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The service connection consists of two components:

• The service drop. This includes usually two, but occasionally three or four, conductors between the consumer and the distribution line; their connections to the distribution lines; and their connections to the entrance of the consumer's residence or business (Fig. 99).

In most cases, the service drop is comprised of overhead conductors. This implies easier and lower-cost construction and permits more flexibility if that is needed after construction of the mini-grid, such as to accommodate a change in the location of the residence on the property or a replacement of a temporary structure located on one part of the property with a more permanent one elsewhere on that property. For these reasons, an overhead service drop is what is assumed in this section. Occasionally, an underground service connection is used, as in the case of the GECO projects implemented by the French in several countries in Africa (see p. 193).

• The service entrance. The service entrance is comprised of the elements necessary to take the electricity from the service drop to inside the customer's premises. Conventionally, the service entrance includes the conductors and associated hardware from the service drop to the meter, the meter, and in some places a disconnecting device. For mini-grids, the meter may be omitted and the service entrance may lead directly indoors to the customer's distribution board or junction

service drop

distribution line service connection

service entrance

housewiring

Fig. 99. The basic components to deliver power from the distribution line to the user.

box, which might then include a current-limiting device as an alternative mechanism to a meter for controlling/monitoring electricity use (see

"Metering", p. 154). Or the meter and the customer's distribution board may both be replaced by a single device, such as the prepayment meters being widely promoted in the Republic of South Africa.

In some countries, the meters or current-limiting devices are placed on the power pole itself, before rather than after the service drop (Figs.100 and 132). Access to these for meter reading or repair may even require the use of a chair or even a ladder. While such a placement minimizes the chances of tampering, this option is not recommended because it makes accurate meter-reading difficult. If current limiting devices are affixed to the pole, this makes replacing or resetting the fuse element or breaker difficult. Furthermore, a consumer might be tempted to climb up the pole to reset the current-limiting device to turn on his power and thereby puts himself at risk by coming into contact with energized lines.

Service drop Conductor type

While bare conductor can be used for a properly designed main distribution grid, it should not be used for service drops for mini-grid systems. Bare energized conductors connected directly to a consumer’s dwelling pose a high risk to electrical hazards to the general public.

Of the conductors described in Chapter VII, multiplex conductor is specifically designed for service drop installation, and hardware exists for deadending, splicing, and connections. When available, it should be used in cases with the level of consumption found in industrialized nations, i.e., hundreds of kWh per month.

However, such high usage is unusual on mini-grids. Therefore, smaller insulated, single-core copper conductor is the most commonly used for service drops. This is especially so in unregulated installations because the installation is inexpensive and literally can be done by anyone who can connect wires.

Unfortunately, these individuals may also have little concern for, or knowledge of, the safety hazards or electrical or mechanical limitations involved.

This type of service drop has the highest failure rate. Because of the conductivity of copper, a fairly small conductor is required to serve the purpose if it is selected on the basis of limiting the voltage drop along the service drop to a specific value. This is even more the case for mini-grids, where consumer demand is typically very low, further reducing the required conductor size. For example, if the voltage drop along a 230-V single-phase service drop were restricted to only 1 % and if several inefficient incandescent lights

Fig. 100. In Thailand, meters are

commonly mounted at all heights on the nearest pole.

and a TV were the only end-uses, then 1-ampere service would provide 230 W which would be more than adequate.^* A copper conductor as small as 0.5 mm² in area would transmit this power a distance of about 30 m and still satisfy these conditions. Alternatively, if each home had only one capacitor-corrected fluorescent lamp which each consumed a total of 30 W (i.e., with losses), a 0.5 mm² service drop could serve 4 homes evenly distributed along 100 m length of service drop strung from house to house. For this reason, the minimum size conductor is restricted more by physical strength requirements than by its current-carrying capacity. Due to the small size of conductors involved (usually less than about 3 mm²), the installations overstress the cable for what it was intended. Long spans will break at the fastening point due to fatigue of the copper as a result of movement caused by wind and mechanical stress.

When compared with copper, aluminum conductor is less expensive but has 60 % more resistance than a copper conductor of comparable size. Furthermore, when used for service drops to meet small power demands, small aluminum conductor faces the same problems noted previously for copper.

In summary, for the typical mini-grid project, individual consumer loads are often very small.

Furthermore, costs must be minimized in order to make electricity more accessible to households in the community. Based on the good conductivity of copper, small conductor could be used but, as mentioned above, the conductor is susceptible to breaking through fatigue. Larger copper conductor might be used simply because of the increased strength that it offers, at an additional cost. One approach for making use of the large current-carrying capacity of copper conductor while avoiding the need to purchase large conductor simply to satisfy strength requirements is to use a homemade duplex option described in Box 8.

This capitalizes on the use of good conductivity of copper conductor and the strength of steel conductor to come up with a cost-effective hybrid. But because the steel wire would be bare, use of this option should be restricted to systems where the neutral conductor is properly grounded. Alternatively, PVC-coated steel fencing wire might be used.

Conductor Sizing

In various countries, national electrical codes have been established to serve as guidelines to be adhered to in order to ensure the design of a safe electrical system. But one has to apply such codes judiciously because they have generally been designed to address conventional needs found in urban areas where constraints are often different from those found in rural areas. For example, minimum conductor sizes have been established by the need to ensure that adequate capacity is available to meet the load that might be expected in urban areas. This is often well in excess of what is found in rural areas and, in these cases, abiding by these guidelines unnecessarily increases the cost of electrification, making it less accessible to rural communities.

As with the sizing of conductors used for the main distribution line, one important factor affecting the size of the conductor used for the service drop is the acceptable voltage drop along this section of line.

This is usually set at no more than 1 to 3 % under maximum consumer load. The acceptable value is somewhat affected by the actual size of the voltage drop already incurred through the distribution line from the powerhouse up to that point.

The size of the service drop conductor required so that the voltage drop at the end of the line does not exceed the desired value (see above) is calculated with the same equations used to calculate the size of the

* It might be noted here that, even in rural homes connected to the national grid, a peak coincident power demand of about 250 W per household is common in many parts of the world (unless the electricity is so heavily subsidized that it encourages unnecessary over-consumption and waste).

Box 8. Homemade duplex service drops for households with small power demands.

The principal justification for using a conductor that is larger than that required to achieve an acceptable voltage drop is to ensure its structural integrity over its lifetime. An alternative approach is to use a much stronger galvanized steel conductor as a messenger wire to provide all the tensioning strength and to support the much smaller insulated copper conductor that is required to serve the small loads typically encountered. In this case, the steel messenger wire would serve two purposes: in addition to supporting one length of copper conductor, it would also serve as the second conductor (the grounded neutral) for this single-phase service drop.

The usual argument against using steel as a conductor is that steel has 11 times the resistance of copper.

Therefore, a steel conductor must have a diameter of slightly more than 3 times that of a copper conductor to have the same resistance. However, this presents no real obstacle because it is still cheaper than the copper conductor it replaces. Furthermore, the principal cost savings from using this homemade duplex is the much smaller and cheaper copper conductor that can be used to serve small loads.

Because of its limited current-carrying capacity, this small duplex conductor should only be used if the supply is current-limited and uses a device such as a PTC thermistor, fuse, or miniature circuit breaker (see alternative approaches to metering, p. 156). Otherwise the current-carrying capacity of the conductor might be exceeded or excessive voltage drops might adversely affect the performance of the consumers' loads.

The preferred insulation for the copper conductor is cross-linked, carbon-impregnated polyethylene (XLP). Otherwise, conventional carbon-impregnated polyethylene would be a very good second choice.

Polyvinyl cloride (PVC) insulation may or may not provide long-term insulation because of the adverse effects of exposure to the UV, rubbing, etc., on this material.

To prepare this duplex conductor for use only with systems with a properly grounded neutral, the steel wire and insulated copper conductor are twisted together either by hand for smaller lengths (perhaps less than 10 m) or perhaps by using a modified twine winder for longer lengths. The copper winds over the steel because the steel is

stiffer. Simply wrapping plastic insulating tape at both ends of the drop is adequate to keep the wires together.

The steel messenger is deadended at each end of the drop by passing it around the insulator, tensioning as much as possible by hand, and then wrapping the end of the steel wire around itself.

Note: The resistivity of standard annealed copper wire is 0.018 ohm⋅mm²/m while that of zinc-coated steel-core wire is 0.19 ohm⋅mm²/m. The resistance of a specific conductor is obtained by multiplying the appropriate figure just given by the length of the conductor and dividing it by its cross-sectional area.

Table 13. Electrical specifications for copper and steel wire.

main distribution conductor (see Table 8, p. 76). However, if small conductors are used (i.e., less than about 10 mm²), the value of inductance x for the conductor is much smaller than its resistance r and the terms “x sin φ” can therefore be neglected. The equation for voltage drop for a single-phase service, then simplifies to the following:

Note that, for these small conductor sizes, the solution to the simplified equation is independent of the power factor.^* To facilitate solving this equation for a service voltage of 230 V, the graph in Fig. 101 was prepared. To use this graph to size a specific service drop, sum the products of the peak coincident load in each home (in watts) along that drop and its distance from the beginning of that service drop (in meters). Look for this value on the horizontal axis and then move up to the point where the line for the desired percentage voltage drop is reached. The required size for a copper conductor is determined by the curve closest to that point. Multiply the area by 1.6 if an aluminum conductor is to be used.

As an example, assume that a home with a peak coincident demand of 200 W is located at 40 m from the beginning of the service drop and a second home with a peak demand of 400 W is located at the end of this 70 m service drop. The value of P x L is (0.20)(0.040) + (0.40)(0.070) is 0.036 kW⋅km. Referring to the table or equation, a copper service drop of 2.5 mm² (or aluminum service drop of about 4.0 mm²) would be required for the voltage drop not to exceed about 1 %.

* As can be seen from the equations for power loss (p. 76 and Table 8), power loss is dependent on the power factor.

The power loss along service drops is inversely proportional to the square of the power factor. For example, doubling the power factor from 0.5 to 1.0 through power-factor correction reduces power loss in the line by a factor of four.

Fig. 101. A graph to calculate the voltage drop at the end of a copper 230-V single-phase service drop serving one or more homes. The area of the conductor associated with each curve is indicated at the top and right of the graph.

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