L’état-major et les trois niveaux d’action militaire

One more issue to consider at this point is whether or not to ground the neutral conductor for those three configurations marked with dashed grounding symbols in Fig. 15. At medium voltages, system

grounding is typically used to protect the electrical system and ensure safe and reliable service. With one conductor firmly bonded to the ground, it permits economies in the construction and use of various line equipment—such as transformers and insulators—by permitting a reduction in the required insulation levels. It increases the effectiveness of lightning arresters by providing a low-resistance path to ground and also somewhat reduces voltage drop along a line by allowing some of the return currents to flow through the earth. It increases worker and public safety, in part by facilitating the detection and

subsequent isolation of any fault to ground that might occur. An example of a fault to ground would be one caused by a phase conductor breaking and falling down to earth. The system ground(s) would provide a return path for the fault current, completing the circuit. If ground resistance is sufficiently low,

* These seem to add to more than the required current. This is not the case because the windings do not supply all the currents at the same time (i.e., the currents are out of phase).

(a)

(b)

generator windings loads

Fig. 17. Each phase conductor supplies one wye-connected load while each delta-wye-connected phase supplies two.

current through the ground loop would trip a breaker. Multi-grounded neutrals attempt to minimize ground resistance and thereby maximize fault currents by providing multiple low-resistance return paths.

However, detecting a fault current even on a medium-voltage system may still be difficult because of the relatively high resistance commonly found between the conductor that has fallen on the ground and the ground itself, even though a fallen conductor can still prove lethal to the touch. It is even more difficult on a secondary system because, from Ohm’s law, the considerably lower voltage implies a considerably lower fault current. Two additional factors reduce the effectiveness of grounding for small systems commonly associated with mini-grids:

• The fault current that can be supplied by a small generator is limited.

• Those installing mini-grids often do not put in the effort required to ensure reliable low-resistance grounds.

For these reasons, fault currents associated with low-voltage mini-grids can be too small to trigger miniature circuit breakers (MCBs) or blow fuses. Therefore, the neutral conductor should not automatically be grounded in the hope that this affords added protection. Grounded mini-grids may actually prove more dangerous. For example, when a person touches an appliance with an internal short to its metal housing, a ground provides a direct path for any fault current through the body to return to the generator. This current may be too small to trigger an MCB or blow a fuse but can be more than

sufficient to place the person at risk. This is discussed in detail in Chapter XI on safety.

Generally, little justification can be found to ground the neutral conductor of an isolated low-voltage mini-grid. More detailed descriptions of grounding and safety are found in Chapter XI. In the case of a mini-grid supplied by a transformer connected to a regional or national grid, the approach to grounding will probably be determined by the national standards in force in the country. And in these cases, the neutral conductor may well be grounded.

The following approaches are suggested for an isolated mini-grid:

• For low-cost, unsophisticated systems, with primarily lighting and entertainment loads that present the user with little chance for touching an energized portion of the circuit, a floating (i.e., ungrounded) system can simply be used. This is safer and less costly than grounding both the system neutral and consumer grounds and bonding the latter to the neutral conductor. And a system ground, even with a properly installed consumer ground, will not give a person accidentally touching the live conductor any protection. Under this latter condition, a floating system at least reduces the magnitude of the current that might flow through someone touching a live component.

• For the occasional consumer who is using other equipment with a metal housing or frame, fault currents through the body can be reduced through the use of a consumer ground. Or for an additional financial outlay, a properly installed and operating RCD will immediately open the consumer circuit when it senses a fault current. But in these cases, the system neutral should nowhere be grounded.

Depending on whether or not the system is grounded, the following actions must be taken:

• If the system (i.e., the neutral conductor used in the system) is not grounded, then all conductors should be treated as live phase conductors. While a neutral conductor might be no more than a few volts above ground, an accidental grounding of any of the phase conductors would raise the voltage of the neutral conductor to the system voltage. Therefore for a floating system, all

conductors should be treated as phase conductors and be adequately insulated. In addition, multi-pole MCBs, which automatically open all conductors when a fault occurs on any one phase, should be used. Fuses might also be used. In this case, these must be used in conjunction with a multi-pole (e.g., a knife) switch mounted on the supply side of the fuse to permit the consumer's circuit, including the fuse, to be isolated manually. This removes the threat from any voltage that might otherwise be present on the line(s) with any fuse still intact and permits the household circuit to be repaired and fuse replaced without fear of shock.

• If a grounded system neutral is used, then multiple grounds should be used along the system. It is also important that all metal surfaces associated with the generating and electrical system in the powerhouse also be bonded to the system ground. Consumers who utilize equipment or

appliances with metal housings should also ground equipment on their premises by bonding it to the grounded neutral conductor on the supply side of the distribution board. Placing a ground rod at the consumer’s service entrance would provide an additional margin of safety.

VII. Conductor

Once the nature of the loading has been determined, the selection of a conductor to most effectively serve consumer load and load growth at minimum cost can be assured by following the standard approach for properly sizing the conductor. In this process, both the voltage drop at the end of the line as well as energy (kWh) losses along the line—both of which depend on conductor size—must be kept within acceptable bounds. This chapter will first briefly review the types of conductor that might be used for the distribution of electricity around a load center and some of their attributes. It will then describe how conductors are sized and installed.