Protective earthing conductor

From Electrical Installation Guide

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 originally provided by bus-trunking.

Connection

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 G54
Have an individual terminal on common earthing bars in distribution boards.

Fig. G54: 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. G55).

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 (seeFig. 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. G55: Direct connection of the PEN conductor to the earth terminal of an appliance

Fig. G56: The TN-C-S scheme

Types of materials

Materials of the kinds mentioned below in Figure G57 can be used for PE conductors, provided that the conditions mentioned in the last column are satisfied.

Type of protective earthing conductor(PE)	IT scheme	TN scheme	TT scheme	Conditions to be respected
Supplementary In the same cable as conductor 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⁽¹⁾	Possible⁽¹⁾ ⁽²⁾	Correct	The PE conductor may be bare or insulated ⁽²⁾ 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⁽⁵⁾	Possible⁽³⁾	PE possible ⁽³⁾PEN possible ⁽⁸⁾	Correct
External sheath of extruded, mineral- insulated conductors (e.g. «pyrotenax» type systems)	Possible⁽³⁾	PE possible ⁽³⁾ PEN not recommended ⁽²⁾⁽³⁾	Possible
Certain extraneous conductive elements⁽⁶⁾such as: Steel building structure Machine frames Water pipes⁽⁷⁾	Possible⁽⁴⁾	PE possible⁽⁴⁾PEN forbidden	Possible
Metallic cable ways, such as, conduits⁽⁹⁾, ducts, trunking, trays, ladders, and so on…	Possible⁽⁴⁾	PE possible⁽⁴⁾PEN not recommended ⁽²⁾⁽⁴⁾	Possible
Forbidden for use as PE conductors, are: metal conduits(9), gas pipes, hot-water pipes, cable-armouring tapes⁽⁹⁾ or wires⁽⁹⁾

(1) 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) 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) 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.
(4) 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.
(5) 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).
(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) 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.
(9) Forbidden in some countries only. Universally allowed to be used for supplementary equipotential conductors.

Fig. G57: Choice of protective conductors (PE)

Conductor sizing

Figure G58 below is based on IEC 60364-5-54. This table provides two methods of determining the appropriate c.s.a. for both PE or PEN conductors.

	c.s.a. of phase conductors Sph (mm²)	Minimum c.s.a. of PE conductor (mm²)	Minimum c.s.a. of PEN conductor (mm²)
			Cu AI
Simplified method ⁽¹⁾	S_ph≤ 16	S_ph⁽²⁾	S_ph⁽³⁾	S_ph⁽³⁾
	16 < S_ph≤ 25	16	16
	25 < S_ph≤ 35			25
	35 < S_ph≤ 50	S_ph/2	S_ph/2
	S_ph > 50			S_ph/2
Adiabatic method	Any size	^{(3) (4)}

(1) Data valid if the prospective conductor is of the same material as the line conductor. Otherwise, a correction factor must be applied.
(2) When the PE conductor is separated from the circuit phase conductors, the following minimum values must be respected:

2.5 mm² if the PE is mechanically protected
4 mm² if the PE is not mechanically protected

(3) For mechanical reasons, a PEN conductor, shall have a cross-sectional area not less than 10 mm² in copper or 16 mm² in aluminium.
(4) Refer to table G53 for the application of this formula.

Fig. G58: Minimum cross section area of protective conductors

The two methods are:

Adiabatic (which corresponds with that described in IEC 60724)

This method, while being economical and assuring protection of the conductor against overheating, leads to small c.s.a.’s compared to those of the corresponding circuit phase conductors. The result is sometimes incompatible with the necessity in IT and TN schemes to minimize the impedance of the circuit earth-fault loop, to ensure positive operation by instantaneous overcurrent tripping devices. This method is used in practice, therefore, for TT installations, and for dimensioning an earthing conductor ⁽¹⁾.

Simplified

This method is based on PE conductor sizes being related to those of the corresponding circuit phase conductors, assuming that the same conductor material is used in each case.
Thus, in Figure G58 for:
Sph ≤ 16 mm² SPE = Sph
16 < Sph ≤ 35 mm² SPE = 16 mm²
Sph > 35 mm²

Note: when, in a TT scheme, the installation earth electrode is beyond the zone of influence of the source earthing electrode, the c.s.a. of the PE conductor can be limited to 25 mm² (for copper) or 35 mm² (for aluminium).
The neutral cannot be used as a PEN conductor unless its c.s.a. is equal to or larger than 10 mm² (copper) or 16 mm² (aluminium).
Moreover, a PEN conductor is not allowed in a flexible cable. Since a PEN conductor functions also as a neutral conductor, its c.s.a. cannot, in any case, be less than that necessary for the neutral.
This c.s.a. cannot be less than that of the phase conductors unless:

The kVA rating of single-phase loads is less than 10% of the total kVA load, and
Imax likely to pass through the neutral in normal circumstances, is less than the current permitted for the selected cable size.

Furthermore, protection of the neutral conductor must be assured by the protective devices provided for phase-conductor protection.

(1) Grounding electrode conductor

Values of factor k to be used in the formulae
These values are identical in several national standards, and the temperature rise ranges, together with factor k values and the upper temperature limits for the different classes of insulation, correspond with those published in IEC 60724 (1984).
The data presented in Figure G59 are those most commonly needed for LV installation design.

k values		Nature of insulation
k values		Polyvinylchloride (PVC)	Cross-linked-polyethylene (XLPE) Ethylene-propylene-rubber (EPR)
Final temperature (°C)		160	250
Initial temperature (°C)		30	30
Insulated conductors not incoporated in cables or bare conductors in contact with cable jackets	Copper	143	176
	Aluminium	95	116
	Steel	52	64
Conductors of a multi-core-cable	Copper	115	143
Conductors of a multi-core-cable	Aluminium	76	94

Fig. G59: k factor values for LV PE conductors, commonly used in national standards and complying with IEC 60724

Protective conductor between MV/LV transformer and the main general distribution board (MGDB)

These conductors must be sized according to national practices

All phase and neutral conductors upstream of the main incoming circuit-breaker controlling and protecting the MGDB are protected by devices at the MV side of the transformer. The conductors in question, together with the PE conductor, must be dimensioned accordingly. Dimensioning of the phase and neutral conductors from the transformer is exemplified (for circuit C1 of the system illustrated in Fig. G65).
Recommended conductor sizes for bare and insulated PE conductors from the transformer neutral point, shown in Figure G60, are indicated below in Figure G61.
The kVA rating to consider is the sum of all (if more than one) transformers connected to the MGDB.

Fig. G60: PE conductor to the main earth bar in the MGDB

The table indicates the c.s.a. of the conductors in mm²according to:

The nominal rating of the MV/LV transformer(s) in kVA
The fault-current clearance time by the MV protective devices, in seconds
The kinds of insulation and conductor materials

If the MV protection is by fuses, then use the 0.2 seconds columns.
In IT schemes, if an overvoltage protection device is installed (between the transformer neutral point and earth) the conductors for connection of the device should also be dimensioned in the same way as that described above for PE conductors.

Transformer rating in kVA (230/400 V output)	Conductor material	Bare conductors			PVC-insulated conductors			XLPE-insulated conductors
	Copper t(s)	0.2	0.5	-	0.2	0.5	-	0.2	0.5	-
	Aluminium t(s)	-	0.2	0.5	-	0.2	0.5	-	0.2	0.5
≤ 100	c.s.a. of PE conductors SPE (mm²)	25	25	25	25	25	25	25	25	25
160		25	25	35	25	25	50	25	25	35
200		25	35	50	25	35	50	25	25	50
250		25	35	70	35	50	70	25	35	50
315		35	50	70	35	50	95	35	50	70
400		50	70	95	50	70	95	35	50	95
500		50	70	120	70	95	120	50	70	95
630		70	95	150	70	95	150	70	95	120
800		70	120	150	95	120	185	70	95	150
1,000		95	120	185	95	120	185	70	120	150
1,250		95	150	185	120	150	240	95	120	185

Fig. G61: Recommended c.s.a. of PE conductor between the MV/LV transformer and the MGDB, as a function of transformer ratings and fault-clearance times.

Equipotential conductor

The main equipotential conductor

This conductor must, in general, have a c.s.a. at least equal to half of that of the largest PE conductor, but in no case need exceed 25 mm² (copper) or 35 mm² (aluminium) while its minimum c.s.a. is 6 mm² (copper) or 10 mm² (aluminium).

Supplementary equipotential conductor

This conductor allows an exposed conductive part which is remote from the nearest main equipotential conductor (PE conductor) to be connected to a local protective conductor. Its c.s.a. must be at least half of that of the protective conductor to which it is connected.
If it connects two exposed conductive parts (M1 and M2 in Figure G62) its c.s.a. must be at least equal to that of the smaller of the two PE conductors (for M1 and M2). Equipotential conductors which are not incorporated in a cable, should be protected mechanically by conduits, ducting, etc. wherever possible.
Other important uses for supplementary equipotential conductors concern the reduction of the earth-fault loop impedance, particulary for indirect-contact protection schemes in TN- or IT-earthed installations, and in special locations with increased electrical risk (refer to IEC 60364-4-41).

Fig. G62: Supplementary equipotential conductors

Protective earthing conductor

From Electrical Installation Guide

Contents

Connection and choice

Connection

Types of materials

Conductor sizing

Protective conductor between MV/LV transformer and the main general distribution board (MGDB)

Equipotential conductor

The main equipotential conductor

Supplementary equipotential conductor

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