7.5.2 - Special regulations for construction sites

Construction site installations are like most others in that they usually rely on earthed equipotential bonding and automatic disconnection for protecting from electric shock. This is the system where an earth fault, which results in metalwork open to touch becoming live, also causes a fault current which will open the protective device to remove the supply within 0.4 s for socket outlet circuits or 5 s for fixed appliances. For construction sites, where the prospective dangers are greater, there are additional requirements. These are:

1. - The times within which disconnection must occur are reduced, except for fixed equipment. Generally the times permitted are much reduced to reflect the more dangerous nature of the construction site. These voltage-related disconnection times are given in {Table 7.1} for TN systems, which are those where the Electricity Supply Company provides an earthing terminal. Where values of maximum earth-fault loop impedance are necessary to check compliance with this requirement, it must be appreciated that the values of {Tables 5.1 and 5.2} no longer apply, and new values must be calculated using the stated supply voltages, and disconnection currents read from {Figs 3.13 to 3.19} using the maximum times from {Table 7.1}. For 240 V circuits with a maximum disconnection time of 0.2 s, maximum permissible earth-fault loop values for various types of protective device are shown in {Table 7.2}. Where the given values of earth-fault loop impedance cannot be met, protection must be by means of RCDs with operating current not exceeding 30 mA. The reduced connection times do NOT apply to 110 V centre-tapped supplies, which are in any case required to be protected by RCDs with an operating current not exceeding 30 mA.

For fixed installations (limited to main switchgear and principal protective devices) the disconnection time is 5 s, so {Tables 5.2 and 5.4} can he used to find maximum earth-fault loop impedance values.

Table 7.1 - Maximum disconnection times for construction site circuits ---------------- (TN systems)
	Supply voltage (Uo)	Disconnection time
	(volts)	(seconds)
	120	0.35
	220 to 277	0.20
	400 and 480	0.05

Table 7.2 - Maximum earth-fault loop impedance values for 240 V construction site circuits to give a maximum 0.2 s disconnection time
Type of protection	Protection rating	Max. loop impedance
	(A)	(ohms)
Cart. Fuse, BS 1361	5	9.60
	15	3.00
	20	1.55
	30	1.00
Cart. Fuse BS 88 pt 2	6	7.74
	10	4.71
	16	2.53
	20	1.60
	25	1.33
	32	0.92
MCB type 1	5	12.00
	10	6.00
	15	4.00
	20	3.00
	30	2.00
MCB type 2	5	6.86
	10	3.43
	15	2.29
	20	1.71
	30	1.14
MCB type 3	5	4.80
	10	2.40
	15	1.60
	20	1.20
	30	0.80
MCB type B	6	8.00
	10	4.80
	16	3.00
	20	2.40
	32	1.50

2. - In other installations the maximum shock voltage is given as 50 V, calculated from the impedance of the protective system to earth in ohms multiplied by the fault current in amperes. This was the basis of {Table 5.3}, but these data do not apply in this case. For construction sites, the value is reduced to 25 V. For example, for a TN system, the impedance of the circuit protective conductor (Zs) multiplied by the current rating of the protective fuse or circuit breaker (In) must not exceed 25.

For a circuit protected by a 15 A miniature circuit breaker type 1, which must carry a current of 60 A to trip in 5 s (see {Fig 3.16}), the impedance of the circuit protective conductor must therefore be no greater than:

Zs	= 25 =	25	Ohms = 0.42 ohms
	In	60

Sockets on a construction site must be separated extra-low voltage (SELV) or protected by a residual current circuit breaker (RCD) with an operating current of not more than 30 mA, or must be electrically separate from the rest of the supply, each socket being fed by its own individual transformer. SELV is unlikely for most applications, because 12 V power tools would draw too much current to be practical. Most sockets are likely to be fed at 110 V from centre-tapped transformers so will comply with this requirement.

Distribution and supply equipment must comply with BS 4363, and, together with the installation itself, must be protected to IP44. This means provision of mechanical protection from objects more than 1 mm thick and protection from splashing water. Such equipment will include switches and isolators to control circuits and to isolate the incoming supply. The main isolator must be capable of being locked or otherwise secured in the 'off' position. Emergency switches should disconnect all live conductors including the neutral.

Cables and their connections must not be subjected to strain, and cables must not be run across roads or walkways without mechanical protection. Circuits supplying equipment must be fed from a distribution assembly including overcurrent protection, a local RCD if necessary, and socket outlets where needed. Socket outlets must be enclosed in distribution assemblies, fixed to the outside of the assembly enclosure, or fixed to a vertical wall. Sockets must not be left unattached, as is often the case on construction sites. Socket outlets and cable couplers must be to BS EN 60309-2. A typical schematic diagram for a construction site system is shown in {Fig 7.6}.

Fig 7.6 - Single-phase distribution system for a construction site to BS 4363 and CP 1017