Isolation Transformer Electrostatic Shielding & Common Mode noises

[tersh]

I have two kinds of isolation transformers. A shell type like this:

And a Toroid isolation transformer like this:

I'd like to ask about how grounding the secondary can eliminate common mode voltage surges. I read this:

http://voltage-disturbance.com/variable-frequency-drive/why-use-an-isolation-transformer/

Grounding the electrostatic shielding can eliminate the common mode surges in the secondary by diverting it back to source and not through the load. But what would happen if you ground one of the secondary outputs of the isolation transformer without grounding the electrostatic shielding (in my Hammond 500w shell type isolation transformer above, the electrostatic shielding is not connected to any lead of the secondary output). How much would it suppress the common mode voltage surge?

And do Toroid isolation transformers have any electrostatic shielding too? If none, then would it work if you ground one of the secondary leads of the Toroid isolation transformer, would this eliminate the common mode voltage noises/surge from transferring from the primary to secondary?

 

[tersh]

I have two kinds of isolation transformers. A shell type like this:

And a Toroid isolation transformer like this:

I'd like to ask about how grounding the secondary can eliminate common mode voltage surges. I read this:

http://voltage-disturbance.com/variable-frequency-drive/why-use-an-isolation-transformer/

Grounding the electrostatic shielding can eliminate the common mode surges in the secondary by diverting it back to source and not through the load. But what would happen if you ground one of the secondary outputs of the isolation transformer without grounding the electrostatic shielding (in my Hammond 500w shell type isolation transformer above, the electrostatic shielding is not connected to any lead of the secondary output). How much would it suppress the common mode voltage surge?

And do Toroid isolation transformers have any electrostatic shielding too? If none, then would it work if you ground one of the secondary leads of the Toroid isolation transformer, would this eliminate the common mode voltage noises/surge from transferring from the primary to secondary?

Note I wasn't referring to any Delta primary or Wye secondary but just plain line to line. Without the electrostatic shielding. Why would common mode voltage surge be attenuated if one of the secondary leg is grounded? If it escapes to the ground. Won't this change the common mode voltage noise to differential mode noise/surge instead?

 

[gar]

181205-2522 EST

tersh:

There is something you don't understand about basic circuits, and I haven't figured out what it is.

Whether a transformer is wound on an EI core or a toroid does not determined whether it can have its secondary electrostatically shielded from the primary or not. How the windings are wound and shielded is the determining factor.

Note I wasn't referring to any Delta primary or Wye secondary but just plain line to line. Without the electrostatic shielding. Why would common mode voltage surge be attenuated if one of the secondary leg is grounded? If it escapes to the ground. Won't this change the common mode voltage noise to differential mode noise/surge instead?

I don't understand this question(s).

.

 

[tersh]

181205-2522 EST

tersh:

There is something you don't understand about basic circuits, and I haven't figured out what it is.

Whether a transformer is wound on an EI core or a toroid does not determined whether it can have its secondary electrostatically shielded from the primary or not. How the windings are wound and shielded is the determining factor.

I don't understand this question(s).

.

Both my 500w shell type and 300w Toroid Isolation Transformers are made by Hammond Manufacturing. But they only indicate the shell type has electrostatic shielding. They didn't give that description to the toroid.

I was asking that without any relationship to the electrostatic shield. Let's say it's not shielding and the capacitive coupling causes the primary common mode voltage noises/surge to transfer to the secondary. Why would grounding the secondary eliminate the common mode voltage noises/surge in line to line connection?

 

[tersh]

Both my 500w shell type and 300w Toroid Isolation Transformers are made by Hammond Manufacturing. But they only indicate the shell type has electrostatic shielding. They didn't give that description to the toroid.

I was asking that without any relationship to the electrostatic shield. Let's say it's not shielding and the capacitive coupling causes the primary common mode voltage noises/surge to transfer to the secondary. Why would grounding the secondary eliminate the common mode voltage noises/surge in line to line connection?

This is the context of the above. A 65 veteran EE advised me to ground one of the 2 floating leads of the 500w isolation transformer to take care of common model voltage noises/surge. But I didn't do it because I want to take advantage of the isolation (where as you know if one lead touches ground, no shock because the secondary is isolated from ground).

I'd like to know what mechanism causes the common-mode voltage surge to be eliminated if one of the floating secondary leads is grounded. I need to be sure the effect is more advantageous than removing the advantage of isolation.

 

[gar]

181206-0834 EST

tersh:

Basically your questions can be answered with a basic understanding of electrical circuit theory. I think the problem of analyzing the problem comes from the interpretation of what certain not well defined words mean.

What does "isolation transformer" mean? To me it means there is no low impedance conductive path from primary to secondary. A transformer with two separate coils falls in this category, an autotransformer does not.

To the basic statement of "isolation transformer" you need to add "with electrostatic shield" if you want to reduce capacitive coupling between windings.

Both EI and toroid core transformers can be auto transformers, or isolation transformers. Any of the combinations can have electrostatic shielding.

If you have an isolation transformer without an electrostatic shield, and one secondary lead is earthed, then does this reduce common mode noise on the primary from coupling to the secondary? Probably not much. There is a lot of distributed capacitance between the primary and secondary windings, and grounding one end of the secondary does not change this much.

Will earthing one end of the secondary solve any problems? Certainly some kinds, but it may create other problems.

Often times I need my scope isolated from the EGC so I use a 2 to 3 prong adapter, or I could use an isolation transformer. Earthing one lead of the isolation transformer secondary would create the same problem the isolation transformer was being used to solve. If I need several thousand volts isolation, then I might use the isolation transformer. If the isolation voltage needs to be higher, then special isolation amplifiers would be used.

.

 

[RumRunner]

I have two kinds of isolation transformers. A shell type like this:

And a Toroid isolation transformer like this:

I'd like to ask about how grounding the secondary can eliminate common mode voltage surges. I read this:

http://voltage-disturbance.com/variable-frequency-drive/why-use-an-isolation-transformer/

Grounding the electrostatic shielding can eliminate the common mode surges in the secondary by diverting it back to source and not through the load. But what would happen if you ground one of the secondary outputs of the isolation transformer without grounding the electrostatic shielding (in my Hammond 500w shell type isolation transformer above, the electrostatic shielding is not connected to any lead of the secondary output). How much would it suppress the common mode voltage surge?

And do Toroid isolation transformers have any electrostatic shielding too? If none, then would it work if you ground one of the secondary leads of the Toroid isolation transformer, would this eliminate the common mode voltage noises/surge from transferring from the primary to secondary?

If you have an electrostatic shielding-- you would be required to have it connected to a reference grounding point. You don't have to have a grounded leg in your secondary to do this. See Art 517-2


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Shielding is basically for the suppressing radio energy (RFI) being broadcast to nearby sensitive equipment.


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Usually, having an ungrounded leg is required in critical areas like anesthetizing rooms.


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You did not give any indication what specific area you intend to use this ungrounded isolation XFMR.


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As gar indicated earthing one lead can have an advantage and in other instance may create problems.


​

To insure the advantage of having an ungrounded isolation XFMR- it has to feed a particular load only. You don't connect other loads other than those intended to be fed from this dedicated power source. (see 517 again)


​

Toroid isolation XFMRs can have shielding. In fact-- they offer easier and effective shielding because of the way they are constructed.

​

Electrostatic shielding has very little impact if at all on the secondary winding. Noises and surges can come from both primary and secondary. The primary is exposed to external disturbances like lightning or poco equipment-- while the isolated secondary winding is exposed to spikes/noises from other equipment being turned off or on.


​

This is the reason you don't want to connect other loads to this isolation XFMR to anything outside the intended load.

​

Surges are like moving targets. These surge sources are so dynamic that you can design one that is good for say 10 to 800V surge and it will just pass less than 800 V and it will smoke over this range.


​

MOV's (metal oxide varistor) are usually installed in some equipment to “supposedly arrest these surges” which honestly are more like snake oil.​

They can fail randomly and nobody even pay attention to them when they fail. No one will even bother replacing the smoked MOV.


​

But, don't get me wrong—in crucial venue where absolute safety that involves life and death situation, this will most likely save lives—but not for assuring that your cup of cappuccino will arrive piping hot.
​

 

[tersh]

If you have an electrostatic shielding-- you would be required to have it connected to a reference grounding point. You don't have to have a grounded leg in your secondary to do this. See Art 517-2​

Supposed you do this in the US with fully intact neutral and grounding and bonding it once a the main panel. But supposed there an open neutral service fault in which the neutral in the service entrance got disconnected. Mike Holt shows the metal box would become electrified because the power has to seek other path outside. Now what would happen if the electrostatic shielding got electrified from the open neutral service fault. Would it damage the transformer?

​

Shielding is basically for the suppressing radio energy (RFI) being broadcast to nearby sensitive equipment.


​

Usually, having an ungrounded leg is required in critical areas like anesthetizing rooms.


​

You did not give any indication what specific area you intend to use this ungrounded isolation XFMR.

​

The purpose of the 500w 240v-120v step down isolation transformer is to run 120v surge protection device like the following.

The VPR of 120v SPD are lower (700v in the case of the following or 500v), while the VPR of 120v SPD are 1200v.

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F

or those Europeans (or Middle Easterners or Chinese) using only 240v. Do they use 240v-120v step down transformers to use the lower VPR 120v SPDs, or do they still use 240v SPDs with higher VPR. Why?

If they use step down transformer to take advantage of 120v SPD, then the VPR would be lower and offer greater protection to the load, right? How many of them do this in practice? What do you think? Any disadvantage of using step down transformer and 120v SPD combo?​

​

As gar indicated earthing one lead can have an advantage and in other instance may create problems.


​

To insure the advantage of having an ungrounded isolation XFMR- it has to feed a particular load only. You don't connect other loads other than those intended to be fed from this dedicated power source. (see 517 again)


​

Toroid isolation XFMRs can have shielding. In fact-- they offer easier and effective shielding because of the way they are constructed.

​

Electrostatic shielding has very little impact if at all on the secondary winding. Noises and surges can come from both primary and secondary. The primary is exposed to external disturbances like lightning or poco equipment-- while the isolated secondary winding is exposed to spikes/noises from other equipment being turned off or on.


​

This is the reason you don't want to connect other loads to this isolation XFMR to anything outside the intended load.

​

Surges are like moving targets. These surge sources are so dynamic that you can design one that is good for say 10 to 800V surge and it will just pass less than 800 V and it will smoke over this range.


​

MOV's (metal oxide varistor) are usually installed in some equipment to “supposedly arrest these surges” which honestly are more like snake oil.​

They can fail randomly and nobody even pay attention to them when they fail. No one will even bother replacing the smoked MOV.


​

But, don't get me wrong—in crucial venue where absolute safety that involves life and death situation, this will most likely save lives—but not for assuring that your cup of cappuccino will arrive piping hot.
​