Residual Current Devices in LV

Collection Technique .......................................................................... Cahier technique no. 114 Residual current devices in LV J. Schonek Building a New Electric World «Cahiers Techniques» is a collection of documents intended for engineers and technicians, people in the industry who are looking for more in-depth information in order to complement that given in product catalogues. Furthermore, these «Cahiers Techniques» are often considered as helpful «tools» for training courses. They provide knowledge on new technical and technological developments in the electrotechnical field and electronics. They also provide better understanding of various phenomena observed in electrical installations, systems and equipments. Each «Cahier Technique» provides an in-depth study of a precise subject in the fields of electrical networks, protection devices, monitoring and control and industrial automation systems. This constantly updated collection can be downloaded from: http://www.technical-publications.schneider-electric.com Please contact your Schneider Electric representative if you want either a "Cahier Technique" or the list of available titles. The "Cahiers Techniques" collection is part of the Schneider Electric’s "Col- lection technique". Foreword The author disclaims all responsibility subsequent to incorrect use of information or diagrams reproduced in this document, and cannot be held responsible for any errors or oversights, or for the consequences of using information and diagrams contained in this document. Reproduction of all or part of a "Cahier Technique" is authorised with the compulsory mention: "Extracted from Schneider Electric "Cahier Technique" no. ..." (please specify). no. 114 Residual current devices in LV Jacques Schonek A graduate of the ENSEEIHT engineering school with a doctorate in engineering from the University of Toulouse, J. Schonek was involved in the design of Telemecanique variable-speed drives from 1980 to 1995. He then went on to manage the harmonic-filtering activity. He is currently working as an expert in Electrical Distribution Applications in the Architectures and Systems group of Schneider Electric. ECT 114 first issue, february 2006 Lexicon Common mode disturbance Natural leakage current Any continuous or transient electromagnetic Current that flows to earth via the insulation, in phenomenon occurring between a live part of a the absence of an insulation fault. power system and earth. May be a transient Neutral system overvoltage, a continuous voltage, an See "System earthing arrangement". overcurrent or an electrostatic discharge. Protective conductors (PE or PEN) Differential mode disturbance Conductors which, according to specifications, Any phenomenon between different live parts of connect the exposed conductive parts (ECP) of a power system, e.g. an overvoltage. electrical equipment and certain other conductive Direct contact parts to the earth electrode. Contact of a person with the live parts of Rated residual operating current I∆n electrical devices (normally energised parts and Value of the residual operating current assigned conductors). by the device manufacturer at which the device Earth-leakage current must operate under the specified conditions. Current that flows from the live parts to earth, in According to construction standards, at 20 °C, the absence of an insulation fault. low voltage residual current devices must Electrisation operate at residual currents between I∆n/2 and Application of voltage between two parts of the I∆n body of a living being. Residual current Electrocution Algebraic sum of the instantaneous values of the Electrisation resulting in death. currents flowing through all live conductors in a circuit at a point of the electrical installation. Exposed conductive part (ECP) Conductive part likely to be touched and which, Residual current device (RCD) although normally insulated from live parts, may Device whose decisive quantity is the residual be energised up to a dangerous voltage level current. It is normally associated with or due to an insulation fault. incorporated in a breaking device. Residual operating current Fault current Id Current resulting from an insulation fault. Value of the residual current causing a residual current device to operate. Indirect contact Contact of a person with accidentally energised System earthing arrangement (SEA) exposed conductive parts (ECP), usually due to Also referred to as the neutral system, earthing an insulation fault. system or earthing arrangement. Standard IEC 60364 stipulates three main types Insulation of earthing arrangements that define the possible Arrangement preventing transmission of voltage connections of the source neutral to earth and of (and current flow) between a normally energised the exposed conductive parts (ECP) to earth or part and an exposed conductive part (ECP) or the neutral. The electrical protection devices are earth. then defined for each one. Insulation fault Touch voltage limit (U ) Break in insulation causing an earth-fault current L Voltage U below which there is no risk of or a short-circuit via the protective conductor. L electrocution. Intentional leakage current Ventricular fibrillation Current that flows to earth via the intentionally A malfunctioning of the heart corresponding to installed components (resistors or capacitors), in loss of synchronism of the activity of its walls the absence of an insulation fault. (diastole and systole). The flow of AC current Live conductors through the body may be responsible for this due Set of conductors for electrical power to the periodic excitation that it generates. The transmission, including the neutral, with the ultimate consequence is stoppage of blood flow. exception of the PEN conductor for which the “protective conductor” (PE) function takes priority over the “neutral” function. Cahier Technique Schneider Electric no. 114 / p.2 Residual current devices in LV Today, residual current devices (RCD) are recognised as the most effective means of protecting life and property against electrical hazards in low- voltage systems. Their selection and optimum use require sound knowledge of the principles and rules governing electrical installations and in particular system earthing arrangements as well as existing technologies and their performance levels. All these aspects are dealt with in this "Cahier Technique", with in addition numerous answers provided by Schneider Electric technical and maintenance departments to frequently asked questions. Contents 1 Introduction p. 4 2 The risks of electrical currents 2.1 Electrisation of persons p. 4 2.2 Fire hazards p. 6 2.3 Damage to equipment p. 7 3 Protection against the risks 3.1 Installation rules p. 8 of electrical currents 3.2 Detection of insulation faults p. 9 4 RCD operating principle and description 4.1 Operating principle p. 11 4.2 Applications p. 11 4.3 Main characteristics p. 12 4.4 Technology p. 13 4.5 Constraints due to the current sensor p. 15 4.6 Special applications p. 17 5 Conclusion p. 22 Appendix 1: Calculation of touch voltages p. 23 Appendix 2: Types of converters and fault-current waveforms p. 25 Appendix 3: Leakage currents for different system earthing arrangements p. 28 Appendix 4: RCD thresholds and power system voltages p. 30 Bibliography p. 31 Cahier Technique Schneider Electric no. 114 / p.3 1 Introduction Compared to other energy sources, electricity current devices) are recognised by international has many advantages, but also many risks. It is standardisation organisations as an effective used on a daily basis by the general public and means to protect life and property. many accidents still occur, resulting in burns, This document will present the subject in three fires and electrocution. steps: Strict installation rules have been set up by c a description of the risks related to electrical international (IEC, CENELEC) and national currents, (e.g. NFPA in the USA and UTE in France,) c an overview of the protection techniques organisations. employed to limit those risks, Dependable protective devices have been c an in-depth presentation of how RCDs designed by carefully analysing the risks and operate. consequences of equipment failures or incorrect use. Among these devices, RCDs (residual 2 The risks of electrical currents 2.1 Electrisation of persons A person subjected to an electrical voltage is electrised. Depending on the level of a) Direct contact electrisation, the person may be subjected to different pathophysiological effects: c disagreeable sensation, Phases c involuntary muscular contraction, c burns, c cardiac arrest (electrocution). These effects depend on various factors, Uc including the physiological characteristics of the person, the environment (e.g. wet or dry conditions) and the characteristics of the current flowing through the body. b) Indirect contact A person may be subjected to an electrical shock in two manners: 3 c direct contact, e.g. the person touches an energised, bare conductor, c indirect contact, e.g. the person touches a metal part of an electrical machine or device with Id Uc an insulation fault. The dangerous aspect is the current (magnitude and duration) flowing through the human body Fig. 1 : Direct and indirect contact. and particularly near the heart. Cahier Technique Schneider Electric no. 114 / p.4 Figure 2 sums up the work of the International the probability of ventricular fibrillation: Electrotechnical Commission on the subject v increases up to about 5% between curves C1 (standard

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