Spectrum Controls 1756sc-IF8u - Manual (Page 110)

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ControlLogix ™ Universal Analog Input Module
The illustration that follows shows each of the three (3) thermocouple
types.
Grounded Junction
Extension Wire
Metal Sheath
Measuring Junction is
connected to sheath
Ungrounded (Insulated) Junction
Measuring Junction is
isolated from sheath
Exposed Junction
Measuring Junction
has no sheath
Isolation
The IF8u 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 IF8u module. If adequate precautions are not taken
for a given thermocouple type, the electrical isolation of the IF8u 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.