Siemens FS720 - Page 67

Mounting and installing extinguishing control panels
Card cage connections (1 sector exting.) FCA2046
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Building Technologies
A6V10210390_m_en_
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Fire Safety
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Step 2: calculate the max. permitted cable resistance
RcableNom = 0.97 * RcableCrit – 0.029 * RLoad
RcableNom
Maximum nominal cable resistance [Ω] during commissioning, both
conductors.
RLoad Load resistance at UloadMin.
When using a 2-way connection box
RcableNom = 0.97 * RcableCrit – 0.029 * (Rload + 220)
Rbox
Internal resistance [Ω] of an externally connected 2-way
connection box (220 Ω).
Examples:
● B04420103 with Vnom 24 V ±10 % and 0.5 A
RcableCrit = (24.8 V - 24 V 0.9) / 0.5 A = 6.4 Ω
RcableNom = 0.97 * 6.4 Ω – 0.029 * (24 V *0.9 / 0.5 A) = 4.96 Ω
● FTEX 30 40 21 with Vnom 24 V ±15 % and Rload 60 Ω
RcableCrit = (24.8 V - 24 V 0.85) / (24 V *0.85 / 60 Ω = 12.9 Ω
RcableNom = 0.97 * 12.9 Ω – 0.029 * 60 Ω = 10.8 Ω