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Discussion Starter · #1 · (Edited)
Hi Guys'


simply speaking, does an Isolation transformer mean that the primary winding is grounded while the secondary is ungrounded ? :)
 

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Depends

A typical Delta/Wye dry type transformer utilized in by commercial industrial industry (not utility) are grounded secondaries.

The primary feed is typically a grounded system but NOT grounded at the Delta Primary in the transformer and the grounded conductor is not connected at the primary only ungrounded phase conductors
 

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Technically all transformer's are isolating, but that is usually not there primary purpose, what is meant when talking about an isolation transformer is that it's sole purpose is to prevent a direct current path from the power source to the device. This can be done for safety purposes or can be used for filtering, grounding is a separate issue and depends on application.
 

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Isolating transformers are the rule. The just are electrically insulated between the primary and the secondary with only magnetic connection. Auto transformers are less common because the primary and secondary are electrically connected and they are generally only allowed for equipment loads rather than general loads. A typical use of an Auto transformer is for voltage correction like a 230 volt motor connected to a 208 volt system. a Boost configured auto transformer might be rated for only 10% of the current rather than hte full load.
Auto transformers are generally smaller for a given rating. they pass harmonic distortions easily too. I think they also pass a lot more fault current as their natural impedance is low.
Now there are some applications of isolating transformers that require the secondary to not be grounded like in some special medical applications.
 

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Now there are some applications of isolating transformers that require the secondary to not be grounded like in some special medical applications.
AND


Industrial facilities under the supervision of trained personnel, though most newer systems we see are impedance grounded.
 

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Discussion Starter · #8 ·
So to end this, An isolated transformer is generally a normal inductive transformer (not an auto transformer), and it has nothing to do with the grounding of the P or the S ?

Am i right ?
 

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So to end this, An isolated transformer is generally a normal inductive transformer (not an auto transformer), and it has nothing to do with the grounding of the P or the S ?

Am i right ?
Usually the primary is grounded,whilst not always at the tranny,
as it could be the instalation that is grounded, (bonded).
And usually the secondary is NOT grounded.

This means that if you touch the secondary hot and ground you will not receive a shock.

This is why they are used to repair televisions and such by repair personnel.

They remove the neutral ground bond.
 

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Discussion Starter · #10 ·
Usually the primary is grounded,whilst not always at the tranny,
as it could be the instalation that is grounded, (bonded).
And usually the secondary is NOT grounded.

This means that if you touch the secondary hot and ground you will not receive a shock.

This is why they are used to repair televisions and such by repair personnel.

They remove the neutral ground bond.

Are you saying that an Isolated transformer is one that is ungrounded on the secondary ?

This brings us back to the beginning
 

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Are you saying that an Isolated transformer is one that is ungrounded on the secondary ?

This brings us back to the beginning

The primary distribution system is a grounded system

the secondary is almost always grounded except in certain circumstances.
 

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Discussion Starter · #12 ·
The primary distribution system is a grounded system

the secondary is almost always grounded except in certain circumstances.

That's what i was thinking about,

Just to be sure on this, does the name (isolated transformer) apply to any induction transformer that is only connected via induction ?
 

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That's what i was thinking about,

Just to be sure on this, does the name (isolated transformer) apply to any induction transformer that is only connected via induction ?
Unless someone can tell me different, as I am not the smartest person here, YES.

But in the data (computer field) when SOME people refer to isolation transformers they are usually referring to Shielded Isolation Transformers
 

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About the only time I've seen them used is to isolate the secondary neutral from the primary neutral on dairy farms. The transformer has a 240 volt primary and a 240 CT secondary. They normally only ground the secondary. They're normally mounted on a pole approx. 15' up. The danger of this type of an installation is that if the primary winding shorts to the transformer case, it won't blow the fuses even though the frame of the transformer is live. They do help with stray voltage issues though.
 

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I think the issue here is really that there is no universally accepted defintion of isolation transformer.

To an electronics person it may mean something different than it does to a power distubution guy.

Under the NEC few transformers are truley isolating as many will share a grounded conductor.
 

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Bob

Years ago a manufacture of Motor generator came out with a motor to a belt driven generator, they claimed this was true isolation. The design called for PVC pipe and a separate electrode system, with an isolated raised floor.
 

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Bob

Years ago a manufacture of Motor generator came out with a motor to a belt driven generator, they claimed this was true isolation. The design called for PVC pipe and a separate electrode system, with an isolated raised floor.
Regardless of it being a transformer or a generator in most cases the NEC will require the grounded conductors to be directly connected to each other via the grounding electrodes.
 

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Regardless of it being a transformer or a generator in most cases the NEC will require the grounded conductors to be directly connected to each other via the grounding electrodes.
Correct but this design did not call for that, in testing we proved that they lost their attempt at 100% isolation, somewhere they had an accidental connection to the base building ground
 

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I believe 250.21 would be an example. If I'm not mistaken, a truly isolated system would be considered separately derived, correct?
 

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So to end this, An isolated transformer is generally a normal inductive transformer (not an auto transformer), and it has nothing to do with the grounding of the P or the S ?

Am i right ?
The part about this that is wrong is the use of the term "inductive" as somehow being the opposite of an autotransformer. An autotransformer is inductive too.

The simplest answer was the correct answer. "Isolated" just means there is no DIRECT ELECTRICAL correction between the primary and secondary, basically then meaning and isolation transformer is not an autotransformer. The connection between primary and secondary is ONLY via magnetic induction. In an autotransformer the primary and secondary are one and the same, but the voltage potential is CHANGED via magnetic induction.

Grounded, ungrounded, shielded, non-shielded, all separate issues.

So to reiterate, there are TWO kinds of transformers; autotransformers and isolation transformers.

And current transformers... no wait, there are THREE, THREE kinds of transformers; autotransformers, isolation transformers and current transformers.

And toroidal transformers... no wait! There are FOUR, FOUR kinds of transformers....

Apologies to Monty Python...:blink:
 
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