Other protective measures against electric shocks

Extra-low voltage is used where the risks are great: swimming pools, wandering-lead hand lamps, and other portable appliances for outdoor use, etc.

The use of SELV (Safety Extra-Low Voltage)

Safety by extra low voltage SELV is used in situations where the operation of electrical equipment presents a serious hazard (swimming pools, amusement parks, etc.). This measure depends on supplying power at extra-low voltage from the secondary windings of isolating transformers especially designed according to national or to international (IEC 60742) standard. The impulse withstand level of insulation between the primary and secondary windings is very high, and/or an earthed metal screen is sometimes incorporated between the windings. The secondary voltage never exceeds 50 V rms.
Three conditions of exploitation must be respected in order to provide satisfactory protection against indirect contact:

• No live conductor at SELV must be connected to earth
• Exposed-conductive-parts of SELV supplied equipment must not be connected to earth, to other exposed conductive parts, or to extraneous-conductive-parts
• All live parts of SELV circuits and of other circuits of higher voltage must be separated by a distance at least equal to that between the primary and secondary windings of a safety isolating transformer.

These measures require that:

• SELV circuits must use conduits exclusively provided for them, unless cables which are insulated for the highest voltage of the other circuits are used for the SELV circuits
• Socket outlets for the SELV system must not have an earth-pin contact. The SELV circuit plugs and sockets must be special, so that inadvertent connection to a different voltage level is not possible.

Note: In normal conditions, when the SELV voltage is less than 25 V, there is no need to provide protection against direct contact hazards. Particular requirements are indicated in Chapter P, Clause 3: “special locations”.

The use of PELV (Protection by Extra Low Voltage)

(see Fig. F21) This system is for general use where low voltage is required, or preferred for safety reasons, other than in the high-risk locations noted above. The conception is similar to that of the SELV system, but the secondary circuit is earthed at one point.
IEC 60364-4-41 defines precisely the significance of the reference PELV. Protection against direct contact hazards is generally necessary, except when the equipment is in the zone of equipotential bonding, and the nominal voltage does not exceed 25 V rms, and the equipment is used in normally dry locations only, and large-area contact with the human body is not expected. In all other cases, 6 V rms is the maximum permitted voltage, where no direct contact protection is provided.

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Fig. F21: Low-voltage supplies from a safety isolating transformer

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FELV system (Functional Extra-Low Voltage)

Where, for functional reasons, a voltage of 50 V or less is used, but not all of the requirements relating to SELV or PELV are fulfilled, appropriate measures described in IEC 60364-4-41 must be taken to ensure protection against both direct and indirect contact hazards, according to the location and use of these circuits.
Note: Such conditions may, for example, be encountered when the circuit contains equipment (such as transformers, relays, remote-control switches, contactors) insufficiently insulated with respect to circuits at higher voltages.

The electrical separation of circuits

(see Fig. F22)

The electrical separation of circuits is suitable for relatively short cable lengths and high levels of insulation resistance. It is preferably used for an individual appliance

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Fig. F22: Safety supply from a class II separation transformer

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The principle of the electrical separation of circuits (generally single-phase circuits) for safety purposes is based on the following rationale.
The two conductors from the unearthed single-phase secondary winding of a separation transformer are insulated from earth.
If a direct contact is made with one conductor, a very small current only will flow into the person making contact, through the earth and back to the other conductor, via the inherent capacitance of that conductor with respect to earth. Since the conductor capacitance to earth is very small, the current is generally below the level of perception. As the length of circuit cable increases, the direct contact current will progressively increase to a point where a dangerous electric shock will be experienced.
Even if a short length of cable precludes any danger from capacitive current, a low value of insulation resistance with respect to earth can result in danger, since the current path is then via the person making contact, through the earth and back to the other conductor through the low conductor-to-earth insulation resistance.
For these reasons, relatively short lengths of well insulated cables are essential in separation systems.
Transformers are specially designed for this duty, with a high degree of insulation between primary and secondary windings, or with equivalent protection, such as an earthed metal screen between the windings. Construction of the transformer is to class II insulation standards.
As indicated before, successful exploitation of the principle requires that:

• No conductor or exposed conductive part of the secondary circuit must be connected to earth,
• The length of secondary cabling must be limited to avoid large capacitance values⁽¹⁾ ,
• A high insulation-resistance value must be maintained for the cabling and appliances.

These conditions generally limit the application of this safety measure to an individual appliance.
In the case where several appliances are supplied from a separation transformer, it is necessary to observe the following requirements:

• The exposed conductive parts of all appliances must be connected together by an insulated protective conductor, but not connected to earth,
• The socket outlets must be provided with an earth-pin connection. The earth-pin connection is used in this case only to ensure the interconnection (bonding) of all exposed conductive parts.

In the case of a second fault, overcurrent protection must provide automatic disconnection in the same conditions as those required for an IT system of power system earthing.

(1) It is recommended in IEC 364-4-41 that the product of the nominal voltage of the circuit in volts and length in metres of the wiring system should not exceed 100,000, and that the length of the wiring system should not exceed 500 m.

Class II equipment

Class II equipment symbol:  

These appliances are also referred to as having “double insulation” since in class II appliances a supplementary insulation is added to the basic insulation (see Fig.F23).

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Fig. F23: Principle of class II insulation level

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No conductive parts of a class II appliance must be connected to a protective conductor:

• Most portable or semi-fixed equipment, certain lamps, and some types of transformer are designed to have double insulation. It is important to take particular care in the exploitation of class II equipment and to verify regularly and often that the class II standard is maintained (no broken outer envelope, etc.). Electronic devices, radio and television sets have safety levels equivalent to class II, but are not formally class II appliances
• Supplementary insulation in an electrical installation: IEC 60364-4-41 (Sub-clause 413-2) and some national standards such as NF C 15-100 (France) describe in more detail the necessary measures to achieve the supplementary insulation during installation work.

A simple example is that of drawing a cable into a PVC conduit. Methods are also described for distribution switchboards.

• For ASSEMBLIES, IEC 61439-1 describes a set of requirements, for what is referred to as “total insulation”, equivalent to class II equipment
• Some cables are recognised as being equivalent to class II by many national standards

Out-of-arm’s reach or interposition of obstacles

In principle, safety by placing simultaneously-accessible conductive parts out-of-reach, or by interposing obstacles, requires also a non-conducting floor, and so is not an easily applied principle

By these means, the probability of touching a live exposed-conductive-part, while at the same time touching an extraneous-conductive-part at earth potential, is extremely low (see Fig. F24).

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Fig. F24: Protection by out-of arm’s reach arrangements and the interposition of non-conducting obstacles

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In practice, this measure can only be applied in a dry location, and is implemented according to the following conditions:

• The floor and the wall of the chamber must be non-conducting, i.e. the resistance to earth at any point must be:

  -  > 50 kΩ (installation voltage ≤ 500 V)
  -  > 100 kΩ (500 V < installation voltage ≤ 1000 V)
Resistance is measured by means of “MEGGER” type instruments (hand-operated generator or battery-operated electronic model) between an electrode placed on the floor or against the wall, and earth (i.e. the nearest protective earth conductor). The electrode contact area pressure must be evidently be the same for all tests.
Different instruments suppliers provide electrodes specific to their own product, so that care should be taken to ensure that the electrodes used are those supplied with the instrument.

• The placing of equipment and obstacles must be such that simultaneous contact with two exposed-conductive-parts or with an exposed conductive-part and an extraneous-conductive-part by an individual person is not possible.

• No exposed protective conductor must be introduced into the chamber concerned.
• Entrances to the chamber must be arranged so that persons entering are not at risk, e.g. a person standing on a conducting floor outside the chamber must not be able to reach through the doorway to touch an exposed-conductive-part, such as a lighting switch mounted in an industrial-type cast-iron conduit box, for example.

Earth-free equipotential chambers

Earth-free equipotential chambers are associated with particular installations (laboratories, etc.) and give rise to a number of practical installation difficulties

In this scheme, all exposed-conductive-parts, including the floor⁽¹⁾ are bonded by suitably large conductors, such that no significant difference of potential can exist between any two points. A failure of insulation between a live conductor and the metal envelope of an appliance will result in the whole “cage” being raised to phase-to-earth voltage, but no fault current will flow. In such conditions, a person entering the chamber would be at risk (since he/she would be stepping on to a live floor).
Suitable precautions must be taken to protect personnel from this danger (e.g. non-conducting floor at entrances, etc.). Special protective devices are also necessary to detect insulation failure, in the absence of significant fault current.

(1) Extraneous conductive parts entering (or leaving) the equipotential space (such as water pipes, etc.) must be encased in suitable insulating material and excluded from the equipotential network, since such parts are likely to be bonded to protective (earthed) conductors elsewhere in the installation.

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Fig. F25: Equipotential bonding of all exposed-conductive-parts simultaneously accessible