Technical information
C-BUS
TM
LIGHTING CONTROL
+
DESIGN GUIDE
C-Bus
TM
Wiring Rules
Schneider Electric C-Bus incorporates a distributed intelligence lighting control system that employs a low voltage network of intelligent input, output
and network communication units to control line rated loads. C-Bus network communications is achieved using proprietary C-Bus protocol over an
unshielded twisted pair LAN cable (Cat-5 UTP). Some simple rules should be observed when specifying or designing a Schneider Electric C-Bus
lighting control system.
Network Topology
Schneider Electric C-Bus lighting control system is comprised of C-Bus devices interconnected using Cat-5 UTP cable to achieve a topology-free
C-Bus network. The lighting control system may incorporate a single network or multiple networks to overcome the following C-Bus network limitations:
• Maximum cable length
• Addressing limitations
• Maximum 2A/36VDC per network
• Reduce network response time (reduce local communications trafc)
• Accommodate physical layout and building structure (network/oor)
When connecting multiple networks together, the following guidelines should be observed:
• Minimize the depth of the network topology for faster, more efcient communications.
• Distribute C-Bus Units on separate networks proportionally to avoid communications and power supply problems. For example if 120
C-Bus Units are required, split into two networks of 60 Units each, rather than 100 Units on the rst network and 20 Units on the second.
Network Size
The maximum number of C-Bus units on any network shall not exceed 100 units. If the lighting control system requires more than 100 units, an
additional network may be connected to the rst network through a C-Bus Network Bridge to allow communications between networks.
• Maximum of 100 C-Bus units per network
• Maximum of 255 networks per system
• No more than 50 DLT keypads on one network because of advanced operations
Network Cable
Maximum cable length is determined by the impedance of the cable used. The resistive and capacitive impedance of network cable varies based on
type. For Cat-5 UTP, the maximum length for reliable communications is 3280 ft.
• Maximum 3280 ft. of Cat-5 UTP cable per network
• C-Bus Network Bridges should be used to split a network into two or more networks when the total cable length required exceeds 3280 ft.
Power Supplies
The C-Bus Power Supply and Output units with integral power supply provide low voltage power to C-Bus devices on the network (36VDC). Current
required to operate C-Bus devices varies, depending on the device. Refer to installation instructions for specic C-Bus devices to determine current
requirements. Each C-Bus Power Supply supplies up to 350mA and C-Bus Output units with integral power supply will supply up to 200mA. Additional
C-Bus Power Supplies may be added to maintain optimum network voltage of 15-36VDC. Distribute power supplies evenly on the network to evenly
distribute voltage drop. All power supplies on a C-Bus network evenly share the network load. Typically, no more than (5) Power Supplies should
be connected to a single network. When utilizing Output Units with an integral power supply rated at 200mA, a maximum of (10) power supplies is
allowed. In no case shall the total available power exceed 2A at 36VDC.
Network Burden
A Network Burden must be connected to a C-Bus network to achieve reliable network communications. The Network Burden is comprised of a
capacitor in series with a resistor. One hardware Network Burden is included with each PC Interface (PCI) and two hardware Network Burdens are
included with each Network Bridge. The Network Burden may be software enabled on most other DIN rail mounted C-Bus units. One, and only one,
Network Burden is required for each C-Bus network. If more than one PCI or Network Bridge is used in a network, all but one Network Burden should
be disabled. For systems with multiple networks connected together via Network Bridges, each network within the
system should have only one Network Burden connected or enabled.
System Clock
The C-Bus network requires one system clock source to operate. This is not a real time clock; however, network communications are synchronized to
this source. The PCI and Network Bridge units may be used to provide the system clock. If more than one of these units is present on a network, the
network automatically determines which is active. A maximum of 3 network clocks should be enabled on a C-Bus network.
Connecting to the C-Bus Network
The C-Bus network is designed to operate at 36VDC. The C-Bus network is optically and/or galvanically isolated from line voltage rated circuits.
When installing C-Bus devices the installer should always follow applicable NEC requirements for Class 2 wiring. Operating at a low voltage of 36VDC
allows electrical work to be performed on the C-Bus side of the system while the system is powered on. Before working on C-Bus network wiring
or devices located in enclosures where Class 1 wiring is present, turn off power to Class 1 circuits. The C-Bus network is short circuit protected to
prevent damage to C-Bus devices if a short occurs. The C-Bus network will not work as long as the network is shorted. C-Bus devices are connected
in parallel on the network. Whether the devices are connected using a daisy-chain topology, a star topology, or a combination of both topologies, the
devices are always connected to the network in parallel to achieve a topology-free network. Homeruns are not recommended for C-Bus networks.
When connecting C-Bus devices to the network cable (typically Cat-5 UTP), care should be taken to observe polarity requirements clearly marked on
each device. It is recommended that installers use a Cat-5 UTP
cable with a distinguishable color, such as pink, to distinguish
C-Bus network cable from other network cable which may be
present. This simplies later servicing of the system by allowing
service personnel to quickly identify C-Bus network cables.
Correct wiring with the twist
being maintained between
the two positive conductors
and their respective negative
conductors.
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