Siemens S5-155U System Manual page 53

Power Supplies for Programmable Controllers and I/Os
Load power supply
Grounding
3 - 24
• • For 24 V DC load circuits, you require a load power supply unit
with safety separation. If an AC plug is used as the isolating
device, the socket must be in the vicinity of the CC and easily
accessible (VDE 0805, 1.7.2).
• • Unregulated load power supply units require a reservoir capacitor
(rated at 200 µF per 1 A load current). Connect the capacitor in
parallel with the output terminals of the load power supply.
• • According to DIN VDE 0113, Part 1, control systems with more
than five items of electromagnetic apparatus require isolation by a
transformer; according to DIN VDE 0100, Part 725, it is
recommended .
• • Load power supplies should be grounded if possible . Provide a
detachable connection to the protective conductor at the load
power supply unit (terminal L- or M) or on the secondary side of
the isolating transformer.
Caution
For ungrounded power supplies, you must provide insulation
monitoring if
• •
hazardous system states can be caused by double faults;
• •
there is no safety separation;
• •
circuits are operated with voltages of more than 120 V DC;
• •
circuits are operated with voltages of more than 50 V AC.
System Manual
C79000-B8576-C199-03