Chapter G

Sizing and protection of conductors

Connection and choice for protective earthing conductor

From Electrical Installation Guide
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Protective (PE) conductors provide the bonding connection between all exposed and extraneous conductive parts of an installation, to create the main equipotential bonding system. These conductors conduct fault current due to insulation failure (between a phase conductor and an exposed conductive part) to the earthed neutral of the source. PE conductors are connected to the main earthing terminal of the installation.

The main earthing terminal is connected to the earthing electrode (see Chapter E) by the earthing conductor (grounding electrode conductor in the USA).

PE conductors must be:

  • Insulated and coloured yellow and green (stripes)
  • Protected against mechanical and chemical damage

In IT and TN-earthed schemes it is strongly recommended that PE conductors should be installed in close proximity (i.e. in the same conduits, on the same cable tray, etc.) as the live cables of the related circuit. This arrangement ensures the minimum possible inductive reactance in the earth-fault current carrying circuits.

It should be noted that this arrangement is provided by construction for busways (busbar trunking systems).


PE conductors must:

  • Not include any means of breaking the continuity of the circuit (such as a switch, removable links, etc.)
  • Connect exposed conductive parts individually to the main PE conductor, i.e. in parallel, not in series, as shown in Figure G55
  • Have an individual terminal on common earthing bars in distribution boards.
Fig. G55 – A poor connection in a series arrangement will leave all downstream appliances unprotected

TT scheme

The PE conductor need not necessarily be installed in close proximity to the live conductors of the corresponding circuit, since high values of earth-fault current are not needed to operate the RCD-type of protection used in TT installations.

IT and TN schemes

The PE or PEN conductor, as previously noted, must be installed as close as possible to the corresponding live conductors of the circuit and no ferro-magnetic material must be interposed between them. A PEN conductor must always be connected directly to the earth terminal of an appliance, with a looped connection from the earth terminal to the neutral terminal of the appliance (see Fig. G56).

  • TN-C scheme (the neutral and PE conductor are one and the same, referred to as a PEN conductor)
The protective function of a PEN conductor has priority, so that all rules governing PE conductors apply strictly to PEN conductors
  • TN-C to TN-S transition
The PE conductor for the installation is connected to the PEN terminal or bar (see Figure G56) generally at the origin of the installation. Downstream of the point of separation, no PE conductor can be connected to the neutral conductor.
Fig. G56 – Direct connection of the PEN conductor to the earth terminal of an appliance
Fig. G57 – The TN-C-S scheme

Types of materials

Materials of the kinds mentioned below in Figure G58 can be used for PE conductors, provided that the conditions mentioned in the last column are satisfied.

Fig. G58 – Choice of protective conductors (PE)
Type of protective earthing conductor(PE) IT scheme TN scheme TT scheme Conditions to be respected
Supplementary conductor In the same cable as the phases, or in the same cable run Strongly recommended Strongly recommended Correct The PE conductor must be insulated to the same level as the phases
Independent of the phase conductors Possible[a] Possible[a][b] Correct
  • The PE conductor may be bare or insulated[b]
  • The electrical continuity must be assured by protection against deterioration by mechanical, chemical and electrochemical hazards
  • Their conductance must be adequate
Metallic housing of bus-trunking or of other prefabricated prewired ducting[c] Possible[d] PE possible[d]
PEN possible[e]
External sheath of extruded, mineral- insulated conductors (e.g. «pyrotenax» type systems) Possible[d] PE possible[d]
PEN not recommended[b][d]
Certain extraneous conductive elements[f] such as:
  • Steel building structure
  • Machine frames
  • Water pipes[g]
Possible[h] PE possible[h]

PEN forbidden

Metallic cable ways, such as, conduits[i]ducts, trunking, trays, ladders, and so on… Possible[h] PE possible[h]
PEN not recommended[b][h]
Forbidden for use as PE conductors, are: metal conduits[i], gas pipes, hot-water pipes, cable-armouring tapes[i]or wires[i]
  1. ^ 1 2 In TN and IT schemes, fault clearance is generally achieved by overcurrent devices (fuses or circuit-breakers) so that the impedance of the fault-current loop must be sufficiently low to assure positive protective device operation. The surest means of achieving a low loop impedance is to use a supplementary core in the same cable as the circuit conductors (or taking the same route as the circuit conductors). This solution minimizes the inductive reactance and therefore the impedance of the loop.
  2. ^ 1 2 3 4 The PEN conductor is a neutral conductor that is also used as a protective earth conductor. This means that a current may be flowing through it at any time (in the absence of an earth fault). For this reason an insulated conductor is recommended for PEN operation.
  3. ^ It must allow the connection of other PE conductors. Note: these elements must carry an indivual green/yellow striped visual indication, 15 to 100 mm long (or the letters PE at less than 15 cm from each extremity).
  4. ^ 1 2 3 4 5 The manufacturer provides the necessary values of R and X components of the impedances (phase/PE, phase/PEN) to include in the calculation of the earth-fault loop impedance.
  5. ^ In the prefabricated pre-wired trunking and similar elements, the metallic housing may be used as a PEN conductor, in parallel with the corresponding bar, or other PE conductor in the housing.
  6. ^ These elements must be demountable only if other means have been provided to ensure uninterrupted continuity of protection.
  7. ^ With the agreement of the appropriate water authorities.
  8. ^ 1 2 3 4 5 Possible, but not recomended, since the impedance of the earth-fault loop cannot be known at the design stage. Measurements on the completed installation are the only practical means of assuring adequate protection for persons.
  9. ^ 1 2 3 4 Forbidden in some countries only. Universally allowed to be used for supplementary equipotential conductors.