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City and Guilds 2330 Certificate in Electrotechnical Technology Level 3 Unit 3 Outcome 1 Session 5 Symptoms 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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Describe typical symptoms of a fault

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Review of previous sessions 

Power circuits are almost all circuits other than lighting and emergency circuits

Control circuits deal with the controlling of motor starters etc.

Lighting circuits deal with lighting.

Fire and emergency lighting circuits are easy to recognise.

Faults occur for many reasons, they need to be understood and rectified.

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 the 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.

In any fault diagnosis process the evidence must be gathered, analysed and interpreted.

When the fault has been found it must be rectified, and re-tested before putting the circuit back into commission.

Care should be taken when turning the supply back on.

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Inherent Faults Inherent faults are faults that are designed into the system.

Complete loss of supply This can occur for one of two reasons ď Ž

There is a power cut, or a fault on the supply system and power has been lost. There is nothing you can do about this.

ď Ž

The installation has been designed badly and there is no discrimination between the supply authorities main cut-out and the consumer’s protective devices.

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Localised Loss of Supply Localised Loss of Supply This will cause part of the installation to lose a supply, not just an individual circuit. This is most likely to be on a sub-main.    

Badly designed overcurrent protection with poor discrimination will lead to localised supply loss. Clients loading their system beyond the design parameters. Overcurrent devices take no account of the nature of the load and switching on of a highly inductive load may cause nuisance operation. RCD’s incorrectly set so that earth leakage becomes a problem.

Nuisance operation of overcurrent devices.   

Poor discrimination. No account taken of the nature of the load. Overload and short circuit devices not properly considered as part of the whole system.

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Transient Voltages Transient Voltages are caused by switching. When equipment containing inductors and capacitors are regularly switched, a short term voltage spike (transient voltage ) is injected onto the system. This voltage can be very high and be capable of causing damage. There is no fault but it can cause problems.

ď Ž

It can cause damage to computer systems, injecting a signal into data cables.

ď Ž

It can cause arcing at switch contacts with consequent overheating.

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Insulation Failure The insulation may fail for a number of reasons.   

Wear and tear. Wrong type of cable have been installed in a particular set of conditions. MIMS cable can be damaged by transient voltages created when it is used in discharge lighting circuits.

Plant, equipment or component failure Inherent faults in plant etc are usually created pre-delivery, or when installed without due care. 

Motor is made to run beyond its design capacity

Overload settings are too high.

Switches too small for load being switched on and off.

Load is too close to control device and causes overheating etc.

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Regulations Regulation 4.1 of the EAWR 89 requires;

All systems shall be at all times of such construction as to prevent, as far as is reasonably practical, danger. This covers a range of responsibilities and is law. Consideration needs to be given to the suitability of the electrical construction, not only for the present, but for the future. Particular consideration must be given to; 

Manufacturer's certified equipment

Likely load and fault conditions

Appropriate protective devices

Fault levels at the point of supply and at other relevant positions

Fault level variations caused by motor

Environmental conditions which affect the mechanical strength of the system

The users requirements

The way in which inspection, testing and maintenance is carried out.

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Faults due to Negligence HASWA74 states that you have a responsibility to yourself and those that your work affects

Ignorance is no excuse It has taken many years to get the current standards to their present state. When work is carried out in such a way that safety is compromised then accidents will happen. It is not an accident though if installing plant, wiring or equipment is done in a way you know is wrong, or in contravention of BS7671:2001 It is unacceptable to decide to leave something out because you can’t be bothered. You must keep up to date with current standards and regulations. It is up to you to know your limitations and to stay within them. No-one knows everything.

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Other Fault Areas Some faults are caused by unforeseen circumstances

Cable connections 

Loose connections – These lead to an increase in heat and hence resistance Different metal connections – Aluminium and copper do not like each other chemically

Glands and seals  

With MIMS cable, if the gland is not sealed properly water can get in and over time a fault is created. With SWA cables glands are sometimes made off with some of the strands not fixed properly or with some of them cut out.

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Other Fault Areas Switches 

Switches incorrectly chosen for the rating of the load. – This leads to heat building up, shortening life and welding of contacts.

Sockets connected incorrectly with links left out, from the front plate to the rear of the box. – Connections that are loose will lead to a build up of heat and lead to discolouration near the pins.

Switchgear has problems with loose and incorrect connections.

Contactors can have wrong coil fitted, poor overload settings, incorrect ratings, loose connections, low oil in dampers, wear on contacts etc.

Electronic devices, when operated beyond their design parameters can cause problems. – The two specific areas are dimmer switches and RCD’s.

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Other Fault Areas 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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Other Fault Areas 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.

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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Other Fault Areas 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 5

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Symptoms of Faults