Electrical Fault Diagnosis and Fixes (part 1)

To diagnose and find faults in electrical installations and equipment is probably one of the most difficult tasks undertaken by an electrician. The knowledge of fault finding and the diagnosis of faults can never be completely 'learned' because no two fault situations are exactly the same. As the systems we install become more complex, then the faults developed on these systems become more complicated to solve. To be successful the individual must have a thorough knowledge of the installation or piece of equipment and have a broad range of the skills and competences associated with the electrotechnical industries.

The ideal person will tackle the problem using a reasoned and logical approach, recognize his own limitations and seek help and guidance where necessary.

The tests recommended by the regulations can be used as a diagnostic tool but the safe working practices described elsewhere must always be observed during the fault-finding procedures.

If possible, fault finding should be planned ahead to avoid inconvenience to other workers and to avoid disruption of the normal working routine. However, a faulty piece of equipment or a fault in the installation is not normally a planned event and usually occurs at the most inconvenient time. The diagnosis and rectification of a fault is therefore often carried out in very stressful circumstances.

Symptoms of an electrical fault

The basic symptoms of an electrical fault may be described in one or a combination of the following ways:

1. There is a complete loss of power.

2. There is partial or localized loss of power.

3. The installation or piece of equipment is failing because of the following:

(a) an individual component is failing;

(b) the whole plant or piece of equipment is failing;

(c) the insulation resistance is low;

(d) the overload or protective devices operate frequently;

(e) electromagnetic relays will not latch, giving an indication of undervoltage.

Causes of electrical faults

A fault is not a natural occurrence; it's an unplanned event which occurs unexpectedly. The fault in an electrical installation or piece of equipment may be caused by:

++ negligence - that's , lack of proper care and attention;

++ misuse - that's , not using the equipment properly or correctly;

++ abuse - that's , deliberate ill-treatment of the equipment.

If the installation was properly designed in the first instance to perform the tasks required of it by the user, then the negligence, misuse or abuse must be the fault of the user. However, if the installation does not perform the tasks required of it by the user then the negligence is due to the electrical contractor in not designing the installation to meet the needs of the user.

Negligence on the part of the user may be due to insufficient maintenance or lack of general care and attention, such as not repairing broken equipment or removing covers or enclosures which were designed to prevent the ingress of dust or moisture.

Misuse of an installation or pieces of equipment may occur because the installation is being asked to do more than it was originally designed to do, because of expansion of a company, for example. Circuits are sometimes overloaded because a company grows and a greater demand is placed on the existing installation by the introduction of new or additional machinery and equipment.

WHERE DO ELECTRICAL FAULTS OCCUR?

1. Faults occur in wiring systems, but not usually along the length of the cable, unless it has been damaged by a recent event such as an object being driven through it or a JCB digger pulling up an underground cable. Cable faults usually occur at each end, where the human hand has been at work at the point of cable inter-connections. This might result in broken conductors, trapped conductors or loose connections in joint boxes, accessories or luminaires.

All cable connections must be made mechanically and electrically secure. They must also remain accessible for future inspection, testing and maintenance. The only exceptions to this rule are when

++ underground cables are connected in a com pound filled or encapsulated joint;

++ floor warming or ceiling warming heating systems are connected to a cold tail;

++ a joint is made by welding, brazing, soldering or compression tool.

Since they are accessible, cable inter-connections are an obvious point of investigation when searching out the cause of a fault.

2. Faults also occur at cable terminations. Regulations require that a cable termination of any kind must securely anchor all conductors to reduce mechanical stresses on the terminal connections.

All conductors of flexible cords must be terminated within the terminal connection otherwise the cur rent carrying capacity of the conductor is reduced, which may cause local heating. Flexible cords are delicate - has the terminal screw been over-tightened, thus breaking the connection as the conductors flex or vibrate? Cables and flexible cords must be suit able for the temperature to be encountered at the point of termination or must be provided with additional insulation sleeves to make them suitable for the surrounding temperatures.

3. Faults also occur at accessories such as switches, sockets, control gear, motor contactors or at the point of connection with electronic equipment. The source of a possible fault is again at the point of human contact with the electrical system and again the connections must be checked as described in the first two points above. Contacts that make and break a circuit are another source of wear and possible failure, so switches and motor contactors may fail after extensive use. Socket outlets that have been used extensively and loaded to capacity in say kitchens, are another source of fault due to overheating or loose connections. Electronic equipment can be damaged by the standard tests described in the regulations and must, therefore, be disconnected before testing begins.

4. Faults occur on instrumentation panels either as a result of a faulty instrument or as a result of a faulty monitoring probe connected to the instrument.

Many panel instruments are standard sizes connected to CT's or VT's and this is another source of possible faults of the types described in points 1-3.

5. Faults occur in protective devices for the reasons given in points 1-3 above but also because they may have been badly selected for the job in hand and don't offer adequate protection or discrimination as described in Section 2 of this guide.

6. Faults often occur in luminaires (light fittings) because the lamp has expired. Discharge lighting (fluorescent fittings) also require a 'starter' to be in good condition, although many fluorescent luminaires these days use starter-less electronic control gear. The points made in 1-3 about cable and flexible cord connections are also relevant to luminaire faults.

7. Faults occur when terminating flexible cords as a result of the flexible cable being of a smaller cross section than the load demands, because it's not adequately anchored to reduce mechanical stresses on the connection or because the flexible cord is not suitable for the ambient temperature to be encountered at the point of connection. When terminating flexible cords, the insulation should be carefully removed without cutting out any flexible cord strands of wire because this effectively reduces the cross section of the conductor. The conductor strands should be twisted together and then doubled over, if possible, and terminated in the appropriate connection. The connection screws should be opened fully so that they will not snag the flexible cord as it's eased into the connection. The insulation should go up to, but not into, the termination. The terminal screws should then be tightened.

8. Faults occur in electrical components, equipment and accessories such as motors, starters, switch gear, control gear, distribution panels, switches, sockets and luminaires because these all have points at which electrical connections are made. It is unusual for an electrical component to become faulty when it's relatively new because it will have been manufactured and tested to comply with the appropriate NEC Standard. Through overuse or misuse components and equipment do become faulty but most faults are caused by poor installation techniques.

Modern electrical installations using new materials can now last longer than fifty years. Therefore, they must be properly installed. Good design, good workman ship and the use of proper materials are essential if the installation is to comply with the relevant regulations.

Fault diagnosis

Before an electrician can begin to diagnose the cause of a fault he must:

++ have a thorough knowledge and understanding of the electrical installation or electrical equipment;

++ collect information about the fault and the events occurring at or about the time of the fault from the people who were in the area at the time;

++ begin to predict the probable cause of the fault using his own and other people's skills and expertise;

++ test some of the predictions using a logical approach to identify the cause of the fault.

Most importantly, electricians must use their detailed knowledge of electrical circuits and equipment learned through training and experience and then apply this knowledge to look for a solution to the fault.

Let us, therefore, now look at some of the basic wiring circuits.

Wiring circuits

LIGHTING CIRCUITS

It’s assumed current demand of points, and states that for lighting outlets we should assume a current equivalent to a minimum of 100W per lamp holder. This means that for a domestic lighting circuit rated at 5A, a maximum of 11 lighting outlets could be connected to each circuit. In practice, it's usual to divide the fixed lighting outlets into two or more circuits of seven or eight outlets each. In this way the whole installation is not plunged into darkness if one lighting circuit fuses.

Lighting circuits are usually wired in 1.0 or 1.5mm cable using either a loop-in or joint-box method of installation. The loop-in method is universally employed with conduit installations or when access from above or below is prohibited after installation, as is the case with some industrial installations or blocks of flats. In this method the only joints are at the switches or lighting points, the live conductors being looped from switch to switch and the neutrals from one lighting point to another.

The use of junction boxes with fixed brass terminals is the method often adopted in domestic installations, since the joint boxes can be made accessible but are out of site in the loft area and under floorboards.

All switches and ceiling roses must contain an earth connection and the live conductors must be broken at the switch position in order to comply with the polarity regulations.

A ceiling rose may only be connected to installations operating at 250V maximum and must only accommodate one flexible cord unless it's specially designed to take more than one. Lampholders must comply with regulations and be suspended from flexible cords capable of suspending the mass of the luminaire fixed to the lampholder.

The type of circuit used will depend upon the installation conditions and the customer's requirements.

One light controlled by one switch is called one-way switch control. A room with two access doors might benefit from a two-way switch control so that the lights may be switched on or off at either position. A long staircase with more than two switches controlling the same lights would require intermediate switching.

One-way, two-way or intermediate switches can be obtained as plate switches for wall mounting or ceiling mounted cord switches. Cord switches can pro vide a convenient method of control in bedrooms or bathrooms and for independently controlling an office luminaire.

To convert an existing one-way switch control into a two-way switch control, a three-core and earth cable is run from the existing switch position to the proposed second switch position. The existing one-way switch is replaced by a two-way switch and connected as shown in Ill. 4.

Ill. 1 Wiring diagram of one-way switch control.

Ill. 2 Wiring diagram of two-way switch control.

Ill. 3 Wiring diagram of intermediate switch control.

Ill. 4 Wiring diagram of one-way to two-way switch control.

SOCKET OUTLET CIRCUITS

A plug top is connected to an appliance by a flexible cord which should normally be no longer than 2m. Pressing the plug top into a socket outlet connects the appliance to the source of supply. Socket outlets therefore provide an easy and convenient method of connecting portable electrical appliances to a source of supply.

Socket outlets can be obtained in 15, 13, 5 and 2 A ratings, but the 13 A flat pin type complying with regs is the most popular for domestic installations in the United Kingdom. Each 13 A plug top contains a cartridge fuse to give maximum potential protection to the flexible cord and the appliances which it serves.

Socket outlets may be wired on a ring or radial circuit and in order that every appliance can be fed from an adjacent and convenient socket outlet, the number of sockets is unlimited provided that the floor area covered by the circuit does not exceed that given.

RADIAL CIRCUITS

In a radial circuit each socket outlet is fed from the previous one. Live is connected to live, neutral to neutral and earth to earth at each socket outlet. The fuse and cable sizes are given in Table 8A of Sub-section 8 but circuits may also be expressed with a block diagram, as shown in Ill. 5. The number of permitted socket outlets is unlimited but each radial circuit must not exceed the floor area stated and the known or estimated load.

Where two or more circuits are installed in the same premises, the socket outlets and permanently connected equipment should be reasonably shared out among the circuits, so that the total load is balanced.

When designing ring or radial circuits special consideration should be given to the loading in kitchens which may require separate circuits. This is because the maximum demand of current-using equipment in kitchens may exceed the rating of the circuit cable and protection devices.

Ring and radial circuits may be used for domestic or other premises where the maximum demand of the current using equipment is estimated not to exceed the rating of the protective devices for the chosen circuit.

Ill. 5 Block diagram of radial circuits.

Ill. 6 Block diagram of ring circuits.

RING CIRCUITS

Ring circuits are very similar to radial circuits in that each socket outlet is fed from the previous one, but in ring circuits the last socket is wired back to the source of supply. Each ring final circuit conductor must be looped into every socket outlet or joint box which forms the ring and must be electrically continuous throughout its length. The number of permitted socket outlets is unlimited but each ring circuit must not cover more than 100m^2 of floor area.

The circuit details are given may also be expressed by the block diagram given in Ill. 6.

Spurs to ring circuits: A spur is defined in Part 2 of the Regulations as a branch cable from a ring final circuit.

Non-fused spurs: The total number of non-fused spurs must not exceed the total number of socket outlets and pieces of stationary equipment connected directly in the circuit.

The cable used for non-fused spurs must not be less than that of the ring circuit. The requirements concerning spurs are given in Sub-section 8 of the On Site Guide but the various circuit arrangements may be expressed by the block diagrams of Ill. 7.

A non-fused spur may only feed one single or one twin socket or one permanently connected piece of equipment.

Non-fused spurs may be connected into the ring circuit at the terminals of socket outlets or at joint boxes or at the origin of the circuit.

Fused spurs The total number of fused spurs is unlimited. A fused spur is connected to the circuit through a fused connection unit, the rating of which should be suitable for the conductor forming the spur but should not exceed 13A. The requirements for fused spurs are also given in Sub-section 8 but the various circuit arrangements may be expressed by the block diagrams of Ill. 8.

The general arrangement shown in Ill. 9 shows 11 socket outlets connected to the ring, three non-fused spur connections and two fused spur connections.

Ill. 7 Connection of non-fused spurs.

Ill. 8 Connection of fused spurs.

Ill. 9 Typical ring circuit with spurs.

Ill. 10 Immersion heater wiring.

WATER HEATING CIRCUITS

A small, single-point over-sink type water heater may be considered as a permanently connected appliance and so may be connected to a ring circuit through a fused connection unit. A water heater of the immersion type is usually rated at a maximum of 3 kW, and could be considered as a permanently connected appliance, fed from a fused connection unit. However, many immersion heating systems are connected into storage vessels of about 150 l in domestic installations, and Sub-section 8 states that immersion heaters fitted to vessels in excess of 15 l should be supplied by their own circuit.

Therefore, immersion heaters must be wired on a separate radial circuit when they are connected to water vessels which hold more than 15 l. Figure 3.10 shows the wiring arrangements for an immersion heater. The hot and cold water connections must be connected to an earth connection in order to meet the supplementary bonding requirements of Regulation 413-05-02. Every switch must be a double-pole (DP) switch and out of reach of anyone using a fixed bath or shower when the immersion heater is fitted to a vessel in a bathroom.

Ill. 11 Possible wiring arrangements for storage heaters.

Ill. 12 Ducted warm air heating system.

ELECTRIC SPACE HEATING CIRCUITS

Electrical heating systems can be broadly divided into two categories: unrestricted local heating and off-peak heating.

Unrestricted local heating may be provided by portable electric radiators which plug into the socket outlets of the installation. Fixed heaters that are wall mounted or inset must be connected through a fused connection and incorporate a local switch, either on the heater itself or as a part of the fuse connecting unit, as shown in Ill. 8. Heating appliances where the heating element can be touched must have a double-pole switch which disconnects all conductors. This requirement includes radiators which have an element inside a silica-glass sheath.

Off-peak heating systems may provide central heating from storage radiators, ducted warm air or underfloor heating elements. All three systems use the thermal storage principle, whereby a large mass of heat-retaining material is heated during the off-peak period and allowed to emit the stored heat through out the day. The final circuits of all off-peak heating installations must be fed from a separate supply con trolled by an electricity board time clock.

When calculating the size of cable required to sup ply a single storage radiator, it's good practice to assume a current demand equal to 3.4 kW at each point. This will allow the radiator to be changed at a future time with the minimum disturbance to the installation.

Each radiator must have a 20A double-pole means of isolation adjacent to the heater and the final connection should be via a flex outlet. See Ill. 11 for wiring arrangements.

Ducted warm air systems have a centrally sited thermal storage heater with a high storage capacity.

The unit is charged during the off-peak period, and a fan drives the stored heat in the form of warm air through large air ducts to outlet grilles in the various rooms. The wiring arrangements for this type of heating are shown in Ill. 12.

The single storage heater is heated by an electric element embedded in bricks and rated between 6 and 15 kW depending upon its thermal capacity. A radiator of this capacity must be supplied on its own circuit, in cable capable of carrying the maximum current demand and protected by a fuse or MCB of 30, 45 or 60A as appropriate. At the heater position, a double-pole switch must be installed to terminate the fixed heater wiring. The flexible cables used for the final connection to the heaters must be of the heat-resistant type.

Floor warming installations use the thermal storage properties of concrete. Special cables are embedded in the concrete floor screed during construction. When current is passed through the cables they become heated, the concrete absorbs this heat and radiates it into the room. The wiring arrangements are shown in Ill. 13. Once heated, the concrete will give off heat for a long time after the supply is switched off and is, therefore, suitable for connection to an off-peak supply.

Underfloor heating cables installed in bathrooms or shower rooms must incorporate an earthed metallic sheath or be covered by an earthed metallic grid connected to the supplementary bonding.

Ill. 13 Floor warming installations.

COOKER CIRCUIT

A cooker with a rating above 3 kW must be supplied on its own circuit but since it's unlikely that in nor mal use every heating element will be switched on at the same time, a diversity factor may be applied in calculating the cable size, as detailed in Table 1A in Sub-section 1 of the On Site Guide.

Consider, as an example, a cooker with the following elements fed from a cooker control unit incorporating a 13A socket:

4 _ 2 kW fast boiling rings _ 8000W 1 _ 2 kW grill _ 2000W

1 _ 2 kW oven _ 2000W Total loading _ 12 000W

Current rating = 12000/250 = 48 A.

When connected to 250V Applying the diversity factor of Table 1A, Total current rating _ 48A First 10 amperes _ 10A 30% of 38A _ 11.4A Socket outlet _ 5A Assessed current demand _ 10 _ 11.4 _ 5 _ 26.4A

Therefore, a cable capable of carrying 26.4A may be used safely rather than a 48A cable.

A cooking appliance must be controlled by a switch separate from the cooker but in a readily accessible position. Where two cooking appliances are installed in one room, such as split level cookers, one switch may be used to control both appliances provided that neither appliance is more than 2m from the switch.

Designing out faults

The designer of the installation can't entirely design out the possibility of a fault occurring but he can design in 'damage limitation' should a fault occur.

E.g. designing in two, three or four lighting and power circuits will reduce the damaging effect of any one circuit failing because not all lighting and power will be lost as a result of a fault. Limiting faults to only one of many circuits is good practice because it limits the disruption caused by a fault. NEC Regulation tells us to divide an installation into circuits as necessary so as to 1 avoid danger and minimize inconvenience in the event of a fault occurring and 2 facilitate safe operation, inspection testing and maintenance.

Finding the electrical fault

The steps involved in successfully finding a fault can be summarized as follows:

1. Gather information by talking to people and looking at relevant sources of information such as manufacturer's data, circuit diagrams, charts and schedules.

2. Analyze the evidence and use standard tests and a visual inspection to predict the cause of the fault.

3. Interpret test results and diagnose the cause of the fault.

4. Rectify the fault.

5. Carry out functional tests to verify that the installation or piece of equipment is working correctly and that the fault has been rectified.

Safe working procedures

1. The circuits must be isolated using a 'safe isolation procedure', such as that described below, before beginning to repair the fault.

2. All test equipment must be 'approved' and connected to the test circuits by recommended test probes which are discussed later in this section. The test equipment used must also be 'proved' on a known supply or by means of a proving unit such as that shown in Ill. 2.49.

3. Isolation devices must be 'secured' in the 'off ' position as shown in Ill. 2.50. The key is retained by the person working on the isolated equipment.

4. Warning notices must be posted.

5. All relevant safety and functional tests must be completed before restoring the supply.

Ill. 14 Flowchart for a secure isolation procedure.

Live testing

The Electricity at Work Act tells us that it's 'preferable' that supplies be made dead before work commences. However, it does acknowledge that some work, such as fault finding and testing, may require the electrical equipment to remain energized.

Therefore, if the fault finding and testing can only be successfully carried out 'live', then the person carrying out the fault diagnosis must:

++ be trained so that he understands the equipment and the potential hazards of working live and can, therefore, be deemed to be 'competent' to carry out the activity;

++ only use approved test equipment;

++ set up barriers and warning notices so that the work activity does not create a situation dangerous to others.

Note that while live testing may be required in order to find the fault, live repair work must not be carried out. The individual circuit or item of equipment must

first be isolated.

Secure isolation of electrical supply

The NEC Regulations are very specific in describing the procedure to be used for isolation of the electrical supply. NEC Regulation tells us that isolation means the disconnection and separation of the electrical equipment from every source of electrical energy in such a way that this disconnection and separation is secure. Regulation tells us that we must also prove the conductors dead before work commences and that the test instrument used for this purpose must itself be proved immediately before and immediately after testing the conductors. To isolate an individual circuit or item of equipment successfully, competently and safely we must follow a procedure such as that given by the flow diagram in Ill. 14.

Start at the top and work your way down the flowchart.

When you get to the heavy-outlined boxes, pause and ask yourself whether everything is satisfactory up to this point. If the answer is yes, move on. If no, go back as indicated by the diagram.

Faulty equipment: to repair or replace?

Having successfully diagnosed the cause of the fault we have to decide if we are to repair or replace the faulty component or piece of equipment.

In many cases the answer will be straightforward and obvious, but in some circumstances the solution will need to be discussed with the customer. Some of the issues which may be discussed are as follows:

++ What is the cost of replacement? Will the replacement cost be prohibitive? Is it possible to replace only some of the components? Will the labor costs of the repair be more expensive than a replacement? Do you have the skills necessary to carry out the repair? Would the repaired piece of equipment be as reliable as a replacement?

++ Is a suitable replacement available within an acceptable time? These days, manufacturers carry small stocks to keep costs down.

++ Can the circuit or system be shut down to facilitate a repair or replacement?

++ Can alternative or temporary supplies and services be provided while replacements or repairs are carried out?

Selecting test equipment

NEC has published notes which advise electricians and other electrically competent people on the selection of suitable test probes, voltage indicating devices and measuring instruments. This is because they consider suitably constructed test equipment to be as vital for personal safety as the training and practical skills of the electrician. In the past, unsatisfactory test probes and voltage indicators have frequently been the cause of accidents, and therefore all test probes must now incorporate the following features:

1. The probes must have finger barriers or be shaped so that the hand or fingers can't make contact with the live conductors under test.

2. The probe tip must not protrude more than 2mm, and preferably only 1mm, be spring-loaded and screened.

3. The lead must be adequately insulated and colored so that one lead is readily distinguished from the other.

4. The lead must be flexible and sufficiently robust.

5. The lead must be long enough to serve its purpose but not too long.

6. The lead must not have accessible exposed conduct ors even if it becomes detached from the probe or from the instrument.

7. Where the leads are to be used in conjunction with a voltage detector they must be protected by a fuse.

A suitable probe and lead is shown in Ill. 2.47.

Regs also tells us that where the test is being made simply to establish the presence or absence of a voltage, the preferred method is to use a proprietary test lamp or voltage indicator which is suitable for the working voltage, rather than a multimeter. Accident history has shown that incorrectly set multimeters or makeshift devices for voltage detection have frequently caused accidents. Ill. 2.48 shows a suitable voltage indicator. Test lamps and voltage indicators are not fail-safe, and therefore reg recommends that they should be regularly proved, preferably before and after use, as described previously in the flowchart for a safe isolation procedure.

The regulations also specify the test voltage or current required to carry out particular tests satisfactorily. All testing must, therefore, be carried out using an 'approved' test instrument if the test results are to be valid. The test instrument must also carry a calibration certificate, otherwise the recorded results may be void. Calibration certificates usually last for a year. Test instruments must, therefore, be tested and recalibrated each year by an approved supplier. This will maintain the accuracy of the instrument to an acceptable level, usually within 2% of the true value.

Modern digital test instruments are reasonably robust, but to maintain them in good working order they must be treated with care. An approved test instrument costs equally as much as a good-quality camera; it should, therefore, receive the same care and consideration.

cont. to part 2 >>