Phase 7: Isolators and Terminal Blocks
In this article we discuss the options we have for isolators, converters, barriers, terminal blocks and other components (index of design phases).
7-Isolators, field terminals and prefabricated cables
Converters, Isolators and Barriers
They are used to convert signal ranges, to electrically isolate two circuits, to duplicate signals, to amplify signals, and so on.
Normally we use the term «signal converter» when we transform one range into another, for example, from 0-10 VDC to 4-20 mA.
The term «isolator» is used when the main objective is to perform a galvanic separation (or electrical isolation) between the input and output circuits.
The term «barrier» usually refers to the intrinsic safety (IS) isolators used to wire signals from the control panel to the hazardous area.
In the case of redundant signals we can use the signal duplicators, mainly with analog signals.
In the market, there are many types and formats available and it is rare that we do not find the solution we are looking for. The important thing from the point of view of design is to correctly perform the selection in each case since the impact on the total cost can be important, especially in the case of large systems. For example, if it is technically acceptable it will be better to use double isolators than simple with the consequent saving of cost and space in the cabinet.
Field terminals (terminals)
The possible options are many and it is difficult to give general rules.
Usually the discussion focuses on the following:
-Connection by screw or spring connection.
In most cases the screw connection is accepted, but not always the spring connection. This one is more suitable when there are vibrations and saves wiring time of the control cabinet.
-Fused or non-fused terminal.
This is a more important point than it seems and should be consulted in the technical specification. There are end users who have clear requirements in this regard.
The reason for using fuses in the terminals is because we want to avoid that a short circuit or other problem in a signal affects the whole module or panel. On the other hand, we know that introducing fuses is to introduce other elements that can fail.
Let’s look at some of the options we have:
A) Use a fuse terminal in each signal individually.
B) Use the fuse terminal for each group of signals or module according to a functional criterion (per equipment or service).
C) Use I/O modules with built-in electronic protection.
D) In the case of the digital outputs we can choose to protect each group with a circuit breaker. This is sometimes used to protect a group of electrovalves.
We can also use several of the options depending on the type of signal and the type of module used.
-Single or double level terminal blocks
It is more common to use single level terminal blocks. The main advantage is when it comes to wiring, both in the manufacturing workshop and in the field. When we use double level is because we want to save space even at the cost of making it difficult to access the terminals.
-Cables with or without terminal tip.
In general, it is technically more convenient to use tips on the cables. There are numerous installations that do not use these tips when using spring terminals.
Pre-fabricated cables for PLCs
It is a good option to consider on many occasions, especially if we want to reduce costs, space and wiring time of the control cabinet.
Finding an advantageous design based on prefabricated cables can be complicated if we have made a functional distribution of I/O signals (based on equipment or field areas) and therefore there may be, in the same module, signals with different wiring (e.g.: 2-wire and 4-wire analog inputs, digital outputs to intermediate relays with voltage or voltage-free contacts, digital inputs with and without intermediate relay, etc.).
In large systems, especially in the oil & gas industry, marshalling cabinets are located between junction boxes and DCS. Within these cabinets field cables are connected and cross wiring is performed to order the signals according to the DCS I/O modules. In this way, it is possible to use prefabricated cables to interconnect the marshalling cabinets with the DCS modules. In large and complex projects this system has many advantages.