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The risks involved include the occurrence of dangerous shock currents and interruptions in the power supply. The definition states that all active components must be isolated from earth or that one point must be connected to earth via an impedance. The exposed-conductive-parts of electrical equipment items on the electrical system are either earthed individually, in groups or collectively with the system's earth also refer to IEC , The system may also be connected to earth via a sufficiently high impedance.
In Germany, this is only applied for measurement or functional purposes. Figure 1: Comparison between an IT system left and a TN system right during a first insulation fault. A distinction must be made between the power source for a "normal" IT system and for a medical IT system. Disconnection in the form of basic insulation is required for the power source. In practice, this usually takes the form of an isolating transformer.
This function can also be performed by a battery, by a standalone photovoltaic PV system or by a mobile power generator. In a medical IT system, the leakage currents during intracardial interventions e. The required isolating transformer is described in IEC In practice, the IT system is frequently known as "the unearthed power supply system". In accordance with IEC , What this means for the IT system in accordance with No touch voltage limitation is considered in d.
This is of particular benefit in the medical sector. The fault current, in the event of a first fault, flows with negligible impedance between a line conductor and an exposed-conductive-part.
Figure 4: Fault current Id with a first insulation fault in the IT system backup circuit diagram. IEC , Subclause It is advisable that a first fault is remedied as fast as practically feasible. Fundamentally, the IT system has a significant advantage in that an insulation fault does not have to be eliminated immediately but can instead by delayed, e. For troubleshooting purposes, measuring pulses are superimposed on the IT system that are in turn picked up and evaluated by measuring current transformers.
The configuration of overcurrent protective devices takes due account of the contents of IEC For IT systems, the following points must be taken into account:. Figure 7: Explanation of the need for an all-pole overcurrent protective device in IT systems.
In acc. However, this requirement is not practicable in allowing RCDs to achieve the desired protective action in IT systems. However, in practice, this is not the case. Even two independent insulation faults at both live conductors or connected equipment items do not cause an RCD to trip because these two faults act like a load.
On closer inspection, however, it becomes apparent that this requirement is technically questionable:. The primary objective governing IT systems is that they should not shut down unexpectedly when an initial fault occurs.
There is therefore no point in using electric arc protective devices for final circuits up to 16 A in IT systems. This includes the following provisions:. Areas used for medical purposes in senior citizen homes and care facilities in which patients receive medical treatment are not therefore included within the scope of DIN VDE VDE , There is no need to use arc fault detection devices AFDDs on circuits that supply electrical consumables where an unexpected interruption on the power supply poses the risk of causing damage.
This applies, for example:. An insulation monitoring device is selected on the basis of the following criteria:. Designation of the corresponding application A distinction is made between insulation monitoring device types based on possible components in the IT system:. A distinction is made between insulation monitoring device types for specific applications:. Important notes for the project planning of IT systems can be found in subclause The response value must be set appropriately for the affected system.
In accordance with the German standard subclause Both parameters are correct in principle and are influenced by the number of consumers and by the quality of the installation e. In practice, the value displayed on the screen of the IMD is used to set an alarm value that is less than this displayed value, and that therefore represents the desired minimum value, leaving sufficient latitude for service and maintenance work.
Also bear in mind here that all important system outputs are also in operation. However, another advantage is that any significant change to the insulation resistance by switching on or off a load or system component is displayed by the IMD, enabling potential weaknesses to be identified.
This can, for example, include motorised winches, elevators and slide valve drives. This requires that the monitored electrical circuits are isolated from all poles of the system. In accordance with IEC , subclause In practical terms, this means that a defined level of loop impedance must be achieved. For IT systems without a neutral conductor, the loop impedance is defined as:. Additionally, it should be stated that symmetrical faults on different live conductors should never cause a fault current that causes a system to shut down.
An RCD can be used for each consumable if and only if the shutdown conditions for overcurrent protection cannot be satisfied, because for example:. IEC subclause This could damage suppression capacitors that are switched to earth.
Ensure that the voltage offset only affects the voltage to earth. No voltage offset between active conductors arises. Single-phase equipment must be configured appropriately, i. In practice, two separate IT systems are frequently set up, one for single-phase loads and one for 3-phase loads. At this point, a comment on the generally applicable comment from IEC sub-clause In such cases, an overload alarm should be considered.
IT systems are always of greatest benefit when they protect against the disconnection of the power supply in the event of a first fault.
The fundamental basis for fault-free and safe operation comprises setting the system up in accordance with the standards and the correct choice of protective and monitoring devices. IEC Low-voltage electrical installations Part 1: Fundamental principles, assessment of general characteristics, definitions. IEC Low-voltage electrical installations Part Protective measures - Protection against electrical shock.
IEC Low-voltage electrical installations Part Protective measures - Protection against overcurrent. Know-how Technology Ungrounded System Ungrounded systems as reflected in the standards Ungrounded System Comparison of system types Advantages of the floating system Ground-fault monitoring Ground fault location Insulation fault location in coupled systems Examples of application Control circuits Insulation resistance - testing, measuring, monitoring Ungrounded systems as reflected in the standards Further notes and requirements for installation and device selection Protection and monitoring devices The 1st fault in the IT system Earthing The power source Network types Questions?
Contact us! Downloads, Links, Products. Network types. The power source. Figure 2: Layout of a medical IT system in acc. The 1st fault in the IT system. Figure 5: Example of touch voltage UT after a first fault. Protection and monitoring devices. Overcurrent protective devices The configuration of overcurrent protective devices takes due account of the contents of IEC For IT systems, the following points must be taken into account: In medical IT systems, it is not permissible to have an overload protective device in the output circuit secondary circuit of the transformer, i.
Therefore, the load current and temperature of the transformer must be monitored, and any variances must be reported IEC , The same message can also be found in IEC , This overcurrent protective device can be dispensed with if, for example, the N conductor is protected from overcurrent on the supply side. A shutdown on all poles also helps to "disengage" all poles in the IT system in accordance with the 5 safety rules At this point, a comment on the generally applicable comment from IEC , Fault current protective devices RCD In acc.
However, no planner can determine these variables with even the best of planning. The problem here is that such discriminating RCDs are also not available.
This applies, for example: to IT systems that were installed to improve security of power supply or for electrical systems for safety purposes in accordance with DIN VDE VDE , in particular in safety lighting systems.
Further notes and requirements for installation and device selection. Selection of insulation monitoring devices An insulation monitoring device is selected on the basis of the following criteria: Maximum nominal voltage Network type, i. Figure Examples for the identification of IMDs.
Requirements on insulation monitoring devices IMDs Important notes for the project planning of IT systems can be found in subclause IMDs must be rated for maximum mains voltage It is advisable to use IMDs that report any interruption in the measuring connections to power circuit conductors and earth. Insulation fault location systems must comply with the requirements of IEC clause 4.
Setting the response values The response value must be set appropriately for the affected system. Figure Off-line monitoring of a motor, e. Correct response to a second insulation fault In accordance with IEC , subclause An RCD can be used for each consumable if and only if the shutdown conditions for overcurrent protection cannot be satisfied, because for example: it is not possible to define the loop impedance precisely cable lengths difficult to estimate, metallic materials close to lines the fault current is so low that the maximum permitted disconnection time when using overcurrent protective devices cannot be achieved the loop resistance is too high to ensure an automatic shutdown and additional equipotential bonding is not possible.
Figure The second fault in the IT system. Summary IT systems are always of greatest benefit when they protect against the disconnection of the power supply in the event of a first fault.
Prüfung elektrischer Anlagen
VDE-Geräteprüfung nach DIN VDE 0701-0702
Ungrounded systems as reflected in the standards
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