Regarding BS 7430

BS 7430:2011 (Code of practice for protective earthing of electrical
installations)
Foreword
Publishing information
This British Standard is published by BSI and came into effect on
31 December 2011. It was prepared by Technical Committee GEL/600,
Earthing.
A list of organizations represented on this committee can be obtained on
request to its secretary.
Supersession
This British Standard supersedes BS 7430:1998, which is withdrawn.
Relationship with other publications
This revision of BS 7430 takes into account changes in legislation
(implementation of the Electricity Safety, Quality and Continuity Regulations
2002 as amended [1]) and changes in and publication of other relevant
industry
standards such as BS 7671:2008+A1, BS EN IEC 61936-1:2011 and
BS EN 50522:2011.
Presentational conventions
The provisions in this standard are presented in roman (i.e. upright) type. Its
recommendations are expressed in sentences in which the principal auxiliary
verb is should.
Commentary, explanation and general informative material is presented in
smaller italic type, and does not constitute a normative element.
The word should is used to express recommendations of this standard. The
word may is used in the text to express permissibility, e.g. as an alternative
to
the primary recommendation of the clause. The word can is used to express
possibility, e.g. a consequence of an action or an event.
Notes and commentaries are provided throughout the text of this standard.
Notes give references and additional information that are important but do
not
form part of the recommendations. Commentaries give background
information.
1 Scope
This British Standard primarily provides recommendations and guidance on
meeting the requirements for the earthing of electrical installations, including:
a) protective earthing of low voltage installations to BS 7671:2008+A1;
b) the interface between LV and HV substations of 11 000/400 V to
BS EN IEC 61936-1:2011 within buildings; and
c) protective earthing and changeover switch arrangements for generators
supplying low voltage installations.
The earthing of a system or installation is generally provided for reasons of
safety.
This British Standard applies only to land-based installations in and around
buildings. It does not apply to:

1) ships, aircraft or offshore installations;
2) earthing of medical equipment [see BS EN 60601 (all parts)];
3) special problems encountered with solid state electronic components;
4) equipment sensitive to static electricity;
5) requirements for functional earthing;
6) earthing of overhead lines between electrical installations; or
7) the internal earthing of equipment.
4 Earthing principles
4.1 Fundamental rule of protection against electric shock
BS EN 61140 gives fundamental principles and requirements which are
common
to electrical installations, systems and equipment or necessary for their
co-ordination, for installations, systems and equipment without a voltage
limit.
BS EN 61140 states that the fundamental rule of protection against electric
shock is that:
hazardous-live-parts shall not be accessible and accessible conductive parts
shall not be hazardous live:
either under normal conditions (operation in intended use, see ISO/IEC
Guide 51, 3.13 and absence of a fault); or
BRITISH STANDARD BS 7430:2011
BSI 2011 5
Licensed Copy: Surrey M5642 ATHENS, University of Surrey, 03/01/2012
21:49, Uncontrolled Copy, (c) The British Standards Institution 2012
under single-fault conditions (see also IEC Guide 104, 2.8).
The system and equipment standards are being amended to comply with
BS EN 61140 so that the fundamental rule is met and to adopt the same
terminology.
In order to achieve the principles within BS EN 61140 within electrical
installations standards, BS 7671:2008+A1 makes certain requirements for the
earthing of an installation and its sub-structure in order to achieve safety.
With
the exception of earth free locations, the main requirement under single fault
conditions is for automatic disconnection of supply with selecting cable
impedances in order to achieve certain touch voltages.
4.2.2 High voltage (HV) power supply systems

NOTE Requirements for earthing HV installations are given in
BS EN IEC 61936-1:2011 and BS EN 50522:2011. See also Clause 5.
4.3 Installation earthing Low-voltage (LV) installation earthing
The purpose of earthing the non-current carrying metalwork of plant and

equipment of a low-voltage installation is to:
a) provide for a low earth fault loop impedance to facilitate automatic
disconnection of supply in the event of a fault to exposed-conductive-parts;
and
b) limit the rise of potential under earth-fault conditions of non-current
carrying metalwork with respect to earth to increase the safety of persons
and animals in proximity to such metalwork.
Extending the earthing arrangement of an installation within a building by use
of protective equipotential bonding of exposed-conductive-parts and
extraneous-conductive-parts increases the safety of persons and animals by
reducing touch voltages between such parts and it can also eliminate
dangerous
sparking.
The performance requirements for earthing of LV installations
(e.g. disconnection times, conductor sizes) are specified in BS 7671:2008+A1.
4.4 Current-using equipment

The purpose of earthing current-using equipment is similar to that for
earthing
electrical installations. However, for certain standards, e.g. those in the
BS EN 60335 series, automatic disconnection of supply is not required in the
event of a fault to accessible conductive parts (see BS EN 60335-1:2002+A14,
Clause 27).
27 Provision for earthing
27.1 Accessible metal parts of class 0I appliances and class I appliances that
may become live in the event of an insulation fault, shall be permanently and
reliably connected to an earthing terminal within the appliance or to the
earthing contact of the appliance inlet. Earthing terminals and earthing
contacts shall not be connected to the neutral terminal.
5 HV/LV interface
Specific requirements that should be followed for high voltage installations
including substations are found in BS EN IEC 61936-1:2011 and
BS EN 50522:2011.Requirements that should be followed for the protection of
LV installations against temporary overvoltages due to earth faults in an HV
supply system are given in BS 7671:2008+A1, Section 442.
NOTE Further guidance on the interface between LV and HV subsystems,
including examples of earthing layouts, is given in Annex A.
6 Low voltage installations

NOTE The earthing requirements for low-voltage installations are specified in
BS 7671:2008+A1. In BS 7671:2008+A1, Chapter 44, requirements for
protection of low voltage installations against temporary overvoltages due to

earth faults in the high voltage system and due to faults in the low voltage
system are detailed. This is considered in Clause 5.
6.1 Automatic disconnection

COMMENTARY ON 6.1
When an earth fault occurs within an installation, dangerous voltages can
appear
between conductive parts, and if these parts are simultaneously accessible
they
might constitute a shock hazard. The severity of any shock experienced by a
person
at risk depends on, among other factors, the type of current (i.e. a.c. or d.c.),
the
magnitude of the voltages and the amount of time which the earth fault is
allowed
to persist.
The earth fault current can also be of such magnitude and duration as to cause
an
excessive temperature rise in the conductors through which it flows, thereby
creating a fire hazard.
BS 7671:2008+A1 includes a number of protective measures which can be
applied for
protection against earth faults. However, this standard only considers the
protective
measure known as automatic disconnection of supply. This is the most
commonly
utilised of the protective measures contained in BS 7671:2008+A1.
Furthermore, the
others do not depend on earthing and have very limited applications.
<<NP>>ADS is a protective measure in which:
a) basic protection is provided by basic insulation of live parts or by barriers
or
enclosures; and
b) fault protection is provided by protective earthing, protective equipotential
bonding and automatic disconnection in case of a fault.
A brief summary of the requirements of BS 7671:2008+A1 for automatic
disconnection of supply is given in 6.2 to 6.11.
Where automatic disconnection of supply is applied, Class I and Class II
equipment may be used.
In designing the protective system of any installation, due account should be
taken of the need to ensure that periodic inspection, testing and maintenance
can be readily and safely undertaken

Regarding BS 7430

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Regarding BS 7430

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BS 7430:2011 (Code of practice for protective earthing of electrical

buildings. It does not apply to:

4.2.2 High voltage (HV) power supply systems

4.3 Installation earthing Low-voltage (LV) installation earthing

The purpose of earthing the non-current carrying metalwork of plant and

4.4 Current-using equipment

6 Low voltage installations

protection of low voltage installations against temporary overvoltages due to

6.1 Automatic disconnection

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