ECN Forum
Posted By: watersparkfalls isolating transformer - 05/29/07 07:21 PM
I am installing an isolating transformer with electrostatic sheild for a music recording studio 240 volt(60 amps) in and 240/120 out. it is in a residential house that has no steel. my question is does this sound like proper application/installation for this job(haven't done x-former in ions)i pull 3-#8's(copper)two phases tied to primary (h1+h4 and h4+h10), one ground (to factory provided lug) from new sub panel(feeds non isolated loads) in music room through 3/4" EMT to jbox,then whip into x-former. at x-former i install a double barrel lug to frame of x-former and tie x2 and x3 from secondary to oneside of lug and feed isolated sub-panel neutral from this bonding point. then bond single lug on frame and run to ground bar of my isolated sub-panel and feed isolated sub-panel from x1 and x4. does this sound right? or did i miss something? thanks in advance,
H2o
Posted By: HotLine1 Re: isolating transformer - 05/30/07 09:51 PM
H2O Sparks:
Check out the Technical Area for schematics. Scott35 runs a tight ship over there, and that should answer all your needs.
John
Posted By: watersparkfalls Re: isolating transformer - 05/30/07 11:51 PM
well i've researched my own question and since this is an isolating transformer i will actually use a tap block for x2 and x3 then intentially ground it to earth via ground rod and pull my isolated neutral from here(thus keeping it free from transient noises and keeping it isolated). for my equipment bond i will pull off frame grounding lug from transformer??
still reading....
H2o
Posted By: watersparkfalls Re: isolating transformer - 05/30/07 11:54 PM
john where is this tech area?
thanks
H2o
Posted By: HotLine1 Re: isolating transformer - 06/03/07 11:14 PM
Go to the main forum page, then scroll down....technical section.......underneath the 'pics' area......
Sorry for the delay; I've been out of town.
John
Posted By: resqcapt19 Re: isolating transformer - 06/07/07 12:50 AM
You can't really isolate it from the utility as the isolating transformer will require a primary EGC that will be connected to the case and the secondary grounded conductor will also have to be bonded to the case.
Don
Posted By: gfretwell Re: isolating transformer - 06/07/07 05:35 AM
Isolation transformers do not have a grounded secondary as a general rule. That is the point. IBM used them for the convenience outlets in all of our old mainframe machines. The secondary was completely floating. That actually accomplished 2 things. We never brought a grounded conductor into the computer room panels so that was the only way to get 120v and it also prevented electrocution or overcurrent in any line to ground contact ... at least the first one anyway. The cases of connected equipment still got grounded via the EGC. Usually the line filters in the first piece of plugged in equipment stabilized the voltage at +/- 60v. We tried to keep the customes out of there but an available NEMA 5-15 will get used.
Posted By: iwire Re: isolating transformer - 06/07/07 08:58 AM
Originally Posted by gfretwell
Isolation transformers do not have a grounded secondary as a general rule. That is the point.


I do not believe that will be NEC compliant.

Posted By: ghost307 Re: isolating transformer - 06/07/07 12:41 PM
Isolation transformers need to have their output referenced to ground just the same way as any other transformer.

NEC says that if it CAN be grounded, it SHALL be grounded.

What doesn't need a ground connection is an "isolated power system" such as is found in a Hospital. But then you've got to have monitoring equipment to tell you when there's a fault in the secondary. There are also a few other very limited exceptions, but isolation transformers aren't among them.
Posted By: gfretwell Re: isolating transformer - 06/07/07 04:10 PM
This was under the covers of listed equipment, not premisis wiring.
Posted By: mikesh Re: isolating transformer - 06/07/07 04:48 PM
An isolation transformer has no electrical connection between the primary and secondary windings. A transformer that is electrically connected from primary to secondary is called an auto transformer.
There is such a thing as an isolated system where there is no ground on the secondary and that type of circuit is often used for specialized medical equipment.
In power distribution the secondary of an isolation transformer must be grounded. Even a delta-delta connection needs a ground to the non-current carrying components and ground detection lights.
Posted By: iwire Re: isolating transformer - 06/07/07 07:22 PM
Originally Posted by gfretwell
This was under the covers of listed equipment, not premisis wiring.


Really?

Originally Posted by gfretwell
IBM used them for the convenience outlets in all of our old mainframe machines. The secondary was completely floating.


If these outlets where intended for 'convenience' it sounds like both a violation and premise wiring.

Just because IBM was huge and did this does not make it right.

I took this thread to be about a audio system in a home, in that case the outlets must be supplied from a grounded system.
Posted By: gfretwell Re: isolating transformer - 06/08/07 03:07 AM
It was in no way connected to the building. How is it premise wiring? This is on the load side of the power plug of a U/L listed appliance. I got that same answer when I asked about other things they did that wouldn't meet NEC muster. It is like the 14ga wires inside a 50a range.
I bet on these studio setups they do the same thing. There is a power plug that feeds a U/L listed "appliance" and that feeds the amps and such.
I agree it would be easier to accept if it wasn't a 5-15 socket.
I suppose IBM would also say this was under the covers and unqualified people were not supposed to be using it.
I also do not see the hazard of not being grounded. This isolation transformer idea is also a very common thing on lab bench power supplies. They say it is safer, The first ground fault is free.
Posted By: JBD Re: isolating transformer - 06/08/07 02:12 PM
In the old days (30 yrs ago) totally isolated receptacles were used in many areas (particularly in lab benchs and computer rooms). These types of installations are no longer acceptable unless they are more than 150V to ground.
Posted By: WESTUPLACE Re: isolating transformer - 06/08/07 03:25 PM
Recording studios offen have balanced supplies. A transformer with a 120 or 240 v primary will have a 120v secondary with a center tap grounded. There is 60v to ground. This is code permitted (have to get my code book to get the chapter) The balanced output reduces noise and hum in equipment. Robert
Posted By: watersparkfalls Re: isolating transformer - 06/12/07 06:42 AM
well i've solved(with some online research...thanks iwire) the isolating part(i think)
i relize now that since the studio is wood frame and my boxes are plastic (even floor boxes) i have essentially isolated my grounding ...since there isnt any metal and i have run all home runs to each and every required iso plug....for which i will install regular receptacles which will be isolated. the only metal will be my emt run to x-former(primary and seconday) and it will be bonded at x-former via bonding bushing (concentric knockout)but will not bond isolated groundbar to panel.
here is a question i have....
this designer will probably want to see those fancy orange iso ground recptacles, not believing that a regular plug is isolated.
but could that be a nec violaion(using orange recep) because lets say i decided to use a brass or stainless plate, it wouldnt be grounded since the yoke isnt(because plastic box)and in some rare and freak way could become energized posing a shock danger.
perhaps im out there and stretching on this one but somehow i feel i should ask, violation or not?
thanks,
H20
Posted By: gfretwell Re: isolating transformer - 06/12/07 03:29 PM
Loop the EGC through the yoke screw on the way to the IG screw. It's still isolated in a plastic box
Posted By: Retired_Helper Re: isolating transformer - 06/12/07 03:55 PM
Why use a metal wallplate? Wouldn't that cause some questions from knowledgeable people? As you mentioned, it could be misconstrued with an IG receptacle. What would be so bad with using a nylon plate, for example? confused
Posted By: gfretwell Re: isolating transformer - 06/12/07 11:50 PM
You are far better explaining to the customer why these are not IG receptacles and just use a good commercial/spec grade.
If they really think they got IG and you didn't do it they may come back at you.
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