contacts. The coil current can be on or off so relays have  two switch positions, and they are double throw switches.  Residual magnetism is a common problem and the contacts  may stay closed or are opened by a slight amount of residual  magnetism. A relay is an electrically operated switch and  is therefore subject to dropout under low system voltage  conditions. Relays allow one circuit to switch a second circuit  that can be completely separate from the first. For example,  a low voltage DC battery circuit can use a relay to switch  a 110-volt three-phase AC circuit. There is no electrical  connection inside the relay between the two circuits; the link  is magnetic and mechanical.  [Figure 9-170] Conductors should be protected with circuit breakers or fuses  located as close as possible to the electrical power source  bus. Normally, the manufacturer of the electrical equipment  specifies the fuse or circuit breaker to be used when installing  equipment. The circuit breaker or fuse should open the circuit  before the conductor emits smoke. To accomplish this, the  time current characteristic of the protection device must fall  below that of the associated conductor. Circuit protector  characteristics should be matched to obtain the maximum  utilization of the connected equipment.  Figure 9-171  shows a  chart used in selecting the circuit breaker and fuse protection  for copper conductors. This limited chart is applicable to a  specific set of ambient temperatures and wire bundle sizes  and is presented as typical only. It is important to consult such  guides before selecting a conductor for a specific purpose.  For example, a wire run individually in the open air may be  protected by the circuit breaker of the next higher rating to  that shown on the chart. of lead, tin, or copper. When the current exceeds the capacity  of the fuse the metal strip heats up and breaks. As a result of  this, the flow of current in the circuit stops. A fuse is placed in series with the voltage source and all  current must flow through it.  [Figure 9-172]  The fuse consists  of a strip of metal that is enclosed in a glass or plastic housing.  The metal strip has a low melting point and is usually made  There are two basic types of fuses: fast acting and slow  blow. The fast-acting type opens very quickly when their  particular current rating is exceeded. This is important for  electric devices that can quickly be destroyed when too much  current flows through them for even a very small amount  of time. Slow blow fuses have a coiled construction inside.  They are designed to open only on a continued overload,  such as a short circuit. A circuit breaker is an automatically operated electrical  switch designed to protect an electrical circuit from damage  caused by an overload or short circuit. Its basic function  is to detect a fault condition and immediately discontinue  electrical flow. Unlike a fuse that operates once and then has  to be replaced, a circuit breaker can be reset to resume normal  operation. All resettable circuit breakers should open the  circuit in which they are installed regardless of the position of  the operating control when an overload or circuit fault exists.  Such circuit breakers are referred to as trip-free. Automatic  Circuit Breakers Fuses Current Limiting Devices  Wire AN gauge copper  Circuit breaker amperage  Fuse amperage 22 20 18 16 14 12 10 8 6 4 2 1 0 5 5 10 10 15 20 30 50 70 70 100 150 150 5 7.5  10  15  20  30  40  50  80  100  125 Figure 9-170.  Relay. Figure 9-172.  A fuse. Figure 9-171.  Wired and circuit protection chart.  9-100