City and Guilds 2330 Certificate in Electrotechnical Technology Level 3 Unit 3 Outcome 1 Session 7 Special precautions B & B Training Associates
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Unit 3
Installation (Buildings and Structures) Fault Diagnosis and rectification
Outcome 2
Select appropriate methods and use tools, equipment and instruments to restore systems equipment and components to working order.
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Outcomes from this session At the end of this session you should be able to
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Recognise situations where special precautions should be applied.
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Review of previous sessions
Control circuits deal with the controlling of motor starters etc, lighting circuits deal with lighting. Fire and emergency lighting circuits are easy to recognise.
The person who is working must be competent to do the task.
Tasks should be planned so that no live working takes place.
An approved voltage tester and a voltage proving unit are required for testing of a potentially live piece of equipment.
There are three key aspects to good fault finding practice, good technical knowledge, good personal experience and a logical approach.
When the fault has been found it must be rectified, and re-tested before putting the circuit back into commission.
Faults can be listed as inherent faults, or faults caused by negligence etc.
Some of the factors which influence the decision as to whether to repair or replace a system or piece of equipment are, labour and capital costs, availability of replacement parts, downtime and impact of the fault on safety, warranties etc.
Factor controlling the rectification of a fault will include access, impact on current workers, labour availability, need for a supply as well as H&S.
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Special Precautions There are specific area where additional precautions need to be considered;
Fibre optic cabling
Static
Electronic equipment
IT equipment
High frequency and capacitance
Storage batteries
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Fibre Optic Cabling Fibre optic is the carrying of information over long distances in thin fibre of very pure glass. The light used is infrared radiation just beyond the red end of the visible spectrum. The infrared is generated by a very small semiconductor made from gallium, aluminium and arsenic.
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Problems Associated with Fibre Optic Cables
Bend radius – When the fibre optic cable is bent too tightly the light does not get reflected internally and a loss of signal occurs.
Joints – There needs to be a perfect end connection to link lengths of fibre optic cable together. This requires special training and equipment.
Looking down the end of a fibre optic cable when the light is on is not wise as damage to the eyes can occur.
Fibre optic cables used for lighting have a different light source and are not used for carrying signals. –
The light comes from a light box some distance away.
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Static Static is caused for a variety of reasons; The electric shock received from static is caused by the built up charge, suddenly discharging when you touch something that conducts. This can have quite a high voltage and can be dangerous to electronic circuits, particularly those containing IC’s (chips). Before working on any circuitry, you should always discharge yourself to some earthed metalwork, or wear a discharge band on your wrist to make sure you are at a low potential relative to the circuit. Capacitance appears in many areas, without a capacitor being present. As long as there are two conductors separated by an insulator, there will be a capacitive effect. MIMS cables after they have been tested often retain sufficient charge to give a shock. Cables run parallel over a long distance gain a certain amount of capacitance. The capacitive effect of windings on a coil is small but present.
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Electronic Equipment Damage to electronic components can occur either through incorrect use of test instruments or static electricity. Static electricity can be eliminated by, before working on any circuitry, you should always discharge yourself to some earthed metalwork, or wear a discharge band on your wrist to make sure you are at a low potential relative to the circuit. The use of an insulation resistance tester on certain components will cause problems. Most electronic components are designed to operate at extra low voltage, the insulation resistance tester commonly injects 500V into the circuit. Correct preparation to ensure there is no static or use of the insulation resistance tester is vital.
IT Equipment. It is never acceptable to switch off a supply where computers are operating without warning to allow for back up of the systems to prevent loss of data. Working where computers operate usually needs to be agreed before hand with the client.
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High Frequency and Capacitance Variations in frequency can cause major problems to items of equipment, and the use of systems that create high frequencies can cause damage. High frequency supplies can cause burns. Some items of office equipment, such as power supplies, can have large capacitors present. The capacitors can contain enough energy to cause a real risk to those working on them. It is necessary to discharge any capacitor before work is carried out on a power supply. Capacitors are in most area of an installation, (power factor correction capacitors), most have a resistor connected across them to ensure the discharge over a short period of time.
Be safe don’t take risks.
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Batteries When an electric current passes through an electrolyte, a liquid that allows ions (electrically charged atoms) to move freely across it, breaks down and hydrogen ions (positive) are collected at the cathode. Sulphate ions (negative) are collected at the anode. The anode and the cathode are just two dissimilar metals. The passing of the current through the electrolyte is called charging. The density of the electrolyte is reduced when a cell gives out a current. The density of the electrolyte is restored when the cell or battery is re-charged. The instrument for measuring the level of charge is a hydrometer and it measures the specific gravity of the electrolyte.
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Simple Cell
If a load is connected between the plates then the current will flow from the anode to the cathode. The delivery of current to a secondary cell is called charging. The delivery of current from the cell to the load is called discharging. Batteries are a collection of cells. There are two types of cell in general use. Primary which is only used once and can not be recharged. Secondary which can be recharged. There are two types of secondary cells lead-acid and alkaline.
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Secondary Cells
The energy supplied from a cell is sufficient only for a limited time as the chemical qualities of electrolyte deteriorate with use.
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Safety When batteries are being charged hydrogen gas is given off and care must be taken to avoid explosions. Rooms that are used for the charging of lead acid batteries should be well ventilated. Regulation 562-01-06 BS7671:2001 states; ‘The location of the source shall be properly and adequately ventilated so
that any exhaust gases, smoke or fumes from the source cannot penetrate, to a hazardous extent, areas occupied by persons’ This lays a duty on the electrician or business to make sure the batteries undergoing charge are not going to cause a problem.
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Alkaline Cells There are two types of alkaline cells in use • •
nickel-iron (NiFe) nickel cadmium (NiCad).
Both are housed in a metal case, the electrolyte is caustic potash and distilled water. The plates are separated by insulated rods and mounted in non-metallic crates to insulate the cell from each other.
Advantages of an alkaline battery •
Strong and can withstand vibration
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Can be left in any state of charge for long periods
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Can stand overload without damaging plates etc
Disadvantages •
Costlier than lead acid End of session 7 End of outcome 1
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