City and Guilds 2330 Certificate in Electrotechnical Technology Level 3 Unit 3 Outcome 1 Session 1 Identification of systems and nature of faults B & B Training Associates
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Unit 3
Installation (Buildings and Structures) Fault Diagnosis and rectification
Outcome 1
Select and apply fault diagnosis and rectification techniques
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Outcomes from this session At the end of this session you should be able to
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Identify electrotechnical systems and equipment, utilising single and three phase supply within installations.
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Main Headings Of Circuits
Power circuits – Ring circuits – Motor circuits – Transformer circuits – Power poles – Others
Lighting circuits – Joint box method – Loop-in method – Three-plate method – Discharge lighting – Other's
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Main Headings Of Circuits
Control circuits – Starters » Star delta » DOL » Electronic » Autotransformer » Rotor resistance – ELV controls » Panels
Emergency circuits – Fire alarms – Emergency lights – Call circuits
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Accessories and Equipment That Can Cause Problems
Switches
Circuit breakers
Socket outlets
Plug tops
Motors
Lights
Others
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Faults That Can Occur The list of faults that can occur is almost limitless. Cables Cables run within the fabric of the building could become damaged by the • insulation being crushed or cut. Short-circuits could occur between conductors or low insulation • resistance could be caused by pressure. There could be causes of crushing by floor boards, cable burns, bends • that are too tight, sharp edges or nails. There are a lot more. •
Cable connections Loose connection • These lead to an increase in heat and hence resistance. This in turn leads • to more heat being generated until either a protective device operates, a cable open-circuits or a fire starts. Different metal connections. • Aluminium and copper do not like each other chemically. When they are in • direct contact they react with each other and there is a resistance increase.
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Faults That Can Occur Glands and Seals The faults depend on the types of cable With MIMS cable – If the gland is not sealed properly, water can get in and over time a fault is created. – The pot can rotate if it is not fitted properly, this in itself can cause a fault if it twists.
With SWA cables – Glands are sometimes made off with some of the SWA strands not fixed properly or worse, with some of them cut-out! – Any reduction in the armouring is a reduction in the overall size of the c.p.c. and is dangerous. – If links are not put in there is a break in the integrity of the c.p.c.
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Faults That Can Occur Switches
Switches incorrectly chosen for the rating of the load leads to heat building up, shortening life and welding of contacts.
Sockets connected incorrectly with the links left out from the front plate to the rear of the box. Loose connections will lead to a build up of heat and will show as discolouration of the pins.
Switch gear will have similar problems as sockets and switches. Also, problems will include incorrect ratings for connected load and short-circuit ratings.
Contactors can have the wrong coil fitted, poor overload settings, incorrect ratings, looses connections, low oil in dampers, wear on contacts etc.
Electronic devices are usually very reliable, but when they have operated beyond their design limitations problems can occur. Two problem areas are dimmer switches and certain RCD’s, when testing using a 500V d.c. insulation resistance tester may have damaged the internal components. Loose connections can also cause overheating and possible damage.
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Faults That Can Occur Instrumentation and Metering The choice of instruments and their associated control gear is important. ď Ž
Calibration needs to be maintained as errors in reading can cause problems, when certain processes automatically occur when certain values are reached.
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Correct procedures for maintenance, particularly when dealing with current transformers. If a current transformer is open-circuited then a very large voltage will appear in the secondary, with every possibility that the winding will be destroyed.
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Wrong choice of instrument for the task will lead to the possible destruction of the instrument, damage to the circuit being measured and danger to the person using the instrument.
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Faults That Can Occur Protective devices
Fuses can be installed incorrectly, wrong fuse wire used, poor discrimination, poor breaking capacity and incorrect use. Poor breaking capacity may lead to the fuse blowing apart and creating shrapnel when a fault occurs.
Circuit breakers can have the incorrect rating, the wrong type and a poor breaking capacity. The poor breaking capacity can lead to the circuit breaker welding the contacts closed which will mean that it is not open under real fault conditions, although it will appear to be fine. The wrong type will lead either to nuisance tripping when inductive loads are turned on, or no operation under fault conditions if the fault current cannot be maintained high enough.
Moulded case circuit breakers have fewer problems as they cannot be easily misapplied, and their breaking capacity is usually very high.
Overloads can be incorrectly set, have the wrong breaking capacity or be the wrong type for the specific installation location.
RCD’s do not operate on overload or short circuit and are an earth leakage or imbalance device. They can have a poor breaking capacity, poor discrimination and nuisance tripping.
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Faults That Can Occur Luminaires Problems that can occur with light fittings
Loose connections causing overheating
Incorrect size of lamp causing overheating, which causes the wires to harden and become brittle.
Shades/fittings that are too heavy causing strain to be put on wires and connections.
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Faults That Can Occur Flexible cables and cords
Loose connections can cause overheating
Wrong rating of flex for a specific set of conditions.
Cores cut out to make it fit, leading to a build up of heat in the cable.
Incorrect use of cord grips leaving the connection to take the strain.
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Faults That Can Occur Portable Appliances and fixed equipment Portable appliances should be tested at a maximum of every 12 months. The quickest way to clear a fault caused by a portable appliance is to unplug it. This will mean that there is no fault as far as the electrical system is concerned. With fixed appliances the appliance is connected directly to the electrical system Care should be given to – Connections – Ratings of protective devices – Regular maintenance – Cleaning.
End of session 1
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