Unit 18. Fuse cutouts

1. Words to remember.

cutout – выключатель, прерыватель

amperage – сила тока (в амперах)

to melt – плавить

widespread interruption – масштабное отключение

a weak spot – слабое звено

overload – перегрузка

rated temperature – номинальная температура

to be endangered – находиться под угрозой

to be exposed in the open – быть установленным снаружи

to disconnect any portion – разъединить любую часть

to leave energized – оставлять под напряжением

a crossarm – перекладина

surge – перенапряжение

2. Read the text. Fuse cutouts

Suppose a circuit is designed to carry 100 A. If the amperage climbs over that limit, it could eventually melt some of the wires and cause widespread interruption of service. To prevent this, a weak spot is intentionally designed into a circuit – a place where overload will register and open the circuit almost immediately. This spot is called a fuse. A fuse consists of a short piece of metal having low melting characteristics which will melt at a rated temperature.

It is amperage flowing through a conductor that sometimes makes the conductor hot to touch. This is what the fuse counts on. Should there be an overload, the fuse melts, thus disconnecting the circuit.

Fuse cutouts go one step further. They may be placed, so as to cut out the section of the circuit that is endangered, allowing the rest of the circuit to remain energized. A primary fuse cutout is connected between the primary lines and the transformer to protect the transformer from overloads and to disconnect the transformer from the primary lines in case of trouble. A primary line fuse cutout can disconnect any portions of a primary circuit supplying several distribution transformers, in case of overload or fault, leaving the rest of the circuit energized.

Although there are several types of fuse cutouts, the principle upon which each is constructed is the same. A fuse ribbon makes a connection between two contacts, either the line and the transformer or the main line and that portion to be protected.

The open-type cutout is essentially the same as the door type, except that the fusible element is exposed in the open. This type cutout is used on distribution circuits operating at voltages over 5000 volts, although it can be used on lower voltage circuits.

When line fuses are used to protect a portion of a primary circuit a repeater fuse may be used. The repeater fuse is usually of the open type and consists of two or three fuses mechanically arranged so that when the first fuse blows and drops, the action places the second fuse automatically in the circuit. If the trouble has been cleared, service will be restored. Should the second fuse also blow, a third is also automatically connected in the circuit; when the last fuse blows, the portion of the circuit is finally de-energized.

All of these fused cutouts are mounted on the crossarm or on the pole by bolts and a steel bracket.

LIGHTNING OR SURGE ARRESTERS

A lightning arrester is a device that protects transformers and other electrical apparatus from voltage surges. These surges can occur either because of lightning or improper switching in the circuit. The lightning arrester provides a path over which the surge can pass to ground, before it has a chance to attack and seriously damage the transformer or other equipment.

The elementary lightning arrester consists of an air gap in series with a resistive element. The voltage surge causes a spark which jumps across the air gap, passes through the resistive element (silicon carbide, for example) which is usually a material that allows a low-resistance path for the high-voltage surge, but presents a high resistance to the flow of line energy. Afterwards, there is no chance of the normal line energy being led into ground.

VOLTAGE REGULATORS

A voltage regulator is generally used to maintain the voltage of a line. The primary feeder voltage generally drops when a large load current is drawn and less voltage is available across the primaries of the distribution transforms. The regulator maintains the voltage at the proper rated value at all times.

The principle of operation of a voltage regulator is somewhat similar to that of a transformer having taps, as previously described. This form of regulator has two fixed windings, a primary (high-voltage) winding connecting in shunt or across a line, and a secondary or low voltage winding connected in series with the line. The secondary or series winding is provided with as many taps as necessary to vary the voltage across this winding. This equipment operates as a voltage regulator by means of a control circuit which automatically changes the tap setting on the series winding, while leaving the voltage applied to the primary (high-voltage) winding alone. The variable voltage in the series winding can thus be added or subtracted from the incoming (or primary) voltage, resulting in an outgoing voltage which can be kept approximately constant even when the incoming primary voltage may vary.

Another type, known as the induction-type voltage regulator, accomplishes the same effect by having the primary coil rotate, changing its position in relation to the secondary coil, which in this case has no taps.

Voltage regulators are either hand or motor-operated. When a motor is used, it is usually automatically controlled by means of relays.