electrical power engineering course

Position The potentiometer The potentiometer can directly measure angular or linear displacements and is the simplest form of position transducer. Potentiometers have finite resolution which is determined by the wire size or grain size of the track. Since they are mechanical devices, they can also suffer from stiction, backlash and hysteresis. Their failure mode also needs consideration; a track break can cause the output signal to be fully high above the failure point and fully low below it. In closed-loop position control systems this manifests itself as a high-speed dither around the break point. The synchro and resolver Figure 4.23 shows a transformer whose secondary can be rotated with respect to the primary. At angle θ the output voltage will be given by eqn 4.10, V o = K V i cos θ (4.10) where K is a constant. The output amplitude is dependent on the angle, and the signal can be in phase (from θ = 270° to 90°) or anti-phase (from θ = 90° to 270°). This principle is th...

Arc and flashover burns Arc burns, also known as flashover burns, are commonly associated with the failure of insulation in electrical equipment, leading to an arc developing in the air between adjacent conductors. A common cause is metal objects, such as screwdrivers, spanners and other foreign objects forming a short circuit between a phase conductor and earth, or across conduc- tors at different voltages. More often than not the initial fault is between two adjacent conductors, such as a phase conductor and earthed metalwork, but the ionized gases created by the fault allows arcs to develop between other conductors. In three-phase systems, the result is often described as a full three-phase flashover. The typical conse- quence is the expulsion from the short circuit of a highly energetic arc and hot gases, with temperatures in the plasma typically exceeding 1000°C. A person in the immediate vicinity of the arc will suffer burn injuries which are often severe and life-threatening....

Equipment installation The code of practice for the installation of hazardous area electrical equipment is BS EN 60079-14 (reference 15R). The guidance in this code of practice is additional to the requirements for non-hazardous area installations. Electrical equipment should, preferably, be located outside a hazardous area or in the lowest risk area available. It should be installed in accordance with its documentation, including any special requirements denoted by the letter ‘X’ at the end of the certificate number. It should also be operated within all of its ratings. The installation should be designed to protect the equipment from adverse environ- mental influences which could jeopardize the method of protection; these include temperature extremes, corrosion, mechanical damage and vibration. Electrical equip- ment, such as pressure transmitters and temperature sensors, connected directly to the process, may become filled with process fluid under failure conditions. The instal- ...

Prevention of indirect contact injuries Some of the techniques used to prevent direct contact injuries will also prevent indirect contact injuries; this includes the use of SELV or PELV supplies and Class II equipment. However, by far the most common method is the earthing of exposed conductive parts, coupled with the installation of fuses or circuit breakers that disconnect the supply in the event of an earth fault, a technique known as Earthed Equipotential Bonding and Automatic Disconnection of Supply (EEBADS) . The EEBADS technique requires the exposed conductive parts of the apparatus and equipment to be earthed through a protective conductor that is connected to the main earthing terminal of the installation. Overcurrent protective devices, such as fuses and cir- cuit breakers are placed in the phase conductors. When an earth fault occurs, connecting a live conductor to exposed metalwork and thereby creating the conditions for indirect contact injuries, the fault current flows...

Precautions against arc and flashover burn injuries Most arc and flashover burn injuries occur during live work on electrical systems. To avoid the hazard, work should be done whenever possible on apparatus that has been made dead and isolated. If this is not possible then only competent people should undertake the work and precautions must be taken to minimize the risk. Depending upon the particular circumstances, these may include the use of: ● insulating screens or barriers between live parts of different polarity and between live parts and earthed metalwork ● insulated tools ● test equipment probes on which only the contact points are bare, with a maximum length of exposed metal of 4 mm, and the rest of the probe is insulated. The leads should have protection against short-circuit current, such as integral fuses or current-limiting features in the test equipment ● personal protective equipment, such as insulating gloves, and heat-resistant face shields and clothing Preca...

Precautions against electric shock and contact burn injuries General principles Shock and contact burn injuries can be prevented first by ensuring that the electrical system is designed, installed and maintained in accordance with sound engineering principles and in compliance with accepted and published standards, and secondly by ensuring that any work on the system is carried out in a safe manner. This section provides some detail on the hardware design principles, safe systems of work being described at the end. Details can be found in reference 16A. Prevention of direct contact injuries Direct contact shock and burn injuries are commonly prevented by ensuring that conductors energized at dangerous voltages cannot be touched. There are, however, some instances where conductors have to be uninsulated and available to touch, such as when live testing work is being done or because they need to be exposed for functional reasons, examples being power pick-up rails on fairground ride...

Principle s of electrical safety Injuries from electricity Every year people are injured or killed as a result of hazardous defects in electrical systems or because they adopt unsafe working practices on electrical systems. The most common types of injury are electric shock and burns, with the burn injuries arising from either current passing through the body or from the effects of arcing and flashovers. In addition to these direct forms of electrical injury, the following secondary types of injury can occur: ● burn injuries and the adverse effects of smoke or fume inhalation from fire of electrical origin ● the effects of an explosion that has an electrical source of ignition ● physical injuries arising from the reaction to electric shock, such as being thrown off a ladder as a result of electric shock and suffering impact injuries from the fall   Electric shock Electric current passing through the body, particularly alternating current at power frequencies of 50 Hz ...

Precautions against explosions The precautions to be taken against electrical ignition of flammable atmospheres are described in Chapter 15. Preventive maintenance and safe systems of work Preventive maintenance A common cause of the types of accidents described in this chapter is electrical systems which are not maintained and which are allowed to degrade into a dangerous condition. Typical examples of dangerous features arising from lack of maintenance are: ● enclosures and assemblies that are broken, allowing access to live parts, or in which the seals have degraded, allowing the ingress of dusts and liquids ● cables with damaged sheathing and basic insulation or with mechanical damage that compromises the protective conductors or creates the conditions for arcing as a result of insulation failure ● corrosion of exposed metallic parts ● low electrolyte levels in battery-based tripping supplies or uninterruptible power supplies ● low levels of insulating oils in switchgea...

Inspection and maintenance All equipment installed in hazardous areas and all parts of intrinsically safe, or similar, installations require periodic inspection to verify the continued integrity of the methods of protection. It is possible for installations to become unsafe due to deterioration, damage or alteration while the installation remains functional. A programme of initial and periodic inspections and, if necessary, remedial action is recommended in BS EN 60079-17 (reference 15S). Inspections are generally initial, periodic or sample. An initial inspection is recom- mended before a new installation or re-installed equipment is brought into service. The objective is to verify that the equipment is suitable for its location, that it has been correctly installed and is in good condition. A periodic inspection is intended to check that the equipment continues to be suitable and a sample inspection can be used to check the effectiveness of the inspection and maintenance arrangeme...

Codes of practice for area classification Codes of practice for area classification usually recommend that area classification is carried out by a team which includes the electrical/control function. Electrical engineers working in industries within the scope of DSEAR should, therefore, have some familiarity with the process of area classification and its meaning. The codes of practice fall into three broad categories; those which deal with principles, those dealing with industry sectors and those covering specific companies, installations or flammable materials. There are too many codes of practice to include in an exhaustive list but the more common ones are mentioned here. Dealing with area classification principles are BS EN 60079-10:2003 (reference 15L) for gases and vapours, and BS EN 50281-3:2002 (reference 15M) for combustible dust. One of the most common industry codes of practice is the Institute of Petroleum Code (reference 15N) which covers gases, vapours and mists. Specif...