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Line J-Y (ST) Y...x 0.8 Number of bus devices Current per bus device Max. voltage drop
0.8 km 60 e.g. 1.5 mA 5.4 V
Table 9.12.1 Maximum voltage drop on the bus line

Baud rate Max. bus capacitance at a baud rate of 9600 Total capacitance of the bus devices + line
60 meters + 0.8 km J-Y (ST) Y ... · 0.8
9600 100 nF
60 · 1 nF + 0.8 km · 50 nF/km
Table 9.12.2 Maximum baud rate depending on the bus devices (in this case meters) and the line capacitance

Surge protective device Part No. Capacitance: core / core Series impedance per core
BLITZDUCTOR XT BXT ML2 BD S 48 920 245 0.7 nF 1.0 Ω
BLITZDUCTOR XT BXT ML2 BE S 24 920 224 0.5 nF 1.8 Ω
BLITZDUCTOR XT BXT ML2 BE S 5 920 220 2.7 nF 1.0 Ω
DEHNconnect DCO SD2 MD 48 917 942 0.6 nF 1.8 Ω
DEHNconnect DCO SD2 ME 24 917 921 0.5 nF 1.8 Ω
DEHNconnect DCO SD2 E 12 917 987 1.2 nF –
Table 9.12.3 Capacitances and series impedances of surge protective devices

capacitances and have an impact on the length of the bus line / If surge protective devices are used, their series resistances
baud rate. and core / core capacitances must be observed (Table 9.12.3).
An M-bus panel has an M-bus standby current of e.g. 375 mA
(250 standard loads of 1.5 mA each) which supplies different Building with external lightning protection system
M-bus devices with different standard loads (e.g. three stand- If a building is fitted with an external lightning protection sys-
ard loads are equivalent to 4.5 mA). The cross-section of the tem, lightning equipotential bonding is required.
copper lines and the sum of the voltage drops in the partial All cores of power supply and information technology cables
sections up to the relevant bus device define the maximum and lines entering or leaving the building are connected to the
length of the bus line (Table 9.12.1). lightning equipotential bonding system via lightning current
arresters. Figure 9.12.2 shows an example of how to protect
Another aspect is the dependence of the maximum transmit- an interconnected M-bus system from lightning currents and
ted baud rate on the total capacitance in the bus segment. surges.
This is shown based on the example of an M-bus panel with a
capacitance of 100 nF at a baud rate of 9600: Building without external lightning protection
system
¨ Type of line J-Y (ST) Y… x 0.8 If no external lightning protection system is installed, surge
¨ About 75 Ω/km, about 50 nF/km for M-bus devices, e.g. protective devices protect the electrical installations and sys-
meters, about 1 nF, about 1.5 mA (Table 9.12.2). tems. Figure 9.12.3 shows an example of how to protect an
interconnected M-bus system from surges.

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