External Wetted Contact Inputs; External Wetted Contact Inputs With 10 To 20 V Field Supply - GE Mark VIeS System Manual

7.10.2 External Wetted Contact Inputs

The thresholds are based on the excitation voltage using optional external resistors, where the terminals are connected to an
internal 12.5K load. There are three types of external wetted contact inputs, depending on source voltage and desired
diagnostic support, as described in the following sections:
• External wetted contact inputs with 10–20 V field supply
• External wetted contact inputs with 20–30 V field supply requiring series resistance
• External wetted contact with line monitoring (open/short detection)

External Wetted Contact Inputs with 10 to 20 V Field Supply

For switches using a field supply of 20 V or lower, the simplest circuit to install is a switch with no resistors. There are no
diagnostics for faults, just a decision at 50% of the specified field voltage for switch open or closed.
10 to 20 V
External Wetted Contact Inputs with 30 to 20 V Field Supply Requiring Series Resistance
For switches using a field supply of higher than 20 V, external resistance is required to limit current to a safe value (where the
current is due to the higher voltage back-feeding the internal 20 V circuity supply rail). It is highly recommended to use the
line monitoring (open/short detection) scheme that uses two resistors and provides full diagnostic coverage for faults.
Alternatively, a single series resistance of 4.7 KΩ ¼ watt can be used as shown in the following diagram solely to limit the
current. To do this, the digit input parameter, LineMonitoring, on the input point should be set to Disable and the parameter
ExtWettingVoltage should be set to 20 V to force a simple 10 V decision threshold. However, these settings will only provide
fault coverage of over-voltage due to a short from the I/O+ screw to field supply. If this fault occurs, the Point Unhealthy
alarm (64–95) and/or the IO Point Input Saturation alarm (2754–2785) will be annunciated.
20 to 30 V
196 GEH-6855_Vol_II
External Wetted Contact Input
Contact
External Wetted Contact Input
Contact
6.8 K
( current limiting )
N / C
PWR _ RET
I O + /
I O - /
N/ C
PWR _ RET
I O + /
I O - /
GEH-6855_Vol_II Mark VIeS Functional Safety Systems Volume II
Public Information
Vsense
12.5K
Vsense
12.5 K