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Important general considerations

Great care must be taken when adding datalogging to railway signalling systems.

The most serious hazard to be considered is the possibility of feeding out a voltage and current that is sufficient to falsely energise a railway signalling relay.

The second most serious is accidentally providing an electrical path through a logger from one part of the signalling system to another, or to earth.

An example would be a connection to monitor rail temperature.  If the sensor used is not isolated, and the datalogger is also not isolated,

the rail will be connected directly to either earth or in the worst case to other parts of the signalling system.  This could result in false feeding of the track circuit.

This is an extremely dangerous situation.

The SA380TX has been designed from the start with this in mind.  Each analogue input and its power supply is individually isolated.

However, even this does not mean that it can be indiscriminately connected to any signalling circuit.

The chance of isolation failure, while extremely low, must still be considered as a possibility.

Analogue and digital inputs of the SA380TX must never be directly connected to

• track circuit tails
• rails
• BX110/B50/B24 power supplies
• any other live signalling circuits

The following sections consider these applications in more detail.  Restrictions are laid out in each section which must absolutely be followed.

The key point to be borne in mind by the design engineer at all times is:

Does the information that is being gained from monitoring this item justify the risk of making the connection?

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SIL 0 software

Software in this product is designed to SIL 0.  It is not a safety critical system and data from an SA380TX must never be used to make safety related decisions.

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Current measurements

Possible applications

• Points motor current
• Track circuit current
  
  

Up to 8 currents may be monitored by a single SA380TX.

The following diagram shows two 4-20mA loop powered current clamps connected to two inputs on an analogue card

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Recommended transducers:

Other types can be used provided that they meet the following requirement:

• The current transducer must be non-intrusive, ie there must be no direct electrical connection between the track circuit / motor current wire and the transducer itself.
  It may of either solid core or split core type and must maintain isolation of 4M Ohms at 250V even in the case of the wire chafing and making direct connection with the housing of the transducer.

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Voltage measurements

Possible applications

• Points motor voltage
• BX110
• B50
• B24

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• 250mA fuses must be inserted in each leg between the voltage being monitored and the transducer.

Recommended transducers:

Other types can be used provided that they meet the following requirement:

• The voltage transducer must contain 3-way isolation to a minimum of 4M Ohms at 250V between input, output and housing.

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Trackwork and point machines

Possible applications:

• Rail temperature
• Rail vibration
• Points load pin
• Clamp lock power pack monitoring
• Points position monitoring

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Sensor requirements:

• Each sensor must have electrical isolation between the 4-20mA output and the trackwork to a minimum of 4M Ohms at 250V.
• 250mA fuses must be provided in each leg of the current loop.
• Only Network Rail approved points load pins may be used.
• When monitoring the pressure in clamp lock power packs, only pressure sensors approved by the manufacturer of the power pack must be used.
• Where sensors are attached to both rails, a risk assessment must be carried out to ensure that there is no risk of a track circuit being false fed

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Other measurements

The 4-20mA analogue input card can be used with many other transducers, including temperature, pressure etc.