Vehicle circuit protection is necessary to prevent damage to the wiring system if a fault should occur that causes excessive current to flow in the circuit (i.e. a short circuit). It essentially forms a ‘weak link’ which gives way when the circuit current exceeds a design limit, and this prevents overheating of the wiring. Car electrical systems have the potential to generate hundreds of amps from their power sources (alternators
and batteries) and it is important that the system is protected from excessive fault currents which could cause a fire. Generally this protection is in the form of fuses, fusible links or circuit breakers:
Fuses
Automotive fuses are available in many different forms and have evolved over the years. In the past, the glass cartridge fuse was commonly used on British- manufactured vehicles (similar in form to mains plug-type fuses (see Figure 6.11)). This basically consists of a short length of tinned wire connected at both ends to metal caps and enclosed in a glass tube. A strip of paper, colour coded and marked with the fuse rating, is placed in the tube adjacent to the wire. When the current through the fuse exceeds the rated limit, the fuse blows (i.e. the wire melts) and the circuit is broken.
The ceramic-type fuse, used in the past on European vehicles, is similar in operation to the glass fuse (see Figure 6.11). The body of the fuse is ceramic and the current conductor is a copper strip formed around the body. Cheaper versions of this fuse are available with plastic bodies, but these should be avoided as they can cause problems in service. If the fuse connection is not tight, heating will occur; this melts the plastic body which deforms and causes an open circuit.
By far the most common type of fuse in use on vehicles today is the blade-type fuse (see Figure 6.12) (also known as the ATO type). These are available in a number of current ratings and can be identified by the colour of their bodies, as shown in Table 6.5.
126 Power distribution Fundamentals of Motor Vehicle Technology: Book 3
In addition mini and maxi versions of this fuse are available to match more closely specific requirements (see Figure 6.13). These fuses have a favourable performance in service for vehicle wiring systems. Their blowing performance is very consistent and therefore it is not necessary to oversize the cabling. Blade-type fuses can be easily accommodated in a compact fuse or relay box and mounted in the vehicle or under the bonnet with minimum space requirements.
Fuse selection
Generally when a fuse blows there is a circuit fault and this should be investigated. A fuse should always be replaced with one of the same value. The fuse protects the wiring and not the consumers! Therefore, the size of the fuse is appropriate to the wire size. Installing a fuse of higher rating will cause the wiring to overheat if a fault occurs and this could lead to a fire in the vehicle.
Fuses are specified according to their stated rating. This could be the continuous current-carrying capacity or the fusing value. The ratio of these two parameters
is known as the fusing factor. Note that glass cartridge fuses are specified according to their fusing value whereas ceramic fuses have their current-carrying capacity stated. For both of these types of fuse, the fusing factor is approximately 2. For example, an 8 A ceramic fuse will blow at 16 A and a 20 A glass fuse will carry a continuous circuit current of 10 A maximum (without nuisance failure).
Blade-type fuses have superior performance in that their fusing factor is much lower (approximately 1.35 at 25ºC) and hence they respond more quickly to an over- current situation. This means that the fuse value can be matched more closely to the circuit current rating and the circuit wire size can be optimised rather than being over-specified. They are rated with maximum circuit current capacity (shown in Table 6.3, page 122).
Another factor to consider is temperature. Fusing action is temperature dependent and the fuse rating must be compensated if high (or low) temperatures occur where the fuse is located. Figure 6.14 shows the temperature re-rating factor to be applied.
For example, a fuse located in an under bonnet fuse box operates at around 60ºC. The circuit current is 10 A. The ideal fuse rating can be calculated as follows:
Ideal fuse = Circuit current
Re-rating factor × Fusing factor
Figure 6.11 Types of fuses Table 6.5 ATO fuse colour code
Colour Current rating (A)
violet 3 tan 5 brown 7.5 red 10 blue 15 yellow 20 clear 25 green 30
Blade type fuse
Capacity (A)
Fusing portion
Terminal
15
Housing
Figure 6.12 Blade-type fuse
Maxifuse Autofuse Minifuse
Vehicle wiring systems 127
If a manual circuit breaker trips, it remains an open circuit and it can be re-set by removing it and inserting a small metal rod to snap the bi-metal strip back into the contact closed position. Circuit breakers are commonly placed in the fuse-box inside the vehicle. They are used to protect high-current circuits which have large in-rush currents, for example motor drive circuits (e.g. power windows, power sunroofs, central locking) and heated screens.
Auto-reset or cycling thermal breakers are used commonly in lighting circuits as in this application a short circuit will cause the light to go on and off repeatedly rather than fail completely. Therefore, the driver should be able to bring the vehicle to rest safely. The basic construction is shown in Figure 6.17.
The current overload heats the bi-metal strip which expands and breaks the circuit connection. With no current flowing, the strip cools and contracts back to its original shape which then closes the contacts and reinstates current flow in the circuit, thus re-setting the breaker automatically. This operation repeats at low frequency whilst excess current is flowing in the circuit.
Rer ating f actor (% r ated current) Ambient temperature (°C) 75 70 –40 –20 0 20 40 60 80 100 120 140 80 85 90 95 100 105 110 115 120
Figure 6.14 Temperature re-rating factor
= 95% 10 A × 0.75 = 14 A
In most cases temperature effects can be excluded as fuses are only available in a certain range of sizes, but if temperature extremes are encountered and the ideal fuse rating calculated is available in a standard fuse size then this fuse should be selected.
Fusible links
A common way of protecting main battery circuits is to use a fusible link. These can be in the form of special fusing wires soldered onto cable ends at the battery connection or as cartridge-type elements mounted in specific fuse boxes located near the battery. They form heavy-duty fuses that will rupture in the event of a catastrophic short circuit and this reduces the risk of a fire if an accident causes the main battery cable to short to earth. Figure 6.15 shows the common types.
Cartridge-type elements are most common and have replaced wired fusible links. These are also known as Pacific-type fuses. They are available in sizes up to 120 A. The internal construction is shown in Figure 6.16. Circuit breakers
Circuit breakers employ a bi-metallic strip to control contacts that carry current in the protected circuit. When a current overload occurs, the strip heats up and bends; this opens the contacts and interrupts the circuit. There are two types of circuit breaker available depending upon the application – the manual reset type and the auto-reset type. The construction of a typical manual re-set type is shown in Figure 6.18.
Figure 6.15 Fusible links
Figure 6.16 Cartridge-type elements
Adjusting screw Adjusting nut Terminal Contact Bimetal Cover
128 Power distribution Fundamentals of Motor Vehicle Technology: Book 3
Cables for automotive applications are generally of the PVC insulated multi-strand type. The thickness and number of strands dictates the current-carrying capacity of the cable. It is important that the cable has the required capacity to prevent overheating and excessive volt drop
K ey P oints K ey P oints
The individual cables are bundled together to form a harness. Normally the colour of the cable is used to identify its purpose or status. A number of standards are used to define the colours used Connectors are used as interfaces or junctions, and terminals are used to connect the cables to components. It is important to use the right connector and that it is fitted to the cable properly either by soldering or crimping
In order to protect the wiring from excessive currents due to short-circuit faults, some form of fuse can be fitted. There are a number of different types of circuit protection and it is important that, if replaced, the same current rating is used. Never replace a fuse with one of higher value!
Alternative circuit protection types are circuit breakers or fusible links. Fusible links are generally used to protect high-current circuits if an accident occurs that causes major damage to the wiring harness. Circuit breakers are re-settable and are used on circuits with high in-rush currents (e.g. motor circuits)
K ey P oints K ey P oints Bimetal Open
Figure 6.18 Manual-reset circuit breaker
6.3
CIRCUIT DIAGRAMS
6.3.1 Introduction
Wiring or circuit diagrams are essential tools when undertaking detailed repairs or fault-finding on vehicle electrical systems. They act as a map for the system so that the route taken by flowing currents to and from components can be seen clearly. This helps considerably in understanding how the system or a component works. In addition they provide information regarding component locations and wiring junctions. The main problem is that, unlike general industrial engineering practice, there is very little standardisation, and hence most manufacturers have their own methodology for creating and presenting wiring diagrams. Therefore, specific manufacturer information must always be sought.
Certain aspects of wiring system technical information follow conventions, for example symbolic representation. Knowledge of these conventions will help when dealing with diagrams which do not conform to standards, as much of the information will be the same. It should also be noted that it will be assumed in any diagram that current flows from left to right and/or from top to bottom.