I've been reading about the EIA-485 standard and they talk about how even with twisted pair wire they need a 3rd connection for common because sometimes the devices need to be isolated. I was thinking about it and I've always taken isolation for granted, but never actually understood why it would be necessary. Why is it that a device would ever need to be electrically isolated from another?
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2 Answers
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There are several reasons why you would want your signals to be electrically isolated between devices.
The main reason I usually think of first is different ground potentials. The simplest way to think about this is if you are connected 2 nodes of a network together that are using different ground references. I have one real life example that I have personally dealt with. We had 2 computer network switches that were connected with a CAT5 cable in 2 adjacent buildings. The ground potential was different enough between each switch that it caused the ports being used to stop working.
Another extension of my previous example is being able to protect a failure in one device to not damage the other. This could be from a current surge, or static, or many other things.
Another reason is to limit the noise in one system from working its way into another system.
And one final reason that I know of is to help prevent electrical shock, especially in marine environments. There are certain items on a boat that will come in contact with water that have a good chance of having a voltage on them. You don't want salt water to be shocking anyone.
There may be other reasons, but this is my extent of knowledge.
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\$\begingroup\$ @Kellenjb - You don't even have to think about boats for prevention against electrical shock. One product may be directly connected to the mains power, while the other may have to be SELV. \$\endgroup\$– stevenvhCommented Jul 1, 2011 at 4:45
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\$\begingroup\$ Ethernet via the typically included transformer by design has quite a good amount of isolation, doesn't it? \$\endgroup\$– kennyCommented Jul 1, 2011 at 12:44
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\$\begingroup\$ @Kenny it had enough isolation to not kill the whole switch, but not enough to not kill the port. \$\endgroup\$– KellenjbCommented Jul 1, 2011 at 12:45
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\$\begingroup\$ @Kellenjb So if they have different ground potentials, then how do you communicate between them... wouldn't they interpret voltage levels differently based on their respective grounds? Did you also use a separate signal for common like in EIA-485? \$\endgroup\$ Commented Jul 1, 2011 at 14:18
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\$\begingroup\$ @Jgord usually things that have different ground potentials use differential signalling so that the ground value doesn't actually matter at all. Also a lot of time light is used as a way to isolate. \$\endgroup\$– KellenjbCommented Jul 1, 2011 at 15:42
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Some environments are so noisy and hostile, big motors come to mind. That you want to isolate the control circuits from the drive circuitry. Also some batteries are made up of a set of batteries and it's best to charge them individually for optimal life.
