1. 5508BESG Services and Utilities Lecture 7
Electrical Installations in Buildings

2. Electrical Services In Buildings
Electrical Protection
Systems for buildings
Electrical Installations
Electrical Design

3. Useful Publications
CIBSE Electricity in Buildings (Guide K), ISBN X
BSRIA Illustrated Guide to Electrical Building Services, BG5/2005.
On site guide to BS7671, the 17th Edition of the Wiring Regulations

4. Protection against shock Protection against overload (over-current)
Safety in electrical installations:
Circuit Protection Devices & Systems
Protection against shock
Protection against overload (over-current)
Protection against physical damage

5. Protection Against Shock
How much electric current flowing through the body does it take to cause significant harm or death?
Electric Current (1 Second contact) Physiological effect
1mA Threshold of feeling, tingling sensation
10-20 mA “Cant let go” –onset of sustained muscular contraction
30-50 mA + Pain , paralysis, loss of consciousness.
0.1A Ventricular fibrillation- death almost instantly
A surprisingly small amount

6. Protection Against Shock
Forms of protection:
Basic Protection Electrical insulation and enclosures and barriers, give protection against contact with live parts. This is used to provide protection against direct contact.
Protective earthing to provide protection against indirect contact
Double or reinforced insulation, provides protection against direct contact.
30mA RCD to give additional supplementary protection against both direct and indirect contact, not used as primary protection.

7. Protection by enclosures and barriers: Ingress Protection (IP Rating System)

8. Protection By Earthing
Every exposed conductive part shall be connected by a protective conductor to the main earthing terminal and from there to the earth.
“Exposed conductive parts” means any metallic parts of the electrical installation and electrical appliances that can be touched.
It also means “extraneous conductive parts” i.e. any metallic part within the building not forming part of the electrical installation that can be touched, including pipework, ductwork, steelwork etc.

9. Local Distribution is by 3 Phase, 4 wires
The Fourth wire is zero voltage
230v
400/230v
400v
230v
400V
230V
400V

10. Protection By Earthing

12. A fuse of about 5 amp may be used
For the previous slide, assume supply is 230 v a.c. and the wattage of the appliance is 1000W. From previous work we know that the current flow in the circuit whilst the appliance is working normally would be :-
A fuse of about 5 amp may be used
The resistance of the human body is in the region of 1000 and therefore the current flow through the person would be :-
i.e. the current would reduce whilst the person was receiving a shock and the fuse would not ‘blow’. 0.23A flowing through the human body could easily be fatal.
Note in this example the figures used are only to illustrate the general principle.

13. Break in insulation causes live to contact exposed metal case of Electrical Appliance

14. Assume 1000w appliance protected by a 5 amp fuse if the “earth loop impedance” (the effective resistance of the entire earth path) is assumed to be 1 the current when the fault occurs would rise to :
a 5 amp fuse would virtually instantly blow.
Note This calculation is not intended to indicate the technique to calculate the current flow under fault conditions but simply to indicate the potential massive surge in current due to a ‘short circuit’ to earth and how this disconnects the supply via the over-current protection.
If the person was in contact with the appliance case at the time the fault occurred they may well not even know that the case become live for an instant due to the current taking the path of least resistance i.e. through the protective conductor to earth (circa 1) rather than through the person to earth (circa 1000).

15. To prevent this all extraneous conductive parts must be “cross bonded” i.e. they must have a protective conductor (earth cable) connected to the main earth terminal

17. Protection from shock By Residual Current Device (RCD)
The use of RCDs are recognised as a means of providing additional protection in the event of failure of the provision for Basic Protection, as an additional means of Fault Protection, and to protect against carelessness by users.

18. Protection against Overload (Over-current Protection)
All cables and flexes are sized to carry the current associated with the load it is connected to, the bigger the load the bigger the cable. If, by virtue of a fault or misuse the cable carries a current higher than it was designed for it will overheat, possibly to the point where a fire results. Over-current protection is designed to prevent this.
The nominal setting of the over-current protective device must be greater than or equal to the design current of the load.
The current-carrying capacity of the cables must be equal to or greater than the nominal setting of the over-current protective device.
All cable must be protected by an appropriately sized and specified over-current protective device.

19. Types of Over-Current Protection Device:
High Breaking Capacity (HBC) fuse links BS 88-6 and BS EN Mainly commercial and industrial use. Give excellent short-circuit current protection.
HBC fuse links BS 1361 House service and consumer unit fuses. Not popular for use in consumer units; however, gives good short-circuit current protection.
MCBs (miniature circuit breakers) BS 3871, now superseded by BS EN CBs Domestic consumer units and commercial/industrial distribution boards. Very popular due to ease of operation. An alternative to, or replacement for, fuse links.

20. Cable and Containment Systems
A cabling system consists of:
Conductor
Insulation
Physical damage protection often called containment.
The most common forms of cable are:

21. Cables
Brown PVC insulated phase (or live) conductor
Blue PVC insulated neutral conductor
Bare earth conductor
Grey or white PVC outer sheath
On domestic systems for small power and lighting PVC sheathed copper conductor cables are normally used. Lead sheathed and rubber insulated cable has long been superseded and, if encountered, must be replaced.
PVC sheathed and insulated twin and earth cable

22. PVC Sheath (insulator) Copper wire (conductor)
Electrical Wiring to BS 7671
PVC Sheath (insulator) Copper wire (conductor)
Pre 2006 Colours
Post 2006 Harmonised Colours
Wiring Colours like voltages have been harmonised across Europe

28. Cable Tray & Basket:
Used as physical protection for the distribution of multiple cables inside and outside non-domestic building. Cable tray is stronger and tends to be used for larger sized cables. Cable basket tends to be used for the containment of large numbers of smaller cables such as data networks, telephone and fire detection circuits.

29. Electrical Installations in Buildings
Electrical Loads:
The loads within a building are all the items that are electrically operated, these can be conveniently categorised as:
Lighting
Small power, socket outlets.
Small power, fixed appliances and specialist items
Lifts and escalators.
HVAC Plant and major specialist plant.

30. LIGHTING CIRCUITS
Usually configured to give Max load of 1kW per circuit for domestic property or 2kW for non domestic
Therefore the number of light fittings per circuit depends on the wattage of the lamps plus the losses from the lamp starting gear.
1000 watts at 230v draws a current of 4.6 Amps
Circuit rating = 5A with a 6A protection device at distribution board/consumer unit
For a non-domestic application a 10 amp protection device is likely to be used because of the higher loads.
Lighting circuits normally located in ceiling voids.
“Loop-in” system is the most common wiring method – minimum possible number of connections

31. LOOP-IN
WIRING
SYSTEM

32. POWER SOCKETS
Should be plentiful to stop overloading (see table on next slide)
Positioning
150mm – 250mm above floor and surfaces
750mm – 900mm (disabled)

33. LOCATION AND RECOMMENDED NUMBER OF SOCKETS
Kitchen 6
Utility Room 3
Living rooms 8
Dining room 4
Master bedroom
Single bedrooms
Study bedrooms
Hall and landing 2
Garage / workshop 1
Bathroom (shaver socket)
LOCATION AND
RECOMMENDED
NUMBER OF
SOCKETS
Connection using 32A rated ring circuits is the most common – ring circuits enables use of smaller cable (2.5mm2)
One ring circuit for upstairs
One for downstairs
Separate circuit for kitchen on newer premises

34. Small Power Usually configured as a ring circuit
Small power circuits can be provided for known and predictable semi-permanent loads eg computer suits or variable and diverse loads eg domestic dwelling.
Usually each ring circuit should cover a MAXIMUM of 100m2 and maximum of 54m of cable.
Protected at 30 or 32 amp. Therefore:
Watts = I x V.
Watts = 32amp x 230V = 7360watts
Typical appliance ratings….
Television 150 W, Vacuum cleaner 750 W , Hair drier 500 W, Microwave oven 850 W, Iron 1000 W, Food mixer, 500 W, Kettle W, Dishwasher 2000 W, Computer 150 W, Hand drier, 2500w, etc
Diversity can be applied, when appropriate, based on
Highest single load + 75% of remaining load

35. RING CIRCUIT

36. RING CIRCUIT LAYOUT IN SINGLE STOREY BUILDING
Steel or plastic conduit buried in wall to protect PVC/PVC conductors.
Conduit contains either individual conductors or twin & earth cable
PVC/PVC conductors incorporating live, neutral and earth (twin and earth) run at ceiling level
Twin Socket Outlet
External wall
Internal wall
Conduit to ceiling level
Consumer
Unit
RING CIRCUIT LAYOUT IN SINGLE STOREY BUILDING

37. Comments and limitations
Typical Cable Sizes
Application
Cable size (mm2)
Current rating (A)
Comments and limitations
Lighting (loop-in)
1.5 6
Up to 1000w light fittings
Immersion heater (radial)
2.5 15
May require larger cable if very long run
Sockets (ring) 32
Max 100m2 floor area and 54m cable run
Cooker (radial)
6.0 or 8.0
32 or 45
Cable specification detailed in manufacturer’s literature
Shower (radial)
6.0, 8.0 or 10.0
Up to 45

38. Typical circuit schedule
Circ. ref.
Type
Current rating (A)
Serving
Cable type/size
Protection
Comments L1
Radial 6
Upstairs lighting
PVC/PVC twin & earth 1.5 mm2
6A MCB
Connection using loop-in method L2
Downstairs lighting P1
Ring 32
Upstairs power
PVC/PVC twin & earth 2.5 mm2
32A MCB
Standard double socket outlets P2
Downstairs power P3
Kitchen power
Double socket outlets for kitchen (give manufacturer’s details) P4
Cooker
PVC/PVC twin & earth 6mm2
Max 15m cable run P5 40
Shower
PVC/PVC twin & earth 10mm2
42A MCB
Toggle switch in bathroom P6
Outbuilding & garden
Steel armoured/PVC twin & earth 2.5mm2
32A RCD
Appropriately rated external sockets to be used

39. SCHEMATIC OF DOMESTIC INCOMING ELECTRICAL SERVICE
Circuits
L1 L2 P1 P2 P3 P4 P5 P6
Combined in Consumer unit
Distribution board
Main isolator switch
Meter
Services fuse and neutral link
Incoming service cable
SCHEMATIC OF DOMESTIC INCOMING ELECTRICAL SERVICE