1. Name the three fundamental sub systems that will normally be identified in a
ID: 2079794 • Letter: 1
Question
1. Name the three fundamental sub systems that will normally be identified in a safety related electrical control system.
2. What is the meaning of minimum object sensitivity in a light curtain system?
What is the resolution value required by standards to qualify for preventing a human hand being undetected when passing through the curtain?
3. What are some of the design factors that are to be taken into account when evaluating a safety control design to see what PL it can achieve? Name at least 3.
4. What is the name given to the degree of confidence for providing safety that may be placed in a safety related control system?
Explanation / Answer
The electrical control system generally provides not only the control but also protection and instrumentation. These three sub-systems have to provide a number of functions.
1)System Control
Electrical control signals enable and trigger essential electrical functions like voltage build-up, load control, and management, normal and emergency shutdown of the plant.
In basic electrical control systems like the designs shown in attached image control is done manually through push buttons and switches. In schemes that are designed for a higher degree of automation, electronic, programmable logic controllers (PLCs) can provide fully automatic start-up and shutdown procedures.
2)Protection
While electricity is the most convenient form of energy presently available, its use involves certain risks.Protection systems have to be designed accordingly to fulfill the specified requirements which can basically be categorized into 2 groups:
i) the protection of human beings (operators, consumers...)
ii)the protection of property (generating equipment, appliances...).
Protection of Human Beings:
Protective measures are normally divided into three categories: basic, direct and indirect protection.
a)Basic Protection: It is ensured by the insulation of all life parts to prevent from a direct contact.
b) Direct Protection
It is ensured by simply placing electrical circuits and installations out of reach and by prevention of direct contact through enclosures, barriers or covers and housing. The degree of protection is best indicated with reference to the international IP classification. In spite of providing basic protection, enclosures, and barriers, accidents still may occur for instance in the case of an insulation failure. In view of these cases, direct protection can be enhanced by the use of Residual Current Operated Circuit Breakers (RCCB) or Earth Leakage Breakers/Relays (ELB/R).
c) Indirect Protection
It is provided by a number of measures briefly mentioned hereunder.
c.1)Earthing: electrical connection of all accessible, conducting parts like covers, frames or housings together and to earth. Also provide effective, automatic disconnection of the supply before a shock is likely to prove fatal.
c.2)Use of Class 11 Equipment: appliances with double insulation, which have no exposed conductive parts.
c.3) Electrical Separation: usually provided by isolating transformers with the secondary side floating (not-earthed). This is, however, limited to certain circuits and appliances only. The first method is mostly applied.
The protection of Property:
It covers the protection of generating equipment, including generator, electrical control system and power cables inside of the plant, the protection of transformers and the distribution system outside of the plant and the protection of consumer appliances.
a)Capacity and Derating Factors for Cables
The permissible current rating of any current carrying device (e.g. generator, cable,..etc) is dependent on the actual layout (e.g. the effective cabling used, i.e., correct cross sections for wires) but also on ambient/atmospheric conditions.
While designing protection for an electrical system, the operating conditions need therefore to be taken into account. Of particular importance are aerating factors for power cables and generators.
Elevated temperatures considerably reduce the rated currents.Bundled cables do not dissipate heat very well, therefore, their rated current drops (or for a given current the cross-section must increase).
b)Coordination of Current Breaking Devices and Conductors
In general, conductors in isolated or small grids should be designed for the maximum power that can be generated.
Still, overload conditions can be created for instance by load imbalance in a three phase system. Such overload or short-circuit currents must be interrupted before they cause temperature rises harmful to insulations, joints, terminations and surroundings of the conductor. There must be proper coordination between the protective device and the rating of the conductor.
On the other hand, this practice takes into account tolerances of fuses and other breaking devices that do not necessarily operate at their rated value.
c)Breaking Capacity and Time/Current Characteristics of Current Breaking Devices
Special attention has to be paid to ensure that a protective device is, in fact, capable of breaking a short circuit current in due time. The designer, therefore, has to assure that the breaking capacity is higher than the possible maximum current, a value that should be ascertained by the generator manufacturer.
d) Lightning Protection
In lightning-prone areas, persons and equipment need to be protected against atmospheric overvoltage. The basic measure for lightning protection is the use of lightning arrestors that divert the surge to the ground.
For high tension transmission systems, it is common practice to place a set of suitably sized surge arrestors at the outgoing lines of a power station and further sets every kilometer of a transmission line. For small low tension systems, 500 V arrestors should be installed close to the equipment.
Adequate earthing must be provided for every arrestor.
3)Instrumentation
The instruments incorporated in the control panel shall basically enable the operator to monitor simple electrical system parameters like voltage, currents, frequency and to record load and energy generation, supply-curves. Furthermore, they should indicate abnormal conditions and facilitate operation (e.g. load prediction and management).
Instruments should be easily readable and ergonomically installed. They should be grouped logically to avoid confusion.It is an instrument part of the control (for instance speed), and any action has to be taken by the operator, she shouldn't have to move at all. All elements of a control system have to be within reach. The degree of instrumentation will mainly depend on the requirements of the owner and on the complexity of the scheme.
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