What Is a Protective Conductor Current? Definition, Flow Path, Limits

What is meant by the term protective conductor current?

Protective conductor current is an electric current appearing in a protective conductor, such as leakage current or electric current resulting from an insulation fault [this term is defined in the IEC 60050-195-2021].

Note 1. Under normal conditions, the protective conductor current is generally equal to the total leakage current of class I electrical equipment whose exposed-conductive-parts are connected to the protective conductor.

Note 2. When an earth fault occurs, earth-fault current flows in the protective conductor.

Features

The regulatory documentation divides all operating conditions of a building electrical installation into normal conditions, when there is no fault in the insulation of live parts and, therefore, a low probability of electric shock, and fault conditions, under which, at least, there is a single fault in the insulation of a live part and there is a high probability of electric shock.

Under normal conditions, an electrical current flowing through the protective conductor is the total leakage current of simultaneously operating class I electrical equipment and is measured in thousandths of an ampere.

Under fault conditions, when a live part is shorted to an exposed conductive part of class I electrical equipment (i.e. when an earth fault occurs), the protective conductor flows earth-fault current, which can reach thousands of amperes in the TN-S, TN-C and TN-C-S types of system earthing.

Both of these currents are qualitatively different – the leakage current always flows through the protective conductor under normal conditions, while the earth-fault current appears in it only under single or multiple fault conditions.

The question arises: how can you measure the current in the protective conductor “generated” by the class I electrical equipment under test, when some of its live part is shorted to an exposed-conductive-part due to a single fault in the basic insulation? That is, in fact, it is necessary to measure the electrical current in the fault mode of the electrical installation of a building.

The current in the protective conductor connected to the exposed-conductive-part of class I faulty electrical equipment will be equal to the earth-fault current. The value of the earth-fault current depends on the type of system earthing to which the electrical installation of a building corresponds, as well as the impedance of the circuit (line conductor – protective conductor) extending from the power supply to the point of measurement – the point of an earth fault. The characteristics of the “tested” electrical equipment do not affect the earth-fault current value. The faulty electrical equipment under these conditions represents only the point at which the earth fault occurred.

It makes no sense to perform such a measurement in a test laboratory, since it must be performed at a specific point of the electrical installation of a building. Therefore, it is not required by IEC 60990.

Moreover, the introduction of IEC 60990 notes that in certain cases it is necessary to measure the protective conductor current of electrical equipment under normal operating conditions. For example, when selecting a residual current protective device.

Section 8 “Measuring of protective conductor current” of IEC 60990 specifies that the installation protective conductor current shall be measured after installation by inserting an ammeter of negligible impedance (for example, 0,5 Ω) in series with the protective conductor. Measurement of protective conductor current is made with the equipment and power distribution system running in all normal operating modes.

The international standard also notes that within any shared earthing system, the protective conductor currents of individual equipment combine in a non-arithmetic manner. Therefore, the protective conductor current of a group of equipment earthed by a single protective earthing conductor cannot be reliably predicted from knowledge of individual equipment protective conductor currents. Consequently, measurements made on individual equipment are of limited use, and the protective conductor current for that group of equipment shall be measured in the shared protective earthing conductor.

Class I electrical equipment in operation always causes some form of leakage current to flow in the protective conductors. Their combination determines the current flowing through the protective conductor, which under normal conditions must be limited in order to ensure reliable protection against electric shock. In addition, the current flowing in the protective conductor should always be compared with the characteristics of the residual current device to avoid false RCD operations under normal conditions due to excessive protective conductor currents.

In the requirements of the IEC standards for low-voltage electrical installations, which contain references to the protective conductor current, it is specifically stated that they are formulated for normal conditions. For example, clause 33.1 “Compatibility of characteristics” of IEC 60364-1, which establishes the general principles of construction of low-voltage electrical installations and the basic requirements for them, states that:

An assessment shall be made of any characteristics of equipment likely to have harmful effects upon other electrical equipment or other services or likely to impair the supply, for example, for coordination with concerned parties. Those characteristics include, for example: … excessive PE conductor currents not due to a fault.

IEC 60364-1

Section 516, “Measures related to protective conductor currents” of IEC standard 60364-5-51, emphasizes that for the purpose of this section, a protective conductor current is a current which flows in the protective conductor when the equipment is fault-free and operating normally. That is, the regulatory requirements for protective conductor current set out in IEC 60364-5-51 apply only to normal conditions.

The requirements of IEC 61140 for protective conductor current are also formulated for normal conditions. Clause 7.6.3 “protective conductor currents” of the international standard specifies that measures shall be taken in the installation and in equipment to prevent excessive protective conductor currents impairing safety or normal use of the electrical installation. Electrical equipment which causes, under normal conditions, a current to flow in the protective conductor of its supply, shall be compatible with protective provisions.

Types of Protective Conductor Currents

Two different protective conductor currents must be distinguished in the regulatory documentation – protective conductor current as applied to the electrical installation of a building (more precisely, as applied to the electric circuit) and protective conductor current as applied to the electrical equipment.

The protective conductor current, as applied to an electric circuit, is the total electric current flowing in the protective conductor of that electric circuit both under normal conditions and under single or multiple fault conditions. It can therefore be defined as in IEC 60990, IEC 61140, IEC 60519-2, IEC 60050-195, IEC 60598-1, IEC 61558-1, IEC 62040-3 and BS 7671. However, this approach to defining the term “protective conductor current” imposes some restrictions on its use. In normative requirements, using this term, it is necessary to specify the conditions for which they are formulated. Therefore, it is more convenient to consider only normal conditions, characterized by the absence of any fault.

The protective conductor current, as applied to electrical equipment, is the electrical current “generated” in the protective conductor by quality class I electrical equipment represented by a single product. The protective conductor current of electrical equipment can only be determined for normal conditions of operation when there is no fault. Therefore, in regulations specifying requirements for various types of electrical equipment, and above all in IEC standard 60990, the term in question should be defined for normal conditions.

Path of the Protective Conductor Current

The protective conductor current can be seen as a form of leakage current. The path the protective conductor current takes depends on the type of system earthing. With TT and IT types of system earthing (Fig. 1), the protective conductor current of class I electrical equipment flows from live parts to the exposed-conductive-parts, through the non-faulty basic insulation. From exposed-conductive-parts, the protective conductor current flows to the earth via protective conductors, the main grounding terminal, earthing conductors and the earth electrode.

Path of the protective conductor current in the TT system
Figure 1. Path of the protective conductor current in the TT system

Figure 1 shows:

  • 1 – earthing arrangement of the power supply;
  • 2 – earthing arrangement of the electrical installation of a building;
  • IPE – protective conductor current.
  • TS – transformer substation;
  • OL – overhead line;
  • UC – underground cable.

If the electrical installation of a building corresponds to the TN-S, TN-C and TN-C-S types of system earthing (Fig. 2), then most of the protective conductor current does not flow into the earth, but through protective conductors and through PEN conductors of the electrical installation of a building and the low-voltage electrical distribution network, to the earthed live part of the power supply.

Path of the protective conductor current in the TN-C-S system
Figure 2. Path of the protective conductor current in the TN-C-S system

Figure 2 shows:

  • 1 – earthing arrangement of the power supply;
  • 2 – earthing arrangement of the electrical installation of a building;
  • IPE – protective conductor current.
  • TS – transformer substation;
  • OL – overhead line;
  • UC – underground cable.

Important! So, the protective conductor current of class I electrical equipment flows along the same conductive path as the earth-fault current. However, unlike the earth-fault current, the protective conductor current depends little on the type of system earthing to which the electrical installation of a building corresponds. Its value is usually in the thousands or hundredths of an ampere.

All class I electrical equipment has some protective conductor current. Standards specifying requirements for specific types of electrical equipment may specify the maximum allowable protective conductor current. If class I electrical equipment has a protective conductor current that does not exceed the reference value, it is considered high-quality electrical equipment that can be operated. Otherwise, the electrical equipment is considered defective and must be repaired or disposed of.

Protective Conductor Current Limits

The limit values for protective conductor currents under normal operating conditions as given by table 1 are applicable to low-voltage current-using equipment supplied at rated frequencies up to 1 kHz.

Rated current of current-using equipment (AC)Maximum protective conductor current for frequencies up to 1 kHz
0 < I ≤ 2 A1 mA
2 A < I ≤ 20 A0,5 mA/A (i.e., for each ampere of rated current of electrical equipment)
I > 20 A10 mA
Table 1 – Maximum protective conductor current for frequencies up to 1 kHz

For current-using equipment for permanent connection intended to be connected to a reinforced protective conductor, product committees should state the maximum values for the protective conductor current, which in no case shall exceed 5 % of the rated input current per phase.

Measurements shall be carried out on equipment as delivered.

In normal use, AC current using equipment shall not generate current with a DC component in the protective conductor that exceeds the values in table 2. This will prevent affecting the proper functioning of protective device(s) or other equipment in the installation.

Rated current of current-using equipment (AC)Maximum protective conductor current (DC)
I ≤ 2 A5 mA
2 A < I ≤ 20 A2,5 mA/A (i.e., for each ampere of rated current of electrical equipment)
I > 20 A50 mA
Table 2 – Maximum protective conductor current for DC

Pluggable electrical equipment with a rated input ≤ 4 kVA shall be designed to have protective conductor current with a smooth superimposed DC current component limited to ≤ 6 mA.

For pluggable electrical equipment with a rated input > 4 kVA and permanently connected electrical equipment independent of the rated input shall contain in the operating manual advice about the protective measure.

In case of DC protective conductor currents > 6 mA, suitable protective devices shall be selected, e.g. RCD type B.

For equipment intended for permanent connection with reinforced protective conductor, the value of the protective conductor current shall be provided by the manufacturer in his documentation and indication shall be given in the instructions for installation that the equipment shall be installed as described in 7.6.4.2 IEC 61140.

For current-using equipment intended for permanent connection and having a protective conductor current higher than 10 mA, provision shall be made for a secure and reliable connection with earth such as described in IEC 60364-5-54.

Clause 7.6.4.2 IEC 61140

Clause 5.1 “Touch current and protective conductor current” of IEC 60950-1 specifies that equipment shall be so designed and constructed that neither touch current nor protective conductor current is likely to create an electric shock hazard.

Table 5A “Maximum current” of IEC 60950-1 sets the maximum protective conductor current equal to 5% of the input current for all stationary equipment subject to paragraph 5.1.7 “Equipment with touch current exceeding 3.5 mA”.

Clause 10.3 “Touch current, protective conductor current and electric burn” of IEC 60598-1 specifies that touch current or protective conductor current that may occur during normal operation of the luminaire shall not exceed the values given in Table 10.3 “limits of touch current or protective conductor current and electric burn” (see figure 3).

Limits of touch current or protective conductor current and electric burn
Figure 3. Limits of touch current or protective conductor current and electric burn (table 10.3 IEC IEC 60598-1-2020)

Clause 18.5.3 “Protective earthing conductor current” of IEC 61558-1-2017 specifies that the protective earthing conductor current shall not exceed the values of table 15.

Extract from Table 15 of IEC 61558-1-2017
Transformers Rated current Max limit (RMS)
Class I transformers fitted with a single or multiphase plug rated up to and including 32 A ≤ 4 А 2 мА
> 4 А but ≤ 10 А 0,5 мA/A
> 10 А 5 мА
Class I transformers intended for permanent connection ≤ 7 А 3,5 мА
> 7 А but ≤ 20 А 0,5 мA/A
> 20 А 10 мА

Note 1. In IEC 61558-1 all class I and class II transformers equipped with a plug according to IEC TR 60083.

References

  1. IEC 60050-195-2021
  2. IEC 61140-2016
  3. IEC 60364-1
  4. BS 7671:2018+A2:2022
  5. IEC 60598-1-2020
  6. IEC 61558-1-2017

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