• ETT Level 3.13: Electrical Protective Devices

    Learn how to properly identify and perform applicable tests to determine functions and characteristics of various electrical protective devices.

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    1. Low-voltage breakers - Confirm and/or apply settings provided by others. Perform applicable tests to determine functions and characteristics of each operating element and expected reaction; evaluate the results. Confirm correct insulation and connection.
      • Circuit Breaker Characteristic Trip Curves and Coordination [PDF] A coordination study is an organized effort to achieve optimum electrical distribution system protection by determining the appropriate frame sizes, ampere ratings and settings of overcurrent protective devices. When an overcurrent occurs in a properly coordinated distribution system, only the protective device nearest the fault will open.
    2. Current relays - Confirm and/or apply settings provided by others. Perform applicable tests to determine functions and characteristics of each operating element and expected reaction; evaluate the results. Relays can include phase overcurrent, current balance, negative sequence, zero sequence, thermal, and ground fault.
    3. Directional and power relays - Confirm and/or apply settings provided by others. Perform applicable tests to determine functions and characteristics of each operating element and expected reaction; evaluate the results. Relays can include directional-power, directional-overcurrent, watt, var, and power-factor types.
    4. Voltage relays - Confirm and/or apply settings provided by others. Perform applicable tests to determine functions and characteristics of each operating element and expected reaction; evaluate the results. Relays can include overvoltage, undervoltage, phase-sequence, and negative-sequence voltage types.
    5. Differential relays - Confirm and/or apply settings provided by others. Perform applicable tests to determine functions and characteristics of each operating element and expected reaction; evaluate the results. Relays can include line, transformer, bus, unit, and rotating machine protection.
    6. Other relay types - Confirm and/or apply settings provided by others. Perform applicable tests to determine functions and characteristics of each operating element and expected reaction; evaluate the results. Relays can include timing, reclosing, temperature, auxiliary, lockout, alarm, and control types.
    7. Fuses - Confirm correct type, rating and application for project use and specifications. Measure fuse resistance and evaluate.
      • Tech Quiz: Fuses [PDF] Tech Quiz #79 Spring 2007 NETA World
      • Fuze Sizing Guide [PDF] This guide is a general recommendation and does not include the many variables that can exist for specific situations such as special local codes, unusual temperature or other operating conditions, N.E.C. demand factors, conductor derating, etc.
      • UL Fuse Classification Chart Safety standards have culminated in the establishment of distinct classes of low-voltage (600 volts or less) fuses, Classes RK1, RK5, G, L, T, J, H and CC being the more important.
      • How to Read A Fuse Time-Current Curve A time-current characteristic curve, for any specified fuse, is displayed as a continuous line representing the average melting time in seconds for a range of overcurrent conditions.

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