SERVICIOS

9.1 General discussion
This chapter deals with insulated power cable protection as well as with busway protection. The primary considerations are presented along with some methods of application.
The proper selection and rating or derating of power cables is as much a part of cable protec- tion as the application of the short-circuit and overcurrent protection devices. The whole scheme of protection is based on a cable rating that is matched to the environment and operat- ing conditions. Methods of assigning these ratings are discussed.
Power cables require short-circuit current, overload, and physical protection in order to meet the  requirements  of  the  National  Electrical  Code®  (NEC®)  (NFPA  70-1999).1 A  brief description of the phenomena of short-circuit current, overload current, and their temperature rises is presented, followed by a discussion of the time-current characteristics (TCCs) of both cables and protective devices. In addition, a number of illustrations of cable systems and typ- ical selection and correlation of protective devices are included.
Because of their rigid construction, busways provide their own mechanical protection. How- ever, they do require short-circuit current and overload protection. A brief discussion of the types of faults on the busways is presented, followed by a discussion of various methods of fault protection.
The general intent of this chapter is to provide a basis for design, to point out the problems involved, and to provide guidance in the application of cable and busway protection. Each specific case and type of cable or busway requires attention. In most cases, the attention is routine, but the out-of-the-ordinary cable and busway schemes require careful consideration.
9.2 Cable protection
Cables are the mortar that holds together the bricks of equipment in an electric system. If the cable system is inadequate, unsatisfactory operation inevitably results. Today’s cables are vastly superior in performance to the cables available just a decade or so ago. But even so, they are not unlimited in power capability and, therefore, need protection to prevent possible operation beyond that capability.
Cables are generally classified as either power or control. Power cables are divided into two voltage classes: 600 V and below, and above 600 V. Control cables include cables used in the control of equipment and also for voice communication, metering, and data transmission.

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The amount of damage caused by the faulting of power cables has been illustrated many times. As  power  and  voltage  levels  increase,  the  potential  hazards  also  increase.  High temperature due to continued overload, nonlinear loads, or uncoordinated fault protection is a frequent cause of decreased cable life and failure. Power cables, internally heated as a result of  their  resistance  to  the  current  being  carried,  can  undergo  insulation  failure  if  the temperature buildup becomes excessive. Proper selection and rating ensures that the cable is large enough for the expected current. Suitable protection ensures that cable temperature rising above ambient does not become excessive. Such protection normally is provided by time-current  sensitive  devices.  In  addition  to  insulation  breakdown,  protection  is  also required against unexpected overload and short-circuit current. Overcurrent can occur due to an increase in the number of connected loads or due to overloading of existing equipment or due to nonlinear loads causing excessive neutral conductor current. 
While the extraordinary temperature of the short-circuit arc produces extensive destruction of materials at a fault location, cables carrying energy to (and from) a fault may also incur ther- mal damage over their entire length if the fault current is not interrupted quickly enough. Depending on conductor size, insulation type, and available fault current, the clearing time of the protection system should be short enough (i.e., coordinated) to stop the current flow before damaging temperatures are reached.
Physical conditions can also cause cable damage and failure. Failure due to excessive heat may be caused by high ambient temperature conditions or fire. Mechanical damage may result in short circuits or reduced cable life and may be caused by persons, equipment, animals, insects, or fungi.
Cable protection is required to protect personnel and equipment and to ensure continuous service. From the standpoint of equipment and process, the type of protection selected is generally determined by economics and the engineering requirements. Personnel protection also receives careful engineering attention and special consideration to ensure compliance with the various codes that may be applicable to a particular installation.
Protection against overload is generally achieved by a device sensitive to current magnitude and duration. Short-circuit protective devices are sensitive to much greater currents and shorter times. Protection against environmental conditions takes on many forms.
Cables may also be damaged by sustained overvoltages such as exist during a ground fault on one phase conductor. Modern cables now bear a rating called percent insulation level (or % IL). This rating is described as follows:
a)     100% IL—Cables that may not be required to operate longer than 1 min in case of ground fault.
b)     133% IL—Cables that may not be required to operate longer than 1 h in case of a ground fault.
c)     173% IL—Cables that may be required to operate longer than 1 h continuously with one phase conductor grounded (manufacturers should be consulted for suitability).