GE Mark VIeS System Manual page 197

External Wetted Contact Inputs with Line Monitoring (Open/Short Detection)
For full diagnostic coverage, the user can enable line monitoring. When line monitoring is enabled, a different circuit is
required with two resistors added to allow detection of a loop break (too little current) or short (too high a current). This adds
two 8.2K ¼ watt resistors in parallel and series with the switch. Even with a 30 V field supply (above normal operation as an
extreme), the resistors limit fault current preventing any impairment of the internal 24 V circuitry supply rail. An additional
limit is for 20 V at the terminals where this is the maximum linear voltage sensed by the channel's A/D path.
External Wetted Contact with Line Monitoring (open/short detection) Wire Detection
10 to 30 V
For detection of open and/or short conditions, there are additional thresholds imposed based on the field voltage entered in the
ToolboxST application and the external resistors. The dual resistor circuit allows for detection of open loop and short to field
supply.
The field supply voltage is entered in the ToolboxST application as the ExtWettingVoltage parameter. Using 24 V as an
example, the thresholds are determined as follows:
• Open circuit at 50% of typical open contact value or 5.06 V (21% of 24 V)
• Contact open/closed decision at average of open and closed values or 12.42 V (51.75% of 24 V)
• Short threshold at average of closed and field level or 19.38 V (80.75% of 24 V) where this is within the 20 V limit of the
signal path's linear range. Therefore, field supplies greater than 24 V have a short threshold that is not supported by the
20 V limit of the path's linear range.
Examples of various field settings for line monitoring (open/short detection) are shown in the following plot, where the
thresholds for open/contact decision/short are shown in dashed lines for entries of 10, 13, 18, 24, and 29 V field supplies for
contacts using two resistors allowing for +/- 10% field voltage variation. The lower solid lines are for the open contact voltage
at the terminals, upper is for the closed contact voltage.
YUAA Universal I/O Modules
8.2K
Contact
+
-
Public Information
N/C
PWR_RET
8.2K
I/O+
I/O-
GEH-6855_Vol_II System Guide 197
12.5K
Vsense