Siemens SIMATIC S7 - Page 103

Fail-Safe CPU and signal module (SM) installation
4.3 Control system wiring guidelines
S7-1200 Functional Safety Manual
Manual, 02/2015, A5E03470344-AA
103
Typical suppressor circuit for relay outputs that switch AC inductive loads
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 (ty
p-
ically metal film). Verify the components meet ave
r-
age power, peak power, and peak voltage
requirements.
①
See table for C value
②
See table for R value
③
Output channel
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 AC suppressor circuit resistor and capacitor values
Inductive load
Suppressor values
I rms
230 VAC
120 VAC
Resistor
Capacitor
Amps VA VA Ω W (power
rating)
nF
.02
4.6
2.4
15000
.1
15
.05
11.5
6
5600
.25
47
.1
23
12
2700
.5
100
.2
46
24
1500
1
150
.5
115
60
560
2.5
470
1
230
120
270
5
1000
2
460
240
150
10
1500
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)
2
dt < 10000 V
2
/μs
Resonant frequency < 300 Hz
Resistor power for 2Hz max switching frequency
Power factor of 0.3 assumed for typical inductive load