Product type
A signal isolator passes a measurement signal from one circuit to another without an electrical connection between them. It is fitted where two ends of a loop sit at different earth potentials, or where a long cable picks up interference that would otherwise appear in the reading. The isolator does not change the value; it removes the conducting path along which a stray current would travel.
A signal isolator passes a measurement signal from one circuit to another without an electrical connection between them. It is fitted where two ends of a loop sit at different earth potentials, or where a long cable picks up interference that would otherwise appear in the reading. The isolator does not change the value; it removes the conducting path along which a stray current would travel.
Two versions are commonly supplied under the same name. An isolator passes a signal from a device that already has its own supply. A distributor also powers the field device, which is what a two-wire transmitter needs, since it takes its supply over the same pair that carries its signal. The two are not interchangeable: a two-wire transmitter connected to a plain isolator is simply unpowered, and a separately powered source connected to a distributor leaves two supplies fighting over one loop.
Selection turns on four questions: whether the field device needs powering, what the input and output signal forms are, what load the output can drive while holding its accuracy, and what isolation the application requires. The input and output forms have to be decided together rather than one at a time, because an isolated signal can only be reproduced in the proportional form of the original.
Isolation is achieved by transferring the signal across a barrier that no conductor crosses, usually by magnetic or optical means. The input signal is converted into a form that can cross the barrier, transferred, and reconstructed on the other side as the same proportional value. Because the two sides share no conductor, a potential difference between them has no path through which to drive a current along the signal cable, which is the mechanism by which an earth loop adds an offset to a reading that is otherwise correct.
Rejection of common-mode interference follows from the same arrangement. A voltage induced on both conductors of an input pair, such as one picked up in a cable tray shared with a drive, arrives as a differential of zero at the receiving end. A filter attenuates such a signal; an isolation barrier removes the route by which it reached the measurement at all. That is why a signal that still drifts when a nearby drive runs is a case for an isolator rather than for a heavier filter.
The output stage decides what the isolated signal can be connected to. A current output drives a load up to a stated resistance, which covers a receiving instrument and the cable to it. A voltage output is a source and therefore requires its load to be above a minimum rather than below a maximum; a load lighter than that takes the output outside its stated accuracy rather than damaging it. Both figures apply at the accuracy the module is rated to, not as absolute limits.
Most of the specification is fixed by these decisions rather than by the accuracy class. Each one constrains the next, which is why they are settled in order.
| Configuration | Options to fix | What it actually decides |
|---|---|---|
| Isolator or distributor | Plain isolation, or isolation with a supply for the field device | Whether a two-wire transmitter can be connected at all. This is decided by the field device, not by preference. |
| Input form | Current or voltage loop, over a defined set of ranges | What the module can accept. A voltage input taken as a current one appears to read low rather than to fail. |
| Output form | Current or voltage, in the proportional form of the input | What the receiving equipment sees. The form is not switchable in the field on most modules, so it is fixed at order. |
| Channel configuration | Single channel, one input to two outputs, or two independent channels | Whether one field signal has to reach two destinations, and how many modules the cabinet depth has to hold. |
| Isolation level | The withstand voltage between input, output and supply | The potential difference the module can sit across, which is what an earth loop or a test voltage imposes on it. |
| Output load capability | Maximum resistance for a current output, or minimum for a voltage output | Whether the module can drive the receiving instrument and its cable while holding the rated accuracy. |
| Mounting width | The width per module on a DIN rail | How many channels fit in the cabinet, which on a large plant is often the deciding factor rather than the specification. |
| Termination and supply | Pluggable terminals, and the supply voltage available in the cabinet | How quickly a module can be changed without disturbing field wiring, and whether it can run from the supply already present. |
| Factor | Why it decides the answer | How to settle it before enquiring |
|---|---|---|
| Whether the field device is powered over the loop | It separates the two versions, and getting it wrong leaves the field device unpowered or two supplies on one pair | State the field device type and how it is supplied, rather than only the signal it produces. |
| Input and output signal forms | They have to be chosen together, because the output can only be the proportional form of the input | Give the source signal and the signal the receiving equipment expects, as a pair. |
| Receiving load | A current output has a maximum load and a voltage output a minimum, both tied to the accuracy figure | State what the signal drives and the length of the cable to it. |
| Number of destinations | A signal required in two places needs a two-output module or a second transmitter | List everything that has to receive the signal, including recorders and indicators. |
| Isolation requirement | The withstand voltage has to exceed whatever potential difference the installation can impose | Describe what the two ends of the loop are connected to, since that is what sets the difference. |
| Cabinet space and rail | Width per channel decides how many fit, and rail type decides whether the module mounts at all | Give the available rail length and the number of channels. |
| Area classification | A loop entering a hazardous area needs equipment certified for it, which the standard version is not | State the area classification and the zone, if any, at the field end of the loop. |
The isolation figure is a withstand voltage, not a working voltage. It describes what the module survives between its three ports, not a potential difference to be applied continuously, and it is not a statement about electrical safety.
These are the types an enquiry for a signal isolator usually turns out to be, and the point at which the choice is genuinely decided.
| Nearest alternative | What separates it from a signal isolator |
|---|---|
| Signal converters | A converter changes the form or the range of a signal, such as a thermocouple to 4-20 mA, and many convert without isolating. Where the two ends of a loop are already at the same potential, a converter is the smaller and cheaper answer; where they are not, isolation is the function that is actually being bought. |
| Remote I/O | Remote I/O moves the conversion and the acquisition to the field and carries the values digitally, so several channels share one connection. An isolator acts on one loop at a time. On a new installation with a digital bus, remote I/O may remove the need for individual isolators; on an existing analogue loop it cannot. |
| PLC systems | A programmable controller reads the signal and acts on it, and some input cards carry channel-to-channel isolation of their own. Where that isolation is already present, a separate module is redundant; where the card isolates only against the backplane, an earth loop between two field devices is still a case for an isolator. |
| Surge protection devices | A surge protector diverts a transient to earth, and an isolator removes the conducting path altogether. The two are complementary rather than alternatives: a protector limits what reaches the cabinet, and an isolator breaks the loop that would otherwise carry a potential difference between two earthed points. |
| Power supply units | A distributor supplies the field device and isolates its signal at the same time, which is what a two-wire transmitter needs. A power supply energises a circuit without passing a signal through. The distinction matters when a transmitter is described as loop-powered: that is a distributor requirement rather than a supply requirement. |
The first reply can be a quotation rather than a set of questions if the enquiry carries these. Where a figure is not known, send the duty description and we will confirm what it implies.
Available products
M2 · Signal Isolators
DIN-rail signal isolators and distributors giving 2000 Vrms isolation between input, output and supply, with 0.1%FS transmission accuracy and 10 ms response.
Also in Industrial Automation & Control
Every type we quote for under Industrial Automation & Control. A type with no link yet is still quoted for - it is described on the Industrial Automation & Control overview, and gets a page of its own as soon as a model is published against it.
Frequently asked
When the problem is a potential difference rather than noise. A filter reduces interference that has already entered the measurement; an isolator removes the path by which it entered. The practical test is what happens when a nearby drive runs, or when a long cable is disconnected at one end: if the reading shifts with the drive or with the earth connection, the loop is carrying a current it should not be carrying, and isolation addresses that in a way no filter does.
Changing between a current and a voltage form is a hardware change rather than a setting on most modules, so the form is fixed when the order is placed. There is a second constraint on the same point: the output can only be the proportional form of the input. A 4-20 mA input can be reproduced as 4-20 mA or as 1-5 V, and it cannot be configured to produce 0-10 V. Both ends therefore have to be specified together rather than settled one at a time.
They describe the loading under which the module holds its stated accuracy. A current output drives up to a stated resistance, which is normally enough for a receiving instrument and its cable. A voltage output requires its load to be above a minimum, because the output is a source: too light a load takes the output outside its accuracy rather than damaging it. Neither figure is an absolute limit, and a load well outside the range is a case to raise rather than to assume.
No. It transfers the value it receives and adds its own error, which is small but not zero. A measurement that is already wrong at the transmitter arrives wrong at the other end of the barrier. Isolation corrects a specific class of error, the one introduced by a shared conductor between two earthed points; it does nothing about a sensor that is mismatched to the process.
Not by itself. Equipment installed on a loop that enters a classified area has to be certified for that area, and the certification is a property of the specific model rather than of the product family. Whether such a version is available is a question to raise with the enquiry, together with the zone and the gas group, rather than something to infer from the standard module specification.
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