Rockwell Automation Safety Guidelines for the Application, Installation, and Maintenance of Solid-State Control - Manual (Page 6)

Publication SGI-1.1 - August 2009
6 Section 2: General Authorized Engineering Information
2.4 Polarity Incorrect polarity of applied voltages may damage solid-state controls. The
correct polarity of solid-state controls should be observed.
Comments: 2.4 —Polarity
In some instances incorrect polarity can cause damage to controlled
equipment or unintended actuation of outputs. This could result in
personal injury due to an unexpected response of the controlled
equipment or process.
See also section 3.3.2.
2.5 Rate of Rise-Voltage or
Current (DV/DT or DI/DT) Solid-state controls can be affected by rapid changes of
voltage or current if the rate of rise (DV/DT and/or DI/DT) is greater than
the maximum permissible value specified by the manufacturer.
Comments: 2.5 —Rate of Rise-Voltage or Current (DV/DT or DI/DT)
The DV/DT rating specifies the maximum rate at which voltage may be
applied to the power terminals of a solid-state device. Voltage applied at a
rate exceeding the DV/DT rating can switch the device ON without an
input signal being applied. Electrical noise with high frequency content is
one source of rapidly changing voltage.
Another common source of high DV/DT is an inductive load that is
switched off faster than the stored energy can be dissipated. This fast
switching produces “inductive kick voltages” that might exceed the
DV/DT limit.
The DI/DT rating specifies the maximum rate at which current flow may
be increased when switching from OFF to ON. Currents that increase
faster than the DI/DT rating cause localized hot spots due to current
crowding in a small area until the entire cross section can become
conductive.
This results in gradual degradation of the device. Subsequent operations
generally result in over dissipation and short circuit failures even under
normal load conditions. The most common situations for high DI/DT are
low load impedance, or capacitance loads.
Manufacturers of solid-state equipment usually include internal means to
limit the rate of rise of voltage and current. Nonetheless, the user should
be aware that additional external means may be necessary to adjust to the
specific conditions of some installations.
2.6 Surge Current Current of a value greater than that specified by the manufacturer can affect
the solid-state control. Current limiting means may be required.
Comments: 2.6 — Surge Current
The manufacturer may specify allowable surge current. Common practice
is to specify the peak sinusoidal current that can be allowed for one-half
cycle at line frequency. The intent behind this practice is to give the user