User guide

Wiring and Cabling
890 USE 108 00 August 2001 137
I/O Module to DIN
Rail Grounding
Requirements
Under certain conditions (painted, coated, or corrosive DIN rails) and with certain I/
O modules, the DIN rail ground may NOT be adequate. The DIN rail resistance
should not exceed 0.1 Ohms.
To check this specification just measure the voltage drop as described and then
calculate the resistance. A constant current of 30A for at least 2 minutes should be
applied between the earthing terminal or contact and each accessible metal part
intended to be earthed. The current should be maintained or adjusted accordingly
during this test to 30A. Any convenient low voltage not exceeding 12V may be used.
The voltage drop should be measured between the points of current flow. Ensure
that the contact resistance between the tip of the measuring probe and the metal
part underneath does not influence the test results.
If the calculated resistance is greater than 0.1 Ohms, we recommend using Tinn DIN
rail models.
The following I/O modules are more sensitive to this type of ground issue: MOT
201/202, ADU 204, ADU 211/212, VRC 200, and CTR 205/212/224.
Other Ground
Contact Points
on the DTA
Backplanes
All DTA backplanes (the DTA 201 and 202 secondary backplanes as well as the
DTA 200) provide a reference ground point on the right side of the backplane. This
connection point may be used as the reference ground for test equipment and for
the BXT 201 cable ground wire in a stacked drop layout.
All DTA backplanes have ground-contact through-holes, so that I/O modules with
ground contact pins can make contact with the DIN rail.
WARNING
Some DIN rails ship with a protective coating that must be removed to
ensure proper grounding.
Failure to observe this precaution can result in severe injury or
equipment damage.