Electrical Installation Design Flashcards
What key factors must be considered in assessing supply details for an electrical installation design?
Live conductors, current, prospective fault current, and earthing arrangement
In a typical single-phase 230 V supply in the UK, how many live conductors are there?
Two
In the UK, what is the voltage for three-phase supplies without a neutral conductor?
400 V
Why are three-phase supplies preferred for larger installations requiring more than 100 A of power?
They are more energy-efficient.
What does the term “nominal voltage” refer to in electrical installations?
The voltage measured between live and earth in the UK
How does the real voltage in an installation relate to the nominal voltage?
The real voltage is always lower than the nominal voltage.
Which type of conductor arrangement is common in larger retail outlets and larger commercial or industrial buildings in the UK?
Three-phase with a neutral
What is the maximum current rating typically associated with small domestic-type installations in the UK?
100 A
In a three-phase and neutral supply in the UK, what are the voltages for line-to-neutral (L-N) and line-to-line (L-L)?
L-N: 230 V, L-L: 400 V
Which type of conductor arrangement is not very common in the UK due to the requirement for all circuits to be three-phase balanced loads?
Three-phase 400 V
According to UK regulations (BS 7671), where can you find diagrams of various conductor arrangements?
Chapter 31
What does the term “external impedance” refer to in the context of earthing arrangements?
Resistance between the installation and the substation transformer
why is it essential to know the supply details before designing an electrical installation?
To ensure the safety of the installation
In the UK, what is the primary difference between a single-phase 230 V supply and a three-phase 400 V supply?
The number of live conductors
What is the purpose of a neutral conductor in a three-phase and neutral supply (L-N : 400 V L-L) in the UK?
To balance the phases and carry unbalanced current
In an electrical installation located far from the substation transformer, how does the distance impact the real voltage compared to the nominal voltage?
The real voltage is lower as distance increases.
What is the purpose of using nominal (normal) values in electrical calculations?
To account for voltage variations due to demand and distance
Which of the following is NOT a common type of conductor arrangement in the UK?
Four-phase
What is the primary reason for using a three-phase supply for larger installations?
It provides better voltage regulation.
What is the maximum demand typically associated with most average-sized houses in the UK?
Less than 100 A
In larger installations with a maximum demand over 100 A, what is often used to distribute the load in the supply?
Three-phase supply
When a building needs a supply over 400 A per phase, what may be installed on-site to provide the required capacity?
A substation transformer
What factors govern the maximum demand for an electrical installation?
The size of the incoming supply cables
Why is it essential to check the supply capacity when adding to an existing electrical installation?
To prevent overload and ensure safety
hen a new building is to be given an electrical supply, what information does the Distribution Network Operator (DNO) need?
site plans, load requirements, and property details
What is the responsibility of the consumer once the electrical supply has been metered?
To pay the electricity bill and maintain the internal electrical system
In newer buildings, what component is often found between the meter and the consumer unit (CU)?
Isolation switch
What is the most common current rating for house supplies in the UK?
80 A
What term is used to describe the maximum demand in electrical installations?
Maximum demand
If a building needs a supply over 400 A per phase, what infrastructure might be added on-site?
Substation transformer
What governs the maximum demand for an electrical installation?
size of the incoming supply cables and the DNO’s service fuse rating
What kind of information does the Distribution Network Operator (DNO) need when providing a new electrical supply to a building?
Load requirements and property details
What is the primary role of a designer in the context of a new electrical installation?
To provide the DNO with the proposed maximum demand for the installation
In the UK, what is the most common earthing arrangement used in electrical installations?
TN-C-S
How does a Distribution Network Operator (DNO) seal their service fuse and meter connections?
With a small wire and a sealing crimp tool
Why are tails insulated and sheathed in electrical installations?
To improve mechanical protection
What is the primary reason for having two layers of insulating material on tails?
To improve protection and reduce the risk of electric shock
In what location should tails have insulation and sheathing extending up to?
B) The point where they connect to the main switch inside the consumer control unit
What type of cable should be used for tails when the consumer control unit (CU) is located away from the meter point?
Steel-wire armour cable
What determines the cross-sectional area (csa) of tails in an electrical installation?
The intended maximum demand or the stipulation of the DNO
According to Regulation 522.8.5, how should meter tails be supported to avoid undue strain on terminations?
By supporting them with clips or cleats
What are the three common circuit arrangements in electrical installations?
Radial power circuits, ring-final circuits, and radial lighting circuits
What is a major advantage of using radial power circuits?
he cable csa is designed to match the load and protective device
In a ring-final circuit, what happens if a fault occurs, such as a loose connection (open circuit)?
The equipment continues to function
What type of circuit should be used for lighting circuits that primarily use energy-efficient LED lighting?
3 A rated circuits
What is the primary purpose of a ring-final circuit in the UK?
To supply socket outlets with conveniently placed outlets
Which of the following statements about ring-final circuits is true?
Ring-final circuits can supply an unlimited number of outlets in a maximum floor area of 100 m2.
What is a “spur” in the context of electrical circuits?
A branch circuit
Why is it recommended to minimize the use of spurs in electrical circuits?
Too many spurs can make fault detection harder
What are some disadvantages of ring-final circuits?
Overloaded circuit conductors may not be detected.
What rating are lighting circuits generally rated at in the UK?
6 A
In what situations might lighting circuits be wired using the “three-plate” method?
C) When using composite cables
What is the advantage of installing the “three-plate” at a switch instead of at a light point?
It reduces the number of connections at luminaires.
How are additional light points typically added to a lighting circuit wired in the “three-plate” method?
By looping off the ceiling rose
What should be considered when selecting the rating for lighting circuits in installations using energy-efficient LED lighting?
The energy consumption of the LED lights
What is the primary advantage of using the conduit method for wiring?
It allows for direct drawing of cables without breaking at light points.
How can lighting circuits be efficiently controlled for energy conservation?
By arranging automated switching devices.
What does diversity take into account when sizing distribution circuits?
The fact that not all loads will be fully loaded simultaneously.
Final circuits are NOT typically subjected to:
Diversity considerations.
which term is used to describe a circuit used for remote switching or monitoring?
Auxiliary circuit.
What is the primary reason for dividing an electrical installation into circuits?
To minimize danger or inconvenience in case of a fault.
What type of wiring system is suitable for distributing large cables around a building or site?
Cable ladder, cable tray, cable brackets.
What is the purpose of essential circuits in an electrical installation?
To provide power to equipment like IT systems in case of power loss.
Which external influence should be considered when selecting a wiring system?
Ambient temperature.
Which wiring system is commonly adopted in industrial premises where the presence of cables is acceptable?
Clipped direct sheathed with armor.
What is the primary purpose of distribution circuits in electrical installations?
To reduce the length of final circuits.
Which of the following is NOT a method for achieving energy efficiency in lighting circuits?
Increasing the number of lighting points.
Which term describes a circuit that requires additional protective measures and separation from other circuits, typically used for fire alarms and emergency systems?
Safety circuit.
In which type of wiring system should cables be enclosed in a conduit or duct when buried underground?
Buried cables.
Which wiring system is suitable for electricity supply distribution in rural areas and is typically mounted on insulators?
On insulators.
What external influence is especially relevant when selecting wiring systems for corrosive environments, such as a plating plant?
Corrosive or polluting substances.
What type of circuit is used for electric vehicle charging equipment in new build housing with accessible parking facilities?
Electric vehicle (EV) charging circuit.
Which factor is NOT typically considered when selecting a wiring system or method of containment?
Cable color.
What is the term for the proper disconnection of protective devices to avoid unnecessary disconnection of unaffected parts of the system during a fault or overload?
Selectivity.
How can selectivity be achieved for fuses in series?
By ensuring the upstream device rating is more than twice that of the downstream device.
Under what conditions is selectivity required for protective devices?
Under normal load conditions and overcurrent conditions.
Why might there be no selectivity between circuit breakers, even if their ratings differ?
Because circuit breakers have a high breaking capacity.
What category is used to classify switchgear usage, indicating the expected duty of the device?
AC-21b.
Why is it important to use devices correctly rated for their expected use as isolators or switches?
To ensure safety and prevent failure.
Why should protective devices be selected carefully for isolation and switching?
To ensure they are rated for their expected use.
In a domestic dwelling, what type of main switch is required in the consumer unit (CU)?
Double-pole main switch.
In which wiring system are insulated cables often used without a sheath, commonly for several small cables?
Cable-trunking systems.
n a three-phase installation with a TT earthing arrangement, how many poles should the main switch in the distribution board (DB) have?
Four poles.
What is the primary difference between a consumer unit (CU) and a distribution board (DB)?
The CU has a main switch that breaks both line and neutral.
Which protective devices are commonly found inside consumer units (CUs) or distribution boards (DBs)?
Residual current breakers with overload (RCBOs) and circuit breakers (CBs).
Why should the main switch in a consumer unit (CU) be rated to switch the full load current?
To ensure safety and prevent failure.
What are DBs classified by, in terms of the number of modules or ways they can accommodate?
Modules and ways.
Which protective devices are typically used for resistive loads such as heating elements and incandescent lighting?
Type B circuit breakers.
What is the limit to the maximum current that an overcurrent protective device (fuse or circuit breaker) can interrupt called?
Rated breaking capacity.
What is the main purpose of the main switch in a consumer unit (CU) within a domestic dwelling?
To isolate both line and neutral conductors.
What is the primary reason why BS 3036 devices are no longer suitable for many installations?
They have low breaking capacity.
What does Ics represent for circuit breakers?
The service short-circuit capacity.
What type of circuit breaker characteristic causes it to operate instantaneously at a specified current?
Magnetic characteristic.
Which term is used to describe the limit to the maximum fault current that an overcurrent protective device (fuse or circuit breaker) can safely interrupt?
Rated short-circuit capacity.
Under what conditions is selectivity not necessary for protective devices?
If no danger or inconvenience arises.
What factor is used to alter the capacity of a conductor based on the ambient temperature of the cable?
Rating factor (Ca)
What is the maximum operating temperature for general thermoplastic insulation?
70°C
How is the current-carrying capacity of a conductor affected by conductor insulation?
Conductor insulation sets the upper limit for current-carrying capacity.
What does a rating factor of 0.5 applied to a cable passing through thermal insulation indicate?
The conductor’s capacity is reduced by 50%.
When is it beneficial to separate larger current circuits from smaller power circuits in terms of grouping?
When circuits have a mixture of sizes and types.
How is the grouping of circuits affected by the number of circuits grouped together?
The more circuits grouped, the lower the current-carrying capacity.
For the provided four circuits, wired in non-sheathed, single-core cable, installed in a trunking, which one(s) can be ignored for grouping purposes?
1.5 mm2 cable with a design current of 2 A
When is a thermal insulation rating factor (Ci) applied to a cable?
When cables pass through thermal insulation.
Which factor is used to determine the conductor’s capacity based on the grouping of circuits?
Rating factor (Cg)
What is the maximum operating temperature for thermosetting insulation?
105°C
In what situation is the Installation method number used instead of the Reference method letter for cable installation?
When flat-profile cables are installed above ceilings or in stud walls
What is the formula to calculate the group rating factor (C) when circuits in a group are not similar in size and type?
C = n / g
If a cable’s design current is 20 A, and it carries a current of 6 A, is it necessary to consider it for grouping purposes?
Yes, because it’s carrying more than 30% of its grouped rating.
Which of the following terms refers to the two detailed ways of describing methods of installation for electrical cables?
Reference methods and Installation methods
What can happen if the method of installation for electrical cables is not chosen correctly?
The cable may run too hot
Which method of installation involves enclosing cables in conduit within an insulated wall?
Method A
If a cable is in contact with wooden joists or a plaster ceiling and covered by thermal insulation exceeding 100 mm thickness, which installation method is used?
Method 101
In the grid method for circuit design, what does Iz represent?
The current-carrying capacity of a cable under specific installation conditions
When determining the maximum voltage drop for a circuit, what are the maximum allowed values for lighting and power circuits, respectively, as stated in Appendix 4 of BS 7671 for a public supply?
3% for lighting and 5% for power circuits
What is the maximum mV/A/m permitted for voltage drop when selecting a cable size based on the maximum voltage drop calculation?
10 mV/A/m
What is the primary purpose of the grid method for circuit design?
To efficiently size conductors for multiple circuits.
What does the rating factor (Cf) of 0.725 apply to in cable installation?
BS 3036 semi-enclosed fuse
Which regulation in BS 7671 requires that a ring-final circuit cable must have a capacity no less than 20 A?
Regulation 433.1.204
Which tables in BS 7671 are used to determine the current-carrying capacity of single-core cables with 70°C thermoplastic insulation?
Table 4D1A
When determining the design current (Ib) for a domestic cooker circuit, what is the value of the factor 10 in the calculation?
It accounts for increased current during starting.
If a cable’s Iz rating is less than the selected protective device’s In rating, what should be done?
Select a larger conductor size (It).
What is the primary purpose of selecting a conductor size based on the maximum voltage drop calculation?
To prevent excessive voltage drop in the circuit.
What is the primary purpose of correctly sizing circuit protective conductors (cpc)?
To ensure the safety of the electrical installation
What does the adiabatic equation help determine when sizing circuit protective conductors (cpc)?
Minimum cpc size for thermal constraints
Which installation method is suitable for a cable in a stud wall containing thermal insulation with the cable not in contact with the wall?
Method 103
Which regulation in BS 7671 requires that the chosen conductors for ring-final circuits must have a capacity no less than 20 A?
Regulation 433.1.204
Total earth fault loop impedance (Zs) is a combination of which components?
Ze and R1
What is the maximum allowed voltage drop for a lighting circuit supplied from a public network, according to Appendix 4 of BS 7671?
3% of the supply voltage
What is the maximum mV/A/m permitted for voltage drop when selecting a cable size based on the maximum voltage drop calculation for a ring-final circuit?
20 mV/A/m
In the grid method for circuit design, which column represents the maximum voltage drop (mV/A/m) for the selected conductor?
Column 11
Which factor allows for the rise in temperature of a cpc during normal operation of live conductors?
Table I3 factor
What is the maximum permissible disconnection time for a final circuit supplying a fixed heater protected by a BS 88-2 device?
0.4 seconds
To determine the suitable cpc size for thermal constraints, which equation is used?
Adiabatic equation
What factor is used to determine the k-value when applying the adiabatic equation for sizing cpc?
Cable type factor
In the design grid method, what does column 13 typically represent?
Line conductor size
What is the purpose of the adiabatic equation in circuit design?
To calculate the thermal constraint of circuit conductors
If a circuit is protected by a 20 A Type B circuit breaker, what is the maximum permissible disconnection time according to BS 7671?
0.4 seconds
If a circuit has a calculated Zs value of 0.68 Ω, is it acceptable if the maximum permitted Zs from Table 41.2 is 1.37 Ω?
Yes, it is acceptable.
Which table in BS 7671 should be referenced to determine the maximum permitted Zs for circuits protected by circuit breakers or RCBOs for all disconnection times?
Table 41.3
What should you check if the calculated Zs exceeds the permitted values from Tables 41.2, 41.3, or 41.4, but the device is an RCBO?
Adequacy of short circuit protection
When researching values for multiple circuits in BS 7671, what is the advantage of finding values for all circuits while on the same table?
It saves time and makes the design process more efficient.
What condition must be met for a circuit to comply with Chapter 43 requirements in terms of coordination?
Ib ≤ In
Which equation is used to calculate R1 + R2 for a circuit when determining the maximum permissible Zs?
R1 + R2 = (mΩ/m) × (length in meters) × (I3 factor)
What is the primary purpose of sizing circuit protective conductors (cpc) correctly?
To reduce the total earth fault loop impedance (Zs)
What is the primary factor that affects the temperature rise of a circuit protective conductor (cpc)?
The ambient temperature of the installation location
What is the purpose of the table I3 factor in the calculation of R1 + R2 for circuit protective conductors?
To account for the rise in temperature of the cpc during normal operation
What is the primary purpose of applying diversity in electrical design?
a) To increase the circuit’s maximum demand
To determine the actual consumption needs
What does the equation Ib ≤ In ≤ Iz represent?
Correct current coordination
What is the significance of the 1.45 factor in overload conditions?
Most protective devices will not disconnect below this current.
When a circuit is subject to overloads, what formula should be applied to determine the permissible current (I)?
Ib = 1.45 × In
In the provided example, if a circuit has a design current of 14 A and is protected by a 16 A device, what would the permissible current be when overloads are considered?
23.2 A
When might a designer compensate for potential overloads in a circuit?
a) When calculating maximum demand
In situations like a hotel conference room with temporary heaters
What is the purpose of ensuring correct current coordination in electrical circuits?
To ensure the protective device is suitable for the circuit’s design current
Which method of applying diversity involves factoring individual final circuits based on likely usage?
Factoring circuits method
which type of accessibility aid in dwellings often requires an uninterruptible power supply (UPS) due to critical medical requirements?
Home medical systems
What does the term “maximum demand” refer to in electrical design?
a) The highest current rating of a protective device
The total consumption of an installation when everything is used simultaneously
Why might a circuit in a hotel be subjected to a higher diversity factor than a domestic dwelling?
To account for the varied facilities within the hotel
What does the factor “0.4” represent in the factoring installations method of applying diversity?
It is applied to dwellings without electric heating as the main form of heating
In the context of maximum demand and diversity, what is diversity based on?
a) The size of the building
he expected operation of the installation
What is the significance of using smart metering in conjunction with load switching devices?
To stop vehicle charging during times of high consumption
When might load control and smart metering be used to manage electrical demand in a domestic installation?
To prevent overloads during peak demand
In the factoring circuits method, what is a factor of “1” applied to a circuit’s design current most likely indicating?
A circuit with no diversity allowed
Which of the following is NOT a reason to calculate maximum demand?
Designing a circuit with high impedance
When is diversity typically applied to maximum demand calculations?
a) To increase the maximum demand value
To determine the actual consumption needs
What does the term “prosumer’s installation” refer to in electrical systems?
An installation capable of consuming and generating electricity
