Breaking of the neutral conductor

If the c.s.a. of the neutral conductor is equal or greater than the c.s.a. of the phase conductor, no specific protection of the neutral conductor is required because it is protected by the phase protection.

Fig. 4.19. The various situations in which the neutral conductor may appear

4.2.3.4. Breaking of the neutral conductor (see Fig. 4.19.)

The need to break or not the neutral conductor is related to the protection against indirect contact

In TN-C scheme

The neutral conductor must not be open-circuited under any circumstances since it constitutes a PE as well as a neutral conductor.

Romanian Electro Trade, Engineering & Consulting 35 In TT, TN-S and IT schemes

In the event of a fault, the circuit breaker will open all poles, including the neutral pole, i.e. the circuit breaker is omnipolar.

The action can only be achieved with fuses in an indirect way, in which the operation of one or more fuses provokes a mechanical trip-out of all poles of an associated series-connected load-break switch.

4.2.3.5. Isolation of the neutral conductor (see Fig. 4.19.)

It is considered to be the good practice that every circuit be provided with the means for its isolation.

4.2.4. Sizing the protective earthing conductor (PE)

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. P.E.

conductors are connected to the main earthing terminal of the installation.

PE conductors must be:

§ insulated and coloured yellow and green (stripes),

§ be protected against mechanical and chemical damage.

4.2.4.1. 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,

§ have an individual terminal on common earthing bars in distribution boards.

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.

Fig. 4.20. Direct connection of the PEN conductor to the earth terminal of an appliance

Romanian Electro Trade, Engineering & Consulting 36 IT and TN schemes

The PE or PEN conductor, as previously noted, must be installed as close by 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. 4.20.).

§ 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 installlation is connected to the PEN terminal or bar (see Fig. 4.21.) generally at the origin of the installation. Downstream of the point of separation, no PE conductor can be connected to the neutral conductor.

Fig. 4.21. The TN-C-S scheme

4.2.4.2. Types of materials

Materials of the kinds mentioned below in figure 4.22. can be used for PE conductors, provided that the conditions mentioned in the last column are satisfied.

Fig. 4.22. Choice of protective conductors (PE)

(1) In schemes TN and IT, fault clearance is generally effected 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 stratagem 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.

Romanian Electro Trade, Engineering & Consulting 37

(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.

4.2.4.3. Conductor sizing

§ Adiabatic method(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.

For any size of the phase conductor:

The c.s.a. of earthing conductor between the installation earth electrode and the main earth terminal:

§ when protected against mechanical damage:

k t S_PE = I⋅

§ without mechanical protection, but protected against corrosion by impermeable cable sheath. Minimum 16 mm² for copper or galvanized steel.

§ without either of the above protections; min. of 25 mm² for bare copper and 50 mm² for bare galvanized steel.

(1) 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.

§ Simplified method

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.

Romanian Electro Trade, Engineering & Consulting 38

ph PE

ph mm S S

S ≤16 ² ⇒ =

2 2

2 35 16

16mm <S_ph ≤ mm ⇒S_PE = mm

35 ² _PE 2^ph

ph

S S mm

S > ⇒ =

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 cable size selected.

Furthermore, protection of the neutral conductor must be assured by the protective devices provided for phase-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 4.23. are those most commonly needed for LV installation design.

Fig. 4.23. k factor values for LV PE conductors, commonly used in national standards and complying with IEC 60724

Romanian Electro Trade, Engineering & Consulting 39 4.2.5. Calculation of Lmax. for a TN-earthed system, using the conventional method

The maximum length of a circuit in a TN-earthed installation is given by the formula:

a

Fig. 4.24. Calculation of L max. for a TN-earthed system, using the conventional method sufficient rapidity to ensure safety against indirect contact.

Correction factor m

Figure 4.25. indicates the correction factor to apply to the values given in figures 4.26., according to the ratio Sph/SPE, the type of circuit, and the conductor materials.

The tables take into account:

Romanian Electro Trade, Engineering & Consulting 40

§ the type of protection: circuit breakers or fuses,

§ operating-current settings,

§ cross-sectional area of phase conductors and protective conductors,

§ type of system earthing,

§ type of circuit breaker (i.e. B, C or D).

Equivalent tables for protection by Compact and Multi 9 circuit breakers (Merlin Gerin) are included in the relevant catalogues.

Fig. 4.25. Correction factor to apply to the lengths given for TN systems(may be used for 230/400 V systems)

Fig. 4.26. Maximum circuit lengths (in metres) for different sizes of copper conductor and instantaneous-tripping-current settings for general-purpose circuit breakers

in 230/240 V TN system with m = 1

4.2.6. Rules for marine electrical cables according Bureau Veritas General

1. All electrical cables and wiring external to equipment shall be at least of a flame-retardant type, in accordance with IEC Publication 60332-1.

2. When cables are laid in bunches, cable types are to be chosen in appliance with IEC Publication 60332-3 Category A, or over means are to be provided such as not to impair their original flame-retarding properties.

3. Where necessary for specific applications such as radio frequency or digital communications systems, which require the use of particular types of cables, the Society may permit the use of cables with do not comply with the provisions of 1 and 2.

Romanian Electro Trade, Engineering & Consulting 41 4. Cables which are required to have fire-resisting characteristics are to comply with the requirements stipulated in IEC Publications 60331.

Choice of insulation

1. The maximum rated operating temperature of the insulating material is to be at least 10⁰C higher than the maximum ambient temperature liable to occur or to be produced in the space where the cable is installed.

2. The maximum rated conductor temperature for normal and short-circuit operation, for the type of insulating compounds normally used for shipboard cables, is not to exceed the values stated in Tab 1. Special consideration will be given to other insulating materials.

3. PVC insulated cables are not to be used either in refrigerated spaces, or on decks exposed to the weather of ships classed for unrestricted service.

Table 1: Maximum rated conductor temperature

Maximum rated conductor

- based upon cross-linked polyethylene halogen free - based upon rubber silicon halogen free

- based upon cross-linked polyolefin material for halogen free cable (1)

(1) Used on sheathed cable only

Choice of protective covering

1. The conductor insulating materials are to be enclosed in an impervious sheath of material appropriate to the expected ambient conditions where cables are installed in the following locations:

- on decks exposed to the weather,

- in damp or wet spaces (e.g. in bathrooms), - in refrigerated spaces,

- in machinery spaces and, in general,

- where condensation water or harmful vapour may be present.

Romanian Electro Trade, Engineering & Consulting 42 2. Where cables are provided with armour or metallic braid (e.g. for cables installed in hazardous areas), an overall impervious sheath means to protect the metallic elements against corrosion is to be provided.

3. An impervious sheath is not required for single-core cables installed in tubes or ducts inside accommodation spaces, in circuits with maximum system voltage 250 V.

4. In choosing different types of protective coverings, due considerations is to be given to the mechanical action to which cable may be subjected during installation and in service.

If the mechanical strength of the protective covering is considered insufficient, the cables are to be mechanically protected (e.g. by an armour or by installation inside pipes or conduits).

5. Single-core cables for a.c. circuits with rated current exceeding 20 A are to be either non-armoured or armoured with non-magnetic material.

Cables in refrigerated spaces

1. Cables installed in refrigerated spaces are to have a watertight or impervious sheat and are to be protected against mechanical damage. If an armour is applied on the sheath, the armour is to be protected against corrosion by a further moisture-resisting covering.

Cables in circuits fore fire alarm, fire detection and fire-extinguishing

1. In general, in circuits intended for fire alarm and detection, emergency fire- extinguishing service, fire telecommunication (e.g. communication between the navigating bridge and the main fire control station), remote stopping and similar control circuits for safety purposes, cables are to be of a fire-resistant type unless:

- the systems are of self-monitoring type or failing to safety, - the systems are duplicated.

2. Cables for services that are required to maintain operation of equipment during a fire (e.g. cables for the general emergency alarm, the public address system when it is the only system to provide the general emergency alarm, the fire- extinguishing medium alarm and their power supplies) are to be of a fire-resistant type.

3. Cables connecting fire pumps to the emergency switchboard shall be of fire-resistant type where they pass through fire risk areas.

Cables fore submerged bilge pumps

1. Cables and their connections to such pumps are to be capable of operating under a head of water equal to their distance below the bulkhead deck. The cable is to be impervious-sheathed and armoured, is to be installed in continuous lengths from above the bulkhead to the motor terminals and is to enter the air bell from the bottom.

Internal wiring of switchboard and other enclosures for equipment

1. For installations in switchboards and other enclosures for equipment, single-core cables may be used without further protection (sheath).

Other types of flame-retardant switchboards wiring may be accepted.

Romanian Electro Trade, Engineering & Consulting 43 Current carrying capacity of cables

1. The current carrying capacity for continuous service of cables given in Tab 2 to Tab 6 is based on the maximum permissible service temperature of the conductor also indicated therein and on an ambient temperature of 45⁰C.

Table 2: Current carrying capacity, in A, in continuous service for cables based on maximum conductor operating temperature of

Table 3: Current carrying capacity, in A, in continuous service for cables based on maximum conductor operating temperature of

Table 4: Current carrying capacity, in A, in continuous service for cables based on maximum conductor operating temperature of

Table 5: Current carrying capacity, in A, in continuous service for cables based on maximum conductor operating temperature of

85⁰C (ambient temperature 45⁰C)

Romanian Electro Trade, Engineering & Consulting 44 2. The current carrying capacity is applicable, with rough approximation, to all types of protective covering (e.g. both armoured and non-armoured cables).

3. Values other then those shown in Tab 2 to Tab 6 may be accepted provided they are determined on the basis of calculation methods or experimental values approved by the Society.

4. When the actual ambient temperature obviously differs from 45⁰C, the correction factors shown in Tab 7 may be applied to the current carrying capacity in Tab 2 to Tab 6.

5. Where more than six cables are bunched together in such a way that is an absence of free air circulating around them, and the cables can be expected to be under full load simultaneously, a correction factor of 0.85 is to be applied.

7. For supply cables to single services for intermittent loads (e.g. cargo winches or machinery space cranes), the current carrying capacity obtained from Tab 2 to Tab 6 may be increased by applying the correction factors given in Tab 9.

The correction factors are calculated with rough approximation for periods of 10 minutes, of witch 4 minutes with a constant load and 6 minutes without load.

Minimum nominal cross-sectional area of conductors

1. In general the minimum allowable conductor cross-sectional areas are those given in tables above.

2. The nominal cross-sectional area of the neutral conductor in three-phase distribution systems is to be equal to at least 50% of the cross-sectional areas of the phases, unless the latter is less than or equal to 16 mm². In such case the cross-sectional of the neutral conductor is to be equal to that of the phase.

Table 6: Current carrying capacity, in A, in continuous service for cables based on maximum conductor operating temperature of

95⁰C (ambient temperature 45⁰C)

Romanian Electro Trade, Engineering & Consulting 45

Table7: Correction factors for various ambient air temperatures Correction factors for ambient air temperature of:

Maximum following conditions with reference to the maximum anticipated ambient temperature:

§ the current carrying capacity is to be not less than the highest continuous load carried by the cable,

§ the voltage drop in the circuit, by full load on this circuit, is not to exceed the specified limits,

§ the cross-sectional area calculated on the basis of the above is to be such that the temperature increases which may be caused by overcurrents or starting transients do not damage the insulation.

Table 8: Corrections factors for short-time loads

½ - hour service 1 – hour service

Sum of nominal cross-sectional areas of all conductors in mm²

Romanian Electro Trade, Engineering & Consulting 46 4. When conductors are carrying the maximum nominal service current, the voltage drop from the main or emergency switchboard busbars to any point in the installation is not to exceed 6% of the nominal voltage.

For battery circuits with supply voltage less than 55 V, this value may be increased to 10%.

For circuits of navigation lights, the voltage drop is not to exceed 5% of the rated voltage under normal conditions.

Table 9: Correction factors for intermittent service Sum of nominal cross-sectional areas of all

conductors in mm² current there is a fall in voltage between the origin of the circuit and the load terminals.

The correct operation of an item of load (a motor; lighting circuit; etc.) depends on the voltage at its terminals being maintained at a value close to its rated value. It is necessary therefore to dimension the circuit conductors such, that at full load current, the load terminal voltage is maintained within the limits required for correct performance.

This section deals with methods of determining voltage drops, in order to check that:

§ they conform to the particular standards and regulations in force;

§ they can be tolerated by the load;

§ they satisfy the essential operational requirements.

4.3.1. Maximum voltage drop limit

Maximum allowable voltage-drop limits vary from one country to another. Typical values for low-voltage installations are given below in figure 4.27.

Fig. 4.27. Maximum voltage-drop between the service-connection point and the point of utilization

Romanian Electro Trade, Engineering & Consulting 47 These voltage-drop limits refer to normal steady-state operating conditions and do not apply at times of motor starting; simultaneous switching (by chance) of several loads, etc.

When voltage drops exceed the values shown in figure 4.27. larger cables (wires) must be used to correct the condition.

Fig. 4.28. Maximum voltage drop

The value of 8%, while permitted, can lead to problems for motor loads; for example:

§ in general, satisfactory motor performance requires a voltage within ± 5% of its

§ in general, satisfactory motor performance requires a voltage within ± 5% of its

In document Designing an Electrical Installation_Beginner Guide (Page 34-151)