in the switchboard.

    To protect the final appliances, it is necessary to install type T3 SPD as close as possible to the protected appliance, because their effective protective distance is lower compared to other types. The most common protection of appliances connected from socket distributions is a socket intended for IT power supply systems with integrated type T3 surge protection according to customer specifications. This way equipped socket protects appliances and the surrounding sockets connected to the same circuit within a distance of 5 m of cable line (including supply cord). The socket contains a red LED fault signalling to control functionality status of the surge protection. In a hospital environment, it must be made of antibacterial material in a colour design corresponding to the importance of the relevant circuit.

    You can learn more about the protection of other parts of the distributions, such as the common TN network electrical distribution, data and communication transfers, antenna systems etc., in other solutions.
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  4. TRANSPORT AND INFRASTRUCTURE
    In railway systems, the equipment in the track is commonly supplied from the ungrounded system (IT) with different voltage levels. Converters’ drives can be powered from 3x 400 V IT distributions, for which T1+T2 HLSA12,5-440/3+1 S IT protection type is suitable. If the equipment is at a protective angle (e.g., in the protective angle of the Electrical Control House’s air-termination system) and there is no risk of a direct lightning current stroke, it is possible to install T2 SPD type, e.g., HSA-440/3+1 S IT in design with status remote monitoring – marked with letter S. Another option is power supply from a single-phase 230 V IT system, where HLSA12,5-275/2+1 IT is installed. Final equipment equipped with electronics sensitive to high-frequency interference in the range of hundreds of kHz to tens of MHz, can be equipped with T3 type surge arrester with EMI/RFI filter. This is serial protection, therefore they are dimensioned for the nominal current of the protected equipment, resp. for its back up fuse. Suitable protection is HSAF16 S IT for 230 V IT voltage with a nominal load current of 16 A, containing remote monitoring. You can learn more about surge protection devices in traction vehicles and equipment directly in railway solutions.
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  5. MINING INDUSTRY
    In the mining industry, where the lives of miners depend on the reliability of electricity supply, the operation of the IT power supply system is no less important than in hospitals. Without the electricity supply would not be possible to mine and transport coal to the ground, but also all supporting technologies cannot work without it, such as water pumping, gas ventilation or corridor lighting. The overground substation supplies electricity for the entire mining complex. From this substation, energy is distributed by core three-phase distributions, most often with a voltage of 3x 6 000 V AC IT, to transformer stations - mining switchboards. In them, high voltage changes to low voltage of different levels, which are usually levels from 3x 400 V IT to 3x 1 000 V IT. An engine is most often final equipment, therefore we will typically encounter only T1 or T2 surge protection type in this application.

    We do not have to consider a direct lightning impulse stroke in underground distribution systems, so T2 type surge protection is sufficient. It is installed in the already mentioned mine transformer stations with a non-explosive termination meeting the criteria for use in gassing mines. Thanks to this there is no need for special surge arresters’ modifications. For example, we will consider the level of 3x 500 V IT for which the product HSA-600/3+1 IT is suitable. If remote monitoring of the surge protection device’s status is required, versions marked as S can be used, i.e., for example HSA-600/3+1 S IT. The potential-free contact of this signalling is then connected to the control system (typically as control PLC input), where it constantly informs the administrator about the surge protection device’s status without the need for physical control in a sealed meter switchboard. There is no need to install additional surge protection stages as the final equipment do not contain sensitive electronics as standard.

    The low-voltage electrical distribution of the ungrounded system is also on the ground and provide supplying of mine elevators, ventilation, etc. These distributions are also at the same voltage as the subsidiary distributions in the mine. But here we have to consider a direct lightning impulse stroke from which, among other things, a combined surge protection type T1 + T2 protects. We will still consider a voltage level of 3x 500 V IT, for which the product HLSA12,5-600/3 + 1 IT is suitable. These products are installed in individual switchboards if the conditions that would allow the installation of T2 surge protection are not met.

    In addition to the IT power supply system, a standard TN-C network is used on the ground, which provides power to other technologies, such as conveyors, machine shops, welding machines and others. It is also important to protect this distribution with suitable surge protection. Another important protection is data and information systems protection. In these applications, we most often encounter RS485 serial line, through which data is transmitted from the monitored devices to the to the controlling dispatch station on the ground. You can learn more about these and other protections in additional solutions.
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