When is an Isolation transformer not an isolation transformer?

Thread Starter

ApacheKid

Joined Jan 12, 2015
377
I want to get an isolation transformer, in my mind this eliminates the need to ground connected appliances and or if they are grounded (as in their plug has a ground pin), there's no path to ground if I accidentally touch something I shouldnt.

Is it enough to just get a large 1:1 transformer - more or less?

I looked at isolation transformer products, on Amazon, I found one that - oddly - seems to not be isolated! it seems one must open it and do a hack to make it fully isolated, why on earth would they call it an isolation transformer if it doesn't isolate?

e.g., one buyers remarks:

1614352127318.png
The main motivation is safety and I will be working on some very old tube radios soon, metal chassis and all that stuff.
 

SamR

Joined Mar 19, 2019
3,180
I got a good deal on mine from Jameco. However, upon inspecting and testing it found that the output ground was not Isolated and was in fact tied to the input safety ground. Easily solved with a pair of wire cutters.
 

dl324

Joined Mar 30, 2015
12,677
I want to get an isolation transformer, in my mind this eliminates the need to ground connected appliances and or if they are grounded (as in their plug has a ground pin), there's no path to ground if I accidentally touch something I shouldnt.
Your logic is faulty. If you touch line voltage and you're grounded, you're going to get shocked (or worse) even if there's no ground wire. The ground wire on appliances protects you from a situation where line voltage comes in contact with the chassis. That should protect you by tripping a GFCI or breaker.

On non-grounded appliances, a GFCI still offers protection by tripping when there's a small mismatch in the line/neutral current. No GFCI, no ground, no protection.


I take it back. Not fully awake yet...
 
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crutschow

Joined Mar 14, 2008
26,735
upon inspecting and testing it found that the output ground was not Isolated and was in fact tied to the input safety ground. Easily solved with a pair of wire cutters.
The purpose of an isolation transformer is to isolate the hot and neutral from the safety ground, not to isolate the safety ground.
That should be left intact because it can help protect you if there's a short in the isolation transformer.
Otherwise there's not a path between the isolation transformer secondary and the safety ground so there's no reason to remove that ground.
Your wire cutters made the setup more dangerous.
 

DickCappels

Joined Aug 21, 2008
7,155
There are many kinds of transformers that offer isolation between windings. For power line frequencies "filament transformers" which are similar to "Bell transformers" and "control transformers". These are ordinary transformers that have a very high resistance between the primary and secondary. They are useful to most of us a "isolation transformers". Other classes of transformers have more carefully controlled leakage current between the primary and secondary -those used in hospital to isolate equipment connected to patients from the line are the most critical of these of which I know. They also have very low capacitance between the windings (to further reduce leakage current) and shielding between windings. Just to let you know.
 

Thread Starter

ApacheKid

Joined Jan 12, 2015
377
The purpose of an isolation transformer is to isolate the hot and neutral from the safety ground, not to isolate the safety ground.
That should be left intact because it can help protect you if there's a short in the isolation transformer.
Otherwise there's not a path between the isolation transformer secondary and the safety ground so there's no reason to remove that ground.
Your wire cutters made the setup more dangerous.
Really? this is confusing...

Surely if the secondary's neutral is grounded then that secondary is connected to the primary's neutral - which is already grounded...?
 

MaxHeadRoom

Joined Jul 18, 2013
22,641
If the secondary is isolated there is no point in a earth ground reference as it has no reference/relation to the secondary conductors.
This is why in industrial equipment where a transformer is used to step down to a 120vac control voltage etc. the GND conductor is connected to one of the secondaries in order to set up a local neutral. and re-reference the earth GND.
Max.
 

crutschow

Joined Mar 14, 2008
26,735
This is why in industrial equipment where a transformer is used to step down to a 120vac control voltage etc. the GND conductor is connected to one of the secondaries in order to set up a local neutral. and re-reference the earth GND.
Really?
If there's no isolation of the secondaries from ground, then it's no safer than the 120Vac from the mains, as far as accidental electrocution.
 

crutschow

Joined Mar 14, 2008
26,735
Surely if the secondary's neutral is grounded then that secondary is connected to the primary's neutral - which is already grounded...?
Not what I said.
I said only the safety ground should be carried across the transformer, not the neutral connection.
They are not the same.
 

SamR

Joined Mar 19, 2019
3,180
isolate the hot and neutral from the safety ground, not to isolate the safety ground.
If the safety ground is tied to the neutral on the primary side and connected to the secondary safety ground how is it isolated from the primary neutral? confused...
 

MaxHeadRoom

Joined Jul 18, 2013
22,641
Really?
If there's no isolation of the secondaries from ground, then it's no safer than the 120Vac from the mains, as far as accidental electrocution.
If you do set up a neutral as #8, then no, there is no isolation, it is done to re-reference the 'Neutral' to earth GND.
If true isolation is required then the isolated secondary should remain as it is with no GND reference.
This is the NFPA for the former, see transformer 1T for the re-reference to GND.
.
 

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Thread Starter

ApacheKid

Joined Jan 12, 2015
377
Now I'm scared to get an isolation transformer, some say its just for devices under test and never for scopes etc, other says its fine for scopes and devices under test - quite bewildering.
 

MaxHeadRoom

Joined Jul 18, 2013
22,641
When using a 'scope' on mains with potential live chassis etc, e.g. equipment with no galvanic isolation from the mains. you need to use an isolation transformer for the 'scope.
Otherwise the GND clip which is at earth GND potential can contact live areas of the equipment.and cause these areas to blow the GND lead etc.
 

crutschow

Joined Mar 14, 2008
26,735
When using a 'scope' on mains with potential live chassis etc, e.g. equipment with no galvanic isolation from the mains. you need to use an isolation transformer for the 'scope
But, if you have a choice, you should use the isolation transformer for the DUT, not the equipment.
That protects you as well as the equipment.
 

Thread Starter

ApacheKid

Joined Jan 12, 2015
377
But, if you have a choice, you should use the isolation transformer for the DUT, not the equipment.
That protects you as well as the equipment.
But when you do that the scope's ground will surely then ground the (isolated) device's ground, defeating the purpose of isolating it. Touching a "live" wire in the DUT will lead to electric shock, whereas if the scope were isolated that could not happen.

Isn't that true?
 

crutschow

Joined Mar 14, 2008
26,735
But when you do that the scope's ground will surely then ground the (isolated) device's ground, defeating the purpose of isolating it
The purpose of isolation to to isolate the mains from ground, not isolate the device from ground.
Touching a "live" wire in the DUT will lead to electric shock, whereas if the scope were isolated that could not happen.
Yes it can.
The scope may be isolated, but you are not.
With the DUT not isolated, touching the "live" to any earth ground will give you a shock.
And as you may know from experience, even wearing rubber soled shoes is not enough to prevent experiencing a shock from a main's wire. :eek:
 

Thread Starter

ApacheKid

Joined Jan 12, 2015
377
The purpose of isolation to to isolate the mains from ground, not isolate the device from ground.
Yes it can.
The scope may be isolated, but you are not.
With the DUT not isolated, touching the "live" to any earth ground will give you a shock.
And as you may know from experience, even wearing rubber soled shoes is not enough to prevent experiencing a shock from a main's wire. :eek:
You get that shock because the other side of that mains wire (neutral) is wired to the earth.

You get a shock because there's a voltage (obviously varying and alternating) between the live and the earth, because the neutral has been tied to that earth, the ground.

If you touch one side of an isolation transformer's secondary and neither end of that secondary is connected to the earth, you will not receive a shock just as you won't if you touch one terminal of a battery, even a 1,000 V battery, just as you do now if you touch the neutral in your house in a wall socket, you won't get a shock (if your house is wired correctly).

This is my understanding anyway, if I'm wrong I'd dearly love to understand why!
 
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