60V |
60kV |
TP48 |
Bonding strap |
10kV |
50kV |
Figure 3 System with transmitter only protected |
4PROTECTION OF THE SENSOR AND TRANSMITTER
The problem of surge protection of the transmitter and sensor is rela- tively easy to solve since it is only necessary to prevent significant voltage differences so as to avoid ignition capable sparks. This can be achieved by using a combination of surge limiting devices, which ef- fectively control the voltage between the signal wires and with respect to the adjacent structure.
A practical solution to this problem is to use a Telematic TP48 (see figure 2) which contains the necessary parallel surge components in an encapsulated block within a stainless steel hexagon bar which can be screwed into the unused cable entry of the transmitter. To achieve suppression against the expected transients it is necessary to use a combination of gas discharge tubes and solid state devices. With the usual test waveform this combination restricts the transient voltage be- tween the circuit and structure to 300 volts which then falls to 60V after two microseconds and the voltage between the signal lines to 60V. It is a matter of some debate as to what transient voltages would be antici- pated on a practical installation with protection but they would not exceed 150V and almost certainly would be considerably less.
To be effective the surge suppressor must be adequately bonded to the structure. Almost all transmitters contained within metallic enclosures have both internal and external bonding connections which can be utilised to ensure adequate bonding. The need for the external bond is reduced if the mounting of the transmitters ensures an effective bond. but if there is any doubt a substantial bond should be used. The size of the bond is largely determined by the need to be mechanically robust. A flat short braid with suitable tags has much to commend it.
This suppression circuit produces in the worst case condition a short 150V pulse across the transmitter isolation and a longer 60V pulse, both of which the isolation will normally reject. Any small transient which is fed by the transformer capacitance to the sensor circuit would be absorbed by the high frequency input filter capacitors of the sensor input circuit.
The results of fitting surge suppression on the transmitter therefore en- sures that there is an adequate level of protection for the sensor and transmitter. However removing the potential difference from the trans- mitter transfers the whole of the potential difference to the isolator as illustrated in Figure 3. Typically an intrinsically safe isolator will with- stand an occasional 5kV transient (the components are routinely tested
at 1.5kV rms) but damage would be expected at 60kV. The usual result of this failure would be damage to the computer interface which would have both cost and operational safety implications. In non hazardous locations it is not unusual for the loss of individual transmitters to be accepted as sacrificial but to protect the computer interface so that the possibility of more complex interacting faults is reduced, and the pos- sibility of the total system being shut down is removed.
The suppressor discussed has a BASEEFA certificate which permits its use in conventional intrinsically safe circuits [it is also Ex d certified]. The level of protection offered has been carefully chosen so that all known two wire transmitters can be adequately protected. The leak- age currents associated with shunt protection devices are controlled so that they do not significantly affect the operational accuracy of the loop.
5PROTECTION OF THE GALVANIC ISOLATOR AND SAFE-AREA EQUIPMENT
The use of surge suppression between the isolator and the computer input interface protects the computer interface and the isolators are then sacrificial. The unspecified damage to the isolators is not however desirable and the better installation is to protect the isolators on the hazardous area side as indicated in figure 4.
The standard solution to this problem is to use the SD32X which would reduce the voltages applied to the isolator to the acceptable levels as indicated and would not significantly affect the operation of the circuit. [Note. There is a version of the suppressor which has a replaceable fuse and isolation link. In this application the fuse it not likely to be blown hence this alternative should only be used if the isolation link is thought to be useful].
The SD series has not yet been certified by BASEEFA as being suitable for connection into intrinsically safe circuits although an application has been made and hence its acceptability is based on it being simple apparatus as defined in the second edition of EN50020 [see Appen- dix B]. It does contain two small inductors which have a combined inductance of 200 microhenries. However the conventional transmitter circuit is powered from a 28 volt 300 ohm source which has permitted cable parameters of 0.13 microfarads and 4.2 millihenries. The per- mitted length of cable is usually restricted to approximately 600 metres by the capacitance requirement and hence a marginal reduction of the permitted inductance to 4 millihenries (equivalent to 4Km) has no ef- fect.
3
