Siemens SIMATIC S7 Functional Safety Manual page 103

Typical suppressor circuit for relay outputs that switch AC inductive loads
①
②
③
If you design your own suppressor circuit, the following table suggests resistor and capacitor
values for a range of AC loads. These values are based on calculations with ideal
component parameters. "I rms" in the table refers to the steady-state current of the load
when fully ON.
Table 4- 9
Conditions satisfied by the table values:
Maximum turn-off transition step < 500V
Resistor peak voltage < 500V
Capacitor peak voltage < 1250V
Suppressor current < 8% of load current (50 Hz)
Suppressor current < 11% of load current (60 Hz)
Capacitor dV/dt < 2 V/μs
Capacitor pulse dissipation : ∫(dv/dt)
Resonant frequency < 300 Hz
Resistor power for 2Hz max switching frequency
Power factor of 0.3 assumed for typical inductive load
S7-1200 Functional Safety Manual
Manual, 02/2015, A5E03470344-AA
See table for C value
See table for R value
Output channel
AC suppressor circuit resistor and capacitor values
Inductive load
I rms 230 VAC
Amps VA
.02 4.6
.05 11.5
.1 23
.2 46
.5 115
1 230
2 460
Fail-Safe CPU and signal module (SM) installation
4.3 Control system wiring guidelines
Ensure that the working voltage of the MOV is at
least 20% greater than the nominal line voltage.
Choose pulse-rated, non-inductive resistors, and
capacitors recommended for pulse applications (typ-
ically metal film). Verify the components meet aver-
age power, peak power, and peak voltage
requirements.
120 VAC
VA Ω
2.4 15000
6 5600
12 2700
24 1500
60 560
120 270
240 150
dt < 10000 V /μs
2 2
Suppressor values
Resistor Capacitor
W (power
rating)
.1
.25
.5
1
2.5
5
10
nF
15
47
100
150
470
1000
1500
103