7.6.2 - Agricultural installations

Of paramount importance where livestock are present is the indication that the levels of separated extra-low voltage (SELV) are too high. Whilst the Regulations do not suggest a safe voltage for animals, simply suggesting a level 'appropriate to the type of livestock', a practical value is likely to be no higher than 25 V. Such systems, as well as those complying with SELV requirements, must be protected to IP44 (ie, protected from solid objects not exceeding 1 mm and from splashing water) or the insulation must be able to withstand 500 V r.m.s. for one minute.

All socket outlets must be protected by residual current device(s) (RCDs) with an operating current of no more than 30 mA. Whilst it is accepted that livestock cannot be protected by earthed equipotential bonding and automatic disconnection (sometimes known as EEBAD) because the voltages to which they would he subjected in the event of a fault are unsafe for them, changes to standard installation requirements do offer some additional protection. The requirements are:

1. - Disconnection times for the operation of protective devices are reduced, usually to half the normal value. The maximum times are voltage related, and are shown in {Table 7.3}.

Table 7.3 - Maximum disconnection times for agricultural circuits for ----------------- livestock (TN systems)
	Supply voltage (Uo)	Disconnection time
	(volts)	(seconds)
	120	0.35
	220 to 277	0.20
	400 and 480	0.05

The application of these reduced connection times leads to the reduced levels of maximum earth-fault loop impedance, shown in {Table 7.4} for 240 V circuits.

2. - Fixed equipment and distribution circuits are permitted to have a disconnection time of 5 s, which is the same as for normal installations, and require the maximum earth-fault loop impedance values shown in {Tables 5.2 and 5.4}. An exception is where the fixed equipment is fed from the same distribution board as circuits requiring disconnection in 0.2 s at 240 V. In such a case Either the resistance of the main protective conductor from the distribution board to the point of connection to the main equipotential bonding system must be low enough to ensure that its volt drop when fault current flows does not exceed 25 V,

or the distribution board must have its exposed conductive parts bonded to all extraneous conductive parts (such as water pipes) in the area.

3. - The maximum of 25 V for the potential difference across the protective conductor under fault conditions stated above is applied to all final circuits. This is half of the level accepted in other installations, so the protective conductor resistance must have half the normal value. Note that where an IT system (usually a generating plant) is used, special requirements apply - these are outside the scope of this Guide, and advice must be sought from a qualified electrical engineer.

4. - Supplementary equipotential bonding must be applied to connect together all exposed and extraneous conductive parts which are accessible to livestock and the main protective system. It is recommended that a metallic grid should be laid in the floor and connected to the protective conductor.

5. - Where an RCD is used it is important to ensure that the earth electrode resistance is not so high that the 25 V level will be exceeded before operation. This can be verified by using the expression

RA =	25
	IAn

where RA = the combined earth electrode and protective conductor resistance,
and An = the rated residual operating current of the RCD.

Application of the expression gives 833 Ohms for a 30 mA device, 250 Ohms for a 100 mA, 83 ohms for a 300 mA and 50 Ohms for a 500 mA. Values higher than 200 Ohms should not be used or there may be instability. The resistance of the protective conductors is usually negligible compared with that of the earth electrode.

The equipotential bonding required will create an earth zone, and special measures are necessary where a circuit fed from this zone extends outside it. If the equipment could be touched by a person in contact with the general mass of earth, disconnection time must not exceed 0.2 s, even for fixed equipment. This means that {Table 7.4} will apply to such circuits.

Fire is a particular hazard in agricultural premises where there may be large quantities of loose straw or other flammable material. A particular fire hazard on agricultural premises is damage to the wiring by rodents gnawing at cables. This effect can be reduced by cable runs which are below ceilings rather than in roof spaces and by the use of steel conduits. The Regulations require the protection of the system by an RCD with an operating current not greater than 500 mA. In practice, a 300 mA rating is likely to be used. This will result in problems of discrimination between this unit and those of lower operating current rating unless the main RCD is of the time delayed type (see {5.9.3}). Care must be taken to ensure that heaters are not in positions where they will ignite their surroundings; a clearance of at least 500 mm is required for radiant heaters.

All electrical equipment must be protected to IP44, and chosen to be suitable to operate under the onerous conditions they will experience. Wiring must be inaccessible to livestock and must be vermin proof. In practice, this will probably mean enclosure in galvanised steel conduit, or the use of mineral insulated cables, Switch and control gear must be to IP44 and constructed of, or enclosed in, insulating material. Switches for emergency use must not be in positions accessible to cattle, or where cattle may make operation difficult. Emergency switches should disconnect all live conductors including the neutral. It may be necessary to omit isolators in some cases to ensure that essential supplies (such as broiler house fans) are not disconnected unintentionally. The likelihood of panic amongst animals when emergencies occur must be taken into account.

Table 7.4 - Maximum earth-fault loop impedance values for 240 V agricultural 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