Fail-Safe CPU and signal module (SM) installation
4.3 Control system wiring guidelines
S7-1200 Functional Safety Manual
Manual, 02/2015, A5E03470344-AA
101
To help prevent unwanted current flows in your installation, the S7-1200 provides isolation
boundaries at certain points. When you plan the wiring for your system, you should consider
these isolation boundaries. Refer to the technical specifications (Page 132) for the amount of
isolation provided and the location of the isolation boundaries. Circuits rated for AC line
voltage include safety isolation to other circuits. Isolation boundaries between 24 VDC
circuits are functional only, and you should not depend on these boundaries for safety.
Guidelines for lamp loads
Guidelines for lamp loads
Lamp loads are damaging to relay contacts because of the high turn-on surge current. This
surge current is 10 to 15 times the steady state current for a tungsten filament lamp. A
replaceable interposing relay or surge limiter is recommended for lamp loads that are
switched a large number of times during the lifetime of the application.
Guidelines for inductive loads
Guidelines for inductive loads
Use suppressor circuits with inductive loads to limit the voltage rise when a control output
turns off. Suppressor circuits protect your outputs from premature failure caused by the high
voltage transient that occurs when current flow through an inductive load is interrupted.
In addition, suppressor circuits limit the electrical noise generated when switching inductive
loads. High frequency noise from poorly suppressed inductive loads can disrupt the
operation of the PLC. Placing an external suppressor circuit so that it is electrically across
the load and physically located near the load is the most effective way to reduce electrical
noise.
S7-1200 DC outputs include internal suppressor circuits that are adequate for inductive
loads in most applications. Since S7-1200 relay output contacts can be used to switch either
a DC or an AC load, internal protection is not provided.
A good suppressor solution is to use contactors and other inductive loads for which the
manufacturer provides suppressor circuits integrated in the load device, or as an optional
accessory. However, some manufacturer provided suppressor circuits may be inadequate
for your application. An additional suppressor circuit may be necessary for optimal noise
reduction and contact life.
For AC loads, a metal oxide varistor (MOV) or other voltage clamping device may be used
with a parallel RC circuit, but is not as effective when used alone. An MOV suppressor with
no parallel RC circuit often results in significant high frequency noise up to the clamp
voltage.
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