1C31125G02 Input module USES guidance

1C31125G02 Input module USES guidance

An individual shield termination is provided at the FTM terminal block for each of the analog inputs and analog outputs. All of the analog inputs and outputs should be wired using shielded, twisted-pair wiring. The shields should be connected to earth ground at one point only as well as terminated at the FTM terminal block; which will typically AC couple the shield to earth ground.Discrete Inputs leaving the cabinet housing the MicroNet should be grouped together based on field signal source location and be shielded as a group. The shield termination should be to the chassis ground on the cabinet housing the MicroNet system; the shield termination should only be to the MicroNet’s cabinet. The Discrete Inputs should also have a common wire (Wetting voltage common), grouped with them inside the shield. The field end of the common wire should be un-terminated, unless wetting voltage is provided remotely in the field. Relay Driver/Discrete Outputs leaving the cabinet housing the MicroNet should also be grouped together based on field relay coil or input location and be shielded as a group. The shield termination should be to the chassis ground on the cabinet housing the MicroNet system; the shield termination should only be to the MicroNet’s Cabinet. The Discrete Output wires should also have a common wire (coil voltage common), grouped with them inside the shield. The field end of the common wire should be un-terminated, unless coil voltage is provided remotely in the field.

Unshielded I/O cables

 may only be used inside the cabinet or for very short distances near the control cabinet. Also short on engine sections of discrete Input & Output cables may be use on the engine/turbine. See AP Note 51204 for a more detailed discussion on shield terminations. The length of exposed wire extending beyond the shield should be limited to 25 mm (1 inch). If the panel that the control chassis and FTMs are mounted on is not at earth ground potential, connect it to earth ground via a 4 mm² (12 AWG) green/yellow wire or braid, keeping the wire or braid as short as possible. The mounting panel should be well grounded to protective earth via the cabinet structure or ground straps that are low RF impedance. Low RF impedance: length not greater than 4 times the cross-sectional circumference of the ground strap. For compliance with EMC standards, it is required that all analog and discrete input / output wiring be separated from all power wiring. Power wiring leaving the cabinet housing the MicroNet may couple noise if not segregated and routed directly against the grounded metal of the cabinet & mounting plates. If segregation is not possible, a line filter of at least 20 dB attenuation is needed.

Signal lines, mainly Analog Outputs 

and Actuator Outputs, send the system ground (DGND) out of the control as part of the field wiring. Take great care in wiring the field devices with cable runs longer than 30 m (100 feet), so that DGND does not become referenced to Protective Earth. When there is significant transient ground bounce and the ground potential at the control and field device is significantly different (significant resistance between the two points), referencing DGND to Protective Earth at the field device can cause system disruptions in some VME modules. A significant potential difference in Protective Earth grounds generally arises when the physical distance between protective earth points is longer than 30 m (100 feet), but may happen any time there is a significant enough inductance and/or resistance. Transient ground bounce is caused when there is a nearby lightning strike, or nearby switched, high current, heavy inductive loads are switched off. The control DGND is referenced to Protective Earth internally to the control via capacitance and local shielded cabling capacitance. If the field device’s Protective Earth is referenced to DGND, transient ground bounce is placed across the control DGND and field device via the low impedance cabling. The transients can significantly disrupt some modules.

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