Circuit Touch Voltage Potential
Touch voltage terms are defined as:‘Prospective touch voltage. Voltage between simultaneously accessible conductive parts when those conductive parts are not being touched by a person or animal.’
‘Conventional touch voltage limit. Maximum value of the prospective touch voltage which is permitted to be maintained indefinitely in specified conditions of external influences.’
‘(Effective) touch voltage. Voltage between conductive parts when touched simultaneously by a person or an animal.’
Touch voltage and electric shock
Touch voltages are liable to occur between simultaneously accessible exposed-conductive-parts and extraneous-conductive-parts in an installation, particularly under conditions of fault within the installation. In this context ‘extraneous-conductive-parts’ may include not only ‘earthy’ metalwork, such as metallic services pipework, but also a surface on which a person is standing which presents no appreciable resistance to the general mass of Earth.
Fig 1 illustrates how touch voltages may occur in an installation forming part of a TN system. The figure shows an item of Class I equipment that has an earth fault, caused by a breakdown of the insulation between the line conductor of the load and the earthed enclosure (exposed-conductive-part) of the equipment. The earth fault is assumed to be accompanied by an open circuit in the load. The earthed enclosure of the equipment, and the metal pipe (extraneous-conductive-part), which is connected to the main earthing terminal (MET) of the installation by a main protective bonding conductor, are simultaneously accessible to a person.
An earth fault current (If) flows, giving rise to a touch voltage between the accessible earthed enclosure of the equipment and the metal pipe. The magnitude of the touch voltage (in volts); will be approximately equal to If (in amperes), multiplied by the impedance of the circuit protective conductor (in ohms), as shown in Fig 1.
Example of how touch voltages occur in an installation forming part of a TN system
Fig 1
During an electric shock, the magnitude of current that flows through the body depends upon the touch voltage and the impedance of the body between points of contact (Ohm’s law). Both touch voltage and body impedance depend on a number of factors, examples of which include the wetness at the point of contact, the surface area of contact, and other impedances in the current path, such as the person’s clothing and footwear, and the general mass of Earth.
For a given current path through the body, the danger to a person depends on the magnitude and duration of the current that flows. Current magnitude and duration cannot categorically be classified into dangerous or safe values. However, the concepts of current thresholds and time/current zones (combined with heart current factor, where applicable), employed in IEC 60479, allow the probable dangerous effects of a current to be predicted.
Touch voltages, magnitude and duration
Most of the parameters that determine the current likely to flow through a person’s body during an electric shock, such as the degree of moisture of the skin and the surface area of contact, are outside the control of an electrical installation designer. However, the designer does have some control over touch voltages and disconnection times when carrying out an electrical installation design.
In order to reduce the risk of electric shock, it is necessary to establish a relationship between touch voltage and disconnection time that could be considered safe for the majority of the population.
Using data such as the time/current zones and values of resistance of the human body given in IEC 60479, a time/body current relationship considered to be safe for the majority of the population was translated by the International Electrotechnical Commission into the two touch voltage/time relationship curves shown in Fig 2. The chosen time/current characteristic lay within a time/current zone designated AC3, which is conducive to no expected organic damage to the body.
Touch voltage duration curves from IEC Technical Report IEC 1200-413
Fig 2
Curve L in Fig 2 relates to particular conditions defined as ‘normally dry situations’, where the surface on which the person at risk is standing presents some resistance to the general mass of Earth and the person has dry or moist skin.
Curve Lp relates to particular conditions defined as ‘wet locations’, where the surface on which the person at risk is standing does not present any resistance to the general mass of Earth and the person has wet skin. The curves can be used for assessing whether a touch voltage may be considered safe. For example, from curve L it can be seen that, for normally dry situations, disconnection of the supply is not necessary for protection against electric shock where the touch voltage is 50 V or less. Similarly, in wet locations, disconnection of the supply is not necessary for protection against electric shock where the touch voltage is 25 V or less, as shown by curve L. The equivalent maximum acceptable value of touch voltage for wet locations is 25 V, as shown by curve Lp. However, irrespective of whether or not disconnection is required for protection against electric shock, disconnection may still be required for protection against thermal effects.
As a further example of the use of the curves, it can be seen from curve L that, for normally dry conditions and a touch voltage of 100 V, it is necessary to disconnect the supply in approximately 400 ms to give protection against electric shock. Similarly, from curve Lp, it can be seen that, for wet conditions and a touch voltage of 100 V, a supply disconnection time of approximately 200 ms is required to give protection against electric shock.
In general touch voltages are unlikely to exceed 100 V provided that the requirements of BS 7671 are met.
Misconceptions about touch voltages
It is a misconception that, where electrical equipment requiring a connection to earth is correctly earthed, it is impossible to experience a sensation of electric shock under earth fault conditions. The correct connection of all exposed-conductive-parts and extraneous-conductive-parts to the MET (Main Earth Terminal) of an installation, as is required by BS 7671, can still lead to the creation of touch voltages and the risk of the sensation of an electric shock in the event of an earth fault.
It is also a misconception that the magnitude of touch voltages cannot exceed 50 V in an installation complying with the requirements of BS 7671. Touch voltages well in excess of 50 V can occur under earth fault conditions in the installation. However, compliance with the requirements of BS 7671 should mean that the magnitude and duration of the touch voltages will be such as not to cause danger.
An example of touch voltages in an installation
Fig 3 below illustrates how touch voltages are caused in an installation forming part of a TN system under earth fault conditions where fault protection is provided by protective earthing, protective equipotential bonding and automatic disconnection in case of a fault).
Example of touch voltages in a TN system
Fig 3
In an installation that meets the requirements of BS 7671, the risk of a person receiving a sensation of electric shock from being in contact with an exposed-conductive-part which has become live under fault conditions is not eliminated. This is because BS 7671 does not place a limit on the impedance of the circuit protective conductor. Touch voltages can be well in excess of 50 V. However, compliance with BS 7671 should mean that the duration of such a voltage is short enough so as not to cause danger.
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