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Question about how to wire an Isolation Transformer
Question about how to wire an Isolation Transformer
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Old 22nd April 2009, 01:17 AM   #21
infinia is offline infinia  United States
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come on folks!! This is an equipment application not a building wiring question. If any codes apply here they would be safety issues addressed by UL/CSA/TUV VDE.
The only way to wire it and still have an ISOLATION transformer is shown in fig 1. Remember at the primary the N is electrically connected to E at the entry panel. If you connect the secondary to E again you have completely eliminated the function. Not to mention any CM noise reduction you are tring to achieve if any.

Anyway for devices shown here http://www.surplussales.com/transfor...onXmers-2.html

The cases are earth grounded and passed through to the output isolated.
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Old 22nd April 2009, 02:20 AM   #22
infinia is offline infinia  United States
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Quote:
Originally posted by Richard Ellis
I would bet the wiring of the output of your IT is currently correct. Did you notice your measurements on the secondary don't add up to your 229Vac?
Think how the circuit works as in electron flow. The measurements from Live or Nuetral to Earth are capacitatively(?) coupled...I think that your DVOM puts such a light load on the circuit...that you can get any reading. I would bet if you loaded the circuit just a little you won't read any voltage at all. Wouldn't be baffling if you had a cheap meter & you tested both 'L' & 'N' ...and it gave you no readable voltage?
You should be able to pull down that reading with microamps & the voltage would drop.to nothing.
___________________________________________Rick... ..........
Hi qpapaq
The secondary voltages you are reading are due to interwinding capacitances between both of your transformer windings and other parasitic/added capacitances to earth forming an AC voltage divider. For medical grade and high common mode isolation transformers the 2 windings are physically seperated ie wound on seperate sides to reduce this "AC leakage" effect. As Rick pointed out this should go away as your load is increased. Idea >You could add a common mode line filter or added Y caps to the secondary to swamp out the interwinding leakage cap.
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Old 22nd April 2009, 02:26 AM   #23
zigzagflux is offline zigzagflux  United States
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Quote:
Originally posted by Iain McNeill
Does the fact that the center tap is grounded mean this scheme will meet code?

edit: P.S. what is the reason for secondary bonding? Is it to do with charge build-up on a floating secondary winding causing flash over?
A grounded center tap, balanced power system is permitted with certain restrictions. The most significant requirement to DIY'ers in simplistic form is the balanced system should only feed the audio rack, no other equipment. You cannot plug in a cord connected lamp, for instance; just that audio gear.

There are a number of reasons to bond the secondary. One that an ungrounded system will tend to exhibit transient overvoltage problems. From IEEE Std 242-7.2.5:

Quote:
If this ground fault is intermittent or allowed to continue, the system could be subjected to possible severe overvoltage to ground, which can be as high as six or eight times phase voltage. This can puncture insulation and result in additional ground faults. These overvoltage are caused by repetitive charging of the system capacitance, or by resonance between the system capacitance and the inductances of equipment in the system.
Granted, this primarily occurs on traditional 480V three phase systems, but it is one of the reasons the Code added in the requirement to bond in almost all systems 50-1000V.

Another reason that comes to mind is that in the event of an equipment ground fault (by far the most common type of electrical equipment failure), the response to a grounded system will usually be operation of the upstream overcurrent device (trip the breaker). This is a good thing, as it prevents the possibility of a device with failed insulation from continuing normal operations without the user knowing. On a practical safety level, the average Joe User could be harmed by failure of this device in any number of ways.
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Old 22nd April 2009, 02:35 AM   #24
zigzagflux is offline zigzagflux  United States
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Quote:
Originally posted by gpapag
zigzagflux
Ref post#14 puzzle
the GFCI on the primary will not trip with any of the 3 occurancies on the secondary.
Exactly, well done. Not that it matters, but I would add that in #3, the 15A breaker would probably trip, but NEVER the GFCI.

Why do I bring this question up? Because those who continue to insist that #1 is the way to go do not understand how ground currents flow:

Quote:
Originally posted by infinia
Remember at the primary the N is electrically connected to E at the entry panel. If you connect the secondary to E again you have completely eliminated the function.
If I 'completely eliminated' the isolating function of the transformer, then why does the GFCI never trip? Even seasoned electricians have difficulty understanding this issue. The answer is that the transformer has indeed done all the isolating necessary; the only other benefit one may be able to obtain (legally) is 115-0-115 balanced power.
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Old 22nd April 2009, 03:09 AM   #25
infinia is offline infinia  United States
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Quote:
Originally posted by zigzagflux


Exactly, well done. Not that it matters, but I would add that in #3, the 15A breaker would probably trip, but NEVER the GFCI.

Why do I bring this question up? Because those who continue to insist that #1 is the way to go do not understand how ground currents flow:



If I 'completely eliminated' the isolating function of the transformer, then why does the GFCI never trip? Even seasoned electricians have difficulty understanding this issue. The answer is that the transformer has indeed done all the isolating necessary; the only other benefit one may be able to obtain (legally) is 115-0-115 balanced power.
Your case number 3 the GFCI is not designed to trip because there are no abnormal earth currents flowing in that simple example of a L-N short. I don't think you understand the purpose of an isolation transformer and you never addressed the cause of the OP original Q. Sure grounding the secondary will fix it but then it defeats the whole purpose of the thing.
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Old 22nd April 2009, 05:04 AM   #26
infinia is offline infinia  United States
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Quote:
Originally posted by zigzagflux


Why do I bring this question up? Because those who continue to insist that #1 is the way to go do not understand how ground currents flow:
Hmm that is a huge underestimation on your part. Not only in my case but most others in this thread as well. But you go ahead an explain away.
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Old 22nd April 2009, 06:50 AM   #27
east electronics is offline east electronics  Greece
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Default yet again one of the bad sides

of our forum ......

often to a simple question in the forum you may find so much theory that will be enough to take you to the moon except the actuall answer .....

Let us also not forget that half of the word application often comes from the word aspect ......

may things that apply in electrical installation laws do not apply for audio and visa versa
earth loops for example may exist a million times in our home and will effect nothing as long as the load is simple lamps ..... but if the load is audio earth loops can create a huge mess )

george .....

i would be very happy to know what i asked above regarding the original post and what made you start this search ....
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Old 22nd April 2009, 10:15 AM   #28
Enzo is offline Enzo  United States
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I guess I am left wondering which side of his 240 would be connected to chassis?
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Old 22nd April 2009, 10:31 AM   #29
infinia is offline infinia  United States
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Default Re: yet again one of the bad sides

Quote:
Originally posted by sakis
of our forum ......

often to a simple question in the forum you may find so much theory that will be enough to take you to the moon except the actuall answer .....



It would not be so bad if it was theory based. What really gets my dander is doggedly held dogma. or to the guy with only a hammer everything appears as a nail head.
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Old 22nd April 2009, 11:06 AM   #30
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by zigzagflux


Emphatically wrong!! The transformer is classified as a separately derived system, and it is required to ground one conductor. This conductor then becomes named the neutral. There are no exceptions to this;
ZIG
you are wrong.
One must never connect the Neutral to Safety Earth.
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