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2365 LEVEL 2 ELECTRICAL INSTALLATION
(DETAILED UP TO DATE 2025)
Who is responsible in the Uk for making sure an installation is adequately earthed The consumer and or the electrical contractor Star point of transformer On the secondary side , neutral and tied down to earth which is accepted at being Zero Volts Earth Fault Loop Path As low a value of impedance as possible Earth Fault Impedance known as
ZS
External Loop Impedance Ze Zs Formula Zs = ZE + R1 + R Earth Fault Current
I = V
Zs The 5 Earthing Systems
TT
TNS
TN-
C-S
IT
TNC
TT
Tera Tera - electricity supply company does not provide any earth , live neutral only
TT System Method of Earth and Max ZE Earth Rod or Electrode Earth Plate or Mesh Max ZE = 21 ohms TNS Tera Neutral Separate Separate Earth and Neutral From Supplier - Separate on Installation Side TNS Earthing Method Supplier neutral earth link as near as practicable to the source supply transformer Consumer Earth Connected to Metal Sheath Of Supply Cable Max Ze = 0.8 ohms
PILCSWA
paper insulated lead covered steel wire armoured IT earth System Isolated Tera Not used on public system only private - common in hospitals No neutral or earth return to source - has to be monitored all times TNC Tera Neutral Combined Not for public use, only private. Combined earth neutral throughout source and installation Why do we need to connect to earth? So sufficient current will flow under earth fault to operate protective device And while fault exists all metalwork shall rise to same potential ( shock prevention) The Means of Earthing The arrangement that connects the general mass of earth with the exposed conductive parts of an installation via the earthing conductor, the MET and the CPC'S Equipotential Zone Anything in installation that conducts electricity and is connected in some way to general mass of earth and is liable to have fault voltages Bonding Connecting all conductive parts together so that touch voltage at any two points is limited Exposed Conductive Parts of electrical equipment or systems that are not live under normal circumstances but may become live under fault
Part conditions Trunking conduit Metal Switches etc
Bs7671 2008 Part 4 Protection (SDAPSIS)
Main Protective Bonding Conductor Connects MET and : Metallic Installation Service Pipes - Water , Gas, Oil Metallic Exposed structural steelwork of building Lightning conductor And other extraneous parts Thermal Constraints The temperature the cpc is capable of carrying for certain times under fault conditions Fusing Factor Fusing Factor = Fusing Current
Current Rating Fusing Current (I2) Minimum current causing fuse to blow Current Rating (fuse) Maximum current the fuse can with stain without blowing Some Fusing Factors Bs3036 1.8 to 2.0 x In BS 1361 1.6 to 1.9 x In BS88 HBC 1.25 to 1.27 x In BS 60898 1.45-1.5 x In IEE Regs Regarding Fuses
- Nominal setting of device is greater than or equal to design current In >= Ib
- Nominal setting is less than or equal to than lowest current carrying capacity of circuit conductors In <= Iz
- The operating current of device I2 is less bran or equal to 1.45 current carrying capacity of conductors. I2 <=1.45 x Iz What 3 Criteria Do Circuit Protective Devices Have a To Meet? Short Circuit Protection Overload Protection Earth Fault Protection Overload Thermal An overload is a situation where a HEALTHY electrical circuit is using more power than it has been designed for. This causes excessive heat. Overcurrent A current exceeding the rated value of the conductors Rated value is current carrying capacity of conductor. Short Circuit A connection of negligible impedance between live conductors. Live conductors means all current carrying conductors under normal conditions and includes neutral. Fault Current A circuit condition in which current flows through an unintended path. A current emanating from a fault. Protective Conductor Current Current appearing in the protective conductor such as leakage or electric current resulting from a fault
shorted out with silver foil Can sometimes be replaced with incorrect fuse rating BS 88 - 2 E&G HBC/HRC High breaking capacity High rupturing capacity
BS88- 2 E&G HBC HRC
Advantages Can discriminate between overload current of short duration and high fault current Simple to observe when fuse blown Consistent and reliable Very accurate BS 88 - 2 E&G HBC HRC Disdvantages Expensive BS 60898 ( replaces 3871 - now obsolete) Circuit Breaker ( most common domestic) BS 60898 Advantages Tripping characteristics preset by manufacturer Trip for sustained overload but not for transient Faulty Circuit Easily Identified Simple Reset Tamper Proof Cheap and multiple units available BS 60898 Disadvantages Have mechanical moving parts Need regular testing to ensure satisfactory operation Ambient temperature affects characteristics Fuse for domestic
BS
BS 88 -
3 - C BS
Fuse Small Commercial
BS 88 - 3
BS 60898
used on central heating systems. Magnetic (Short Circuit) As current increases so does the generated EMF which trips mechanism. In Nominal Current Current setting of device Ib Design Current of Circuit Iz Current carrying capacity of any of the circuit conductors I The current which ensures the effective operation of the device (Usually thermal trip) Ia Current that causes the effective operation of the protective device within the required time (Usually magnetic - instantaneous trip) RCD Residual Current Device Protects against Earth Leakage Only and must NOT be used as so,e means of protection Only acts on a fault to Earth. A fault to Neutral would not trip device as it would appear as a Load. RCD type O No built in time delay RCD type S Selectable time delay , vary when necessary RCD type G Small in built time delay to prevent nuisance tripping RCD operation Only operates on a fault to Earth RCBO Residual Current Circuit Breaker With Overload Protection Earth leakage and thermal protection.
RCD Testing Must be tested by operating test button every 3 months RCD Operating Currents and Times A current not exceeding 30mA And an operating time of 40mS at a residual current of 5 times it's operating current ( 150mA) RCD Other sizes 100mA ,200mA,300mA and 500mA 300mA and 500mA used to protect property rather than life (Fire) Equipotential Bonding Electrical connection maintaining various exposed conductive parts and extraneous conductive parts at substantially the same potential Called Protective Equipotential Bonding when used for purpose of safety Statutory Regulation Legally Binding Implications For Non Conforming With Statutory Regs Prosecution, Fine, Imprisoment, Prohibition Notice, Improvement Notice , Dismisal, Injury , Loss Earnings, Death, Loss Reputation Haswa Health Safety Work Act 1974 Employer and Employes have duty of care to their own and each other's safety EAWR Electricity AT Work Act 1989 Primarily concerned with electrical safety applying to work, activities and systems. Electricity AT Work Act 1989 Simply a statement of name and when came into force
