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

25 Aug 2014  | Peter Wilson

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Part 1 started a look at grounding: when to consider it, how chassis materials affect it, and the problem of ground loops. Part 2 discussed power supply returns and I/O signal grounding. Part 3 covered inter-board interface signals, star grounding, and shielding.

The safety earth
A brief word is in order about the need to ensure a mains earth connection, since it is obvious from the preceding discussion that this requirement is frequently at odds with anti-interference grounding practice. Most countries now have electrical standards which require that equipment powered from dangerous voltages should have a means of protecting the user from the consequences of component failure. The main hazard is deemed to be inadvertent connection of the live mains voltage to parts of the equipment with which the user could come into contact directly, such as a metal case or a ground terminal.

Imagine that the fault is such that it makes a short circuit between live and case, as shown in figure 20. These are normally isolated and if no earth connection is made the equipment will continue to function normally but the user will be threatened with a lethal shock hazard without knowing it. If the safety earth conductor is connected then the protective mains fuse will blow when the fault occurs, preventing the hazard and alerting the user to the fault.

Figure 20: The need for a safety earth.

For this reason a safety earth conductor is mandatory for all equipment that is designed to use this type of protection, and does not rely on extra levels of insulation. The conductor must have an adequate cross-section to carry any prospective fault current, and all accessible conductive parts must be electrically bonded to it. The general requirements for earth continuity are:

The earth path should remain intact until the circuit protection has operated.

The impedance should not significantly or unnecessarily restrict the fault current.

As an example, EN 60065 requires a resistance of less than 0.5 Ω at 10 A for a minute. Design for safety is covered in greater detail in Section 9.1.

Wiring and cables
This section will look briefly at the major types of wire and cable that can be found within typical electronic equipment. There are so many varieties that it comes as something of a surprise to find that most applications can be satisfied from a small part of the range. First, a couple of definitions: wires are single-circuit conductors, insulated or not; cables are groups of individual conductors, separately insulated and mechanically contained within an overall sheath.

Wire types
The simplest form of wire is tinned copper wire, available in various gauges depending on required current-carrying capacity. Component leads are almost invariably tinned copper, but the wire on its own is not used to a great extent in the electronics industry. Its main application was for links on printed circuit boards, but the increasing use of double-sided and multi-layer plated-through-hole boards makes them redundant.

Tinned copper wire can also be used in rewirable fuselinks. Insulated copper wire is used principally in wound components such as inductors and transformers. The insulating coating is a polyurethane compound which has self-fluxing properties when heated, which makes for ease of soldered connection, especially to thin wires.

Table 1.2 compares dimensions, current capacity and other properties for various sizes of copper wire. In the UK the wires are specified under BS EN 13602 for tinned copper and BS EN 60182 (IEC 60182-1) for enamel insulated, and are sold in metric sizes. Two grades of insulation are available, Grade 1 being thinner; Grade 2 has roughly twice the breakdown voltage capability.

Wire inductance
We mentioned earlier that any length of wire has inductance as well as resistance. The approximate formula for the inductance (L) of a straight length of round section wire at high frequencies is

L = K × l × (2.3 log10 (4l/d)—1)µH

where: l and d are length and diameter respectively, l >> d and K is 0.0051 for dimensions in inches or 0.002 for dimensions in cm.

This equation is used to derive the inductance of a 1 m length (note that this is not quite the same as inductance per meter) in Table 1.2 and you can see that inductance is only marginally affected by wire diameter. Low values of inductance are not easily obtained by adding cross-section and the reactive component of impedance dominates above a few kiloHertz whatever the size of the conductor.

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