Fuses
A fuse is a device for opening a circuit by means of a conductor designed to melt when an excessive current flows through it.
Two types are commonly used: the rewirable fuse (Fig. 86) and the cartridge fuse.
The principal feature of a rewirable fuse is that, once the fuse has blown, the fusing element or wire can be easily replaced at minimum cost. While these fuses may be convenient, low-cost, and popular, a principal disadvantage is that any inexperienced person can replace the blown fuse wire with one of incorrect size or one made of ordinary wire. Such an action completely negates the purpose of the fuse to open the circuit when current reaches an unsafe level and places the system in jeopardy. In one site visited, the continually blowing fuse was replaced by progressively larger fuse wire. In the end, the generator overheated and burned from the overload (p. 202).
Another disadvantage is that this type of fuse does not discriminate between a momentary high current that is acceptable (e.g., due to a motor starting) and a continuous overload current that must be
interrupted. It also is not precise, because the actual fusing current depends on the ambient temperature and the length of the fusing element. Furthermore, the minimum current for the fuse to blow might be considerably (e.g,, two times) higher than its current rating, making it possible for the line being protected
Fig. 86. A rewirable fuse is screwed to the ceramic cover that is then snapped over the ceramic base, completing the circuit.
to operate at a considerably higher current than it was designed for. The fuse can also deteriorate over time, causing nuisance interruptions of the circuit.
To address some of these drawbacks, the cartridge fuse was developed. In this design, the fusing element or wire is enclosed within a cartridge made of ceramic or glass and is less susceptible to deterioration in service. By being manufactured under controlled conditions, its current rating is more precisely known.
Miniature circuit breakers (MCBs)
A circuit breaker is an electro-mechanical device that is designed primarily to automatically open a circuit when currents in excess of its design rating pass through (Fig. 87).
Under normal conditions, a mechanism within the breaker holds the contacts in the closed position. The contacts are automatically separated when the release mechanism in the breaker is operated by magnetic and/or thermal means.
A magnetic breaker is tripped when excess current activates a solenoid. This pulls an iron slug into the solenoid's coil and collapses the attached tripping linkage to open the contacts.
Such breakers have a very quick reaction time. A thermal breaker is tripped with excess current heats a bimetallic strip.
The resultant deflection trips the release mechanism. Because of the time required to heat the bimetallic strip, reaction times tend to be slower. This might be more appropriate on a circuit with a motor, because a brief initial peak current demand in excess of the breaker's rated current is part of the normal operating cycle of a motor. A magnetic breaker used under these circumstances might trip each time an attempt is made to start the motor. Some breakers can contain both types of activation.
An ordinary switch is designed to make or break a current not greatly in excess to its normal rated current.
A breaker can also be used to open a circuit manually, such as when work is undertaken on the circuit it controls (e.g., the housewiring). However, a breaker is capable of disconnecting a much larger fault current. Ordinary switches would spark excessively under similar conditions, possibly damaging the switch or even starting a fire.
While it is costlier than a fuse, a circuit breaker provides numerous advantages:
• It is easy to use and considerably more precise and more sensitive than a fuse.
• It can also be quicker acting; when small overload currents occur, the circuit breaker is likely to operate before the fuse blows.
• It can be tripped by a small sustained overload current but not by a harmless transient overcurrent such as due to the switching surge which accompanies the ignition of a fluorescent lamp.
• The breaker on a faulty circuit is easy to detect, because this is indicated by the position of the switch, and the breaker cannot be switched on as long as the fault condition remains.
• It can more conveniently be used as a switch when repairs have to be done to the circuit. It can be reset manually after a fault has been corrected, and no stock of fuses is necessary.
Fig. 87. A selection of circuit
breakers. (Source: Airpax Protector Group, Cambridge, MD)
• It is factory-calibrated and cannot readily be changed.
• Under fault conditions, breakers positively disconnect all poles of the circuit it controls
The required capacity of the circuit breaker (or fuse) depends on its function. An overcurrent device at the powerhouse (or transformer, if the mini-grid is connected to a larger network) would be used to protect the generator or transformer from being overloaded. The capacity of an overcurrent device placed on the consumer's premises would depend on its function. If it is to ensure that the current does not exceed the capacity of the housewiring, then the size of the device would be set by this capacity. If it is to limit consumer-drawn power to a specific limit that determines his tariff, then the device would be sized according to this limit. For example, a household that has subscribed to a 50 W service would have a breaker that would trip if the demand goes significantly beyond this limit.
Residual current devices (RCDs)
As is discussed below (p. 133), even very small currents can prove fatal. These currents are much smaller than those that can be detected by the standard fuses and MCBs discussed above. The RCD is a
specialized form of circuit breaker developed to detect small fault currents that can pose a threat to humans.
An RCD, also called a ground-fault circuit interrupter (GFCI), is a device that is inserted in the circuit and located between the power supply and the circuit along which protection is sought, usually on the premises of the consumer (Fig. 88). This device is an automatic switch that senses the current into the circuit to be protected (Ii) and compares it with the current out of this circuit (Io). Under normal operating conditions, these two currents should be equal, and the switch maintains the supply.
However, under fault conditions, such as when a person touches the live conductor, a portion of the current passing through the RCD into the protected circuit would then pass
through that person (Il), leak into the ground, circumvent the RCD, and return through the ground back to the supply either through a system ground if there is one, through any fault, or simply through capacitive coupling between the circuit and the ground (p. 135). As soon as the RCD senses a difference (∆) between the incoming and outgoing currents, it trips and isolates the protected circuit.
An RCD operates by detecting the difference in current flowing into and out of the protected circuit, independently of how well the generator is grounded or whether it is grounded at all. But incorrectly grounding the consumer circuit can prevent an RCD from detecting fault currents.
Fig. 88. Potentially dangerous currents leaking through a person (IB) will cause the current in (Ii) and current out (Io) of the circuit to be unequal, forcing the RCD to open the circuit.
For example, if the neutral conductor were grounded on the consumer side of the RCD
(Fig. 89a) and the generator ground is poor or nonexistent, a lethal fault current could flow through a person and return to the neutral conductor through this consumer ground. Since current both in and out of the RCD could then be roughly equal, the RCD may not detect the fault. If a consumer ground is used and if this is
connected to the neutral conductor, this connection must be on the supply side of the RCD (Fig. 89b).
Actually, the neutral conductor can be grounded any number of times between the RCD and the power supply and not adversely affect the operation of the RCD.
If the metal frame of a piece of electrical equipment is bonded to the consumer ground (represented by the dashed line in Fig. 89b), any leakage current to the frame caused by an internal fault would also cause the RCD to trip before it is even touched by an person.
While an RCD is always a useful device for protecting household members against accidental shock, this can be a relatively expensive device. In the U.S., single-pole
RCDs incorporated in dual power outlets are available for about $10. These are preset to trip at 4 to 6 mA. In the U.K., two-pole RCDs rated to trip at 10 mA cost roughly $70. The least expensive units are those tripping at 30 mA but still cost about $40. Because the danger from shock is minimal if loads are limited to lights and double-insulated appliances, RCDs for individual households are not essential.* For more affluent consumers who are likely to use other appliances such as refrigerators, cookers, and machine tools, the use of RCDs should be considered, especially since these individuals can probably easily cover the additional cost involved.
* Double-insulated appliances are those where the wires inside the appliance are insulated, where terminals are normally not in contact with the inside of any metal casing, and where any metal casing in enclosed in a plastic housing. These include appliances such as radios, TVs, and some power tools.
Fig. 89. Proper placement of the RCDs is critical if these devices are to operate properly. Incorrect placement of the consumer ground may prevent the RCD from detecting a fatal body current (a). If the consumer ground is bonded to the system ground (larger dashed line), this should be done on the supply side of the RCD (b).
Ii RCD
Tripping of an RCD indicates a fault condition that must be corrected in order to remove the hazard that it will likely continue to present to equipment or people.
If the RCD resets and the person in the household knows what caused the tripping, the mini-grid operator can then be informed, isolate the supply for that premise only, and repair or remove the faulty appliance.
If the cause for the tripping is unknown, this must be investigated further by the system operator:
• If an RCD can be reset, this implies a fault that is temporary in nature, possibly caused by someone touching a faulty appliance. If the consumer does not know which appliance is causing that tripping, the system operator must investigate further. One way to accomplish this is to install a (temporary) consumer ground electrode if one is not already installed. All appliances should be disconnected from the protected circuit being checked. Each appliance is then connected, one at a time, to the circuit, the chassis is grounded through the consumer ground electrode, and then the appliance is switched on. If the RCD trips either when the appliance is connected or when it is switched on, the culprit load has been found.
• If the RCD still does not reset, the fault is probably permanent in nature and located in the equipment or cabling. The technician would first disconnect both output leads from the RCD. If the device can be reset, the problem is probably not a faulty device. He would then reconnect the RCD and progressively isolate further sections of the circuit, by temporarily disconnecting both conductors to those sections being checked until disconnecting one section or appliance allows the RCD to be reset. This indicates that the fault is located in the last disconnected section.
In both cases above, once the culprit has been found, it is necessary to find the source of the problem so that it can be repaired. A close inspection of the wiring and insulation may locate the cause of the fault.
An ohmmeter might also be of some use.