Correct earthing practice, from the home to modern industrial and commercial installations, is critical for both safety and correct operation of plant and equipment. James Hunt finds out about the issues and techniques in this article originally published
People are in contact with the earth most of the time. Therefore, if a person touches objects charged at a different voltage from earth, there is a shock hazard. Earthing connects all such objects to the general mass of earth, so providing a path for fault currents and removing potential differences between earth and earthed parts. Moreover, fault currents can flow and operate protective devices.
Designing a safe earthing system means providing the easiest and shortest path for the fault current without exposing a person to electric shock, incurring downtime or exceeding electrical equipment limits. Such a system depends on the duration and magnitude of the fault current and soil resistivity.
Linking the electrical supply system to earth is often carried out at the supply transformer. Here, the neutral conductor is connected to the earth electrode (or buried cable metal sheath).
For decades, earthing issues have mostly been concerned with protection against injury and damage through the effects of electric shock and lightning strike. Today, earthing is becoming still more important for the larger installations, because of modern electronic equipment's sensitivity to electromagnetic disturbances. Therefore, buying CE-marked equipment is not necessarily sufficient to ensure that a whole installation is compliant. Says ERA Technology: 'Most of the time, the equation CE + CE = CE doesn't work because of problems of equipment connection to the installation's bonding network.' Everything from the common bonding network of a building (structural metal parts) right down to details of the bonding of a cable's screen, is important in this context. ERA Technology runs seminars on this and other earthing topics (www.era.co.uk).
Moreover, with power systems becoming ever more complex, system fault levels have also risen, resulting in more careful design of earthing systems. Equipment operating frequency and bandwidth are always increasing so that functional safety has become an important issue. Modern earthing techniques have to cope.
In the case of plant where there may be a risk of explosion / fire hazard, earthing can be divided into system and equipment earthing. The former is essential for proper system operation; the latter concerns both plant and personnel safety. Moreover, it is essential that equipment earthing controls the build up of static electricity, so reducing risk in potentially hazardous environments.
Earthing service pipes in the home:
Traditionally, especially in domestic installations, earthing has been achieved through bonding to the building's metal water supply pipes. However, the use of metal water service pipes for electrical earthing is no longer regarded as the safest method.
Regulation 542-02-04 (BS7671: 1992, Requirements for Electrical Installations) states: 'The metalwork of gas, water, or other service shall not be used as a protective earth electrode. This requirement does not preclude the bonding of such metalwork as required by Regulation 413-02'.
Regulation 413-02-02 states "In each installation, main equipotential bonding conductors (see table) complying with Section 547 shall connect to the main earthing terminal for that installation's extraneous-conductive-parts including the following: water service and gas installation pipes; other service pipes and ducting; central heating and air conditioning systems; exposed metallic structural parts of the building; the lightning protective system'.
Where an installation serves more than one building, the requirement is applied to each. It is also necessary to apply equipotential bonding to metallic sheaths of communication cables.
Regulation 547-02-02 states 'The main equipotential bonding connection to any gas, water or other service shall be made as near as practicable to the point of entry of that service into the premises. Where there is an insulating section / insert at that point, or a meter, the connection shall be made to the consumer's hard metal pipework and before any branch pipework. Where practicable the connection shall be made within 500mm of the meter outlet union or at the point of entry to the building if the meter is external'.
It is clear, therefore, that all metalwork entering a building must be bonded to its main earth terminal. Similar recommendations are illustrated in BS6651: 1999, Protection of structures against lightning and also in BS7430: 1998, Earthing. Moreover, Clause 19.3.9 of BS6651: 1999 and Clause 21.4 of BS7430: 1998. Both advocate bonding of all metallic services entering or leaving the structure to the main earth termination.
In conclusion, using buried services as a building's main earth electrode will not comply with the requirements of BS7671: 1992. However, metallic pipework within a building must be bonded to the main earthing terminal of that building**.
Today, many modern buildings have plastic pipes and these may make for a safer electrical installation. One reason why bathrooms and kitchens can be dangerous is the presence of earthed metal water pipes. If a person touches a live part and - at the same time - an earthed metal pipe, the result could be fatal. Therefore, the contention is that a home with many earthed metal pipes is potentially less safe than an earth-free environment. This is where plastic pipe installations may lead to greater electrical safety.
All earthing conductors should be protected against corrosion and all bonding connections must be accessible and labelled 'Safety Electrical Connection - Do Not Remove'.
Protective Multiple Earthing (PME):
The advantage of PME is that neutral is bonded to earth. Therefore any phase-to-earth fault is a phase-to-neutral fault, giving a low earth-fault loop impedance. The resulting high fault current operates protective devices quickly. The neutral and earth conductors must be separate within the installation. Electrical contractors should note that PME supplies must not be used on construction sites. Moreover, the Electricity Supply Regulations forbid the use of PME supplies to caravan sites.
Lightning strike:
If buildings / installations suffer direct lightning strikes (sometimes equivalent to a very damaging 200kA in 200ms), the earth-seeking current will be dispersed through all available paths to earth - intentional or otherwise. Much of that current will exit via the power distribution system, causing a great deal of damage to installations and associated electronic equipment. There may also be significant risk to human life. Lightning conductors should be bonded to the installation earth using a conductor no larger in cross-sectional area than that of the earthing conductor.
Specialist companies such as Furse (through, for example, one of its distributors such as Keison) can supply a wide range of products and services aimed at reducing or eliminating potential harm to people and equipment through lightning strike or earthing problems.
More help:
Electrical contractors requiring more information can contact the Association of Technical Lightning & Access Specialists (ATLAS), the national representative employers' organisation for companies engaged in the steeplejack, lightning conduction, earthing design businesses and more or its members. The Association is the leading, independent professional voice of this very specialist sector. Email [email protected].
The Copper Development Association may also be able to help (www.cda.org.uk).
* Ref: Paul - Institution of Electrical Engineers - 'Earthing Plastic Pipes'
** Information provided by Furse.
Main earthing definitions (from Cisco practices):
Earth - The conductive mass of the earth, the electric potential of which at any point is taken as being equal to zero. Note that words 'ground' and 'earth' are interchangeable.
Earth electrode - Conductive part or a group of such in intimate contact with, and providing electrical connection to earth.
Earthing network - That part of an earthing installation restricted to the earth electrodes and interconnections.
Main earthing terminal - Terminal for the connection of protective conductors, including equipotential bonding and functional earthing conductors (if any) to the means of earthing.
Earthing conductor - Protective conductor - connects main earthing terminal to the earth electrode.
Equipotential bonding - Electrical connection - places exposed conductive and extraneous conductive parts at largely equal potential.
Equipotential bonding conductor - Protective conductor - ensures equipotential bonding.
Neutral conductor (N) - Conductor connected to a system's neutral point - must contribute to transmission of electrical energy.
Protective conductor (PE) - Conductor sometimes required to protect against electric shock. It may electrically connect to: exposed conductive and extraneous conductive parts; main earthing terminal; earth electrode; earthed point of source or artificial neutral.
Essential terminology:
Earthing letter classifications are as follows:
- First letter gives supply earthing type (eg: I - supply system not earthed; T - one or more points of supply directly earthed).
- Second letter gives earthing arrangement (eg: T - all exposed conductive metalwork connected directly to earth; N - all exposed conductive metalwork connected directly to earthed supply conductor provided by REC).
- Third and fourth letters show earthed supply conductor system arrangement (eg: S - separate neutral / earth conductor systems; C - neutral / earth combined).
So, for example, a TN-C system combines neutral and earth conductors throughout an installation, as well as for the supply, while.a TN-C-S: PME (Protective Multiple Earth) has a combined neutral and earth conductor (earth lead).
This article was originally published in Electrical Times magazine (issue August 2004) by Highbury Business Communications - a Voltimum UK Media Partner.
Contact: Boris Sedacca
Electrical Times
Highbury Business Communications
Nexus House, Azalea Drive, Swanley, Kent BR8 8HY
Tel: 01322 660070
Fax: 01322 616376
Email: [email protected]
Web: www.hhc.co.uk
And: www.electricaltimes.co.uk