1kA (10μS) |
15m |
15μH |
1.5kV |
Figure 4 Intrinsically safe circuit fully protected |
The system should be designed so that when the surge current is di- verted the voltage drop across the bonding conductor does not create a large voltage across the isolator. Figure 4 gives an illustration of a satisfactory system. With the currents and distances indicated the iso- lator is still subjected to a 1.5kV pulse and hence the importance of keeping the interconnection as short as possible cannot be over em- phasised.
The use of a second suppressor on the circuit means that the intrinsi- cally safe system is now indirectly bonded at two points. The sequence in which the suppressors begin to conduct is quite complex since it does depend on how the potential difference between the two earths develops. The sustained situation which is the least desirable is that the transmitter protector requires 60 volts to conduct and the computer protector 30 volts to conduct. Hence there would need to be at least 90 volts between the two earths before a significant current could flow
within the intrinsically safe circuit. During this short time the circuit is not intrinsically safe but the equipment at either end of the line is oper- ating within its rating. Any hazard which does exist is in the cable and is in the Zone 1, or Zone 2 location. It is a smaller hazard than that which would exist without the protection and hence is a desirable ac- ceptable solution.
6PROTECTION OF SUPPLIES AND SIGNALS FROM EXTERNAL SOURCES
If the mains supply to the system is subject to lightning surges then the operational integrity and safety of the system can be adversely af- fected. An obvious invasion route for the intrinsically safe system is via the isolator supply which is derived either directly or indirectly from the supply. The intrinsic safety certification process assumes that the power
Signal Suppressor | Data link |
| |
Mains supply |
|
Mains Filter |
|
suppressor |
|
Figure 5 Adequately protected system | |
4 |
|
