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Circuit designer's guide to safety earth, wiring/cables

25 Aug 2014  | Peter Wilson

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A useful rule of thumb is that the inductance of a one inch length of ordinary equipment wire is around 20 nH and that of a one centimeter length is around 7 nH. This factor becomes important in high-speed digital and RF circuits where performance is limited by physical separation, and also in circuits where the rate-of-change of current (di/dt) is high.

Equipment wire
Equipment wire is classified mainly according to its insulation. This determines the voltage rating and the environmental properties of the wire, particularly its operating temperature range and its resistance to chemical and solvent attack. The standard type of wire, and the most widely available, is PVC insulated to BS 4808 which has a maximum temperature rating of 85°C. As well as current ratings at 25°C you will find specifications at 70°C; these allow for a 15°C temperature rise, to the maximum rated temperature, at the specified current. Temperature ratings of 70°C for large conductor switchgear applications and 105°C to American and Canadian UL and CSA standards are also available in PVC. PTFE is used for wider temperature ranges, up to 200°C, but is harder to work with.

Other more specialised insulations include extra-flexible PVC for test leads and silicone rubber for high temperature (150°C) and harsh environments. Many wires carry military, telecom and safety authority approval and have to be specified on projects that are carried out for these customers.

Table 1.3 is included here as a guide to the electrical characteristics of various commonly available PVC equipment wires. Note that the published current ratings of each wire are related to permitted temperature rise. Copper has a positive temperature coefficient of resistivity of 0.00393 per°C, so that resistance rises with increasing current; using the room temperature resistance may be optimistic by several per cent if the actual ambient temperature is high or if significant self-heating occurs.

Wire-wrap wire
A further specialised type of wire is that used for wire-wrap construction. This is available primarily in two sizes, with two types of insulation: Kynar, trademark of Pennwalt, and Tefzel, trademark of Du Pont. Tefzel is the more expensive but has a higher temperature rating and is easier to strip. Table 1.4 lists the properties of the four types.

Cable types
Ignoring the more specialised types, cables can be divided loosely into three categories: power; data and multi-core; RF.

Power cables
Because mains power cables are inherently meant to carry dangerous voltages they are subject to strict standards: in the UK the principal one is BS 6500. International ones are IEC 60227 for PVC insulated or IEC 60245 for rubber insulated. These standards have been harmonized throughout the CENELEC countries in Europe so that any equipment which uses a cable with a harmonized code number will be acceptable throughout Europe.

BS 6500 specifies a range of current ratings and allows a variety of sheath materials depending on application. The principal ones are rubber and PVC; rubber is about twice the price of PVC but is somewhat more flexible and therefore suitable for portable equipment, and can be obtained in a high-temperature HOFR (heat and oil resisting, flame retardant) grade. The current-carrying capacities and voltage drops for DC and single-phase AC, and supportable mass are shown in Table 1.5.

Unfortunately, American and Canadian mains cables also need to be approved, but the approvals authorities are different (UL and CSA). Cables manufactured to the European harmonized standards do not meet UL/CSA standards and vice versa. So, if you intend to export your mains-powered equipment both to Europe and North America you will need to supply it with two different cables. The easy way to do this is to use a CEE-22 6-amp connector on the equipment and supply a different cable set depending on the market. This practice has been adopted by virtually all of the large-volume multinational equipment suppliers with the result that the CEE-22 mains inlet is universally accepted. There are also several suppliers of readymade cable sets for the different countries!

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