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An analog-signal isolator is a signal converter that maintains electrical (galvanic) isolation between its input and output circuits. To be precise, the electrical path between the input and output circuits is eliminated by design. This isolation is done by means of either signal transformers or a combination of power transformers and opto-isolators. The level of isolation may range from a few hundred volts to as much as kilovolts. Since Signal isolators also break the direct electrical (galvanic) path between two or more loop points. They are used to protect against dangerous measured-variable voltages and provide increased protection from surges and spikes.
In addition, low-level input transmitters, like isolated millivolt transmitters and isolated RTD transmitters, etc, are used to provide input to output isolation.
Types of Isolators
There are three main types of isolators, which are as follows:
1. Four-wire isolators
2. Loop-powered isolators
3. Two-wire isolator transmitters
1. Four-wire isolators
Four-Wire Isolators consists of six terminals, excluding the ground terminal -- two for input, two for output, and two for the power supply. They need power supply of 115 VAC or 24 VDC. In some DC powered units, the output and power negative may be common. Four-Wire isolators are highly versatile in accepting most types of analog inputs and provide most types of analog outputs.
Downside
The disadvantages of Four-wire isolators are that the requirement of power supply and their size may be larger than a loop-isolator.
2. Loop-powered isolators
Loop Isolators are highly compact devices that do not require an external operating power supply. They are also powered by the input loop. These are extremely convenient to use for isolating current input signals such as 4-20 mA or 10-50 mA.
Disadvantage
- The input of local powered isolator requires current signal. Also, they need extra input voltage drop to provide for the self-power operation.
Two-Wire Transmitters are also used as isolators when the input is 4-20 mA or high level voltage such as 1-5 VDC.
Why Isolation is required?
Isolation is required because it prevents erroneous readings or incompatible connections caused by ground loops when it is kept open. It also provides safety from high voltage inputs. Usually, ground loops can be present wherever there is multiple grounding of the measuring, or the controlling or monitoring equipment. Due to the presence of these ground loops, there is a high possibility that errors would be introduced frequently, thus, affecting readings in process instruments. Beside, the output signals would get affected transiently by other non-related plant electrical activity.
An isolator solves the problem by opening up the ground loop. The following are some of the common causes of ground loops:
1. Due to electro-magnetic induction, a common-mode voltage introduced into input leads
2. Since electrical currents are present in a finite ground-plane resistance, a common-mode voltage generated in the ground circuit
3. Need for multiple grounding in series-connected instruments in a loop
Rule-of-Thumb Guidelines for Isolation:
1. It is always advisable to use an isolated transmitter for safety reasons if a sensor is operating at more than 30 VAC or 42 VDC potential
2. In an instrumentation loop without isolators, there can be only one ground or common. If there are multiple instruments or devices that need grounding, then all except one will need an isolator.
3. Using an isolated transmitter is advisable in case if grounding of RTD sensor or T/C sensor would occur.
4. In general, a current signal-series loop will need isolators more often than a parallel-voltage connection scheme with common ground for multiple devices.
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