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Old 07-16-2012, 09:00 PM   #1
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240v/120v Isolation Transformer


I am intrigued by a post in another thread about a 240v/120v isolation transformer.

I assume this is a transformer that takes the 240v two conductors and changes it into 120v with a isolated nuetral.

Is this a common practice? Is there a benefit? Is it cheaper to operate or just a waste of money? How does it actually work?

Is this best for computers etc.?

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Old 07-16-2012, 09:12 PM   #2
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240v/120v Isolation Transformer


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Originally Posted by bcgfdc3 View Post
I am intrigued by a post in another thread about a 240v/120v isolation transformer.

I assume this is a transformer that takes the 240v two conductors and changes it into 120v with a isolated nuetral.

Is this a common practice? Is there a benefit? Is it cheaper to operate or just a waste of money? How does it actually work?

Is this best for computers etc.?
I read that also, and have no idea why he would use this... Unless of course it is wired improperly and trying to achieve some type of isolated grounding technique...

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Old 07-16-2012, 11:33 PM   #3
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240v/120v Isolation Transformer


As a person who works on electronic stuff once in a while, I have one of these supplying a two-wire outlet on my workbench.

When I'm working on something potentially lethal (like a TV with the back off), having both conductors ungrounded is a safety measure. I can grab any one thing (except the high-voltage CRT circuit) and be completely safe.

It also protects some of my test equipment, most notably the oscilloscope. Most scopes have one of the probes grounded, and if you're measuring something that involves the AC power circuit (very common in older sets), the incoming power to the set needs to be isolated from ground.

Rob

P.S. This is most likely a blatant code violation, but compliance would absolutely cause destruction of equipment and hazard to personnel. Therefore, we violate the code.
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Old 07-17-2012, 01:37 AM   #4
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240v/120v Isolation Transformer


Quote:
Originally Posted by micromind View Post
As a person who works on electronic stuff once in a while, I have one of these supplying a two-wire outlet on my workbench.

When I'm working on something potentially lethal (like a TV with the back off), having both conductors ungrounded is a safety measure. I can grab any one thing (except the high-voltage CRT circuit) and be completely safe.

It also protects some of my test equipment, most notably the oscilloscope. Most scopes have one of the probes grounded, and if you're measuring something that involves the AC power circuit (very common in older sets), the incoming power to the set needs to be isolated from ground.

Rob

P.S. This is most likely a blatant code violation, but compliance would absolutely cause destruction of equipment and hazard to personnel. Therefore, we violate the code.
I concure.

In most cases the tranformer is going to have 1000v or more isolation....

But what is of more value is the filtering....at least to someone like me. High freq signals do not travel well through a transformer....so between the isolation and low freq response, you can get a nice clean AC source to power your electronics.

For reference, look up HiPot.
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Old 07-17-2012, 03:27 AM   #5
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240v/120v Isolation Transformer


most notably the oscilloscope. Most scopes have one of the probes grounded,
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Old 07-17-2012, 05:00 AM   #6
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240v/120v Isolation Transformer


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Originally Posted by micromind View Post
As a person who works on electronic stuff once in a while, I have one of these supplying a two-wire outlet on my workbench.
Same here for working on switching power supplies. You can't place any grounded equipment leads on the AC hot side unless you want a little excitement.
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Old 07-17-2012, 05:06 AM   #7
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240v/120v Isolation Transformer


I understand the need for isolating the power supply for what you guys described, but the OP read about isolation transformers in another thread, and that person was using them for a completely different purpose.


Quote:
Originally Posted by Lee Daughton View Post
I have all my computers and TV behind a 240/120 isolation transformer , 240v input ,120v output , negating the afformentioned floating neutral problem.

Last edited by stickboy1375; 07-17-2012 at 05:09 AM.
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Old 07-17-2012, 05:11 AM   #8
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240v/120v Isolation Transformer


To accurately answer your question,
we need to know the context of the original story.
Isolation transformers have some uses.
But a general comment could be mis constrewed.
What was the original story about ?
can you post a link ?
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Old 07-17-2012, 05:46 AM   #9
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240v/120v Isolation Transformer


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Originally Posted by dmxtothemax View Post
can you post a link ?
This was the original post, it was in this thread... 2 TVs fried: Electrical vs. Comcast Coax Cable Problem

Quote:
Originally Posted by Lee Daughton View Post
I have been reading the posts here since I have a similar problem. Some posts insinuate that you have a "Floating" neutral at the pole, however if you did you would , then you would have some lights in your house that would appear brighter than normal and some that would appear dimmer than normal and switching lights on randomly would cause the lights to change in illuminance , which I would think that you'd notice. 240v appliances would be totally uneffected since loss of an incoming phase conductor would cause a total loss of power to any 240v device. As one poster noted that the cable pedistal caught fire , yea the neutral floated and since the cable system was properly "Bonded" (technically more code recognized description) ,the cable shield tried to carry the neutral current to the pedistal and then to electrical system ground peg at the pole. This condition could happen in a structure with minimal supervision and small electrical loads such as a refrigerator. The cable shield is of seriously insufficient size to carry any substantial amount of current and would deffinently overheat and possible catch fire. As another poster noted the "NEC" in articles 820 and 830 address bonding and grounding of cable systems amongst other issues pertaining to cable installation. Cable system grounding and bonding is an issue which generrally gets neglected and causes many pieces of burnt equipment. In general it takes 2 wires to create an electrical or signal circuit , 1 wire being the center signal conductor ,and 1 wire (Shield) as the other conductor. Since the center conductor voltage uses the shield as its reference 0v to develop a potential difference (Voltage) it is absolutely necessary that the shield be bonded and grounded throughout the system to prevent widely fluctuating voltages that may occur much like a floating neutral. In my situation we suffered a lightning strike near my residence killing my expensive wide screen. Since it was under warranty , Samsung ,(cant say enough good things about my TV and their service) , sent out a service tech to repair. I had initialy considered a power surge as the culprit , however the tech said the motherboard was burnt and not the power supply. I really wasn't surprised because I have all my computers and TV behind a 240/120 isolation transformer , 240v input ,120v output , negating the afformentioned floating neutral problem. After the tech got my TV working I found that only the HDMI output of my converter box ,not the S video and RCA audio outputs which I was using temporarily on my old TV , had failed ,this led me to beleive that issue was created by a surge on the incoming cable wire. Drove to my local cable company store and swapped the box and all was fine again. Had a lighting storm again last night and my wide screen died exibiting the same blink codes as the previous time. Wont know about the converter box till the tech repairs my wide screen since it's the only device I have that works on an HDMI feed. An interesting note: We have 2 TV cards in our computers through DTA's and my wife's computer TV was on and unaffected where as the cable converting box and my wide screen were turned off since I saw the storm coming and didn't want a repeat of the same issue as the first time. The DTA's are connected to TV cards in our computers and solidly grounded by a 3wire cord to the electrical system ground conductor where as the converter box and TV are connected by 2wire ungrounded factory cables. The cable company should have grounds on their system and a ground , technically a bond, wire should also be connected to your electrical system grounding conductor at your main service to a terminal connector at the cable entrance location. Floating cable grounds may also cause slow internet connections since the foam dielectric , the white insulating material around the center signal wire , can build up a capacitive charge and inhibit internet speeds. In other words a loose cable shield ground can run the gammit from annoying to disastrous. Fortunately for me , I live in an area with a Electrical Inspection Agency with a thorough knowledge of electrical codes and a special meter which can check grounding integrity. I'll be calling them next to request a check of my cable grounding integrity. You can bet your last dollar I'll get to the bottom of my problem and chew some "Butts" clean off before I'm done. You'll probably have your local cable provider try to pull the old NEC 90.2(B)(4) code which they will contend applies to any area exterior to your home and thusly makes their exterior wiring code exempt. Thats a tricky exemption and doesn't normally fly in our jurisdiction. Typically it applies to their office spaces and server farms. If you don't have a solid local electric inspection authority, a reputable electrical testing agency can make the ground continuity test for a fee.
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Old 07-17-2012, 06:57 AM   #10
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240v/120v Isolation Transformer


Quote:
Originally Posted by bcgfdc3 View Post
I am intrigued by a post in another thread about a 240v/120v isolation transformer.

1) I assume this is a transformer that takes the 240v two conductors and changes it into 120v with a isolated nuetral.

2) Is this a common practice?
3) Is there a benefit?
4) Is it cheaper to operate or just a waste of money?
5) How does it actually work?

6) Is this best for computers etc.?
IMO from the info above.

1) Yes
2) No
3) Only in rare cases. It does however make doom and gloom technical type people feel more secure.
4) See answer #3
5) A transformer with no connection between input and output windings.
6) See answer #3

Last edited by zappa; 07-17-2012 at 07:30 AM.
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Old 07-17-2012, 09:54 PM   #11
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240v/120v Isolation Transformer


Most electricains would not use an isolation tranny to fix problems
like this original story problem.
Better to sort out the route cause of the problem,
multipile earths and or neutrals are bound to cause problems,
better to use ONE proper earth line and ONE proper neutral line.

Isolation trannys were never intended to fix poor wiring,
they were just used to provide extra protection from
electrocution when working with servicing tools or power tools.

So the isolation tranny would protect the stellitte set up
but the problem in the rest of the wiring system still exists
and other problems are bound to arise if it is not correctted.
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Old 07-18-2012, 05:10 PM   #12
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240v/120v Isolation Transformer


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Originally Posted by micromind View Post
It also protects some of my test equipment, most notably the oscilloscope. Most scopes have one of the probes grounded, and if you're measuring something that involves the AC power circuit (very common in older sets), the incoming power to the set needs to be isolated from ground.

Rob

P.S. This is most likely a blatant code violation, but compliance would absolutely cause destruction of equipment and hazard to personnel. Therefore, we violate the code.
With the scopes, I invert one channel and add both, changing from a single ended input to a balanced differential input.
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Old 07-18-2012, 08:26 PM   #13
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240v/120v Isolation Transformer


Isolation transformers are required for use in many hospital and medical care facilities applications, such as operating rooms and such.

They can completely isolate the end user from potential shock hazards, ground-faults, etc.

I saw one that was used to solve a heated floor tile circuit that kept tripping out the GFCI unit in the thermostat. It was either that or a jackhammer to the tile floor. No problems with the circuit since the transformer was installed.
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Old 07-18-2012, 10:31 PM   #14
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240v/120v Isolation Transformer


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Originally Posted by kbsparky View Post
Isolation transformers are required for use in many hospital and medical care facilities applications, such as operating rooms and such.

They can completely isolate the end user from potential shock hazards, ground-faults, etc.

I saw one that was used to solve a heated floor tile circuit that kept tripping out the GFCI unit in the thermostat. It was either that or a jackhammer to the tile floor. No problems with the circuit since the transformer was installed.
This is a prime example of what I was warning about !
Obviously there is a leak somewhere !
This would trip out the efci,
using an isolation tranny would stop the trips BUT
they dont stop the leak!
sooner or later it will get worse and some one will get zapped.
Better I think to find and fix the leak.
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Old 07-19-2012, 04:01 PM   #15
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240v/120v Isolation Transformer


How can they get zapped if the isolation transformer prevents folks from getting shocked? Hospitals use them in operating rooms for safety.

Why would it not be a safe alternative to use such a setup?

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