PEM Conductor: Definition, Examples of Systems, Requirements

PEM Conductor: conductor combining the functions of both a protective earthing conductor and a mid-point conductor [defined in the IEC 60050-195:2021].

Note. In the Russian Federation , the term “PEM conductor” is defined differently: PEM conductor – electrical conductor combining the functions of both a protective earthing conductor and a mid conductor [3].

A PEM conductor, like other protective conductors, is not classified as a live part. However, a PEM conductor is a current-carrying conductor that is counted in the total number of conductors of an electrical circuit, network or system. It does not refer to line conductors.

The requirements described in IEC 60364-4-41 411.4.2 for TN systems require that the mid-point of the power supply be earthed. If the earthed mid-point is a live part of a DC power supply, a PEM conductor can be connected to it. That is, in low-voltage DC power installations, or parts thereof, the functions of the protective earthing conductor and the mid-point conductor can be combined in a single conductor. This conductor is referred to as the combined protective earth conductor and mid-point conductor. However, it is better known by its short name “PEM conductor” or simply “PEM”.

Examples of Systems that Use PEM Conductors

In TN-C and TN-C-S three-wire DC electrical systems, PEM conductors can be used. Figure 1 and 2 show examples of electrical systems that use PEM conductors.

Figure 31J – TN-C d.c. system
Figure 31J – TN-C d.c. system [2]

NOTE. Additional earthing of the PEL conductor or the PEM conductor in the electrical installation may be provided.

Figure 31K – TN-C-S d.c. system
Figure 31K – TN-C-S d.c. system [2]

Requirements for PEM Conductors

  1. Since the PEM conductor is charged with the functions of protection against electric shock performed by a protective earthing conductor, it must first meet the requirements for a protective conductor. Above all, the continuity of the electrical circuit of the PEM conductor must be ensured. Therefore, switching devices such as fuses, circuit breakers, disconnectors, etc. must not be included in its circuit.
  2. Secondly, the PEM conductor has the function of transmitting electrical energy. Therefore, it must meet the requirements for the mid-point conductor. The dual function of the PEM conductor must impose the same restrictions on its performance as the PEN conductor. Therefore, the minimum cross-sectional area of the PEM conductor in IEC 60364-5-54 is set to be the same as that of the PEN conductor – 10 mm2 for copper and 16 mm2 for aluminum.

Color and alphanumeric identification.

A PEM conductor, when insulated, shall be identified by GREEN-AND-YELLOW coloured insulation throughout its length and with BLUE coloured markings at its terminations and points of connection of the PEM conductor.
The BLUE coloured markings at the termination and points of connection may be omitted provided one of the following two conditions is met:

  • in electrical equipment, if relevant requirements are included in specific product standards or within a country;
  • in the case of wiring systems, for example those used in industry, if decided by the relevant committee.

If confusion with a PEN or PEL conductor is likely, the alphanumeric designation given in 7.3.6 [3] shall be indicated at the terminations and points of connection.

PEM conductor identification

The alphanumeric identification of a PEM conductor shall be “PEM”.

References

  1. IEC 60050-195:2021
  2. IEC 60364-1
  3. IEC 60445-2021