What is the objective of earthing?
It is to ensure all exposed and extraneous conductive parts associated with the electrical installation are at earth potential which is 0V.
How is earthing done and to achieve what purpose?
Earthing is done by providing a connection with much less resistance than the design value capable of carrying the expected max fault current. To achieve the purpose of dissipating unwanted electrical surges efficiently.
Is one point between generator and the consumer earthed or multiple?
Mutliple points are earthed (grounded).
List another objective that earthing covers?
It prevents any part of equipment (except live parts) from reaching a dangerously different potential than its surroundings.
Ensures sufficient fault current flows safely for proper operation of protective devices.
Limits overvoltages between neutral/line and ground.
Suppresses dangerous earth potential gradients which could cause electric shock or incorrect operation of control and protection devices.
What are the two types of Earthing?
Neutral Earthing and Equipment Earthing.
Describe the two types of Earthing.
Neutral earthing connects the system neutral to earth which ensures system security and protection.
Equipment earthing connects non-current carying parts of equipment to earth which ensures personnel safety and protection against lighting.
To achieve effective earthing, what factors must be considered?
Earth Resistance, size and shape of the earth conductor, soil characteristics where the electrode is buried, electrode design, current density at the electrode surface and ground potential in the vicinity of the electrode.
What factors influence soil resistivity?
Porosity which describes the size and number of voids in the material and water content which if high and filled with dissolved minerals and salts, sharply reduces resistivity.
Give an example of something that has a low soil resistivity and one that has a high soil resistivity.
See water has very low resistivity and rock has very high resistivity.
Do we want low or high soil resistivity for effective earthing? Why?
We want low soil resistivity because it allows electrical current to flow easily and safely to the earth path instead of making non-circuit components live in the case of a fault.
Describe the function of earthing electrodes. List a key requirement too.
They are the part of the earthing system that touches the ground directly to release earth leakage currents safely.
Key requirements are that they must have sufficient csa to carry large fault currents for a short period, should last for years, have good electrical conductivity and resist corrosion in various soil conditions.
What materials can earth rods be made out of and what forms can they take?
Materials: copper, stainless or galvanised steel and cast iron.
Forms: Vertical rods, plates or horizontal conductors.
List 3 reasons as to why Earth rods are common.
They are cheap and easy to install - minimal excavation needed for installation - various range of lengths and materials.
What is the most common earth rod installation method and how does it work?
The most common is the 3 point of Fall of Potential Method and it works by using 3 electrodes: 1 for volts, 1 for current and 1 earth electrode being tested (this one needs proper sensitivity and low resistance). This measures the resistance of the earth electrode accurately.
Draw the most common earth electrode method.
Please view power point: Week 9 Earthing pg 18 for image.
What is one way to reduce earth resistance using rods?
Connect them in parallel with good spacing outisde the resistance area of the others. This is because resistance is inversely proportional to the number of rods.
Which earthing design method makes RCDs unable to detect fault current?
TN-C systems because earth and neutral wires are combined in one conductor.
Which earthing design system uses the most wires and perhaps the most expensive?
TN-S becuase the neutral and earth conductors are completely separate all the way through; from both the consumer and the producers side.
Which earthing design method is used most in Ireland and why?
TN-C-S. This is becuase both neutral and earth are combined on the supply side of the system however, on the consumer’s side they are separate. This allows RCD to detect faults effectively.
Which earthing design system uses an electrode on both the consumer and the supply side?
TT.
Which earthing system has its source isolated from earth or connected to earth through a high impedance making all exposed conductive parts connected to an independant earth?
IT.
What is an earthing conductor and what is it also referred to?
Also referred to as the earthing lead, it connects the installation’s main earthing terminal to either the earth electrode or the earth terminal provided by the Electical supply company. It is a critical component of the protective system.
What is a circuit protective conductor, CPC?
It is a network of conductors that links all exposed conductive parts of the installation and connects them to the main earthing terminal. It can also include the earthing conductor and/or the equipotential bonding conductors.
What is the purpose of bonding?
Bonding reduces the risk of electric shock to anyone who might touch two separate metals parts when a fault occurs in the electrical installation. This minimises the voltage difference that could otherwise exist.
