Protection of Electrical Networks
Suitable for engineers, technicians, designers and operators working with electrical networks, this book discusses network structures and grounding systems, as well as problems that can occur in networks.
TABLE OF CONTENTS Protection of Electrical Networks
Chapter 8: Overcurrent Switching Devices
Overcurrents in an electrical network may be caused by a short-circuit or an overload. The switching devices used to clear these overcurrents are circuit-breakers and fuses.
In this chapter we will define the electrical specifications of fuses and circuit-breakers.
Since low and medium voltage specifications are different, they will be dealt with in separate sections.
Definition
Prospective short-circuit current: this is the short-circuit current in steady-state operating conditions that would occur if the impedance of the switching device was zero.
8.1 Low voltage circuit-breakers
Rated (1) operating voltage U e
This is, or these are, the voltage(s) at which the device can be used.
Rated (1) current I n
This is the maximum value of the current that a circuit-breaker, fitted with a tripping relay, can withstand at an ambient temperature specified by the manufacturer and in compliance with the specified heating limits.
Nevertheless, a circuit-breaker can be used at higher ambient temperatures if it is derated. Thus, a circuit-breaker with a rated current of 125 A will only be able to withstand 117 A at 50 C and 109 A at 60 C.
Frame-size rating of a circuit-breaker
When a circuit-breaker is fitted with several tripping relays with different current ratings, the frame-size of the breaker matches the highest current rating of the tripping relays with which it can be fitted.
Overload relay trip current setting I rth or I r
With the exception of small circuit-breakers (Multi 9 type for Schneider circuit-breakers), which are easily replaced,...
Copyright ISTE Ltd 2006 under license agreement with Books24x7
