Siemens CPU 313C Hardware And Installation Manual page 208

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The following measures must be taken on all transition points 1 <-> 2 and higher:
• Set up local equipotential bonding at each subsequent lightning protection zone
transition.
• Include all lines (also metal conduits, for example) in the local equipotential
bonding of all subsequent lightning protection zone transition points.
• Include all metal installations located within the lightning protection zone in the
local equipotential bonding (for example, metal part within lightning protection
zone 2 at transition 1 <-> 2).
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We recommend fine-wire fusing for following elements:
• All 1 <-> 2 and greater lightning protection zone transitions
• All cables that run within a lightning protection zone and are longer than 100 m
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Always use the Blitzductor VT, type AD 24 V SIMATIC for the 24 VDC power
supply module of the S7-300. All other surge protection components do not meet
the required tolerance range of 20.4 V to 28.8 V of the S7-300 power supply.
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You can use standard surge protection components for the digital I/O modules.
However, please note that these only permit a maximum of 26.8 V for a rated
voltage of 24 VDC. If the tolerance of your 24 VDC power supply is higher, use
surge protection components with 30 VDC rating.
You can also use Blitzductor VT, type AD 24 V. Note that input current can
increase if negative input voltages are generated.
12-22
S7-300 Automation System, Hardware and Installation: CPU 31xC and CPU 31x
A5E00105492-03