Siemens S5-135U/155U appendix Short-Circuit Protection and Fusing

Digital Input/Output Modules

With isolated input/output modules the supply voltage is applied separately to the individual modules.

Reference Potential

e.g.430

DI

0 V

e.g.431

DI

0 V

e.g.451

DQ

0 V

e.g.453

DQ

0 V

L I

I

L

I L I

L

I L

I L

I L

L Q Q Q L L Q Q

L Q L Q

L Q L Q

and Connection

_

_

_

_

_

+

_

+

+

+

+

+

to Protective

+

_

Conductor (PE)

1L_

1L+

2L+

2L_

AC

+

AC

+

230V

_

230V

_

DC

DC

24V

24V

Figure 8-13 Feeding of Isolated Input/Output Modules from Two Power Supply Units

With isolated modules, the inputs or outputs can be fed from two separate power supply units in isolation groups created by the internal distribution in the module.

Note that when inputs or outputs from two isolated groups are connected to one power supply unit, isolation between the groups is defeated.

8.2.6Short-Circuit Protection and Fusing

With digital output modules for DC voltage, fuses are provided on the module, in addition to the electronic short-circuit protection, for the wiring cables and to protect the module. The fuses also provide protection against reversal of supply voltage connections and can only be replaced at the factory.

The electronic short-circuit protection guaranteed in the technical specifications applies to a resistance which is lower than the specified, maximum permissible line resistance.

In the event of a short-circuit, two to three-times the rated output current flows briefly at the output before the switched electronic short-circuit protection becomes effective. When selecting the load power supply units, therefore, allow for the increased short-circuit current as well as all connected output loads (observe the coincidence factor). With unregulated load power supplies, this overcurrent is generally ensured. With regulated load power supplies, especially at low output power levels (up to 20 A), you must allow for a corresponding overcurrent.

8-24	System Manual
	C79000-G8576-C199-06