Galvanically & optically isolation on PLC I/O modules


Thread Starter

Andrew Ng

Can anyone tell me what do you mean by galvanically & optically isolated in a PLC I/O module? When, Where & Why do we use each of these isolations.

Alok Varshney

Galvanic or optical isolation in I/O cards relates to the technology used to provide the isolation and changeover between the field and the PLC internal circuit. Field voltages are converted to internal TTL voltage (usually 5V DC) either by transformers (galvanic) or optocouplers (optical). This provides the isolation between field and the channel internal circuitry. The idea of any isolation remaining the same - to protect the circuit from problems externally. As a end user it does not matter (to me) whether it is optical or galvanic as long as there is one of the two. The thing to remember, which not many people realise, is that isolation on I/O cards can be in 3 possibilites - channel to field, channel to channel and channel to ground. The more isolation you get from the manufacturer, the higher the price. Usually all manufacturers have various models to choose from for the same functionality but differing isolation capabilites. Hope the helps you in some way. good luck.
The previous reply has good information as to indicate that manufacturers offer different models including channel-channel, field-channel, etc. It should be noted that channel-channel isolation on for instance an analog input module is of no use if the customer wires the commons of both points to the same power supply common. In this case you should not buy the more expensive module since you will defeat its purpose by the way you wire your field devices.
This is the answer to this question I gave last August in regard to the definition of galvanic isolation is:

"I'm not sure that there is a recognized definition for galvanic isolation in regards to electrical signals and circuits, but one definition might be:

Galvanic Isolation - Electrical isolation of a type that no direct electrical connection exists between the two sides being isolated. Common
forms of galvanic isolation are optical and transformer.

Some times the term "physical barrier" is substituted for "no direct electrical connection."

An alternate definition which I have seen is:

Galvanic Isolation - A means of interconnecting two circuits which prevents an unacceptable current from flowing as a result of potential
difference (both AC and DC) between the circuits.

Optical isolation uses light as the isolating means and converts the input signal to light and then converts the light back to the output signal. It is a form of galvanic isolation."

In regards to the usage of galvanic isolation or any isolation for that matter, isolation is generally used to isolate the detrimental effects of two different circuits being connected together. The most common reason isolation is used to prevent is a ground loop. Isolation is also sometimes used to prevent noise and surges from passing between circuits. In some cases an isolating device become the "fuse" to prevent damage to a more expensive device.

In regards to usage with PLC I/O modules, it is again isolation between circuits. For four wire devices(analyzers, scales, etc) that source
current, isolation is required to isolate the signal/ground of the four wire device from the other circuits on the I/O module.

Many people use isolated I/O modules as a matter of good engineering practice. Some of the reasons are future use of the I/O point is unknown, better isolation from noise and surges, less likely to have a ground loop by mistake, etc.

Bill Mostia
William(Bill) L. Mostia, Jr. PE
Excellence in Dependable Automation
[email protected](b)
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Galvanic isolation means Electrically isolated.
Optically isolated means the galvanic (electrical) isolation is by optics. If you have a PLC output card with relay outputs and each relay output contact is wired to terminals, then the PLC internal circuit and relay contacts are Galvanically isolated.

Optically isolated means the PLC internal circuit and external circuit components are coupled by OPTICAL isolation such as a LED/photo transistor or LED/Triac combinations. This is ANOTHER form of galvanic isolation.

There are also i/o cards available that are galvanically or optically isolated (from PLC circuit) and galvanically isolated from one another (ex- a 2 channel 4-20 ma analog input card with optically isolated inputs and also galvanically isolated inputs) Hope this helps.

Great piece of information, i want to know little more.

While i was going through specification of Siemens S7 300 BL100 DI card, It was written that "Inputs are electrically isolated in pair of 8."

What does it mean?
Although I'm not familiar with that particular hardware, I would interpret that as meaning that the inputs are isolated from the underlying system (PLC), but are not mutually isolated one from another.

In other words, eight inputs may share the same common connection, but that common connection is isolated from the PLC common ("ground") and also is isolated from the common of any other group of eight inputs on the system. So, each group of 8 inputs exists in its own isolated world.

Ken Crater