Phase 2: Selection of I/O modules (part B)
In this article (part B) we discuss other design factors of the selection of Input / Output Modules such as intrinsic safety, electrical isolation, redundant I/Os, etc. (index of design phases)
2B-Hardware selection of input / output modules
G) Electrical isolation
Normally there are different levels of isolation in the input / output modules (channel-to-channel, between groups of channels, between channel and ground, etc.). The standard modules are not isolated channel by channel, but in most cases the isolation between groups of signals is usually sufficient. It is usually a good practice not to mix the common terminal of the modules with the “ground”. The modules with galvanic separation among channels have a high cost so it is necessary to analyze it well. If we only need to isolate a few signals we can install external isolators.
If the technical specification is very demanding in terms of electrical isolation we must analyze in more detail than usual what is the best technical solution and at what cost (modules with galvanic isolation, external isolators and / or intermediate relays, etc.)
The modules with diagnostic functions of the internal faults offer us an interesting plus but we have to pay. When the down time of the machine or process is important then it will be worth paying this extra cost, so we can avoid or greatly reduce down time. Nowadays many manufacturers already incorporate standard diagnostics even in the standard modules.
I) -SIL certification
If our application requires a safety PLC we must use the fail safe modules whose cost is much higher. These modules incorporate many diagnostic functions and have a redundant internal structure with logic 1oo2D or 2oo3D.
The same goes for the interposing fail-safe relays because they are special and certified relays that have a very high cost.
If we also have redundant I/Os the cost of the solution will increase significantly.
J) Extreme conditions
Many manufacturers have special modules to work in extreme conditions, for example, at high temperature. Their cost is much higher and should only be used when they are really required.
In many cases the control panel is located in a conditioned room. In any case, the operating temperature of the control system as a whole is usually defined in the technical specification. It is a very important data that we must never forget in the whole design process.
K) -Hazardous area – Intrinsic safety solutions
Hazardous areas with risk of explosion are classified in different categories. The typical case is found in some areas within refineries or gas plants. This means the design of the safety PLC or the DCS must fulfill a series of demanding technical requirements. A very common solution is to use intrinsic safety I/Os, this implies either intermediate barriers or intrinsic safety modules available in some manufacturers. These applications require a much higher level of training of the design engineer.
There are other options for designing the electrical panel inside a classified area such as pressurized cabinets or “explosion proof”. In any case, in the case of control cabinets based on PLCs it is frequent to use the intrinsic safety solution that has some advantages such as, for example, the possibility to place the control cabinet in a safe area without danger of explosion.
In large installations, such as in refineries, another solution is to pressurize the control and panels rooms. This requires the use of field cables and instruments suitable for hazardous areas.
L) -Switching frequency
Applications of motion control require special modules that can work at high speed. In digital signals the DC ranges allow more switching speed and are more recommended for fast applications. Depending on the case we can use standard or special modules. If we use high-frequency inputs (eg, capable of reading KHz signals), special caution must be taken with the electrical noise of other power cables.
M) -Redundant Inputs / Outputs
The concept of redundancy is broad and should be analyzed slowly. There are several types of redundant architectures (TMR, QMR, etc.), each with its advantages and disadvantages. On the other hand, we must distinguish between redundancy to increase safety and redundancy to increase availability.
The customer specification must define the number of input / output signals of each type, which are redundant and the type of redundancy or logic (1oo1 for single signals without redundancy, 1oo2 / 2oo2 for redundant signals and 2oo3 signals).
Although good design practices advise using redundant controllers and redundant power supplies whenever redundant I/Os are used, this is not always possible because of a cost issue. That is, we can find designs that use redundant I/Os and 2oo3 logic, but with a single CPU (very often this is due to the CPU redundancy limitations of the PLC model used).
When designing a redundant system we must take into account the “Mean Time Between Failures” (MTBF) values provided by the manufacturer. Typically, this time will decrease as the module’s complexity increases, that is, we will encounter very high times in input and output modules and much lower in CPUs.
Redundancy of I/Os (hardware) and / or redundancy of field instruments.
Another aspect to be analyzed is the redundancy of the instruments and field elements, since the redundancy of I/Os does not usually coincide with the redundancy in the field. It is common to have non-redundant instruments or electro valves and to specify redundant I/Os. Here we can enter into the discussion of the probability of failure of the PLC hardware versus the probability of failure of the sensor or field device.
N) – Remote signal
Depending on the distribution of instruments and equipment in the field it may be interesting to use remote I/O racks next to the field elements. In these cases the usual goal is to save wiring. In critical applications (nuclear, airports, etc.) the objective can be to get very high availability by having the redundant I/Os in racks separated several hundred meters.
In large installations it is a good option to distribute the controllers and I/O by zones, which communicate with each other and with the servers through the control network.
O) -Spare I/Os
The percentage of spare signals is not always defined in the specification. A good practice is to keep this percentage between 10 and 20%.