1 Scope
This standard specifies types and safety technical requirements of system earthing; the purpose is to guarantee the safety of people and equipment.
This standard is applicable to power grid of which the nominal system voltage is AC 220/380V.
2 Normative References
The provisions contains in the following normative documents which, through reference in this text, shall constitute provisions of this Standard. For dated reference, subsequent amendments (excluding correction) to, or revisions of, any of these publications do not apply. However, parties that enter into an agreement according to this standard are encouraged to study whether the latest edition of these documents can be used. For undated references, the latest edition of the referenced document is applicable to these specifications.
GB/T 4776-2008 Electrical Safety Terminology
GB 13955 Installation and Operation of Residual current operated protectors
GB 16895.3-2004 Electrical Installations of Buildings-Part 5-54: Selection and Erection of Electrical Equipment-Earthing Arrangements, Protective Conductors and Protective Bonding Conductors (IEC 60364-5-54: 2002, IDT)
3 Terms and Definitions
For the purposes of this standard, the following terms and definitions apply.
3.1
Electrical installation
The combination of associated electrical equipment in order to reach one or several specific purposes.
3.2
Exposed conductive part
Easily-touched conductive part and part which is non-live but may be electriferous in the situation of faults.
[GB/T 4776-2008, definition 3.1.10]
3.3
Extraneous conductive part
Conductive part which is not a part of the electrical installation but liable to be led into the potential (generally earth potential).
[GB/T 4776-2008, definition 3.1.11]
3.4
Neutral conductor (symbol N)
Conductor which is connected to neutral point of the system and capable of providing transmitable electric energy.
[GB/T 4776-2008, definition 3.3.3.11]
3.5
Protective conductor (symbol PE)
Conductor that adopts protective measures used for avoiding electric shock in the situation of faults.
[GB/T 4776-2008, definition 3.3.3.10]
This standard refers to conductor electrically connected to any of the following parts:
——Exposed conductive parts;
——Extraneous conductive part;
——Main earthing terminal or main earthing conductor;
——Earthing poles;
——Power supply earthing point or artificial neutral point.
3.6
PEN conductor
Conductor which simultaneously has the function of protective earthing conductor and neutral conductor.
[GB/T 4776-2008, definition 3.3.3.13]
Note: acronym PEN is a combination of protective conductor symbol PE and neutral conductor symbol N.
3.7
Earthing conductor
Conductor used for electrical connection between equipment, device or system set point and earthing pole and in possession of low impedance.
[GB/T 4776-2008, definition 3.3.3.9]
3.8
Touch voltage
Accidentally occurred voltage when human body simultaneously touches two points.
[GB/T 4776-2008, definition 3.2.4]
Note 1: According to the custom, this term is only used in the aspect related to protection against indirect contact.
Note 2: In some cases, the touch voltage value might be obviously affected by the impedance of human who touches these parts.
3.9
Prospect live touch voltage
The possible maximum touch voltage where faults with negligible impedance occurs in electrical installations.
3.10
System earthing
Earthing of a certain point (generally, neutral point) at side of system power supply.
3.11
Protective earthing
Arranged one point or multipoint earthing on equipment, device or system out of safety purpose.
[GB/T 4776-2008, definition 3.3.2.3]
3.12
Main equipotential bonding
MEB
Electrical connection which makes the potentials for conductive part of exposed conductor and conductive part outside electrical installation basically equal.
3.13
Supplementary equipotential bonding
SEB
Connect the conductive part between two objects directly with conductor to make their potentials approximately equal.
4 System Earthing Types
The type of system earthing takes Latin alphabet as the code number; the meaning is:
The first alphabet shows the relationship of the power end and earth.
T——the power end has a point earthing directly;
I——none of the live parts of the power end are earthed or a point is earthed through impedance.
The second letter shows the relation between the exposed conductive part of the electrical installation and earth;
T——where the exposed conductive part of electrical installation is directly earthed, this earthing point in independent from earthing point of power end in electrical;
N——the exposed conductive part of electrical installation has direct electrical connection with the earthing point of power end.
The alphabet following strigula (-) is used to express the combination situation of neutral conductor and protective conductor:
S——neutral conductor and protective conductor are separate;
C——neutral conductor and protective conductor are syncretic.
System earthing has the following kinds of types.
4.1 TN system
A point of the power end is directly earthed; the exposed conductive part of electrical installation is connected to this earthing point through protective neutral conductor or protective conductor.
According to the combination situation of neutral conductor and protective conductor, the type of TN system has the following three kinds:
a) TN-S system: neutral conductor and protective conductor of the overall system are separate (See Figure 1).
