In article , Andy wrote:
On Dec 2, 12:03 pm, (GregS) wrote:
In article , "Dave Plowman (News)"
wrote:



In article ,
GregS wrote:
In article , sbnjhfty
wrote:
I'm looking for an isolation transformer for repairing small (less
than 1000W) power supplies. I see loads of them on ebay and some are
medical grade and inexpensive. Are these usable for what I want?
Other than that, any tips on where to get a good deal on one? I
don't want to spend a load as this is hobby work only.

Just an 'isolation transformer" means NOTHING for what you want. Most
all isolation transformers sold have the secondary tied to ground. You
need a service isolation transformer. You can usually get inside and
remove a secondary "neutral" connection.

That's interesting. I've never seen any transformer with the secondary
tied to ground - unless as part of an installation. The word 'isolation'
means just that. Sure you're not thinking of an auto transformer?

I have been thinking about it. The MAIN purpose of an isolation transformer
is to make it non isolated by making a NEW neutral close to the device
being powered. The main purpose of an isolation transformer is noise control.

Now we have us tecks who all our lives have learned isolation and that stuff.
I taliked to the people at Tripplite and verified ALL their isolation
transformers ARE grounded. Read the paragraph, and yet it says above,
"Complete
line
Isolation"http://www.tripplite.com/en/products/model.cfm?txtSeriesID=325&EID=1
3...
Also look
throughhttp://www.tripplite.com/en/keyword-search.cfm?q=isolation%20transforme
r
The isolator you buy from MCM is very unlikely to have the secondary
grounded, but
I really can't verify that.

In reading PC Power Protection by Mark Waller, he
pointed out their is a NEC requirment for this grounding.
I have not found an exact description in the code. There is about 75
references
in that book, but no index marks. Man, I should sell that book !!!! Big
Bucks.

greg

On an isolation transformer (single phase), the primary side is always
grounded because the neutral leg is always bonded in the distribution
panel. This is a code requirement. If the secondary is also grounded
(I am pretty sure this is also a requirement), the two sides are still
electrically isolated. The current leaving the secondary hot leg must
return to the neutral (grounded) leg of the secondary. There is no
path from the secondary hot to the primary neutral. The secondary has
to leave the hot leg and return on the secondary neutral leg. The
secondary current cannot go any where except through secondary loop.
Any noise on the high side (from nearby VFDs or other) must pass
through the primary winding in order to induce a current onto the
secondary. The winding itself is a low pass filter, so much of the
noise on the primary is dissipated as heat.

I would agree with this except you also state then, there is no path from
secondary hot to primary neutral, which makes no sense in your
explanation. In electronic servicing you don't want to ground the
secondary making a new neutral. You got to get away from the code to do it.

greg