Author Topic: Question: Isolation transformer, Ground floating or Ground shorted to Neutral  (Read 7859 times)

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Offline Gtx21Topic starter

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For an Isolation transformer, I have disconnected the (edit) isolation side ground pin from the earth-ground.

Now that the ground pin on the isolation side isn't connected to anything, should I leave it floating or should I short it Isolation-side-Neutral?

Edit: The reason I ask this is because I have seen other transformers a transformer that has all 3, isolation-side neutral, earth-ground, isolation-side ground pin, shorted together.
 
Edit:  I'll try and be more clear.
Code: [Select]
     |    +-------------------------+
     |    |                         |       +-----+
Wall |===>|  Isolation Transformer  | ----->| DUT |
     |    |                         |   ^   +-----+
     |    +-------------------------+   |
                                        +--+
                                           |
Previously, Earth ground from the wall     |
went to the DUT with the scondary winding  |
output here: ------------------------------+

It is this connection that I severed.
   
« Last Edit: September 02, 2018, 10:46:03 pm by Gtx21 »
 

Online Monkeh

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You should reconnect it properly.
 

Offline ArthurDent

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It appears that you haven't got the concept of what the purpose of an isolation transformer is. There is no 'isolated ground' rather the one ground from the U shaped ground pin on the 3-wire input cord is connected to the transformer core and also to the U shaped pin on the 3-wire outlet connector on any isolation transformer that is designed to meet code. Only the secondary winding is isolated from ground.
« Last Edit: September 02, 2018, 01:14:08 pm by ArthurDent »
 

Offline Gtx21Topic starter

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You should reconnect it properly.

I don't want the ground pin on the isolation side connected to earth ground.
 

Offline IanB

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I don't want the ground pin on the isolation side connected to earth ground.

Then you don't want to be safe. Why don't you want to be safe?
 

Offline helius

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Why would you ever believe it would be safe to make the DUT chassis live by connecting it to "neutral" (which is not neutral at all, but simply one side of a transformer secondary)? This is manifestly unsafe, and makes shock more likely when you are manipulating live circuits in the DUT. If you are not manipulating live circuits in the DUT, you should not be using an isolation transformer.

Earthing the DUT chassis is safer than leaving it floating, but it still won't protect you from insulation faults. If you believe that either choice will protect you from shock, you are wrong.
 

Offline Gtx21Topic starter

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I don't want the ground pin on the isolation side connected to earth ground.

Then you don't want to be safe. Why don't you want to be safe?

The transformer is grounded.  The isolation transformer case is grounded.  Only the DUT will not be grounded.  By leaving it connected, I have an earth-ground on my DUT which can cause me probing issues.
 

Offline Gtx21Topic starter

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Why would you ever believe it would be safe to make the DUT chassis live by connecting it to "neutral" (which is not neutral at all, but simply one side of a transformer secondary)? This is manifestly unsafe, and makes shock more likely when you are manipulating live circuits in the DUT. If you are not manipulating live circuits in the DUT, you should not be using an isolation transformer.

Earthing the DUT chassis is safer than leaving it floating, but it still won't protect you from insulation faults. If you believe that either choice will protect you from shock, you are wrong.

There is no DUT chassis in this case.  Just a test board.  I"m not sure what you mean by manipulating live circuits.  I am probing live circuits.
« Last Edit: September 02, 2018, 10:22:52 pm by Gtx21 »
 

Offline IanB

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The isolation transformer has two outputs, L1 and L2, both of which are hot. If you connect either of L1 or L2 to the ground pin of the output receptacle, then you will be making anything connected to that pin become "hot". If you do that, then why are you using an isolation transformer at all? You might just as well connect the DUT directly to the mains.

If you have an isolation transformer, then you should not mess with its internal wiring. You should leave it as provided. Otherwise, maybe you do not understand what you are doing and you should take a step back until you do.
 

Offline Benta

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Umm... the purpose of an isolation transformer is to have a DUT supply setup that is potential-free.

The input side, chassis and core of the isolation transformer should be grounded. The secondary side is supposed to float. Any grounding on that side will happen through your test equipment or DUT.


 

Offline Gtx21Topic starter

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Re: Question: Isolation transformer, Ground floating or Ground shorted to Neutral
« Reply #10 on: September 02, 2018, 10:42:14 pm »
The isolation transformer has two outputs, L1 and L2, both of which are hot. If you connect either of L1 or L2 to the ground pin of the output receptacle, then you will be making anything connected to that pin become "hot". If you do that, then why are you using an isolation transformer at all? You might just as well connect the DUT directly to the mains.

If you have an isolation transformer, then you should not mess with its internal wiring. You should leave it as provided. Otherwise, maybe you do not understand what you are doing and you should take a step back until you do.

There are more than 1 kind of isolation transformer.  I have seen 3 different internal configurations for them.  Some leave the Isolation-side ground pin floating, some leave the isolation-side ground pin grounded to earth-ground.  The 3rd I don't understand what they are doing but they have the Isolation Neutral pin shorted to the Isolation Ground Pin then shorted to Earth-Ground.  So inconsistency exists.
 

Offline Gtx21Topic starter

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Re: Question: Isolation transformer, Ground floating or Ground shorted to Neutral
« Reply #11 on: September 02, 2018, 10:43:33 pm »
Umm... the purpose of an isolation transformer is to have a DUT supply setup that is potential-free.

The input side, chassis and core of the isolation transformer should be grounded. The secondary side is supposed to float. Any grounding on that side will happen through your test equipment or DUT.

This was my belief and why I disconnected the Isolation Side Ground pin from Earth-Ground.
 

Offline Benta

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Re: Question: Isolation transformer, Ground floating or Ground shorted to Neutral
« Reply #12 on: September 02, 2018, 10:46:03 pm »
Umm... the purpose of an isolation transformer is to have a DUT supply setup that is potential-free.

The input side, chassis and core of the isolation transformer should be grounded. The secondary side is supposed to float. Any grounding on that side will happen through your test equipment or DUT.

This was my belief and why I disconnected the Isolation Side Ground pin from Earth-Ground.

After seeing your edit on the OP, we're on the same page.
 

Offline Bud

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Re: Question: Isolation transformer, Ground floating or Ground shorted to Neutral
« Reply #13 on: September 02, 2018, 11:20:18 pm »
Umm... the purpose of an isolation transformer is to have a DUT supply setup that is potential-free.

The input side, chassis and core of the isolation transformer should be grounded. The secondary side is supposed to float. Any grounding on that side will happen through your test equipment or DUT.

Can you post a maybe handwritten diagram ( as a quick measure) explaining where you will have a potential in the DUT if you connect its chassis ground to the mains ground. I am just trying to understand your concern.
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Offline Bud

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Re: Question: Isolation transformer, Ground floating or Ground shorted to Neutral
« Reply #14 on: September 02, 2018, 11:25:25 pm »
Umm... the purpose of an isolation transformer is to have a DUT supply setup that is potential-free.

The input side, chassis and core of the isolation transformer should be grounded. The secondary side is supposed to float. Any grounding on that side will happen through your test equipment or DUT.

This was my belief and why I disconnected the Isolation Side Ground pin from Earth-Ground.

Say you debug a TX circuit in a transmitter as the DUT. You key the transmitter in and its chassis become hot because of RF currents. You touch the chassis and it burns you. To remediate that you must ground the transmitter chassis. What do you do now  ?
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Online tautech

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Re: Question: Isolation transformer, Ground floating or Ground shorted to Neutral
« Reply #15 on: September 03, 2018, 02:35:01 am »
As Monkeh said, reconnect it.

Think about what an isolation transformer primarily offers
Isolation of each conductor WRT mains Ground

That's what keeps you safe-----r.
The 2 current conductors are still lethal WRT each other but no longer to mains ground where N + E are tied together at the switchboard and in Powerco circuits.

As has been mentioned the isolated output is now Live and Live but again only WRT each other.

Play it safe out there in EE land.
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Offline sibeen

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Re: Question: Isolation transformer, Ground floating or Ground shorted to Neutral
« Reply #16 on: September 03, 2018, 03:11:59 am »
The 3rd I don't understand what they are doing but they have the Isolation Neutral pin shorted to the Isolation Ground Pin then shorted to Earth-Ground.  So inconsistency exists.

In this case the transformer is not being used for isolation but is being used to construct a clean Neutral - Earth bond.
 

Offline IanB

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Re: Question: Isolation transformer, Ground floating or Ground shorted to Neutral
« Reply #17 on: September 03, 2018, 03:59:49 am »
This was my belief and why I disconnected the Isolation Side Ground pin from Earth-Ground.

The idea of the ground pin on a socket/receptacle is that any exposed metal parts connected to it should be tied to the earth ground in your local environment, putting them at the same potential as you, and therefore making them relatively safe to touch. This remains as true on the output side of an isolation transformer as it does on the input side.

None of the live parts of the device under test should have any connection to this safety ground. They should be completely isolated, and the isolation transformer exists to preserve this isolation.
 

Offline Gtx21Topic starter

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Re: Question: Isolation transformer, Ground floating or Ground shorted to Neutral
« Reply #18 on: September 03, 2018, 10:44:08 pm »
This was my belief and why I disconnected the Isolation Side Ground pin from Earth-Ground.

The idea of the ground pin on a socket/receptacle is that any exposed metal parts connected to it should be tied to the earth ground in your local environment, putting them at the same potential as you, and therefore making them relatively safe to touch. This remains as true on the output side of an isolation transformer as it does on the input side.

None of the live parts of the device under test should have any connection to this safety ground. They should be completely isolated, and the isolation transformer exists to preserve this isolation.
 
This is an excellent point.  I think I’m getting tripped up on specific instance based needs.
My goal at this very moment is to reduce probing risk.   
 
Now, if the earth ground for the DUT only goes to the casing, I think I’m on board.   If the earth ground for the dut is shorted to the DUT neutral, or used in some way internally, then I have a problem.
 
The particular instance I ran into is where a heat sink was earth-grounded via a support screw to the case bottom, which was connected to the Emitter of a BJT, thus bring the Neutral to Earth Ground.   In this situation, I got lucky and realized it before I started probing.  Had I not, poof goes something.
 
Maybe I’ll just spend the $300 for some differential probes.
 

Offline Bratster

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Re: Question: Isolation transformer, Ground floating or Ground shorted to Neutral
« Reply #19 on: September 03, 2018, 10:49:19 pm »
The micsig dp10013 probes are around $130 on eBay right now.

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Offline IanB

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Re: Question: Isolation transformer, Ground floating or Ground shorted to Neutral
« Reply #20 on: September 03, 2018, 10:55:29 pm »
If the earth ground for the dut is shorted to the DUT neutral, or used in some way internally, then I have a problem.
 
The particular instance I ran into is where a heat sink was earth-grounded via a support screw to the case bottom, which was connected to the Emitter of a BJT, this bring the Neutral to Earth Ground.   In this situation, I got lucky and realized it before I started probing.  Had I not, poof goes something.

If the situation you describe occurs, then the device you are looking at has an electrical fault. No part of the live wiring of a device is permitted to be connected to earth ground. There are insulation testers that exist to check for this issue. They apply a high voltage between the combined L/N conductors of the device and the earth/ground conductor/chassis/casing and test for leakage current. A high resistance of 1 MΩ or more must exist for the device to pass the test, otherwise the device is unsafe and must not be used.

Rather than investing in a differential probe, it might be better to invest in an insulation tester so you can check for device faults before working on them.
 

Offline perieanuo

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Re: Question: Isolation transformer, Ground floating or Ground shorted to Neutral
« Reply #21 on: September 04, 2018, 03:26:42 pm »
Ground shorted to neutral, NEVER! Bad bad practice.Saw this in Romania.



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Online Monkeh

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Re: Question: Isolation transformer, Ground floating or Ground shorted to Neutral
« Reply #22 on: September 04, 2018, 03:40:40 pm »
Ground shorted to neutral, NEVER! Bad bad practice.Saw this in Romania.

Ponder how neutral becomes neutral. It is, in fact, normal practice - when you want a neutral. There is no neutral without it.
 

Offline perieanuo

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Question: Isolation transformer, Ground floating or Ground shorted to Neutral
« Reply #23 on: September 04, 2018, 03:49:39 pm »
Ground shorted to neutral, NEVER! Bad bad practice.Saw this in Romania.

Ponder how neutral becomes neutral. It is, in fact, normal practice - when you want a neutral. There is no neutral without it.
Neutral is never neutral.Usual in triphasic systems every phase is distributed to 1/3 of the customers hoping it gets the network balanced and the neutral to 0, but every triphase network is unbalanced, so Neutral is not 0,never ever.It's almost zero [emoji4]
Ground is earth plus 2-4 ohms, end of story,protects you from dying.Ground is mandatory imho everywhere, not only in wet rooms and dangerous electrical equipment.
So keep the ground connection continuous.
Regards,pierre


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Online Monkeh

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Re: Question: Isolation transformer, Ground floating or Ground shorted to Neutral
« Reply #24 on: September 04, 2018, 03:54:13 pm »
Ground shorted to neutral, NEVER! Bad bad practice.Saw this in Romania.

Ponder how neutral becomes neutral. It is, in fact, normal practice - when you want a neutral. There is no neutral without it.
Neutral is never neutral.Usual in triphasic systems every phase is distributed to 1/3 of the customers hoping it gets the network balanced and the neutral to 0, but every triphase network is unbalanced, so Neutral is not 0,never ever.It's almost zero [emoji4]
Ground is earth plus 2-4 ohms, end of story,protects you from dying.Ground is mandatory imho everywhere, not only in wet rooms and dangerous electrical equipment.
So keep the ground connection continuous.
Regards,pierre

 


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