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Isolation Transformer Potential to Earth Ground

chopnhack

Apr 28, 2014
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I am somewhat confused by the schematic below. On the isolation side of the transformer, the hot lead is shown as having a 0 V potential to earth ground? If I was to probe the hot lead to earth ground I would have no potential?? Can someone clarify this for me, I can kind of see how this could be possible, but some part of me is saying, ?

Thanks!

iso_zpsebefa338.jpg
 

(*steve*)

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Showing 0V is a bit wrong. Assuming your meter has a reasonable impedance (reasonably low) then it will read 0V because there is no circuit to allow current to flow. If it has a really high input impedance then any capacitance may allow some small current to flow and it could read something (a realy high input impedance meter won't even read zero when the probes are not connected to anything).

The whole point is that you can make any single point a ground reference without causing large currents to flow.
 

davenn

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The whole point of isolation transformers is that the output IS isolated from earth that is .... the earth DOES NOT form part of the circuit
it means that you can touch either the phase and neutral individually and not get zapped

where as the input side / and your house mains cabling, there is no isolation as can be seen by the 120V between phase and earth
That is because the neutral is ties to earth at the point where the mains cabling enters the house at the fuse/meter panel

cheers
Dave
 

KrisBlueNZ

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The important characteristic of the isolated side of an isolation transformer is that the whole winding is completely isolated from everything.

Voltage appears across the winding, i.e. between the Phase and Neutral inputs to the powered appliance, but this supply is fully floating relative to everything else.

It's like the AC version of a battery. Imagine you're holding a 1.5V battery in your hand. You would measure 1.5V if you connected your voltmeter straight across the battery, but if you measure voltage from either terminal of the battery relative to some other point - the ground outside, for example - you won't measure any voltage, because the battery is fully isolated.

This is what they're trying to convey in the diagram by showing "0V" between each side of the isolated winding and earth. They're showing that if you measure voltage between those points, ideally, you won't get a reading.

That diagram causes confusion because it's not possible to measure those three voltages, as marked, simultaneously. The two measurement connections that are marked "0V" will only show 0V if measured separately. If you measure them simultaneously, using two multimeters, they will not (cannot) both measure 0V simultaneously.

Even if you measure them independently, as Steve pointed out, in real life you will probably measure some voltage between each side of the secondary and ground, especially if you use a voltmeter with high input impedance, because of leakage and coupling.

Down here, at least, isolating transformers must not have any connection to the earth pin on the isolated output socket. I can't explain why the ground is shown connected all the way through. Here, that would be illegal.
 

chopnhack

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It's like the AC version of a battery. Imagine you're holding a 1.5V battery in your hand. You would measure 1.5V if you connected your voltmeter straight across the battery, but if you measure voltage from either terminal of the battery relative to some other point - the ground outside, for example - you won't measure any voltage, because the battery is fully isolated.

Gold star, mate - that really clarifies it for me, thanks!

Down here, at least, isolating transformers must not have any connection to the earth pin on the isolated output socket. I can't explain why the ground is shown connected all the way through. Here, that would be illegal.

Ground is common, so how is the ground pin grounded in your isolated output socket?
 

KrisBlueNZ

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Ground is common, so how is the ground pin grounded in your isolated output socket?
It's not grounded; it's not connected to anything at all.

I don't know the rationale behind the choice between connecting or breaking the ground connection on the output socket. I found a couple of discussions on the subject though:

http://electronics.stackexchange.co...g-the-ground-wire-in-an-isolation-transformer
http://www.dslreports.com/forum/r23982314-Isolation-transformer-secondary-Ground-questions

The posted answers say (unanimously, I think) that isolating transformer products that you buy are normally made with the ground pin on the output socket connected straight through to the ground pin on the input plug, but that for full and proper isolation, there should be no connection.

davenn and Steve may be able to explain the reasons...
 

(*steve*)

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Explain? Well, I can speculate...

I agree that with an earth connection the device being powered is not completely isolated. And that may be justification enough if you want complete isolation (and it's an isolation transformer).

Practically, do I always want complete isolation?

Let's assume my equipment has an earth leakage problem. If I use the isolation transformer with an earth connection, the symptom (tripping ELCB's) stop, so I can try to diagnose the problem.

However, if I am using mains powered test equipment (let's say an oscilloscope), I can connect the earth lead to somewhere that *should be* isolated from ground, but due to the fault is not. This may cause a significant current to flow in the ground wires and through the protective ground. This is similar to the type of problem which would cause us to reach for an isolation transformer, but the kicker here is that we're already using one.

If you're using the isolation transformer for some other purpose, and you're not operating test equipment on it, or if you're 100% sure there's no ground fault, then the protective earth connection may (will?) enhance safety.
 

chopnhack

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It seems that the consensus from the article is that its useful to sometimes have complete isolation including no ground on the isolation side - just a floating loop and at other times its useful to have the grounds tied together. A little ahead of my experience at the moment, I hope when I get around to using an o-scope I will remember and put this to use! Thanks all :)
 
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