Rockwell Automation Safety Guidelines for the Application, Installation, and Maintenance of Solid-State Control - Manual (Page 10)

Publication SGI-1.1 - August 2009
10 Section 3: Application Guidelines
with the particular hazard(s) through use of special features such as
automatic transfer of control functions to redundant devices in the event
of failure of primary controls; alarm circuits and diagnostics to signal and
identify failures that require repair in order to maintain redundancy;
emergency power sources with automatic transfer upon loss of primary
power source; or other appropriate features.
3.1.4 Overcurrent Protection
To protect triacs and transistors from shorted loads, a closely matched short
circuit protective device (SCPD) is often incorporated. These SCPDs should
be replaced only with devices recommended by the manufacturer.
Comments: 3.1.4 —Overcurrent Protection
Even a closely matched short-circuit protective device (SCPD) will
generally protect a solid-state device only against shorted loads, an
accidental short to ground, or a phase-to-phase short. Depending upon
the application, additional protective measures may be needed to protect
the solid-state devices against small to moderate overcurrents. Consult
with the manufacturer if necessary.
3.1.5 Overvoltage Protection
To protect triacs, SCRs and transistors from overvoltages, it may be advisable
to consider incorporating peak voltage clamping devices such as varistors,
zener diodes, or snubber networks in circuits incorporating these devices.
Comments: 3.1.5 — Overvoltage Protection
See section 2.7.
3.2 Circuit Isolation
Requirements 3.2.1 Separating Voltages
Solid-state logic uses low level voltage (e.g., less than 32V DC) circuits. In
contrast, the inputs and outputs are often high level (e.g., 120V AC) voltages.
Proper design of the interface protects against an unwanted interaction
between the low level and high level circuits; such an interaction can result in a
failure of the low voltage circuitry. This is potentially dangerous. An input and
output circuitry incorporating effective isolation techniques (which may
include limiting impedance or Class 2 supplied circuitry) should be selected.
Comments: 3.2.1 —Separating Voltages
For specifications of Class 2 circuitry, refer to Article 725 of the National
Electrical Code, NFPA 70.