Earthing/bonding Flashcards
What is the difference between earthing and bonding
Earthing connects exposed conductive parts to ground and bonding connects extraneouse conductive parts to ground
What is the difference between exposed and extraneous conductive parts
Exposed conductive parts are external metal part of electrical equipment that you would not normally expect to become live for example the casing on white goods.
Extraneouse conductive parts are exposed metal surfaces that are not connected to electrical appliances such as metal pipework and structural steel
Describe an IT earthing system
The system neutral may be connected to ground through a resistor or may not be connected to earth at all. The earth connection on the appliances is connected to the ground through a TT electrode.
What are the advantages and disadvantages of an IT earthing system.
No harmonic signals on the earth line. This is useful for supplying sensitive equipment. Useful for generator supplied installations where there is an increased risk of earth faults.
What is the advantage of TNCS over TNS
Reduced cost of extra earth. If the earth conductor on a TNS breaks then it creates a discontinuity in the system. This would result in high potential of the neutral conductor on the load side of the fault. This is not a problem with TNCS because the general mass of earth can act as a neutral.
What is a TNC earthing system and why is it not common.
The neutral and earth are combined within the installation. This can lead to high neutral potential in the event of a very unbalanced system. RCDs will not function with this system.
What is touch voltage?
It is the voltage between extraneouse conductive parts and exposed conductive parts made live by a fault.
What is basic protection
Insulation of live parts, placing of live parts behind a barrier or enclosure. Using obstacles to prevent unintended approach to live parts. Placing obstacles out of reach.
What is fault protection
This is protection against shock resulting from contact with exposed conductive parts. This is done by limiting the touch voltage by bonding and limitation the duration of the fault by disconnection of protective devices.
List 5 measures of general protection
Bonding, automatic disconnection of supply, class 2 (double insulated) equipment, isolating transformer, SELV/PELV
Explain ground potential rise
Ground potential rise is when large amounts of electrical current enter the earth. This is usually because of a substantion or lightening strike. This results in a series of concentric circles of different electrical potentials. A cow is able to bridge these concentric circles with its long body. This results in a potential difference between the animals front and hind legs which can result in electricution.
