Galvanic Isolation - Beijer Electronics BoX2 extreme Installation Manual

Additional Installation Tips
6.4

Galvanic Isolation

The operator panel has galvanic isolation against the 24 V DC power supply
but no galvanic isolation between the communication ports for RS-232,
RS-422/RS-485 and USB. Only the Ethernet- and RS-485 connection have
galvanic isolation.
Operator panel
RS422
PC
*
When a PC is connected to the operator panel, the internal 0 V (GND) of the
panel is connected to the protective ground via the PC.
A number of USB devices can have the shield connected together with the
protective ground. Here, the 0 V (GND) of the operator panel is connected to the
protective ground when, for example, a USB memory stick, keyboard, or similar
device is plugged in.
If a number of units are connected that have a 0 V and a ground connection, and
these are connected to various grounding points, there is a substantial risk of
problems. Grounding currents go through communication cables, the rear plate
of the controller, internally in the operator panel, and can cause errors.
Use external units to improve communication and achieve galvanic isolation.
Westermo has good industry-standard insulators that are also insulated from the
24 V DC feed.
Note:
It is very important to make sure that the 24 V feed in the external insulation unit is not
connected to one of the communication outlets. If it does not have 100% insulation
against the 24 V feed, disturbances and grounding currents from the 0 V on the 24 V
side disrupt the communication.
Using this type of unit solves one problem but creates a larger problem! A substandard
installation may work now, but problems may arise when other devices are connected.
26
RS232 USB
* *
*
Different ground potential
Modular controller
Power CPU COM COM2
* * *
*
= Internal 0 V (GND) connection
Beijer Electronics, MAEN276
Printer
*
PC
*