SECTION 5 PROTECTION OF PERSONS AND EQUIPMENT WITHIN BUILDINGS
5.5 PROTECTION OF PERSONS WITHIN BUILDINGS
The principal objective of measures for the protection of persons within buildings is to prevent hazardous potential differences between conductive parts with which the person(s) may be in contact. This is normally achieved by applying equipotential bonding between any conductive path into and out of the building, i.e. the entry points and exit points referred to in Clause 5.4.
If such bonding has been installed it does not matter if a person is subject to an EPR with respect to distant earth as all conductive materials in the vicinity will be at approximately the same potential.
An important consideration in the installation of equipotential bonding is how to install such bonding without adversely affecting the operation of the various services involved, particularly the protection systems associated with the respective systems. This is explained further in Clause 5.5.2.
AS 4262.1 provides further information on the protection of users of telecommunications equipment from overvoltages that may exist between a user environment and
5.5.2 Installation of equipotential bonding
In general, if made of copper, bonding conductors shall have a cross-sectional area as given below, unless they are main current-carrying conductors of the LPS. The typical dimensions of the main current-carrying conductors of a LPS are given in Table 4.6.
All possible entry and exit points for the lightning discharge should be electrically bonded together in as direct a manner as practicable. The route taken by the bonding conductors is important. If incorrectly routed the bonding conductors themselves may damage other circuits or equipment by induction or side-flashing as currents of the order of tens of kiloamperes and voltages of the order of several thousand volts with respect to remote earth may be involved. Consequently, bonding conductors should not be grouped with other cables that are sensitive to induction unless the other cables are also bonded to the LPS. If the bonding conductor is long (some tens of metres) it shall be considered as an impulse transmission line, in which mode the protection afforded by the bonding will be limited.
Some specific recommendations applicable to bonding of the entry and exit points referred to in Clause 5.4 are given below:
NOTE: Appendix E provides additional information on earthing and bonding.
(a) Rooftop antennae and communications hardware The bonding conductor should be attached to the most substantial part of the structural metal supporting the equipment consistent with it fulfilling the requirements of an air terminal for the LPS of a building. The bonding conductor to the antenna or communications hardware should be insulated to at least the level required in AS/NZS 3191, if run within the building, but may be uninsulated if run externally. The cross-sectional area of the bonding conductor should be not less than 16 mm2 if made of copper.
(b) The electricity supply service entry There are two distinct considerations that apply.
Firstly, the electrical installation earth should be bonded to the LPS earth termination network with a copper conductor of not less than 6 mm2 cross-sectional area.
Secondly, SPDs should be installed for each active conductor of the electricity supply service. Where an SPD is mounted on, or in, the building, its earthing system should be bonded to the LPS by a conductor having a cross-sectional area of not less than that utilized for its own earthing conductor. Where SPD equipment is separated from the building (e.g. mounted on a customer’s electricity supply service pole), the SPD earth should not be used as the earthing termination for the building LPS, however, the LPS earth termination network and the SPD earth may be bonded together, if desired.
(c) The telecommunications service entry This may be either aerial (overhead) or underground. If aerial, the service should be regarded as a potential entry point for lightning and an SPD should be fitted, subject to the requirements of the telecommunications regulatory authority*. The telecommunications service earthing system shall be bonded to the LPS earth termination network.
If the telecommunications service is underground, the service will act essentially as an exit point for lightning. In this mode it may be necessary to fit an SPD to the service to provide a bonding point for potential equalization.
The bonding conductor should have a cross-sectional area of not less than 6 mm2 if made of copper.
* In Australia, these requirements are set out in AS/ACIF S009, Installation requirements for customer cabling.
(d) Metallic water supply and sewerage systems Metallic water supply and drainage systems should be bonded to the LPS and connected to the electricity supply service earth. However, some water supply authorities fit insulating spacers or ferrules for galvanic corrosion control at customers’ installations. These may require bridging by an SPD as determined in consultation with the water supply authority. Bonding conductors to these services should have a cross-sectional area of not less than 4 mm2 if made of copper. Metallic piping systems associated with fire sprinklers, water, hot water or flammable liquid, that are unavoidably in contact with the exposed conductive parts of wiring enclosures, cable components or other electrical equipment shall be connected to such equipment by means of an equipotential bonding conductor.
If calculation or local experience indicates that the water supply system is of very low resistance to earth (e.g. less than 0.5 Ω), it may well be the principal exit point for the lightning impulse. In such circumstances, consideration should be given to upgrading the current-carrying capacity of the bonding conductor between the LPS earth and the water supply system to a cross-sectional area of not less than 35 mm2 if made of copper.
(e) Other service lines Specific considerations may apply for some structures. For example, a radio telephone tower should be bonded to its associated equipment building; similarly, a pump station should be bonded to an elevated water tower. For both examples given, the bonding conductor is likely to carry the full lightning current and should therefore have a cross-sectional area of not less than 35 mm2 if made of copper.
(f) Building earthing systems Buildings frequently have several earthing systems that may be installed independently at different times. These include the electricity supply service earthing system, the telecommunications earthing system (sometimes more than one), the LPS earth termination network and other special purpose earthing systems.
It is generally desirable to bond all such earthing systems but there may be specific reasons for not doing so. Direct-current-carrying earths, e.g. older telex systems, should usually be isolated to prevent corrosion damage to other services and earths.
In such cases consideration should be given to bonding these earths through a galvanic isolator, to facilitate the protection of persons from lightning surges. This type of isolator can be used where there is a corrosion-based objection to bonding, e.g. copper-based earths to galvanized iron earths or structures, of which the latter would suffer galvanic corrosion. If 50 Hz or audio frequency bonding is not needed, a gas discharge arrestor may serve the purpose. Bonding conductors between earthing systems should have a cross-sectional area of not less than 4 mm2 if made of copper.
(g) The LPS earth termination network Where an LPS is in place all of the services described in Items (a) to (f) should be bonded to the LPS earth termination network.
Caution should be applied when bonding the LPS earth termination network to cathodically-protected earthing systems, such as cathodically-protected concrete reinforcing of fuel tanks. Bonding to such systems will require galvanic isolation as in (f) above.
(h) Bonding of conductive parts Where a reinforced concrete pad, reinforced concrete walls or metallic building components, e.g. a metal roof, are used as part of an LPS, these should be interconnected with each other and bonded to the LPS. This bonding should be performed regardless of whether the building is in a high risk lightning area.
5.6 PROTECTION OF EQUIPMENT