Shell Electrical Engineer Handbook

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  10.06.24 - 030 1 Theory Electrical engineering Protecting electrical installations This lesson examines the protections necessary in an electrical installation. The properties of each type of protection are given. Based on these properties, theapplication range for each of the protection systems is examined. In addition,this lesson contains warnings and advice for proper use of these protections. Contents of the lesson 1   Electrical safety2   Safety fuse3   The importance of an earth wire4   Thermal protection5   Electromagnetic maximum protection6   Earth leakage protection7   Control current blocking switch The copyright in this material is vested in Shell Global Solutions International B.V., The Hague, The Netherlands and Shell Netherlands Raffinaderij B.V. All rightsreserved. Neither the whole or any part of this document may be reproduced, stored in any retrieval system or transmitted in any form by any means (electronic,mechanical, reprographic, recording or otherwise) without the prior written consent of the copyright owner.  Theory / 10.06.24 - 030 2 Lesson 1. Electrical safety In an electrical installation, limits must be imposed on the magnitude of currents.The flow of excessive currents can be prevented by using various types of  protections.If no precautions are taken:-   large damage to the installation may occur;-   generation of heat may cause fire;-   the installation can become live, due to which contact can lead toelectrocution.Commonly used protections are:-   safety fuse;-   thermal protection;-   electromagnetic protection. 2. Safety fuse A safety fuse is a device containing a wire or strip which melts if the currentexceeds a certain value for a certain time.They are used for instance to protect a wire from an excessive current, whichmight cause it to become too hot. They also ensure the voltage is disconnected inthe case of short circuit or earth leakage.Safety fuses can be divided into two groups: screw plug fuses and short circuit-resistant fuses.Screw plug fuses consist of a cartridge fuse in a screw cartridge holder. 5578-030-001  Figure 1Cartridge fuse of a screw plug fuse The principle of operation is as follows (see figure 1): Normally, most of the current flows through the fuse wire. Should this wire blow, the current has to pass through the detector wire which will then also blowand let go of the telltale.- consequences of  inadequate safety - safety fuse - screw plug fuse - cartridge fuse - detector wire  Theory / 10.06.24 - 030 3 The cross section of the fuse wire determines the current the safety fuse wasmade for.To prevent a cartridge holder from being fitted with a higher current cartridge,the bottom of the holder has been fitted with a collar of a certain diameter. Thecartridge for the highest current also has the largest diameter, so it will not fitinto a smaller collar (see figure 2). 5578-030-002  Figure 2Cartridges and cartridge holder in a safety fuse 25 A cartridge holders are always used. There are also cartridge holders with amaximum value of 63 A. They can hold 35, 50 and 63 A cartridges.The maximum value of the fuse cartridge used is limited by the cross section of the cable used in the installation.In some cases it is desirable for a safety fuse to be able to resist a higher currentfor a short time, particularly in switching on motors which have a start-upcurrent several times the rated load current  I  n . In that case so-called slow fusesare used, which do not blow in the case of a short-lasting high start-up current.- fuse wire - collar - cartridge holder - slow fuse  Theory / 10.06.24 - 030 4 Safety fuses do not react to a small overload; the cut-out time would then be toolong. They are therefore used only as a short circuit protection. Question 1What is the purpose of the detector wire? Short circuit resistant safety fuses Figure 3 shows the design of a short circuit resistant safety fuse.These safety fuses are designed for cutting out big short circuit currents withoutthe risk of the safety fuse exploding. For that reason, they are sometimes calledexplosion-proof safety fuses.Because of the shape of the connecting strips, the name blade fuse or bladecartridge is also used.  Figure 3Short circuit resistant safety fuseaits componentsbafter assembly Safety fuses may only be replaced under no current conditions. For blade fuses,a special plastic handle must be used. Only the electrician is allowed to do this,however.- short circuit protection - explosion-proof  fuse cartridges - blade fuse
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