symptoms of faults

B & B Training Associates Ltd 2005 1

City and Guilds 2330City and Guilds 2330

Certificate in Electrotechnical Certificate in Electrotechnical TechnologyTechnology

Level 3Level 3Unit 3 Outcome 1 Session 5Unit 3 Outcome 1 Session 5

Symptoms of faultsSymptoms of faults


Unit 3 Unit 3 Installation (Buildings and Installation (Buildings and

Structures) Fault Diagnosis and Structures) Fault Diagnosis and rectificationrectification

Outcome 1Outcome 1Select and apply fault diagnosis and

rectification techniques


Outcomes from this sessionOutcomes from this session

At the end of this session you should be able to

• Describe typical symptoms of a fault


Review of previous sessionsReview 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.


Inherent FaultsInherent FaultsInherent faults are faults that are designed into the system.Complete loss of supplyThis can occur for one of two reasons 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. The installation has been designed badly and there is no discrimination

between the supply authorities main cut-out and the consumer’s protective devices.


Localised Loss of SupplyLocalised Loss of Supply

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.

Localised Loss of SupplyThis 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.

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.


Transient VoltagesTransient VoltagesTransient 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.

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

It can cause arcing at switch contacts with consequent overheating.


Insulation FailureInsulation FailureThe 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.

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.

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


RegulationsRegulationsRegulation 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.


Ignorance is no excuse

Faults due to NegligenceFaults due to NegligenceHASWA74 states that you have a responsibility to yourself

and those that your work affects

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.


Other Fault AreasOther Fault AreasSome faults are caused by unforeseen circumstancesCable 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.


Other Fault AreasOther Fault AreasSwitches 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.


Other Fault AreasOther Fault AreasProtective 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.


Other Fault AreasOther Fault AreasLuminaires 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.


Other Fault AreasOther Fault Areas

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.

Portable Appliances and fixed equipmentPortable 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.

End of session 5

symptoms of faults

Documents

good fault

supply system

emergency lighting circuits

discharge lighting circuits

localised supply loss

structures fault diagnosis

short circuit devices

localised loss of supplythis