An isolation transformer doesn't have either one of it's output legs tied to ground. So if you touch either ONE of the leads coming out of the isolation transformer, you won't be electrocuted because you won't be completing a circuit. Now if you should come in contact with BOTH leads... :(

Here's a more detailed explanation:

A generator at the power company has two leads coming off of it. The generator produces 120v across the two leads. Rather than running two wires all the way to your house, they connect one lead to ground, the other they send to your house where it is labeled as "HOT". All of the NEUTRAL wires within your home are also connected to the ground. So when you plug a light into an outlet, you complete an electrical circuit where the electricity flows from the generator, through the HOT wire, through the lamp, through the ground, and back to the generator.

Because you are almost always in contact with the ground (in some fashion), if you ever touch a hot wire, you become a completed circuit, where the power flows from the generator, through the HOT, through YOU, through the ground, and back to the generator.

Now if you've ever touched a HOT wire (and felt that really weird tingly sensation) and didn't get electrocuted, it's because you were ONLY touching a HOT wire. To complete the circuit, the electricity had to flow not only through you, but your shoes, the carpet, the wood framing of your house, and the concrete to complete the connection to ground. All these other things acted as great insulators and didn't allow enough current to flow through you to electrocute you.

But not if you place a hand on your plugged in washing machine and the other to a HOT wire, you will be fried. That's because washing machine was GROUNDED, which means there is a wire that connected the body of the washing machine directly to ground. The only resistance between the HOT and ground is now your fingers, arms, heart, etc).

Now of course the reality of how these things work is much more complicated. For example, the power between the generator and your house goes though a series of step-up and step-down transformers. Generators don't just have two wires coming out of them either. They usually have three sets of wires with each set generating AC power that is 120 degrees out of phase with the other two leads (three phase power). The way you get 240v circuits in your home is that two of the three HOT leads from the power pole go to your house. Etc.

"reg transformers". The term is meaningless. There are isolation transformers and autotransformers. Isolation transformers have at least two windings, primary and secondary. Isolation transformers do not internally have any part of the secondary windings connected to the primary windings. With autotransformers, the winding(s) are connected so as to act as a single winding which is connected to both the supply and output conductors, with some of those connections being at different points. The same winding is thus both the primary and secondary. Often external connections are made to isolation transformers to apply them to special situations that may result in an interconnection of the primary and secondary windings. This is not by any means a complete explanation, but it is a direct answer to your question.

Bus, you are correct sir! Here's the deal. My range hood pooped it's drawers Xmas day. In looking at the wiring dia, I see an Isolation Trans and an Autotrans. Since I'm plunging ahead blindly, I ordered both. Sure would like to know what they do! Problem is this: 3 level halogen lighting won't work at all. 3 speed blower, speed LED light up but no blower. 120 volt to hood, changed bulbs, etc. still no luck.

An isolation transformer can provide some protection against ground faults, depending on how it is connected in the circuit. An autotransformer often is used as a low cost way to get some voltage different from the supply. It provides no extra shock hazard protection. I have installed lots of range hoods, seldom had to repair one, and do not recall seeing any with transformers. Just guessing, the autotransformer might have a number of taps that provide three different voltages for the lighting levels, with each level being connected to that portion of the transformer by the switch. What is the voltage rating of the lamps? Or the part number of the lamps?

§ hookoodookoo:
Here's a more detailed explanation:
A generator at the power company has two leads coming off of it. The generator produces 120v across the two leads. Rather than running two wires all the way to your house, they connect one lead to ground, the other they send to
your house where it is labeled as "HOT". All of the NEUTRAL wires within your home are also connected to the ground. So when you plug a light into an outlet, you complete an electrical circuit where the electricity flows from the generator, through the HOT wire, through the lamp, through the ground, and back to the generator.§

not quite right. You get 240 from the POCO on the low side of the transformer, the winding is center tapped so you have 2 120 volt feeds and 1 240 volt feed to the house via a total of 3 wires.

The power never flows through the ground. It flows from one hot to the other (240) or from one hot to the neutral (120)

Attaching the neut to ground isn;t actually neccessary to be functional but it affords benefits when it is used.

an isolation transformer is typically a 1:1 transformer (not always and doesn;t have to be), meaning it does not change voltage. They are built differently than a typical transformer to preven unwanted electrical noise being passed onto the secondary side.

Not sure why an isolation x-former is used on a vent hood.

As bus driver posted, the autotransformer is probably used to provide variable voltages. If it is converted to DC voltage, it could be used to drive different speeds of a DC motor or it could allow multiple voltages for a lighting situation as well.

...not quite right. You get 240 from the POCO on the low side of the transformer, the winding is center tapped so you have 2 120 volt feeds and 1 240 volt feed to the house via a total of 3 wires.

The power never flows through the ground. It flows from one hot to the other (240) or from one hot to the neutral (120)

Attaching the neut to ground isn;t actually neccessary to be functional but it affords benefits when it is used. ...

I know my post wasn't exactly right, I even included the disclaimer this what I was describing was a simplification of what really goes on. After all, some of the details can get mind boggling if you don't understand what is going on. After all, the details include the fact that when you see those three thick wires on a power pole, they are all "HOT" wires. The small one is the neutral and has a voltage potential equal to earth ground. Now that's not a potential of zero, it's just a reference point as all voltages are really a measurment of a voltage relative to a reference point. Hence when a bird lands on a HOT wire, he doesn't get electrocuted because the only voltage potential around is the one wire he is touching. The birds body aquires the same voltage potential, but like the saying goes, it's not voltage that kills you, it's current. When the bird only touches a single voltage reference point, no current can flow so he doesn't get electrocuted.

But then there's the other odd things, like the fact that the power company attempts to balance the load on the three phases of those three HOT wires. Because the three wires are 120 degrees out of phase and the loads are balanced, no current actually flows through the neutral. But there is no way to try to easily explain completing a circuit AND that current doesn't flow through the neutral because that contradicts looking at a single phase in a persons home where you have two wires running to a light bulb (HOT and NEUTRAL) and obviously there is current running through both wires when the light is on.

But I didn't want to get into all that gibberish and only wanted to find a way to try to explain it to someone who hasn't studied electricity.

"The way you get 240v circuits in your home is that two of the three HOT leads from the power pole go to your house. Etc."

Now that is just totally wrong. For a typical residence, only one of the power company hots connects to the transformer. As pointed out earlier, the transformer is tapped at each end (240V) and at the center (neutral). There is no neutral that comes from the power company prior to the transformer.

Bus gave the correct answer for the OP.

*I might add that the POCO transformer, as manufactured, is a true isolation transformer.* "Isolation" transfomers can be step up, step down, or have equal input and output voltages. By means of multiple or tapped secondary windings, several different output voltages can be produced by just one transformer. The better quality TV sets of the 1950's and early 1960's used such as their internal power transformers. *To complete the thought of the first sentence in this response, the primary and secondary windings of the typical single phase residential distribution transformer are tied together at one end and in turn tied to the grounded conductor of the POCO primary lines.* Here the local linemen call that the Neutral. I am not sure that terminology is universal for that particular conductor. Here every guy wire is also bonded to that conductor and every pole has a grounding coil on the lower end of the pole, also bonded to the grounded conductor. Some POCOS are eliminating that coil on new poles. The principal ideas I intend in this post are in the sentences beginning and ending with asterisk.

"The way you get 240v circuits in your home is that two of the three HOT leads from the power pole go to your house. Etc."

I messed up that one...

You get 240 from the POCO on the low side of the transformer, the winding is center tapped so you have 2 120 volt feeds and 1 240 volt feed to the house via a total of 3 wires.

...but that one isn't quite right either, or a little confusing as I read it.

So to be more specific, the transformer closes to your house gets feed by one of the three legs of the three phase power produced by the power company. The other side of that transformer is the 240 volt/center tap mentioned above.

But what you get with those three wires feeding your house is (1) the center tap, usually tied to ground, used as the neutral, (2) the high end feed from the transformer, and (3) the low end feed from the transformer.

Mathematically, you can think of it as the neutral, tied to ground, is the zero reference point. The high feed gives you +120 volts while the low feed gives you -120 volts. So when you connect something from neutral to either high/low feed, you get 120 volts (+ or - doesn't matter because everything is an AC current, i.e. the current is either following back-n-forth, or forth-n-back). But when you connect something from high to low, you get 240 volts.

the transformer closes to your house gets feed by one of the three legs of the three phase power produced by the power company. The other side of that transformer is the 240 volt/center tap mentioned above

By my house each transformer is fed by two of the three phase wires. So I assumed this meant that each of the 120 lines they tapped for the service laterals was fed by a separate phase?

Pole transformers for 7.2 kV (about the lowest distribution voltage used now) and higher can run either line to line between two phases of the 3 available, or line to neutral.
There can be problems with circulating currents with multiple line to line transformers present.
This also varies by area. In many very dry places a run neutral is used even on lower distribution lines.
Any two winding transformer provides isolation between the primary and secondary sides. An isolation transformer is designed to provide a defined level of insulation (breakdown voltage) between the two sides and typically has a defined capacitance between the windings to further reduce voltage coupling. Many isolation transformers even have screening between the primary and secondary windings to further limit capacitive coupling between the windings.
In audio engineering an Âisolation transformer is used to reduce noise coupling from one circuit to another by eliminating a direct connection. These rarely have a rated isolation voltage for safety but do have ratings for isolation as a function of frequency and any DC current present (the DC current can cause core saturation and result in distortion of the coupled AC waveform).
Medical equipment normally uses very carefully designed and tested isolation transformers to try and prevent any stray currents from flowing in a patient (invasive electrical connections to a patient often have very low impedance to the body).
While a high voltage is used in an external defibrillator, only a few volts are needed in surgery using cardiac paddles that touch the heart directly.

""You get 240 from the POCO on the low side of the transformer, the winding is center so you have 2 120 volt feeds and 1 240 volt feed to the house via a total of 3 wires.

...but that one isn't quite right either, or a little confusing as I read it.
So to be more specific, the transformer closes to your house gets feed by one of the three legs of the three phase power produced by the power company. The other side of that transformer is the 240 volt/center tap mentioned above.

But what you get with those three wires feeding your house is (1) the center tap, usually tied to ground, used as the neutral, (2) the high end feed from the transformer, and (3) the low end feed from the transformer."

No, you have one winding on the low side with 3 taps total. The center tap, which becomes the grounded conductor to either of the other taps will provide 120 volts. from tap to tap of the ungrouinded conductors you get your 240. That is how/why you have 2 legs that are 120 volts to ground and then your 240 is leg to leg.

No, you have one winding on the low side with 3 taps total. The center tap, which becomes the grounded conductor to either of the other taps will provide 120 volts. from tap to tap of the ungrouinded conductors you get your 240. That is how/why you have 2 legs that are 120 volts to ground and then your 240 is leg to leg.

We're saying the same thing, I just might have used the wrong terminology as what I called "high end feed" and "low end feed" (meaning feeds to the house) is what is getting called "legs" above. But I can also see now where my wording was confusion too since when people talk about low side v high side of a transformer, they are usually talking about the "high side winding" (i.e. the winding with the higher voltage") and the "low side winding" (i.e. the winding with the lower voltage).

Of course for those really interested, read "How Stuff Works" on the subject, they've got some visuals there.

http://science.howstuffworks.com/power.htm

Page 9 covers the transformers at the house we're talking about here.

http://science.howstuffworks.com/power9.htm

Here is a link that might be useful: How Stuff Works - Electrical Power