The function of the IS ground is to provide a secure, high-integrity, low-impedance path through which fault currents will flow, while minimizing voltages seen in the hazardous area. The most likely source of high-voltage invasion is from the local distribution transformer feeding the control system and, practically speaking, the IS ground is there to shunt fault current from such an invasion back to the neutral of this transformer. It, therefore, has to be of low impedance to be the preferred path for the fault current (while the barrier fuse blows).
Why Use IS?
Now that we know the basics of the various components of an IS circuit and the associated restrictions regarding its installation, what are some of the arguments for using intrinsic safety that explain why it is so widely used in other parts of the world?
- Cost—IS systems do not require lockable fused isolators, protected cable, special glands or explosion-proof enclosures. These lead to not only higher initial costs, but also require additional time whenever the junction box or the device needs to be opened or closed.
- Use of unarmoured cable—The system is electrically, not mechanically protected, though mechanical protection may be desired for other reasons, such as crush resistance.
- Fault-tolerance—IS is the only technique that remains safe after faults develop in cables and fallible components.
- Live maintenance—IS is the only technique that permits live working without gas clearance certificates for all area classifications.
- Personnel safety—Extra-low voltages and currents mean there is minimal risk of injury in the event of contact with bare wires.
IS "works" by reducing the power going to the field. The simpler alternative which uses diodes/resistors, is called a passive zener barrier. If a barrier introduces too much voltage drop due to too high a resistance being added to the network by the barrier, resulting in insufficient power to drive the output full scale, then you may need to use an isolator. Since isolators are separately powered, they do not present as large a load to the loop.
This happens on occasion for analog-output devices such as valves. The figure on the left shows the differences between these two alternatives.
Is intrinsic safety part of your future? Only you can decide. However, as you can see, there are a number of reasons that you should consider it when designing a new installation. With our increasing focus on safety, intrinsic safety installations reduce the overall risk of explosion through human error.
Ian Verhappen P.Eng. is an ISA Fellow, ISA Certified Automation Professional and a recognized authority on Foundation fieldbus and industrial communications technologies.Verhappen leads global consultancy Industrial
Automation Networks Inc. Check out his Google+ profile.