Leads and Cable Terminals
GeneralInstrument leads are normally made of a copper wire conductor with plastic insulation. Conductors are described as single-core, multi-core or stranded depending on their construction.
A single-core conductor consists of a single wire. Since this type of lead is stiff, it may break if subjected to vibration, making it unsuitable for automotive use.
A multi-core conductor consists of 2-7 wires. Although this type of lead is more flexible than the single-core type it is usually unsuitable for automotive applications.
A stranded conductor consists of more than 7 wires. The thinner and more numerous the individual wires, the softer and more pliable the lead, making it ideal for automotive wiring.
Resistivity
Like other power supply system components, leads possess a certain resistance. The factors which determine the resistance of a lead are the material, length and cross-sectional area, and - to some extent - the temperature. Resistivity is defined as the resistance of a conductor with a length of 1 m and a cross-sectional area of 1 Sq.mm at a temperature of +20 °C.
Denoted by the Greek letter p (rho), resistivity is expressed in the units ohm Sq.mm / m. The length, area and resistivity of the material may be used to calculate the resistance of a lead at a temperature of +20 °C, using the expression:
R(ohm) = p x length (m) / area (Sq.mm)
Example: What is the resistance (R) of a copper lead with a length of 10 m and a cross-sectional area of 1.5 Sq.mm? The resistivity (p) of the material is 0.0174 ohm Sq.mm / m
Substituting this value in the expression gives R = 0.0174 x 10 / 1.5. Thus, the resistance of the lead is 0.116 (ohm) at +20 °C.
The passage of current through a resistance generates heat, while the resistance itself causes a voltage drop in the circuit. The heat produced by a high current flowing in a lead may be sufficient to melt the insulation and cause a short-circuit. Heat may cause a problem in short leads even if the voltage drop is not especially high.
Voltage drops are more associated with long leads. In a 12 V system, the total voltage drop between power source and consumer must not exceed 0.4 V, consisting of 0.3 V max. on the positive side and 0.1 max. on the negative side.
Specification of cross-sectional area
The nomogram offers the simplest means of specifying the cross-sectional area. The required area may be read off directly using the length of the lead and the current or power to be conducted, assuming that leads are of the type(s) recommended by Volvo. Since the area indicated by the chart represents the minimum permissible size, a smaller conductor must not be used.
Example: Calculate the required area of a 6 m long lead carrying a current of 5 A in a 12 V system. Join the 6 m and 5 A points on the left and right-hand scales, and read off the size (in this case approx. 1.3 Sq. mm) on the middle scale. Use a 1.5 Sq.mmconductor.
NOTE: When connecting additional components to the system, where connection is not made directly to the power source (battery), the supply and ground leads at the point of connection should also be checked to ensure that the sizes are adequate to carry the increased load. When carrying out this check, all components which may impose a simultaneous load on the lead must be totalled before reading the nomogram.
Running leads
When installing extra leads in the car, it is important that they be run correctly:
- The leads must be sufficiently long to avoid damage to the wiring, while ensuring that the installation is not made unduly complicated or future fault tracing more difficult.
- The leads must be properly secured and positioned so that they cannot become pinched or chafed either by fixed or moving components.
- The wiring must be protected by plastic sleeving.
- The leads must be provided with rubber grommets at all points of passage through body panels or similar components.
Cable terminals
1 Insulation strain reliever
2 Ridges
3 Locking tag
4 Cable terminal with strain reliever clamps both conductor and insulation
The importance of eliminating unnecessary voltage drops when specifying conductor sizes is matched by the importance of selecting cable terminals of the correct type. To ensure satisfactory electrical contact between a conductor and terminal, the latter must be of the correct size for the lead, and must be crimped onto the prepared conductor end.
Crimping is carried out using a purpose-designed tool to squeeze the terminal onto the lead in such manner that the conductor material is deformed, thereby ensuring solid connection between the two. When correctly carried out, the voltage drop in a crimped connection is negligible.
Where possible, the cable terminal should incorporate an insulation strain reliever. Since it also clamps the insulation, this type reduces the risk of conductor breakage. Of the various types of cable terminal available, the following are the most common; Flat pin, Round pin, Round or fork lug.
Flat pin
Flat pin terminals are available either with or without ridges and locking tags. The purpose the ridges is to increase the force required to join and separate the pin and its mating sleeve, reducing the risk of inadvertent disconnection.
Ridges should not be used in cable terminals with more than two flat pins since the force required to connect and disconnect the two elements will then be excessive. The purpose of the locking tag is to hold the pin in position when it is used as part of a connector. To remove a flat pin from a connector, insert a small screwdriver sufficiently far under the pin to release the locking tag and enable the pin to be withdrawn. Since the connection and disconnection forces in a multi-pin connector may be excessive even when flat pins without ridges are used, round pins are usually preferable.
Round pin
Round pins cannot carry the same high electrical load as flat pins.
However, since round pins are often used in connectors, they are also provided with locking tags. Removal from a connector may be carried out using a tool as follows:
- Press lead sufficiently far into connector to release locking tag.
- Insert tool sufficiently far to enable locking tag to move past locking position.
- Withdraw pin and lead from connector.
Round or fork lug
Cable terminals of the round or fork lug type are designed to ensure satisfactory contact in cases in which the leads are frequently disconnected and reconnected.
Materials and surface treatment
Three main types of material and surface treatment combinations are used in cable terminals:
- Tinned brass.
- Tinned phosphor bronze.
- Silver-plated phosphor bronze.
The material and surface treatment used in a particular case are selected on the basis of the environmental and reliability requirements.