What Is a Touch Voltage? [Definitive Guide]

We are all familiar with the dangers of electricity, but few people understand what touch voltage is and why it can be so dangerous. Touch voltage is an electrical hazard that occurs when a person comes into contact with a live part of a circuit. It’s important to be aware of this danger because even though touch voltage may seem harmless at first glance, it can quickly cause serious injury or death if not addressed properly.

Touch Voltage Definition

The term ‘touch voltage’ is officially defined within the IEC 60050-195-2021 as:

voltage between conductive parts when touched simultaneously by a human being or livestock.

IEC 60050-195-2021

Note 1 to entry: The value of the touch voltage is influenced by the impedance of the human being or the livestock in electric contact with these conductive parts.

IEC 60728-11, IEC 61140, IEC 61557-1 and IEC 62282-5-1 define the term “(effective) touch voltage” in the same way as IEC 60050-195.

The technical report IEC 60479-5 defines the term “touch voltage” from IEC 60050-195. A note to this definition explains that:

The touch voltage may be different from the open-circuit voltage between those conductive parts.

IEC TR 60479-5-2007

According to IEC 61557-1-2019, the term in question has the following short symbol: Ut.

Touch Voltage Meaning

When a person or animal touches conductive parts with different electrical potentials at the same time, he or she will be subjected to a voltage that is referred to in regulations as a touch voltage. Under these conditions, an electric current will flow through the human (animal) body that can cause fatal electric shock, result in serious electrical injury, or cause mechanical injury.

If a person (animal), having an electrical connection to earth, touches any live part, he will also be subject to a touch voltage. An electric current will also flow through the human (animal) body, the magnitude of which depends on the touch voltage and the impedance of his body.

Human (animal) contact with live parts usually occurs under single or multiple fault conditions. For example, when a fault in the insulation of live parts makes them accessible to touch. However, it is most likely that a person is touched by an exposed-conductive-part of class 0 or class I electrical equipment that has become live because of a fault in the basic insulation of some hazardous-live-part. It is possible, but less likely, for a person to come into touch with the conductive enclosure of live class II electrical equipment when the double or reinforced insulation of a hazardous-live-part is faulty.

Protection Measures Against Touch Voltage

In order to reduce touch voltage in the electrical installations of buildings, protective bonding is carried out. In this process, exposed-conductive-parts of class I electrical equipment are connected together by protective conductors, and extraneous-conductive-parts are connected by protective bonding conductors.

Under conditions of increased probability of electric shock, when class I electrical equipment is used, for example in rooms of a building with conductive floors and walls, characterized by high humidity, temperature and other unfavorable conditions, supplementary bonding is carried out. When this is carried out, the exposed-conductive-parts of class I electrical equipment are connected to extraneous-conductive-parts by means of protective conductors of the supplementary bonding.

The protective bonding is usually used in conjunction with other precautions, e.g. the automatic disconnection of the power supply. In this case, the equipotential bonding system firstly creates an artificial conductive path for the earth-fault current to flow. Secondly, it reduces the touch voltage until a protective device is triggered which disconnects the distribution or final electrical circuit with class I faulty electrical equipment.

Prospective Touch Voltage

Prospective touch voltage is a voltage between simultaneously accessible conductive parts when those conductive parts are not being touched by a human being or livestock.

The prospective touch voltage is the voltage between the conductive parts accessible to simultaneous touch when those parts are not touched by a person or animal. The term “prospective touch voltage” describes the maximum voltage value between specified conductive parts. If a person (animal) touches these conductive parts, the magnitude of the touch voltage may decrease compared to the value of the prospective touch voltage.

In order to reduce the prospective touch voltage in the electrical installations of buildings, protective equipotential bonding is carried out, and in rooms of the building characterized by an increased probability of an electric shock, such as bathrooms, supplementary equipotential bonding is also carried out.

The voltage between an exposed-conductive-part energized by a fault in the basic insulation of a hazardous-live-part and the earth or a conductive surface where a person might be present is also a prospective contact voltage. Its value depends on the type of system earthing to which the electrical installation of a building corresponds.

Calculation of Prospective Touch Voltages

Let’s estimate the values of the prospective touch voltages for the most common electrical distribution system, which is the electrical installation of a building connected to a low-voltage electrical distribution network consisting of a step-down transformer substation and an overhead line or underground cable.

If the basic insulation of a class I hazardous-live-part of electrical equipment is faulty and a short circuit occurs to an exposed-conductive-part, in the electrical installation of a building corresponding to the TT type of system earthing, the earth-fault current flows from the live part to the exposed-conductive-part.

From the exposed-conductive-part, the electric current flows through the protective conductor, the main earthing terminal, the earthing conductors and the earthing electrode to the local earth. Through the earth, the earth-fault current flows to the earthing electrode of the earthing arrangement of the neutral of transformer installed in the 10/0.4 kV transformer substation (See Figure 1 of the article “Earth fault current“).

Consider the simplified circuit of the TT system shown in Figure 1. The earth-fault current flows in a closed loop formed by the impedances of the phase conductor of the distribution line, the phase and protective conductors of the electrical circuits of the electrical installation of a building, the earthing arrangements of the power supply and the electrical installation of a building, and the power supply.

Simplified substitution diagram of the TT system
Figure 1. Simplified substitution diagram of the TT system

Figure 1 shows:

  • ZL DL is the impedance of the phase conductor of the distribution line from the low-voltage switchgear of the transformer substation to the input terminals of the electrical installation of a building;
  • ZL EIB – impedance of phase conductors of distribution and final electrical circuits from the input terminals of the electrical installation of a building to the point of earth fault;
  • ZPE EIB – impedance of protective conductors of distribution and final electrical circuits from the main earthing terminal of the earthing arrangement of the electrical installation of a building to the point of earth fault;
  • ZEA PS – impedance of the power supply earthing arrangement;
  • ZEA EIB – impedance of the earthing arrangement of the electrical installation of a building;
  • IEF – earth fault current;
  • UTP EIB – prospective touch voltage in the electrical installation of a building;
  • UTP E is the prospective touch voltage relative to earth;
  • 1 – exposed-conductive-part of class I hazardous electrical equipment;
  • 2 – earth;
  • 3 – main earthing terminal of the earthing arrangement of the electrical installation of a building.

The value of the prospective touch voltage in the electrical installation of a building UTP EIB equals the voltage drop across the protective conductors of the ZPE EIB electric circuits from the earth fault point 1, located in the exposed conductive part of the class I hazardous electric equipment, to the main earthing terminal 3:

UTP EIB = ZPE EIB × IEF, where IEF is the earth fault current, A.

The prospective touch voltage in the electrical installation of a building will be low for two reasons:

  • Firstly, the impedance of the protective conductors of the electrical installation of a building is usually less than 1 ohm.
  • Secondly, the earth-fault current in a TT system is usually less than a few amperes.

The value of the prospective touch voltage relative to earth UTP E is equal to the sum of the voltage drop across the protective conductors of the electrical circuits of the electrical installation of a building ZPE EIB and the voltage drop across the earthing arrangement of the electrical installation of a building ZEA EIB from the main earthing terminal 3 to earth 2:

UTP E = (ZPE EIB + ZEA EIB) × IEF.

As the sum of the impedances of the phase conductors of the distribution line, the phase conductors, and the protective conductors of the electrical circuits of the electrical installation of a building is substantially less than the sum of the impedances of the power supply earthing arrangement and of the electrical installation of a building, the prospective touch voltage relative to earth can be approximated as follows:

UTP E ≈ ZEA EIB × IEF ≈ Uo × ZEA EIB / (ZEA EIB + ZEA EIB ), where Uo is the nominal voltage of the phase conductor relative to the earth, V.

For example, if the nominal voltage of the electrical installation of a building is 230/400 V, the impedance of the earthing arrangement of the neutral of the transformer of the transformer substation is 4 Ω, and the impedance of the earthing arrangement of the electrical installation of a building is 10 Ω, then the value of the prospective touch voltage relative to the earth is approximately equal:

UTP E ≈ 230 V × 10 Ohm / (4+10) Ohm ≈ 164 V, where 230 V is the nominal phase voltage.

The value of the prospective touch voltage relative to earth depends on the ratio of the impedances of the earthing arrangements of the power supply and the electrical installation of a building.

If the impedance of the earthing arrangement of the power supply decreases, and if the impedance of the earthing arrangement of the electrical installation of a building increases, the prospective touch voltage to earth increases.

According to IEC 60364-4-41, in electrical installations of buildings having a TT type of system earthing, residual current devices are generally used as protective devices within the automatic disconnection of the power supply. The impedance of the earthing arrangement of the electrical installation of a building may therefore be greater than 100 Ohms.

If the impedance of the earthing arrangement of the neutral of the transformer is 4 ohms, and the impedance of the earthing arrangement of the electrical installation of a building is 100 ohms, then the value of the prospective touch voltage relative to earth will be approximately equal to the phase voltage:

UTP E ≈ 230 V × 100 Ohm / (4+100) Ohm ≈ 221 V.

In contrast to the TT system, in the TN-C-S system the earth-fault current does not flow mainly in the earth, but along the PEN conductor of the distribution line (see Fig. 2 of “Earth fault current“).

That is, the predominant part of the earth-fault current flows in the closed loop formed by the impedances of the phase conductor and the PEN conductor of the distribution line, the phase conductors and the protective conductors of the electrical circuits of the electrical installation of a building, and the power supply (Fig. 2).

The sum of the impedances of the earthing arrangements of the power supply and the electrical installation of a building is many times the impedance of the PEN conductor of the distribution line in parallel to which they are connected. Therefore, a small part of the earth-fault current flows through these two resistances.

The phase conductor and the PEN conductor of the distribution line from the transformer substation to the electrical installation of a building are generally of the same length and cross-section. The lengths and cross-sections of the phase and protective conductors of the distribution and final circuits from the input terminals of the electrical installation of a building to the point of earth fault are also, as a rule, equal.

Consequently, the impedances of the phase conductor and the PEN conductor of the distribution line, as well as the phase conductors and protective conductors of the electrical installation of a building, are equal to each other.

Therefore, in the event of an earth fault, the voltage drop across the impedances of the PEN conductor of the distribution line and the protective conductors of the electrical installation of a building will be approximately half of the phase voltage – 115 V.

Simplified substitution diagram of the TN-C-S system
Figure 2. Simplified substitution diagram of the TN-C-S system

Figure 2 shows:

  • ZL DL is the impedance of the phase conductor of the distribution line from the low-voltage switchgear of the transformer substation to the input terminals of the electrical installation of a building;
  • ZL EIB – impedance of phase conductors of distribution and final electrical circuits from the input terminals of the electrical installation of a building to the point of earth fault;
  • IEF – earth fault current;
  • ZPEN DL is the impedance of the PEN conductor of the distribution line from the low-voltage switchgear of the transformer substation to the input terminals of the electrical installation of a building;
  • ZPE EIB – impedance of protective conductors of distribution and final electrical circuits from the input terminals of the electrical installation of a building to the point of earth fault;
  • ZEA PS – impedance of the power supply earthing arrangement;
  • ZEA EIB – impedance of the earthing arrangement of the electrical installation of a building;
  • UTP EIB – prospective touch voltage in the electrical installation of a building;
  • UTP E is the prospective touch voltage relative to earth;
  • 1 – exposed-conductive-part of class I hazardous electrical equipment;
  • 2 – earth;
  • 3 – input terminal of the electrical installation of a building, on which the PEN conductor of the distribution line is separated into protective and neutral conductors of the electrical installation of a building of a phase conductor and PEN conductor of the distribution line, as well as phase conductors and protective conductors of the electrical installation of a building.

The value of the prospective touch voltage in the electrical installation of a building, corresponding to the TN-C-S type of system earthing, is equal to the voltage drop across the protective conductors of the distribution and final electrical circuits from the earth fault point 1, located in the exposed conductive part of the class I faulty electrical equipment, to the input terminal 3, at which the PEN conductor of the distribution line is separated into the protective and neutral conductors of the electrical installation of a building:

UTP EIB = ZPE EIB × IEF.

The value of the prospective contact voltage of the electrical installation of a building depends on the ratio of the impedances of the PEN conductor of the distribution line and the protective conductors of the electrical circuits of the electrical installation of a building. If these resistances are equal, the value of the prospective touch voltage of the electrical installation of a building is approximately one-fourth of the phase voltage:

UTP EIB  ≈ Uo × 0.5 × 0.5 ≈ 230 × 0.25 ≈ 57,6 V.

If the impedance of the PEN conductor of the distribution line is half the impedance of the protective conductors of the electrical installation of a building, the value of the prospective touch voltage of the electrical installation of a building will be approximately two-sixths of the phase voltage:

UTP EIB ≈ Uo × 1/2 × 2/3 ≈ 230 × 2/6 ≈ 76,7 V.

In the limit it can reach half of the phase voltage – 115 V, if the impedance of the PEN conductor of the distribution line is zero, for example, when the electrical installation of a building is connected directly to a transformer substation built into the building:

UTP EIB ≈ Uo × 1/2 × 1 ≈ 230 × 1/2 ≈ 115 V.

The prospective touch voltage relative to earth is equal to the sum of the voltage drop across the protective conductors of the electrical circuits of the electrical installation of a building and the voltage drop across the earthing arrangement of the electrical installation of a building from the main earthing terminal to earth 2. The latter depends on the voltage drop across the PEN conductor of the distribution line and the ratio of the impedances of the earthing arrangements of the power supply and the electrical installation of a building. The prospective touch voltage relative to earth can be calculated as follows:

UTP E = (ZPEN DL × ZEA EIB / (ZEA PS + ZEA EIB) + ZPE EIB) × IEF.

On the one hand, the value of the prospective touch voltage relative to earth depends on the ratio of the impedances of the PEN conductor of the power supply line to the protective conductors of the building installation.

On the other hand, it depends on the ratio of the impedances of the earthing devices of the power supply system and the building installation. With equal impedances of the PEN conductor and the protective conductors of the electrical installation of a building, on the one hand, and the impedances of the earthing arrangements of the power supply and the electrical installation of a building, on the other hand, the prospective touch voltage relative to earth is approximately three-eighths of a phase voltage:

UTP E ≈ Uo × 1/2 × (1/2 ×1/2 +1/2) ≈ 230 × 3/8 ≈ 86,3 V.

If the impedance of the PEN conductor of the distribution line is half the impedance of the protective conductors of the electrical installation of a building, and the impedance of the earthing arrangement of the power supply is also half the impedance of the earthing arrangement of the electrical installation of a building, the prospective touch voltage relative to earth will be higher:

UTP E ≈ Uo × 1/2 × (1/3 × 2/3 + 2/3) ≈ 230 × 8/18 ≈ 102,2 V.

The maximum value of the prospective touch voltage relative to earth is half of the phase voltage – 115 V, if the electrical installation of a building is connected directly to a transformer substation which is built into the building. In this case, the prospective touch voltage relative to earth is equal to the prospective touch voltage in the electrical installation of a building.

The same value of the prospective touch voltage relative to earth will be present when a earth fault has occurred at the origin of the electrical installation of a building. In this case, the prospective touch voltage of the electrical installation of a building is zero. The prospective touch voltage relative to earth can also reach half the phase voltage if the electrical installation of a building does not have an earthing arrangement.

Conventional Touch Voltage Limit

Conventional touch voltage limit is a value of the maximum prospective touch voltage that is permitted to be maintained indefinitely at specified conditions of external influences.

The term ‘conventional touch voltage limit’ is officially defined within the IEC 61557-1-2019 as:

maximum value of the touch voltage which is permitted to be maintained indefinitely in specified conditions of external influences and is usually equal to 50 V AC, RMS or 120 V ripple free DC.

IEC 61557-1-2019

According to IEC 61557-1-2019, the term in question has the following short symbol: UL.

The conventional touch voltage limit sets the value of the maximum prospective touch voltage that can occur in the electrical installation of a building for an unlimited period of time. This voltage must not normally exceed the upper limit of the extra-low voltage of 50 VAC and 120 VDC. However, if electrical equipment is used in an environment with an increased risk of electric shock, the specified maximum values of prospective touch voltages are generally reduced to reduce the probability of electric shock.

References

  1. IEC 60050-195-2021
  2. IEC 61557-1-2019

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