Installation guide

18 PV Clinics Installation Manual 21/07/11 01.00 01/12
Fig. 6 Interconnection of the 3 wires in ‘2 wire’ RS485 communication
Weidmuller’s Transclinics use a 3 wire system so connect all the devices using the three signal wires A (or
+), B (or -) and GND (or ref) using internal cable conductors (do not use the cable shield for any of these
connections).
5.2.2 Isolated vs Non-isolated Devices
All RS485 device inputs have an internal 0V reference which is compared to the signals A and B when read-
ing the signal data. If common mode voltages on the A and B signal wires compared to this internal refer-
ence exceed the signal voltage (around 12V) then the circuit will not be able to read the data. This is why the
reference signal (via the signal GND wire) must be connected throughout the network.
Non-isolated input circuits still have an electrical connection to the other parts of the circuit (like the power
supply for example). This means that noise currents in other circuits and voltage differences around the plant
can disturb the internal 0V reference, produce currents in the Signal GND, and prevent proper communica-
tion.
Weidmuller’s Transclinic RS485 circuits have isolated inputs which allow far higher levels of common mode
noise on the signal lines without disturbing the signal transmission. For this reason, Weidmuller recommends
that only Transclinic devices are connected together and that slave devices from other manufacturers are
connected using a separate network.
The Master device (control room PC or Signal converter) on a Transclinic only network can have non-
isolated inputs if proper shielding is used, if the environment is not excessively noisy and if the device circuit
is properly designed. Using a suitable isolated power supply for the Master device can also improve input
circuit isolation (but only for noise induced through the Power Supply). Even so, Weidmuller recommends us-
ing a master device with fully isolated inputs.
5.2.3 Cable Shields
Cable shielding is mainly used to protect the inner conductors that carry the data signals by absorbing any
induced voltages. It is normally recommended that the shielding be connected to a clean earth in order to
discharge these induced overvoltages.
Use a single earth point for the shield: PV Systems generally cover a large area and earth points around
the plant will be at varying potentials. Connecting the shield at two earth points means that considerable cur-
rents can flow through the shielding which may corrupt the signal. These ‘ground loops’ frequently occur in
photovoltaic installations. Consequently Weidmuller recommends that the cable shield must have continuity
throughout the entire bus but the shield drain must only be connected at a single point for the entire line.
Keep Shield and signal wires electrically separate: Never use the shield to connect the reference signal
(Signal GND) and be careful that there are no indirect connections between the shield and the signal wires
that it protects. For example, never connect the shield to an SPD that is being used to protect the data sig-
nals and never connect the shield to a Transclinic device.
Use a clean and electrically independent communications ground point: The communications ground
must theoretically be completely independent of other grounds in the system. In reality this does not usually
happen as overvoltages from other equipment may be induced through the ground (such as inverters) or
through nearby lightning discharges or switching noise (for example). If in doubt, connect the shield to com-
munications ground using a gas arrester. This eliminates overvoltages introduced in the shield via the
ground.
5.2.4 Mixed devices on Transclinics networks
As well as the configuration of the transmission speeds, parity, etc., Weidmuller cannot know how equipment
from third party manufacturers will operate; if they are suitably isolated devices; or how they obtain the signal