Spectrum Controls 1746sc-NI8u - Manual (Page 126)

114 SLC 500 ™ Universal Analog Input Module
Ungrounded (Insulated) Junction
Measuring Junction is
isolated from sheath
Exposed Junction
Measuring Junction
has no sheath
Isolation The NI8u module provides 12.5 VDC electrical isolation channel to
channel, 500 VDC electrical isolation channel to chassis ground, and 500
VDC electrical isolation channel to backplane. Care must be taken when
choosing a thermocouple type, and connecting it from the environment
being measured to the NI8u module. If adequate precautions are not
taken for a given thermocouple type, the electrical isolation of the NI8u
module may be compromised.
Grounded Junction Thermocouples
As shown in the illustration that follows, the shield input terminals are
connected together, which are then connected to chassis ground. Using
grounded junction thermocouples with electrically conductive sheaths,
removes the thermocouple signal to chassis ground isolation of the
module. This is inherent to the thermocouple construction. In addition,
if multiple grounded junction thermocouples are used, the module’s
channel to channel isolation is removed since there is no isolation
between signal and sheath, and the sheaths are tied together. It should be
noted that the isolation is removed even if the sheaths are connected to
chassis ground at a location other than the module, since the module is
connected to chassis ground.
For grounded junction thermocouples it is recommended that they have
protective sheathes made of electrically insulated material (e.g. ceramic),
or the metal protective sheaths be floated. The metal sheaths would need
to be floated with respect to any path to chassis ground or to another
thermocouple metal sheath. This means the metal sheath must be
insulated from electrically conductive process material, and have all
connections to chassis ground broken. It should be noted that a floated
sheath may result in a less noise immune thermocouple signal.