IT systems The basis for reliable power supply FA01en/01.2004
IT systems The basis for reliable power supply in critical areas The advantages of sophisticated industrial systems can only be of use, if high availability of power supplies and time and costeffective fault location and clearance is guaranteed. Already when selecting the appropriate power supply system and the associated protective and monitoring devices, the basis for reliable personnel protection and protection of property and fault-free operation can be established. Electrical safety for power supplies A reliable EMC-friendly electrical installation ensuring the highest degree of safety for persons and equipment is of primary importance for all the persons responsible for industrial systems, hospitals and office buildings. The safety concept for electrical installations therefore must ensure the safety of both personnel and equipment improve the operating continuity contribute to the efficiency of the electrical system. In order to achieve this, power supply systems and the associated protective and monitoring equipment should be carefully selected to allow comprehensive protection of people and the electrical system against hazards caused by electric current fast indication and response to critical operating conditions in electrical installations the reduction of maintenance costs and expenses due to failures equipment data management according to customer requirements. How to choose the right power supply system The following aspects should be considered when selecting the appropriate power supply system and the associated protective measures in accordance with IEC 60364-4-41: likelihood of insulation faults; basic protection and fault protection; continuity of power supply; technical and economic possibilities; values that proved to be successful in practice. A detailed description of the power supply systems with earth connection is given in IEC 60364-3.The three main types of distribution systems are the TN system, the TT system and the IT system. In TN systems, one point is directly earthed; the exposed conductive parts of the installation are connected to that point by protective conductors. In TT systems, also one point is directly earthed; the exposed conductive parts of the installation are connected to earth electrode which are electrically independent of the earth electrodes of the system. In IT systems (be careful not to mix up the term IT systems which means isolated systems with information technology systems which are also called IT systems) all active parts either are insulated from earth or one point is connected to earth through a sufficiently high impedance. The exposed conductive parts of the electrical installation either are earthed individually or collectively. In order to guarantee sufficient protection of persons and property, always the coordination of the earth connection and the characteristics of protective conductors and the type of distribution system is required. The permissible protective measures are specified in IEC 60364-4-41. In earthed systems (TN, TT systems), usually residual current devices (RCD) and overcurrent protective device are used, whereas in IT systems insulation monitoring devices are used almost everywhere. Figure 1: Types of systems according to IEC 60364-3 On principle, in electrical installations, the protection of persons must be high on the priority list. But more and more importance also is attached to the availability of electrical energy. The following aspects should be considered when the electrical installation is disconnected due to an insulation fault: Persons are at risk, for example, due to a sudden failure of the lighting system; disconnection of equipment which is intended to guarantee operating safety. Risk of economic loss due to high costs incurred due to interruption to production, especially in areas where restarting is tedious and expensive; data loss increased expenses due to disturbances and destruction in installations or equipment. Moreover, sensitive loads can be disturbed by high fault currents. Disconnection may cause overvoltages and/ or electromagnetic influences which may lead to functional disturbances or even to damage to sensitive devices. Therefore, with regard to the availability of the power supply, in particular, the behaviour of power supply systems in case of a first fault is of importance. 2 FA01en/01.2004
Comparison between earthed and unearthed power supply systems IT systems (unearthed systems) either are supplied from an isolating transformer or an independent voltage source, such as a battery or a generator.the pecularity of these systems is that no active conductor is directly connected to earth. In the event of a shortcircuit to exposed conductive parts or to earth, only a small fault current will flow, owing to the lack of a return path, dependent on the value of the insulation resistances R F and the capacitance C e of the conductors to earth. The difference between the earthed system and the unearthed system in the case of an insulation fault becomes clear by comparing the figures 2 and 3. On the occurrence of a direct earth fault R F, an earth leakage current I d equal to the short circuit current I K will flow in earthed systems (TN/TT systems). The series-connected fuse blows, and the power supply is cut off (figure 3). By way of contrast, the unearthed system (IT system) (figure 2). It is obvious that in the event of an insulation fault 0 < R F < only a limited capacitive current flows through the line to earth capacitances. The series-connected fuse will not react in this case, which means that the power supply is guaranteed in case of a single fault to earth. As far as the safety of the power supply is concerned, the IT system offers the most advantages. That is the reason why it is used in a lot of areas where a maximum of reliability and safety of the power supply is required. These are for example control circuits according to EN 60204: 1998-11, power supply systems for medically used rooms according to DIN VDE 0107: 1994-10, mobile generators according to DIN VDE 0100-551: 1997-08 and other application fields. Apart from this, IT systems with insulation monitoring are increasingly used in industrial systems with variable-speed drives, in complex production systems or electronic data processing systems, because an unexpected failure of the power supply may cause considerable costs. When operating an IT system, it has to be considered that a first fault changes an initially unearthed system (IT system) into an earthed system (TN or TT system) and that a further fault may lead to the tripping of circuit breakers and hence lead to disconnection. Experience has shown that the single-pole fault (first fault) is the most likely type of fault to occur (> 90%) and the risk of hazards due to a second fault are regarded to be very unlikely. This fact is taken into account in the standard IEC 60364-4-41 where it is recommended that a first insulation fault should be eliminated with the shortest practical delay. Figure 2: IT system with insulation monitoring (IMD = insulation monitoring device) Figure 3: TN system with earth fault R F FA01en/01.2004 3
Advanced information through insulation monitoring According to IEC 60364-4-41, an IT system must always be equipped with an insulation monitoring device. The insulation monitoring device, which is connected between the active conductors and earth, superimposes a DC measuring voltage onto the system. On the occurrence of an insulation fault, the measuring circuit between system and earth closes via the insulation fault R F, so that a DC measuring current Im proportionate to the insulation fault is produced. This DC measuring current causes a voltage drop which is evaluated by the electronic circuitry. If this voltage drop exceeds a certain value, which is proportional to the insulation resistance, an indication is given via alarm LEDs and alarm contacts.the small system leakage capacitances C e existing in the system are only charged to the value of the measuring DC voltage and do not influence the measurement after the brief transient response (figure 2). A detailed description of the requirements for an insulation monitoring device is given in IEC 62557-8: 1998-05. Thanks to the insulation monitoring device, the operator of the electrical installation is provided with the necessary advance information which gives him effective support for preventive maintenance measures (figure 5). Modern measuring principles for modern electrical loads The measuring principle described before, can be used when the connected loads exclusively are AC consumers. However, recently there had been changes in the structure of the electrical loads. In many cases there are drives or loads with switched-mode power supplies (e. g. personal computers, electronic control gear). Whilst on the one hand there are the advantages of limited power loss, smaller dimensions, and less weight, on the other hand problems may arise through harmonic components caused by switched-mode power supply units and influence by DC leakage current. Insulation monitoring devices using the measuring principle of superimposed measuring DC voltage may cause false alarms due to DC components. The reason for that is that in case of a fault these extraneous voltages occur additionally to the measuring voltage and therefore either lead to an increased measuring current and hence to increased response sensitivity or lead to lower measuring current what means that tripping is prevented (figure 6). High system leakage capacitances, which often exist in the form of interference suppressor filters (EMC) between system and the earth, are also a source of interferences for insulation monitoring devices with DC measuring voltage. When switching the IT system on, for the DC measuring voltage these capacitances represent a low-ohmic connection to the earth so that a high measuring DC current (load current for C e ) will flow which results in an alarm message through the insulation monitoring device. Figure 4: functional principle of an insulation monitoring device Figure 6: influence of extraneous DC voltages on the measuring voltage Figure 5: advance information through insulation monitoring 4 FA01en/01.2004
In order to eliminate this influence on the insulation measurement caused by DC measuring voltages and system leakage capacitances, modern insulation monitoring devices work with a pulsed measuring voltage. This measuring principle responds to system leakage capacitances with variable clock times, taking the load curve of C e into consideration. The value of the occurring extraneous DC voltages is determined within one measuring cycle and in this way can be considered accordingly when the insulation resistance is detected. In practice, that means that neither extraneous DC voltages nor high system leakage capacitances can no longer influence the measuring results and in this way allow precise determination of the insulation resistance. The latest, international standard IEC 61557-8: 1998-05 specifies the requirements for insulation monitoring devices which are used in systems up to AC 1000 V and DC 1500 V, independent of the measuring principle. In order to fulfil the requirements of modern power supplies with regard to electrical safety and operating continuity, new insulation monitoring devices have been developed. The new AMP Plus measuring principle allows to determine the insulation resistance precisely, even in extended systems and in systems with variablespeed drives. Comprehensive information about the current state of the electrical installation via text display guarantees the necessary advance information. Besides, there is the possibility of transferring measuring data via the fieldbus to higher-level systems or of filing measuring data in a memory, in order to register events including date and time. Another version which can be extended to a complete insulation fault location system is available, allowing time and cost-saving location of faulty circuits. Figure 7: insulation monitoring device A-ISOMETER IRDH375 (Bender, Grünberg) FA01en/01.2004 5
Summary Nowadays, the increasing complexity of industrial systems places extremely high demands on the reliability of power supply, where even a short power failure may be expensive due to production stoppage and malary function. With the application of IT systems a tool is available that effectively helps to solve this problem. Written by Dipl.-Ing. W. Hofheinz Managing director Dipl.-Ing. W. Bender GmbH & Co. KG Londorfer Straße 65 35305 Grünberg Germany Literature IEC 61557-9: 1999-09 Electrical safety in low voltage distribution systems up to 1000 V a.c. and 1500 V d.c. Equipment for testing, measuring or monitoring of protective measures Part 9: Equipment for insulation fault location in IT systems EN 61557-9: 1999-11 Electrical safety in low voltage distribution systems up to 1 kv a.c. and 1.5 kv d.c. Equipment for testing, measuring or monitoring of protective measures Part 9: Equipment for insulation fault location in IT systems (IEC 61557-9: 1999) IEC 60364-4-41: 1992-10 Electrical installations of building; Part 4: Protection for safety; chapter 41: Protection against electric shock IEC 60364-5-53 Second edition: 1994-06 Electrical installation of buildings Part 5: Selection and Erection of electrical euquipment Chapter 53: Switchgear and Controlgear, clause 532.: Insulation monitoring devices EN 60204-1: 1997-12 Safety of machinery Electrical equipment of machines Part 1: General requirements (IEC 60204-1: 1997)" IEC 60363-3:1993-03 Electrical installations of buildings; Part 3; Assessment of general characteristics DIN VDE 0100-551 (VDE 0100 Teil 551): 1997-08 Electrical installations of buildings Part 5: Selection and erection of eletrical equipment Chapter 55: Other equipment Section 551: Low-voltage generating sets (IEC 364-5-551: 1994); German version HD 384.5.551 S1: 1997 DIN VDE 0107 (VDE 0107):1994-10 Erection of low-voltage installations Requirements for special installations or locations Part 710: Medical locations W. Hofheinz: Protective measures with insulation monitoring, 2nd Edition VDE-Verlag, Berlin Note: This technical information and the articles and illustrations which it contains are protected by copyright. Reproduction, translation, microfilming and storage in electronic systems, in particular for commercial purposes, are prohibited without the publisher's consent. We provide no warranty or guarantee for erroneous or omitted entries. All data is based on manufacturer's information. All logos and product designations are registered trademarks of the respective manufacturer. Dipl.-Ing. W. Bender GmbH & Co. KG Postfach 1161 35301 Grünberg Germany Londorfer Straße 65 35305 Grünberg Germany Tel.: +49(0)6401 / 807-0 Fax: 807 259 E-Mail: info@bender-de.com www.bender-de.com Right to modifications reserved!