In Electrical Engineering terms, earth (or ground) is the point of reference in an electrical circuit from which the voltages are estimated. The earthing system or to our friends over the pond; grounding system also has the function of providing a common return path for electric current through a physical connection to the geology. In an electrical installation, an earthing system or grounding system electrode connects specific parts of that installation with the Earth’s conductive surface for safety and functional purposes.
Purpose of an Earthing System (or Grounding System)
Electrical circuits connect to earth (ground) for a number of reasons. An Earthing system provides: (See What’s the difference between Earthing and Bonding)
- Personal protection – living beings in the vicinity of substations by not exposing to unsafe potentials under steady-state or fault conditions. (see step potential and touch potential risks)
- Electrical system operational protection
- Potential (voltage) grading earthing
- Electromagnetic pulses protection
- Lightning protection
- A sufficiently low impedance to facilitate satisfactory protection operation under fault conditions. (see stray current)
- Voltage protection, within reasonable limits under fault conditions (such as lightning, switching surges or inadvertent contact with higher voltage systems), and ensure that insulation breakdown voltages are not exceeded, i.e. insulation co-ordination.
- Graded insulation in power transformers.
- Voltage limiting to earth on conductive materials which enclose electrical conductors or equipment.
Lesser well-known reasons for earthing include:
- To stabilise the phase-to-earth voltages on electricity lines under steady-state conditions, i.e. by dissipating electrostatic charges.
- A means of monitoring the insulation of the power delivery system.
- Eliminate persistent arcing ground faults.
- To ensure that a fault which develops between the high and low voltage windings of a transformer can be detected by the primary protection.
- Provide an alternative path for induced current and thereby minimise the electrical ‘noise’ in cables.
- Provide an equipotential platform on which electronic equipment can operate.
Earthing System Geology
Generally speaking, the earthing system needs to achieve a low impedance connection with the geology. So that it can disperse or collect current to or from the ground. Which, in turn, means a voltage rise doesn’t reach a level that could cause harm.
The Function of an Earthing System
Within installations, an earth connection is also necessary to ensure the correct operation of equipment. – For example, electronic devices, where an earthed shield may be required. It is essential to consider the earthing within a whole installation as one complete system. Why? Electrons can’t read!
No. Seriously, designing an earthing system to typically provide two safety functions.
The first, to prevent a shock due to different potentials on exposed metalwork – achieved by bonding. A connection to the ground through the use of the earthing electrode also limits the build-up of static electricity. Ideal when handling flammable products or electrostatic-sensitive devices.
The second function of the earthing system is to ensure that, in the event of an earth fault. Any fault current occurring can return to the source in a controlled manner. I mean by managing the return path avoiding damage to equipment or injury to people.
A sufficiently low-impedance earthing system ensures that the portion of the returning earth fault current can flow to operate protective devices correctly. Initiating circuit breakers or fuses to interrupt the flow of current successfully.
APPROVED EARTHING
At the risk of stating the obvious. Electricity supply to a customer that hasn’t got earthing to an approved or accepted standard carries a disproportionate risk – business risk, and human risk. Not just to the people within the facility, but to a wider area which could affect innocent 3rd parties nearby.
An incorrectly designed or installed earthing system that fails to control the fault energy within known permissible limits (defined by what the average human body can tolerate), puts lives at a very real risk of injury/death and can also cause damage to equipment.
Your earthing system should always be:
- Designed by a proven competent designer, i.e. someone qualified
- Designed and installed to an accepted practice such as IEC 50522, BS 7430, IEEE Std.80, etc. (legal requirement) (See earthing standards)
- Installed by a proven competent installer
- Verified and validated after installation, i.e. confirmed safe, fit-for-purpose
- Monitored or tested throughout its life at regular intervals to make sure it’s still doing a good job of protecting people
Methods of Earthing
Unearthed or Insulated System
This method does not have a deliberate, formal connection to the earth. There may be some high impedance connections for instrumentation; for example, the coil of a measuring device.
Under normal conditions, the capacitance between each phase and earth is substantially the same. The outcome is to stabilise the system with respect to earth. On a three-phase system, the voltage of each phase to earth is the star voltage of the system. Therefore, the neutral point (if any), is at, or near, earth potential.
Earthed Systems
An earthed system has at least one conductor or point (usually the neutral or star point) intentionally connected to the earth. On three-phased systems, usually making the connection to earth at the star-point or neutral of the transformer.
Adopting earthing, in this manner, if there is a need to connect line-to-neutral loads to the system, i.e. to prevent the neutral voltage fluctuating significantly with the load. The earth connection reduces voltage fluctuation and unbalances, which would otherwise occur. Another advantage is that using residual relays to detect faults before they become phase-to-phase faults. Thus reducing fault currents, and damage on other parts of the electrical network.
There are two main types of Earthed System:
- Impedance Earthed System;
- and low-impedance (solidly) Earthed System.
Impedance Earthing System
Resistors and reactors inserted in the connection between the neutral point and earth. Usually, to limit the fault current to an acceptable level.
In practice, to avoid excessive transient over-voltages due to resonance with the system shunt capacitance, inductive earthing needs to allow at least 60% of the 3-phase short circuit capacity to flow for earth faults. This form of earthing has lower energy dissipation than resistive earthing.
Petersen Coils
Arc-suppression coils (ASC’s), also known as Petersen coils or ground fault neutralisers, can be used as the earth connection. These are tuned reactors, which neutralise the capacitive current of the healthy phases so that any fault current is of low magnitude.
Due to the self-clearing nature of this earthing it is useful in certain circumstances on medium voltage overhead systems, for example, those which are prone to a high number of transient faults and have many earthed points.
The use of auto-reclosing circuit breakers has mostly taken over from ASC’s within high and medium voltage systems. However, due mainly to improvements in the equipment available and protective system sophistication, there is increasing interest in ASC’s. Their ideal application is for overhead line systems, which have a high number of earthed points (e.g. transformers), and many connected customers. There cannot be too much single-phase line or cable, as this compromises the performance of the scheme.
Resistance earthing is more commonly used because it can allow the fault current to be limited and dampen transient overvoltages. In distribution systems, particularly those at 11 kV, it is common to find 750 A, 1000 A or 1500 A Liquid Earth Resistors (LER’s) or the more common stainless steel type resistors installed in various combinations to limit the earth fault current.
Low Impedance (solidly) Earthed System
The low-impedance earth system is the most common arrangement, particularly at low voltage. Here the neutral/earth connection is made through a robust connection with no impedance intentionally added. The disadvantage of this arrangement is that the earth fault current usually is high, but the system voltages remain suppressed or low under fault conditions.
LV Earth systems
Having dealt with the earthing available on a Power System above, let’s consider the low voltage earthing system briefly.
Standard definitions for the connections are:
T: Terre, direct connection to the earth.
N: neutral.
C: combined.
S: separate.
The main types are:
TN-S
TN-S The incoming supply has a single point of connection between the supply neutral and earth at the supply transformer. The supply cables have separate neutral and earth protective conductors (S.N.E.). Generally, the neutral conductor is a fourth ‘core’, and the earth conductor forms a protective sheath or PE conductor. The customer may have an earth terminal connected to the sheath of the service cable or a separate earth conductor.
TN-S was pretty much the standard arrangement in the UK, before the introduction of protective multiple earthing (PME or TN-C-S) systems.
TN-C-S
TN-C–S Earthing the supply neutral at several points. The supply cables have a combined neutral and earth metallic outer sheath with a PVC covering (CNE cables). The combined neutral earth sheath is the PEN (protective earth neutral), conductor.
The supply within the customer’s premises would usually be TN-S, i.e. the neutral and earth would be separate, linked only at the service position. When combing the neutral and earth within the premises, then the system is TN-C.
PNB
PNB Protective Neutral Bonding is a variation of the TN-C -S system in that, providing the customer with an earth terminal which connects to the supply neutral, but the neutral is connected to earth at one point only. Typically at or near to the customer’s supply point. This arrangement is reserved for use when a single customer has its own transformer.
The remaining two systems are:
TT
TT This is a system where the supply is earthed at one point only, but the cable sheaths and exposed metalwork of the customer’s installation are connected to earth via a separate electrode which is independent of the supply electrode.
IT
IT This is a system having no direct connection between live parts and earth, but with its exposed conductive parts of the installation earthed. Sometimes a high impedance connection to earth is provided to simplify the protection scheme required to detect the first earth fault.
Earthing arrangements within the UK and many other countries are required to conform to BS 7671. This standard is based upon the latest 18th edition of the Institution of Electrical Engineers Regulations for Electrical Installations. The Electricity Safety, Quality and Continuity Regulations do not apply, so an earth connection is not a statutory requirement and unearthed systems (such as the IT above) are permitted.
Key Point
The underlying principle is first to take all reasonable precautions to avoid direct contact with live electrical parts, and secondly to provide measures to protect against indirect contact. The latter involves effective earthing and bonding, and a system of protection which removes the fault condition. The principle is more commonly known as protective bonding.
There are some locations where special earthing arrangements are necessary such as
- Mines,
- Quarries,
- Petrol filling stations,
- Lightning Protection
- and Lift installations.
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