Electricity on Construction Sites
G. Parvin
Revised by R.A. Hardy (Electrical Design Engineer) Revised by Eur Ing Geoffrey Stokes
BSc(Hons), CEng, FIEE, FIBSE (Principal Engineer, NICEIC)
Electrical installations for constructional purposes are not in many instances afforded the time and attention warranted. Relative to the main contract the value of the constructional electrics is low; but if sufficient attention is given to this discipline prior to the commencement of site works, considerable expense will be spared.
The term ‘temporary’ is often used for electrical installations for constructional purposes and this term conjures up visions of a length of twin and earth cable con- nected into a 30 A single-phase and neutral switch-fuse, trailing across rough ground eventually disappearing into a 13 A metal-clad socket-outlet mounted on a pattress.
The ‘installation’ is carefully engineered to fulfil all the site electrical requirements for a modest price!
Fortunately, due to Electricity at Work Regulations 1989 and the efforts of the Health and Safety Executive, equipment manufacturers and consulting engineers have greatly reduced the numbers of serious accidents resulting from improper use of equipment or the use of inferior quality products, thereby ensuring that the methods we employ in the design and application of products enable the UK to be proud of the ‘Safety at Work’ situation.
It is the responsibility of the main contractor to ensure that the construction pro- gramme is adhered to, and he will require the assurance that the electrical system is suitable to provide reliable power distribution, whether the contract period is over six months or six years, and involving a 2 kVA supply or a 2 MW supply.
The equipment and designs discussed in this chapter are for use on low voltage (l.v.) systems only, although it is realised that occasionally it is necessary to accept supplies at higher voltages, such as 11 kV and 33 kV. Such intake voltages involve the use of h.v. switchgear and transformers which are outside the scope of the British Standards and Codes of Practice mentioned herein.
This chapter has been prepared to assist the electrical designer and contractor to apply Regulations and Codes of Practice in the compilation of a comprehen- sive electrical distribution system which will provide power for all of his site requirements.
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EQUIPMENT DESIGN AND MANUFACTURE
British Standard 4363,Specification for distribution units and electricity supplies for construction and building sites was published in 1968 and re-written in 1998 and pro- vides an invaluable base against which to manufacture construction site distribution equipment. It is quite common for supply authorities in the UK to be asked to provide 500 kVA at l.v. which entails the installation of 800 A switchgear at the supply intake point.
There are a number of important aspects involved in the general design of distribution units which are discussed below. BS 7375: 1996: Code of Practice for Distribution of electricity on construction and building sites provides a wealth of guidance for engineers, contractors, builders and others involved in provision of equipment suitable for use is such arduous site conditions.
Mechanical
Robust construction is necessary to enable the equipment to fulfil its on-site require- ments, as exposure to rough handling, accidental vehicular nudging and unit repo- sitioning are common occurrences. In addition to this, the environmental conditions experienced on construction sites require careful consideration. Equipment must be capable of continuous operation in coastal locations without any additional pro- tection; similarly, tropical climates must be catered for with the humidity and sand- storms that accompany them. Additional measures are necessary for very hostile environments, such as the use of filters, anti-condensation heaters, etc., but the basic design of the unit remains unaltered.
Electrical
Prevention of direct contact with live parts is achieved by insulation and barriers or enclosures. Protection against indirect contact is effected by efficient earthing and careful selection of protective devices to provide automatic disconnection.
Ruggedness, reliability, versatility and safety of operation are all combined to provide an assembly which will operate satisfactorily for long periods in onerous environments.
RANGE OF EQUIPMENT
The range of equipment likely to be found on a typical construction complex is detailed below.
Incoming supply assembly (ISA)
The purpose of the ISA on a construction site is primarily to accept the Electricity Distributor’s main cable, afford metering equipment space and consumer’s main protection. The ISA comprises two compartments each with its own means of
access to authorised personnel. The metering compartment houses the supply authority’s equipment and can be sealed. The consumer’s main switch compartment comprises the main isolator (lockable in the OFF position) and protective equip- ment. Sufficient facilities are afforded for the maximum cable size likely to be terminated.
If the metering equipment is housed within a substation or the authority’s own enclosure, then the ISA provides the main site protection and control only. Figure 5.1 shows typical forms of ISA.
Incoming supply and distribution assembly (ISDA)
The ISDA comprises a compartment for the incoming feeder cable and metering equipment; a main/isolation compartment for main control; and an outgoing com- partment housing sub-circuit control and protection devices.
Incoming feeder compartment
The incoming feeder compartment provides facilities for the incoming cable termi- nation and the Electricity Distributor’s cut-out. When called for, the facility must exist for incorporation of current transformers (CTs) and metering equipment, as well as potential fuses. All the equipment contained in this section belongs to the supply authority and access is restricted to their authorised personnel.
Fig. 5.1 Incoming supply assembly.
Main switch compartment
The main switch compartment contains a moulded case circuit breaker (mccb), air circuit-breaker (acb), or a switch-fuse. It is normal practice to interlock the compartment door with the main switch to prevent access to live equipment. Simi- larly a lockable dolly is recommended for securing it while in the OFF position.
Termination facilities require particular attention and must be capable of accepting cables of capacity commensurate with the rating of the ISDA and in line with BS 5372.
Outgoing feeder compartment
The number of outgoing circuits depends on the particular application, but a minimum quantity of twelve should normally be provided for. Loads from this section may vary from perimeter lighting to large tower cranes or sub-main distri- bution units. Another extremely important consideration is the prospective fault level at the incoming terminals of the ISDA and this information must be known at an early stage of the contract. It is conceivable that the ISDA will be positioned adjacent to the substation and therefore the protective devices incorporated should reflect the high fault level associated with the main supply transformer.
Multisection removable gland plates assist when making provision for outgoing circuits, and the termination of protective and control devices must be provided behind a removable cover plate to enable safe connection of equipment to be made while adjacent circuits are energised. Figure 5.2 shows a typical ISDA.
Fig. 5.2 Incoming supply and distribution assembly.
Mains distribution assembly (MDA)
When metering equipment is installed elsewhere, the ISDA then becomes an MDA (Fig. 5.3). The assembly consists of two sections, incoming and outgoing. It is not economically viable to utilise a single size for MDAs, and therefore they are normally embraced in the following ratings: 100–300 A, 400–600 A, 600–800 A, 800–1250 A and above 1250 A. (IEC recommendations may adjust the foregoing capacities to 125 A, 315 A, 400 A, 630 A, 800 A and 1250 A.)
The positioning of an MDA within a distribution network generally depends on its size and application; it may serve as the major distribution device or be supplied from an ISDA or upstream larger MDA. To this end versatility is of paramount importance while maintaining safety of operation.
Incoming feeder compartment
Accessibility and ease of termination are prime factors of the MDA incoming feeder compartment. Distance between gland plate and cable termination point requires special attention, for, unlike the ISDA, it is more likely that the supply cable will be PVC/SWA/PVC and ease of glanding and terminating will need the stated space of BS 5372.
Fig. 5.3 400 A mains distribution assembly.
If the incoming feeder compartment door is lockable, the handle of the main iso- lator should be operable externally to the enclosure, and preferably interlocked with the access panel or door. As with the ISDA the main isolator handle should be lock- able in the OFF position.
Outgoing feeder compartment
It is recommended that provision is made in the outgoing feeder compartment of an MDA to provide facilities to include at least twelve circuit-breakers, and with the variety of loads which can be introduced on to a construction site this quantity is certainly not overspecified. To reduce overall length it is normal practice to fit mcbs or mccbs in both the back and front of an MDA, and additional breakers can be incorporated with ease at a later date if space is left for this purpose. A low-level neutral bar assists when connecting to outgoing circuit-breakers, allowing them to be installed safely and quickly. When residual current devices (rcds) are fitted as additional protection against electric shock, it is important to ensure that this neutral conductor passes through the device to an individual neutral connector. Outgo- ing supplies utilise armoured or sheathed cables although the use of socket-outlets should not be discounted and to this end the enterprising manufacturer designs and builds an MDA which suits both methods of distribution. Figure 5.3 includes two socket-outlets.
The purpose of the standards relating to construction site electrical distribution equipment is to provide a system which can be used on another site. A certain MDA may be suitable for application where fault levels are low, but one must guard against the possibility of the unit, after it has completed its term of application, being transported to a larger site with possibly a prospective fault current of 26 kA. The check list in Table 5.1 highlights the information required to assess the suitability of an MDA for a particular application.
Table 5.1 Check list/questionnaire for site distribution equipment.
(1) Location/atmospheric conditions.
(2) Trip rating of devices.
(3) Polarity of devices.
(4) Bolted or switched neutral.
(5) Main supply details, voltage and frequency, system protection, etc.
(6) Full or half size neutral.
(7) If a fault level is not specified try to establish:
(a) transformer rating,
(b) whether the transformer feeds the distribution unit direct,
(c) if the answer to (b) is no, what other switching device is interposed between?
(d) size and type of cable(s) between transformer and distribution unit, (e) distance between transformer and distribution unit.
(8) Size and type of cables – emphasise if aluminium cable is being used.
(9) Front or rear acces.
(10) Enclosure protection, environmental.
(11) Are glands supplied by cable contractor?
(12) Instrumentation requirements.
(13) Any restrictions on dimensions.
(14) Label details (if known).
Transformer assembly (TA/1/3)
The MDAs distribute the mains voltage direct to the larger electrical loads or to the TAs which are located at load centres wherever possible. They operate from a 400 V three-phase supply or a 230 V single-phase supply and step down the voltage to 110 V. The standard design of a TA comprises a double-wound transformer housed in a weatherproof enclosure and fitted with socket-outlets and protective devices.
The star point of the secondary side of the three-phase unit is earthed and the single- phase unit secondary is centre-tapped to earth, reducing the potential to earth to 63.5 V and 55 V respectively. The widespread use of transformers providing power at 110 V centre or star-point earthed has resulted in a considerable reduction in injuries due to electric shock since the early 1970s.
There are two main types of TAs: 1 kVA to 25 kVA continuously rated, and 0.5 kVA to 3 kVA power tool rated. It is usual to associate power transformers with final distribution on a construction complex, and the power tool rated portable transformers with smaller projects, such as service industries and house building.
Figure 5.4 shows the types of TAs.
The same basic parameters apply to each type of transformer: 110 V centre- tapped or star-point earthed. Secondary distribution is provided by 110 V socket outlets to BS EN 60309 (BS 4343). The quantity of socket-outlets available on a TA can vary slightly, depending on company standards. Table 5.2 identifies the
Fig. 5.4 Typical transformer assemblies.
more common arrangements currently in use. Full flexibility is important and the availability of 110 V imperative. When recommending the number of socket- outlets it is better to err on the side of too many, as long as adequate protection is provided.
Armoured and non-armoured cables
It is important that supplies for construction and building sites meet the require- ments of section 604 of BS 7671:Electrical Requirements for Electrical Installations andBS 7375: 1996:Code of Practice for distribution of electricity on construction and building sites. It is important to note the requirements peculiar to wiring systems which include the following.
Wiring systems must be selected and arranged such that terminations are not sub- jected to strain, unless the terminations are specially designed to accommodate the likely strains. Cables that transverse roadways or walkways must be adequately protected against mechanical damage. Cables and flexible cords must be suitable for their intended use and, where the operating voltage exceeds 63.5 V, must incor- porate a continuous metal sheath or armour that must be earthed. Other than for fixed wiring, the armouring of such cables must be in addition to a protective con- ductor core within the cable.
Armoured cables operating at a voltage exceeding 12 V but not normally exceed- ing 63.5 V must have an oversheath of PVC or of an oil-resisting, flame-retardant (O&FR) compound; for example, non-flexible armoured cables may be of the wire- armoured type with conductors insulated with PVC or XLPE with an oversheath Table 5.2 Number of socket-outlets related to transformer rating with normal protective arrangements.
Socket-outlets
Protection
Rating (kVA) number and rating (A) primary secondary
0.5–3 (PTR)* 2 ¥16 mcb or fuse
3 2 ¥16 mcb or fuse
5 (1-phase) 4 ¥16; 1 ¥32 mcb† mcb†
5 (3-phase) 3 ¥16 (1-ph) 1¥32 (3-ph) mcb† mcb†
10 (1-phase) 6 ¥16; 2 ¥32 mcb† mcb†
10 (3-phase) 5 ¥16 (1-ph) 1 ¥16 (3-ph)
1 ¥32 (1-ph) mcb† mcb†
25+(3-phase) According to specific
requirements‡ mcb† mcb†
* Power tool rated.
†All mcbs are to BS EN 60898. It is permissible to supply two 16A single-phase socket-outlets from one double-pole circuit breaker of 20A rating, provided that the cable attached to the associated plug top is suitably sized.
‡The wide variety of socket-outlets possible prevents a typical arrangement being specified.
of PVC. Non-armoured cables operating within this voltage range must be insulated and sheathed with general purpose (GP) or heat-resisting elastomers such as vul- canised rubber insulated GP cables or O&FR cables, both to BS 6007.
Flexible cords and cables, with a conductor cross-sectional area (csa) of not less than 1.5 mm2, may be the armoured type with oversheath of heavy-duty O&FR to BS 6708. Such cables must be suitable for correct anchoring at accessories and switchgear. Where operating on reduced low voltage, cables and cords must incor- porate a tough-rubber or PVC outer sheath that is water, oil, abrasion and impact resistant and suitable for the ambient temperatures prevailing. Cables with PVC insulation and sheathing to BS 6004 must not be used as flexible cables or cords. All cables that are likely to be moved in normal use must be the flexible type.
PVC and XLPE steel-wire armoured cables are suitable for fixed wiring provided they are properly supported throughout their length and terminated in appropriate glands. Cable glands play another important role in providing a connection to earth for the armouring and for this reason, if for no other, the gland must be sufficiently tightened and kept free of undue strains.
The csa of the circuit protective conductor must be not less than that of the largest associated phase conductor.
Where cables are laid underground either for site supplies or as part of the per- manent installation, their routes must be properly recorded on a map or site plan.
The metal armouring or sheath of cables must be adequately earthed irrespec- tive of whether a protective conductor is provided in the form of a separate core within the cable. This will normally be achieved by the proper glanding of the cable together with gland tag, nut, bolt and washers with a short length of suitably-sized copper protective conductor to the earthing terminal within the switchgear. On no account should an earthing and bonding clamp to BS 951, or other similar device, be applied to the metal sheath or armouring for this or any other purpose. Such misuse of the clamp is likely to damage the cable and would be considered unreli- able for earthing purposes.
Where site conditions are particularly arduous, the designer would wish to con- sider the additional benefits of monitoring the protective conductor (see BS 4444:
Guide to electrical earth monitoring and protective conductor monitoring) of some or all of the distribution and final circuits.
Transformer supply cabling
It is normal practice to use PVC insulated armoured cables to connect and MDA to TAs: however, for flexibility some TAs may be fitted with an appliance inlet and socket-outlet for connection and loop on of a flexible armoured supply cable. This arrangement enables speedy re-siting of TAs with minimum inconvenience.
110 V distribution equipment
A particularly important aspect of the sub-distribution within the concentrated working areas of a building under construction is the availability of 110 V socket- outlets. This power is required for general lighting, safety lighting and small tool operation. It is necessary to provide multiples of socket-outlets for this purpose, and the following types of equipment are available.
Socket-outlet assembly 110 V single-phase (SOA/1)
The single-phase outlet unit is intended to be supplied from a 32 A single-phase source which in turn is derived from the local TA. The SOA/1/4 has four 16 A socket outlets, each pair being protected by a 20 A dp mcb; the SOA/1/6 has six socket outlets similarly protected. The SOA/1 units are in a portable and free-standing form, the cable being heavy duty TRS or equivalent.
Socket-outlet assembly 110 V three-phase (SOA/3)
The three-phase SOA/3 is not as common as the SOA/1/4–6, but the SOA/3/4–6 is normally provided with an additional 32 A three-phase and earth socket outlet for looping to a similar unit on a rising main system. This system of supply is more per- manent and the flexibility is achieved by extensions to the system. Main control and protection is derived from a 32 A tp mcb and socket-outlet on the local TA, and the attached four-core TRS cable is fitted with a matching plug.
Extension outlet assembly (EOA)
EOAs are available in both single-phase and three-phase designs, and are desig- nated EOA/1 and EOA/3 respectively.
The versatile EOA/1/4 is intended, as the name implies, as an extension to the main single-phase 110 V distribution system and comprises a portable weatherproof enclosure fitted with four 16 A dp and earth weatherproof socket-outlets to BS 4343.
The flexible cable is connected via a plug top to an available socket-outlet on the SOA/1/4–6, the SOA/3/4–6 or directly into the local TA. Some 2 kW is available at this point and a 20 A mcb protective device affords control.
Extension leads
Re-siting EOAs can be carried out by non-skilled instructed personnel and the use of extension leads speeds up this operation. Each extension lead comprises a length of heavy duty rubber insulated and sheathed flexible cable fitted with a 110 V con- nector (portable socket-outlet), and matching plug.
DESIGN OF SYSTEM
This section provides a practical guide to the design of a distribution scheme for electrical services to plant, machinery, power tools, lighting and welfare facilities associated with construction sites.
Consultation with construction personnel
Before commencement of any design work the electrical design engineer needs to consult the main contractor’s personnel to determine the methods they intend to use for the project construction. Various drawings should be made available, includ- ing an overall site plan detailing adjacent public roads, typical floor plan and an elevation drawing.
When the electrical designer becomes involved in a project of this nature much pre-planning by the main contractor will have been carried out. Underground