Causes of explosions
Three conditions must be present for an explosion to take place:
p Combustible material - gas, vapours, dust etc.
p Oxygen.
p Electrical or thermal energy - of sufficient value.
Intrinsic safety measures are used in the signal circuits used for measurement, control, regulation and monitoring in order to guarantee safety and proper function. Isolator modules are used in addition to input and/or output modules of the PLCs or the process control system (PCS) which guarantee explosion protection of the signal circuits in the event of errors.
The interface modules from Pepperl & Fuchs establish the isolation between the hazardous areas and the safe areas within a system. They are electronic devices in accordance with DIN EN 50 014 and they are installed in a safe area. Intrinsically safe and nonintrinsically safe circuits are either galvanically isolated or zener barriers. The primary functions of interface modules are the transmission of signals from the hazardous area to the safe area and vice versa. In addition, the connected field devices are supplied with power.
Voltage clamping
Pepperl & Fuchs zener barriers interface intrinsically safe and nonintrinsically safe circuits. They are designed to transmit electrical signals bidirectionally and to limit the electrical energy which can be transferred from the safe area to the hazardous area in a fault condition.
The function of 'zener diode barriers' is dependent on the breakdown voltage of the zener diode. If this voltage is exceeded during normal operation, these barriers may be damaged.
Should the voltage be exceeded, for example, because of a fault condition in the safe area, the diodes begin to conduct and the fuse will blow, thereby preventing the transfer of energy to the hazardous area.
Zener barriers are characterised by their voltage, resistance and polarity, for example 10 V, 50 W, positive polarity. This information refers to the zener voltage Uz and the combined resistance of all barrier components. With this they present the safety characteristics. The values displayed on the ID plate refer to the 'worst case' data for Uz and Ik as determined by the certification. Ik is obtained from the division of Uz through the R1 resistor. It should be noted that these values do not refer to the operating area of the zener barriers.
Finding ways to cut installation costs, simplify wiring and reduce downtime has always been a priority. To achieve this, Pepperl & Fuchs developed the KF series of power rail mounting transformer isolated barriers. These units have gold-plated contacts on the base that allows them to pick up power from a power rail that fits directly into a standard 35 mm DIN rail.
Power is sent through the rail by a feed module which delivers 24 V d.c. and is protected by a 4 A fuse. Each barrier is then automatically supplied with power simply by snapping it onto the rail. 'Daisy chain' wiring between modules is thus elimiunated.
Another advantage of the KF series is the removable connection blocks with self-opening terminals. The removable terminals are coded and allow the equipment to be replaced or serviced under power, then when reconnecting, the coding ensures that cross wiring cannot take place. These removable screw terminals will cater for cables up to 2,5 mm2.
Other options
Remote process interface (RPI) is an interface system for conditioning conventional measurement and control signals in sensors, actuators and field devices on one hand and a process control system or a PLC on the other. Connection to the control system is made via a fieldbus interface. The signals are galvanically isolated and are suitable for intrinsically safe and nonintrinsically safe signals with Zones 1 and 0.
The RPI, I/O modules are simply snapped onto a rail much like that found in the system mentioned under 'Recent developments'. However, here the Pepperl & Fuchs power/bus rail is used for both powering the individual devices and for transferring signals. In addition to the two conductors for power supply, the power rail has three other conductors on which the measurement signals are transmitted in digital form via an internal serial data bus from the I/O devices to a gateway and vice versa. This gateway, in turn, provides the interface to an external standard bus; from here the signals are either transmitted or received from the control system. By selecting the gateway, adaptation to the various bus systems (for example, Profibus or Modbus) as well as to various process control systems is possible.
P&F Products
(011) 609 8188
www.pandf.co.za
| Tel: | +27 10 430 0250 |
| Fax: | 086 756 8741 |
| Email: | [email protected] |
| www: | www.pepperl-fuchs.co.za |
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