2365 LEVEL 2 ELECTRICAL INSTALLATION DETAILED EXAM QUESTIONS WELL ANSWERED LATEST
UPDATE 2026
Who is responsible in the Uk for making sure an installation is adequately earthed - Answers The
consumer and or the electrical contractor
Star point of transformer - Answers On the secondary side , neutral and tied down to earth which is
accepted at being Zero Volts
Earth Fault Loop Path - Answers As low a value of impedance as possible
Earth Fault Impedance known as - Answers ZS
External Loop Impedance - Answers Ze
Zs Formula - Answers Zs = ZE + R1 + R2
Earth Fault Current - Answers I = V
---
Zs
The 5 Earthing Systems - Answers TT
TNS
TN-C-S
IT
TNC
TT - Answers Tera Tera - electricity supply company does not provide any earth , live neutral only
TT System
Method of Earth and Max ZE - Answers Earth Rod or Electrode
Earth Plate or Mesh
Max ZE = 21 ohms
TNS - Answers Tera Neutral Separate
Separate Earth and Neutral From Supplier - Separate on Installation Side
TNS Earthing Method - Answers 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
TN-C-S - Answers Tera Neutral Combined Separate
(Protective Multiple Earthing )
Supplier link Neutral Earth close to source transformer and at points throughout distribution system.
Also Earth Neutral link at consumer intake
Max Ze = 0.35 ohms
Loop path known as PNE
(Protected earthed neutral)
Where can TNCS not be used - Answers Petrol stations
Caravan parks
Some construction sites
Cable used on TN-C-S system - Answers Concentric
What is the danger of a broken neutral on a TNCS system - Answers Shock Risk
If neutral is broken then Neutral will become Live to Earth either side of the break .
More danger of this with overhead supplies.
Why are there multiple earthing points (PME) on a TNCS system - Answers If this is not done then a
fault to Neutral on one system would cause a shock risk to all other installations installed on that
system.
Cable Used on TNS - Answers Split Concentric
Replaced old PILCSWA
paper insulated lead covered steel wire armoured
IT earth System - Answers Isolated Tera
Not used on public system only private
No neutral or earth return to source - has to be monitored all times
TNC - Answers Tera Neutral Combined
Not for public use, only private.
,Combined earth neutral throughout source and installation
Why do we need to connect to earth ? - Answers 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 - Answers 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 - Answers 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 - Answers Connecting all conductive parts together so that touch voltage at any two points is
limited
Exposed Conductive Part - Answers Parts of electrical equipment or systems that are not live under
normal circumstances but may become live under fault conditions
Trunking conduit
Metal Switches etc
Extraneous - Answers Parts that are not part of electrical equipment or systems but may still
introduce a potential path to earth
Gas pipes, water pipes
Structural steel work , ducting
Bonding Clamps - Answers BS951
Safety Electrical Connection Do Not Remove
Mains Services Bonding - Answers Must be not more than 600mm from main shut off valve and
before any junction in pipework.
Bonding Clamp Colour Codes - Answers Red EC14 dry non corrosive
Blue Ec15 corrosive humid
Green Ec16 corrosive humid big
Circuit Protective Conductor - Answers A protective conductor connecting exposed conductive parts
of equipment to the main earthing terminal
What Role does the C.P.C play in A.D.S ? - Answers It maintains equal potential between all exposed
conductive parts
Disconnection Times - Answers 0.2 seconds for circuits rated at or below 32A on installations forming
TT systems
0.4 seconds for circuits rated at or below 32A on installations forming TN systems
1 second for distribution circuits or circuits exceeding 32A on installations firming TT systems
5 seconds for distribution circuits or circuits exceeding 32A on installations forming TN systems.
What situations require additional protection by R.C.D ? - Answers Socket Outlets rated at 20A or less
and used by ordinary persons
Mobile equipment rated at 32A or less and used outdoors
Special locations such as bathrooms, saunas , swimming pools and construction sites
Circuits with cables buried in walls or partitions to a depth less than 20mm and not protected by an
earthed armouring covering.
Bs7671 2008 Part 4 - Answers Protection
(SDAPSIS)
-
Main Protective Bonding Conductor - Answers Connects MET and :
Metallic Installation Service Pipes - Water , Gas, Oil
Metallic Exposed structural steelwork of building
Lightning conductor
, And other extraneous parts
Thermal Constraints - Answers The temperature the cpc is capable of carrying for certain times under
fault conditions
Method 1 Thermal Constraints - Answers Table 54.7
Method 2 - Answers Adiabatic Equation
S=√I2 x T
---------
K
Using Adiabiatic - Answers Calculate Fault a Current
I = V/Zs (Zs = Ze +r1 +r2)
Get T from table and then plug info into √I2 x T
--------
K
Fusing Factor - Answers Fusing Factor = Fusing Current
------------------
Current Rating
Fusing Current - Answers Minimum current causing fuse to blow
Current Rating (fuse) - Answers Maximum current the fuse can with stain without blowing
Some Fusing Factors - Answers 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 B 3-5 x In
C 5 -10 x In
D 10-20 x In
IEE Regs Regarding Fuses - Answers 1. Nominal setting of device is greater than or equal to design
current In >= Ib
2.Nominal setting is less than or equal to than lowest current carrying capacity of circuit conductors In
<= Iz
3. 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 ? - Answers Short Circuit Protection
Overload Protection
Earth Fault Protection
Overload - Answers 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 - Answers A current exceeding the rated value of the conductors
Rated value is current carrying capacity of conductor.
Short Circuit - Answers A connection of negligible impedance between live conductors. Live
conductors means all current carrying conductors under normal conditions and includes neutral.
Fault Current - Answers A circuit condition in which current flows through an unintended path.
A current emanating from a fault.
Protective Conductor Current - Answers Current appearing in the protective conductor such as
leakage or electric current resulting from a fault
BS3036 - Answers Semi Enclosed Rewireable Fuse
White 5A
Blue 15A
Yellow 20A
Red 30A
Green 45A
BS3036 Advantages - Answers No moving parts
Low cost of replacing element
UPDATE 2026
Who is responsible in the Uk for making sure an installation is adequately earthed - Answers The
consumer and or the electrical contractor
Star point of transformer - Answers On the secondary side , neutral and tied down to earth which is
accepted at being Zero Volts
Earth Fault Loop Path - Answers As low a value of impedance as possible
Earth Fault Impedance known as - Answers ZS
External Loop Impedance - Answers Ze
Zs Formula - Answers Zs = ZE + R1 + R2
Earth Fault Current - Answers I = V
---
Zs
The 5 Earthing Systems - Answers TT
TNS
TN-C-S
IT
TNC
TT - Answers Tera Tera - electricity supply company does not provide any earth , live neutral only
TT System
Method of Earth and Max ZE - Answers Earth Rod or Electrode
Earth Plate or Mesh
Max ZE = 21 ohms
TNS - Answers Tera Neutral Separate
Separate Earth and Neutral From Supplier - Separate on Installation Side
TNS Earthing Method - Answers 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
TN-C-S - Answers Tera Neutral Combined Separate
(Protective Multiple Earthing )
Supplier link Neutral Earth close to source transformer and at points throughout distribution system.
Also Earth Neutral link at consumer intake
Max Ze = 0.35 ohms
Loop path known as PNE
(Protected earthed neutral)
Where can TNCS not be used - Answers Petrol stations
Caravan parks
Some construction sites
Cable used on TN-C-S system - Answers Concentric
What is the danger of a broken neutral on a TNCS system - Answers Shock Risk
If neutral is broken then Neutral will become Live to Earth either side of the break .
More danger of this with overhead supplies.
Why are there multiple earthing points (PME) on a TNCS system - Answers If this is not done then a
fault to Neutral on one system would cause a shock risk to all other installations installed on that
system.
Cable Used on TNS - Answers Split Concentric
Replaced old PILCSWA
paper insulated lead covered steel wire armoured
IT earth System - Answers Isolated Tera
Not used on public system only private
No neutral or earth return to source - has to be monitored all times
TNC - Answers Tera Neutral Combined
Not for public use, only private.
,Combined earth neutral throughout source and installation
Why do we need to connect to earth ? - Answers 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 - Answers 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 - Answers 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 - Answers Connecting all conductive parts together so that touch voltage at any two points is
limited
Exposed Conductive Part - Answers Parts of electrical equipment or systems that are not live under
normal circumstances but may become live under fault conditions
Trunking conduit
Metal Switches etc
Extraneous - Answers Parts that are not part of electrical equipment or systems but may still
introduce a potential path to earth
Gas pipes, water pipes
Structural steel work , ducting
Bonding Clamps - Answers BS951
Safety Electrical Connection Do Not Remove
Mains Services Bonding - Answers Must be not more than 600mm from main shut off valve and
before any junction in pipework.
Bonding Clamp Colour Codes - Answers Red EC14 dry non corrosive
Blue Ec15 corrosive humid
Green Ec16 corrosive humid big
Circuit Protective Conductor - Answers A protective conductor connecting exposed conductive parts
of equipment to the main earthing terminal
What Role does the C.P.C play in A.D.S ? - Answers It maintains equal potential between all exposed
conductive parts
Disconnection Times - Answers 0.2 seconds for circuits rated at or below 32A on installations forming
TT systems
0.4 seconds for circuits rated at or below 32A on installations forming TN systems
1 second for distribution circuits or circuits exceeding 32A on installations firming TT systems
5 seconds for distribution circuits or circuits exceeding 32A on installations forming TN systems.
What situations require additional protection by R.C.D ? - Answers Socket Outlets rated at 20A or less
and used by ordinary persons
Mobile equipment rated at 32A or less and used outdoors
Special locations such as bathrooms, saunas , swimming pools and construction sites
Circuits with cables buried in walls or partitions to a depth less than 20mm and not protected by an
earthed armouring covering.
Bs7671 2008 Part 4 - Answers Protection
(SDAPSIS)
-
Main Protective Bonding Conductor - Answers Connects MET and :
Metallic Installation Service Pipes - Water , Gas, Oil
Metallic Exposed structural steelwork of building
Lightning conductor
, And other extraneous parts
Thermal Constraints - Answers The temperature the cpc is capable of carrying for certain times under
fault conditions
Method 1 Thermal Constraints - Answers Table 54.7
Method 2 - Answers Adiabatic Equation
S=√I2 x T
---------
K
Using Adiabiatic - Answers Calculate Fault a Current
I = V/Zs (Zs = Ze +r1 +r2)
Get T from table and then plug info into √I2 x T
--------
K
Fusing Factor - Answers Fusing Factor = Fusing Current
------------------
Current Rating
Fusing Current - Answers Minimum current causing fuse to blow
Current Rating (fuse) - Answers Maximum current the fuse can with stain without blowing
Some Fusing Factors - Answers 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 B 3-5 x In
C 5 -10 x In
D 10-20 x In
IEE Regs Regarding Fuses - Answers 1. Nominal setting of device is greater than or equal to design
current In >= Ib
2.Nominal setting is less than or equal to than lowest current carrying capacity of circuit conductors In
<= Iz
3. 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 ? - Answers Short Circuit Protection
Overload Protection
Earth Fault Protection
Overload - Answers 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 - Answers A current exceeding the rated value of the conductors
Rated value is current carrying capacity of conductor.
Short Circuit - Answers A connection of negligible impedance between live conductors. Live
conductors means all current carrying conductors under normal conditions and includes neutral.
Fault Current - Answers A circuit condition in which current flows through an unintended path.
A current emanating from a fault.
Protective Conductor Current - Answers Current appearing in the protective conductor such as
leakage or electric current resulting from a fault
BS3036 - Answers Semi Enclosed Rewireable Fuse
White 5A
Blue 15A
Yellow 20A
Red 30A
Green 45A
BS3036 Advantages - Answers No moving parts
Low cost of replacing element
