Installation guide

14 PV Clinics Installation Manual 21/07/11 01.00 01/12
4. Surge protection
4.1 Best practice for PV Plants
Surge Protection Devices (SPDs) protect your equipment from various common fault conditions that occur in
PV plants. Larger PV installations cover a considerable surface area, typically in a highly exposed location,
and normally projected plant life spans are very large, so protection is essential. You might think that equip-
ment designed for such environments would have in-built protection but there are technical reasons why this
is not generally the case. Important among these is that SPDs are frequently destroyed by large surges so
separate devices (preferably with plug-in modules) that can be quickly replaced after an incident are essen-
tial.
4.2 Common mode and differential voltages
The Transclinic has Isolated RS485 inputs which means that the signal wires +, - and GND are not electrical-
ly connected to the rest of the device. This means that is a voltage is applied to all wires equally no current
will flow. This type of voltage is called common mode voltage. Isolated RS485 inputs provide good protection
from common mode voltages.
Differential voltages are those that arrive at the device down one signal wire (+, - or GND) only. The actual
RS485 data is a differential signal but limited to the voltages allowed by the standard. If a large differential
voltage arrives at the device will flow through the input components and damage will occur. SPDs for com-
munications circuits limit differential voltages and most also limit the common mode voltage (to protect de-
vices with non-isolated inputs).
Use of twisted pair cables and the complete electrical separation of the signal and shield connections is es-
sential to prevent differential noise and the subsequent damage to components. Conversely, connecting the
signal GND to any earth (or to the cable shield/drain can easily damage the equipment).
Differential voltages can also be induced in the PV Array wiring unless the wires are run parallel to each oth-
er. As the diagram below shows, large loops that can pick up noise (shown in red) are easy to create.