Mid-Point Conductor (M): Definition, Meaning, Examples of Use

In this article we will look at what a mid-point conductor is, examples of its use in various systems, and its purpose. We will focus separately on the color and alphanumeric identification of the mid-point conductor.

What Is a Mid-Point Conductor (M)?

Mid-point conductor: conductor electrically connected to the mid-point and capable of contributing to the distribution of electric energy [defined in the IEC 60050-195-2021].

Examples of Use

Based on the definition, it follows that mid-point conductors can occur in 3-wire DC systems. Figure 1 shows an example of the TN-S system, Figure 2 shows the TN-C-S system, Figure 3 shows the TT system, and Figure 4 shows the IT system, which uses mid-point conductors.

TN-S DC 3-wire system
Figure 1 – TN-S d.c. 3-wire system (based on figure 31H of IEC 60364-1)
Figure 31K – TN-C-S d.c. system
Figure 2 – TN-C-S d.c. system (based on figure 31K of IEC 60364-1)
TT DC three-wire system
Figure 3 – TT d.c. 3-wire system (based on figure 31L of IEC 60364-1)
DC IT system three-wire
Figure 4 – d.c. IT system 3-wire (based on figure 31M of IEC 60364-1)

The mid-point conductor is used in DC electrical circuits, where it performs functions equivalent to those assigned to the neutral conductor used in AC electrical circuits. The mid-point conductor has an electrical connection to the middle live part of the power supply (mid-point). It, along with the pole conductors, is used in electrical installations of buildings to provide electrical power to DC electrical equipment.

Color and Alphanumeric Identification of the Mid-Point Conductor

Mid-point conductor shall be identified by the colour BLUE. In order to avoid confusion with other colours it is recommended to use an unsaturated colour BLUE, often called “light blue” [2].

Where a mid-point conductor is present, the colour BLUE shall not be used for identifying any other electrical conductor. In the absence of a mid-point conductor within the whole wiring system, the colour BLUE may be used for identifying a conductor with any other purpose, except as a protective conductor.

[2]

Bare conductors used as mid-point conductors shall be either coloured by a BLUE stripe, 15 mm to 100 mm wide in each unit or enclosure and at each accessible position, or coloured BLUE throughout their length.

NOTE. In IEC 60079-11, the colour BLUE is used for the marking by colour of terminals, terminal boxes, plugs and sockets of intrinsically-safe circuits.

The alphanumeric identification of a mid-point conductor shall be “M” [2].

List of notes concerning particular conditions in certain countries:

  • In Canada, the colour identification WHITE or NATURAL GREY for the mid-point or neutral conductor is used as a replacement for the colour identification BLUE.
  • In Japan, the colour identification WHITE or NATURAL GREY for the mid-point or neutral conductor is used as a replacement for the colour identification BLUE.
  • In the Russian Federation, the BLUE colour should be used only for identification of the neutral conductors , the mid conductors and the earthed line conductors.
  • In the United States , the colour identification WHITE or NATURAL GREY for the mid-point or neutral conductor is used as a replacement for the colour identification BLUE.
Pole Conductor and mid-point conductor color and alphanumeric identification
Colours, alphanumeric notations and graphical symbols used for identification of conductors and terminals [2]

References

  1. IEC 60050-195-2021
  2. IEC 60445-2021