Earthing

This Technical Measures Document refers to codes and standards applicable to earthing of plant.

Related Technical Measures Documents include:

The relevant Level 2 Criteria are 5.2.1.11(64)f and 5.2.1.13.

General principles

Earthing can classified in two ways:

System earthing;
Equipment earthing.

System earthing is essential to the proper operation of the system, whereas equipment earthing concerns the safety of personnel and plant. A key function of equipment earthing is to provide a controlled method to prevent the build up of static electricity, thus reducing the risk of electrical discharge in potentially hazardous environments. Generally, a resistance to earth of less than 10⁶ Ω.m will ensure safe dissipation of static electricity in all situations.

Flammable liquids transfer

The major hazard involved with the transfer of flammable liquids is the build up of static due to charge separation with potential for discharge resulting in fire and subsequent loss of containment. Certain non-polar liquids can be charged, e.g. while flowing through pipelines. Detectable and hazardous charges must be expected if the specific resistance of the liquid exceeds 10⁸ Ω.m.

The potential for accumulation of static charges may strongly increase if the liquid contains a non-miscible component or a suspended solid. Examples include:

Crystallisation processes in toluene;
Quantities of water in toluene.

With the presence of a second phase, velocities less than 1 m/s should be employed.

Measures that can be employed to reduce these hazards include:

Ensure that the pipe transferring the liquid is completely filled to exclude the formation of explosive mixtures;
Wherever possible ensure no contaminants / solids are present;
Utilise inert gas blanketing;
When transferring flammable liquids by 'blowing across' use an inert gas;
Avoid mechanical mixing or agitation of low conductivity liquids wherever possible;
Use of ball valves with earthed metal spheres;
Employ low transfer velocities. For only partially filled pipes, or pipes which discharge into containers, the velocity is to be limited as follows:

For chargeable esters: maximum 10 m/s;
For mineral oil products (e.g. gasoline, petrol, kerosene, paraffin, jet fuel) and for other chargeable liquids (excluding carbon disulphide and ether):

Nominal pipe diameter, mm	40	50	80	100	200	400	600
Velocity, m/s	7.0	6.0	3.6	3.0	1.8	1.3	1.0
Quantity, l/min	600	800	1100	1600	3500	10000	17000

If these velocities are adhered to, no hazardous charges will be generated within homogenous liquids. But when suspensions of crystals in non-conductive liquids are conveyed, hazardous charges may always be generated, even at velocities below 1 m/s.

For ether and carbon disulphide in pipelines up to a diameter of 25 mm, the maximum velocity should not exceed 1m/s. Larger pipes require lower velocities;
A general rule for all homogeneous liquids (except carbon disulphide and ether) and all pipelines: at velocities below 1 m/s, no dangerous charges will be generated;
Flanges should be earth bonded;
Use sub-surface dip pipes or bottom entry filling when discharging into vessels;
Ensure regular inspection and testing of earth bonding.

Powder transfer

Further information can be found in the Technical Measures Document on Explosion Relief.

Powder transfer can be carried out by several different methods:

Screw conveying;
Vacuum transfer;
Pneumatic conveying;

There are two distinct types of pneumatic conveying used for powder transfer, namely low pressure / dilute phase or high pressure / dense phase. Low pressure / dilute phase systems tend to employ high system velocities ranging from 10 to 25 m/s, whereas high pressure / dense phase systems tend to employ low system velocities ranging from 0.25 to 2.5 m/s.

Intensive charging of the conveyed material and pipeline is possible during pneumatic powder transfer potentially resulting in:

Electrostatic discharge between conductive parts (e.g. between metal flanges and a part of the steel structure of the building);
Entrainment of considerable charges into receiving containers.

Powders can be divided into three groups depending upon the volume resistivity of the material of which the particles are composed. These groups are:

Low resistivity powders, e.g. metals having volume resistivities up to about 10⁶ Ω.m;
Medium resistivity powders, e.g. many organic powders, such as flour, having volume resistivities in the approximate range 10⁶ Ω.m to 10⁹ Ω.m;
High resistivity powders, e.g. certain organic powders, many synthetic polymers and some minerals, such as quartz, having volume resistivities above about 10⁹ Ω.m.

Measures that reduce these hazards include:

Ensure pipelines used for pneumatic conveying are made from metal with good earth bonding. Resistance to ground for all conductive components should be < 10 ohms;
Ground all operators loading powder so that their resistance to ground is < 1 x10⁸ ohms;
Avoid use of insulating coatings on the inner surfaces of metal containers and pipelines;
Use plastic flanges with plastic transfer lines;
Avoid use of coating or sheathing on pipelines constructed of insulating material;
Use antistatic plastic or paper bags in or around flammable gases, vapours or dusts having minimum ignition energies of < 4 mJ;
Discharge powder into the container or silo via intermediate loading equipment, e.g. a cyclone fabricated from conductive material to reduce velocities and earth charge. (Alternatively rotary valves, bag dump hoppers or scroll feeder systems can be employed).

Offloading

Stringent precautions are required to prevent accumulations of static electricity and to give protection against lightning. Standard copper strip (25 mm x 3 mm section or equivalent) is usually employed for the main earthing system. This should be connected to at least one copper-earthing rod that has been tested and shown to have a total resistance to earth of <10 ohms.

The operator should employ a bulk loading and offloading procedure. This should include written instruction that state when offloading flammable liquids, the driver must first connect the tanker to the earthing connection at the off-loading point. The electrically conducting discharge hose can then be connected to the liquid intake point on the storage. The electrical resistance between the two couplings on a flexible hose must not be higher than 10⁶ ohms.

Temporary storage

Before temporary storage is brought on line for storage of flammable liquids or explosible powders, an assessment of earthing provision with associated earth testing should be undertaken. This should encompass the storage vessel and all supporting ancillary equipment.

Flexible pipelines

When flexible hoses are employed, measures that can be adopted include:

Where velocities exceed 1m/s hoses should be made of conductive material or non-conductive material with embedded fine wire mesh. The mesh should be bonded to the metal flanges or coupling of the hose;
If a metal hose with a liner is employed, the metal mantle and flanges or couplings must be bonded to each other;
The electrical resistance between the two couplings must not be higher than 10⁶ ohms. This resistance is to be measured at regular intervals;
Use of ball valves with earthed metal spheres.

Status of guidance

Existing codes of practice provide comprehensive information with respect to best practice for earthing of plant and equipment.

Codes of Practice relating to earthing

HS(G)176 The storage of flammable liquids in tanks, HSE, 1998.
Paragraph 90 provides guidance of the earthing of flexible hoses.
Paragraph 96 requires that all dip rods and tubes should be earthed and where appropriate an earthing lead for connection to a road or rail tanker should be fitted.
Paragraphs 134 to 136 give guidance on earthing requirements for the tank structure.
HS(G)140 Safe use and handling of flammable liquids, HSE, 1996.
Paragraph 33 recommends the earthing of flexible hoses.
Paragraphs 47 to 52 provide guidance on prevention of electrostatic charging and friction sparking by earthing.
LPGA COP 1 Bulk LPG storage at fixed installations. Part 1 : Design, installation and operation of vessels located above ground, LP Gas Association, 1998.
Supersedes HS(G)34 Storage of LPG at fixed installations, HSE, 1987.
Part 1, section 5.2 provides guidance on earthing requirements for LPG plant. It requires that electrical resistance to earth should be less than 1 x 10⁶ ohms and compliance with BS 5958 Parts 1 and 2.
Part 1, section 7.3.3.8 requires that delivery vehicles are earthed by use of a connecting lead to the earthing point of the fixed installation or by provision of a vehicle earth tread plate and/or a connecting lead to the vessel bonding connection.
L16 Design and Construction of vented, non-pressure road tankers used for the carriage of flammable liquids: Road Traffic (Carriage of Dangerous Substances in Road Tankers and Tank Containers) Regulations 1992, HSE, Legislation Series L16.
Paragraph 15 gives guidance on earthing of road tankers.
L17 Design and Construction of vented, non-pressure road tankers used for the carriage of carbon disulphide: Road Traffic (Carriage of Dangerous Substances in Road Tankers and Tank Containers) Regulations 1992, HSE, Legislation Series L17.
Paragraph 12 gives guidance on earthing of road tankers.
L18 Design and Construction of vacuum operated road tankers used for the carriage of hazardous wastes: Road Traffic (Carriage of Dangerous Substances in Road Tankers and Tank Containers) Regulations 1992, HSE, Legislation Series L18.
Paragraph 46 gives guidance on earthing of road tankers.
L19 Design and Construction of vacuum operated road tankers used for the carriage of deeply refrigerated gases: Road Traffic (Carriage of Dangerous Substances in Road Tankers and Tank Containers) Regulations 1992, HSE, Legislation Series L19.
Paragraph 58 gives guidance on earthing of road tankers.
BS 3492 : 1987 Specification for electrically bonded road and rail tanker hose and hose assemblies for petroleum products, including aviation fuels, British Standards Institution.
BS 5345 : Part 1 : 1989 Selection, installation and maintenance of electrical apparatus for use in potentially explosive atmospheres (other than mining applications or explosives processing and manufacture) – Part 1: General recommendations, British Standards Institution.
Has been superseded by BS EN 60079-14 : 1997 and will be withdrawn on 1 December 1999.
BS 5345 : Part 4 : 1977 Selection, installation and maintenance of electrical apparatus for use in potentially explosive atmospheres (other than mining applications or explosives processing and manufacture) – Part 4: Installation and maintenance requirements for electrical apparatus with type of protection 'i'. Intrinsically safe electrical apparatus and systems, British Standards Institution.
Paragraph 16.1 provides guidance on earth connections required for intrinsic safety. It provides information on the requirement for earth connections to preserve the integrity of an intrinsically safe system (e.g. diode safety barrier earth, a transformer screen earth, a barrier relay frame earth) and the methods to achieve it in detail. It also discusses the requirement for mechanical protection in places where the risk of damage is high.
Paragraph 16.2 provides guidance that states the requirements for Other connections to earth, to minimise the risk of invasion of the intrinsically safe system by any currents or voltages from the point of connection to earth.
Has been superseded by BS EN 60079-14 : 1997 and will be withdrawn on 1 December 1999.
BS 5908 : 1990 Code of practice for fire precautions in the chemical and allied industries, British Standards Institution.
Section 10, Paragraph 58.3.1. provides guidance on the requirements for resistance to earth. It recommends a resistance to earth of less than 1 MW to ensure adequate charge dissipation. In order to ease maintenance problems, it is recommended that the resistance to earth should be less than 10 W. It also notes that there may be applications, such as cathodic protection, where such low resistance to earth is undesirable. Regular monitoring checks are recommended. It also discusses the use of antistatic or conductive floors and footwear. Full details are given in BS 2050.
BS 5958 : 1991 Code of practice for the control of undesirable static electricity. Part 1: General considerations, British Standards Institution.
Section 4, Paragraph 13.2.2 recommends that conductors in good contact with earth have a resistance to earth far less than 10⁶ W;
Section 4, Paragraph 13.3.5. provides practical guidance that an appropriate value for the maximum resistance to earth from all parts of fixed metal equipment is 10 W, although a resistance up to 10⁶ W can be accepted, provided that it can be maintained. It also discusses where special earthing connections are required, for example, where equipment is mounted on insulating supports, or if a high resistivity contamination may develop across a joint;
Section 4, Paragraph 13.3.5. recommends the avoidance of high resistivity materials (polymers) for the construction of plant for use in the presence of flammable mixtures. If they are to be used safety measures should be developed for each individual plant to reduce the hazard to an acceptable level;
Section 4, Paragraph 13.4.1 provides guidance on the important features in the design of earthing devices.
BS 5958 : 1991 Code of practice for the control of undesirable static electricity. Part 2 Recommendations for particular industrial situations, British Standards Institution.
Paragraph 20 provides the guidance for the safe operation of metal pneumatic conveying systems, including minimum ignition energies that may result in a hazard.
BS 6651 : 1992 Code of practice for the protection of structures against lightning, British Standards Institution.
BS 7430 : 1998 Code of practice for earthing, British Standards Institution.
BS EN 1127-1 : 1998, Explosive atmospheres – Explosion prevention and protection – Part 1: Basic concepts and methodology, British Standards Institution.
Paragraph 6.4.7. requires bonding of all the conductive parts that could become hazardously charged and earthing them. When non-conductive materials are present, hazardous levels of charging of the non-conductive parts and materials, including solids, liquids and dusts shall be avoided.
FS6023: 1989 Explosible dusts: The elimination of ignition sources, Fire Protection Association, 1989.
Recommendations are provided on the bonding and earthing of items of plant, including replacement of plastics and rubber materials.