Questions on Isolation Panel

[Do You Like Ham]

In a typical 208v to 120v isolation transformer installation, where the 208v is comming from a 3-phase service, is the 120v secondary still 120 deg out of phase line-to-line?
As I understand it (trying to digest http://stevenengineering.com/tech_support/PDFs/45HIPS.pdf), there is no neutral (e.g., where the two conductors on the 120v secondary connect to a receptacle), these are actually two hot/line conductors... which is why they have special color coding (page 8) is that right?

I'm trying to digest the calculations in http://www.pglifelink.com/pdf/isolated-power-explanation.pdf

On page 3 after diagram 6, the value 1201000 is in the denominator... where did this number come from?

Related post I found: http://forums.mikeholt.com/showthread.php?t=51993
In that 277 to 120 case, it says "59.5 Volts on a "phase"", I'm curious what was meant by that. What would be the voltage on a "phase" be in my 208 to 120 case? Is it 69.3 (120 / sqrt(3))? My first reaction would have been that the voltage would have been indeterminate since it is isolated (thus, nearly infinite impedence?)

Thanks, much... I've never worked with one of these scenarios before.

 

[Do You Like Ham]

After some further contemplation and research, I'll try to answer some of my own questions here for some ... and hopefully, someone can confirm.

In a typical 208v to 120v isolation transformer installation, where the 208v is comming from a 3-phase service, is the 120v secondary still 120 deg out of phase line-to-line?

I don't believe so... the secondary has a single 'continuous' circuit induced current. Someone could probably describe this better.

I'm trying to digest the calculations in http://www.pglifelink.com/pdf/isolated-power-explanation.pdf

On page 3 after diagram 6, the value 1201000 is in the denominator... where did this number come from?

This must be an error... from http://www.ceb.cam.ac.uk/research/groups/rg-eme/teaching-notes/resistors-and-capacitors-in-parallel
Z = RXc / sqrt(R^2 + Xc^2)
In this case
R=1000
Xc = 1.2 x 10^6

Znum = 1.2 x 10^6 x 1000
Zden = sqrt(1000^2 + (1.2 x 10^6)^2) = 1,200,000.417 = 1.2 x 10^6 and some change... looks like they just added the values, instead of properly applying the theory.

As I understand it (trying to digest http://stevenengineering.com/tech_support/PDFs/45HIPS.pdf), there is no neutral (e.g., where the two conductors on the 120v secondary connect to a receptacle), these are actually two hot/line conductors... which is why they have special color coding (page 8) is that right?

I think so?

Related post I found: http://forums.mikeholt.com/showthread.php?t=51993
In that 277 to 120 case, it says "59.5 Volts on a "phase"", I'm curious what was meant by that.

May be anyone's guess.. may have been measured at some point in the line, where what they were actually measuring was a voltage division?

 

[lielec11]

As I understand it (trying to digest http://stevenengineering.com/tech_support/PDFs/45HIPS.pdf), there is no neutral (e.g., where the two conductors on the 120v secondary connect to a receptacle), these are actually two hot/line conductors... which is why they have special color coding (page 8) is that right?

I think you're wrong on this one here. If you're referring to an isolation panel like the ones used in operating rooms, there is most definitely a neutral, otherwise how do they supply 120V single phase loads in the OR? The neutral/ground is "floating" as it is not connected to the main grounding system, nor is it separately derived. Correct me if I'm wrong...

 

[GoldDigger]

In what is called a technical power system, the supply comes from a center tapped 120V winding with the center tap solidly grounded. The receptacle "hot" and "neutral" contacts are each at 60V to ground in opposite phase.
If it were used or even just brought to the outlet, the center tap would be a true neutral. But it is not. The two ungrounded conductors st the 120V outlet are thus both hot. Just like a 240V only outlet in a 120/240 3 wire system.

Tapatalk!

 

[cuba_pete]

Isolated Power vs. Isolated Ground vs. Balanced Power

Isolated Power vs. Isolated Ground vs. Balanced Power

There are a couple of different systems in which the naming convention is similar. Check out the Balanced Power link.

The "neutral" is a center tapped secondary, just like the typical split-phase system. So, yes, in that system there are two "phases" 180 degrees out-of-phase.

Please don't hit me up with calling them two "phases"...that's why I put it in quotes.

 

[Shoe]

Note: typically OR isolation panels are ungrounded. There is no solid reference to ground or a "neutral" tie to ground. A balanced system will often appear to show about 60V to ground, but not necessarily. The two-pole breakers from the isolation panel will supply the 120V (line-to-line).

 

[lielec11]

Note: typically OR isolation panels are ungrounded. There is no solid reference to ground or a "neutral" tie to ground. A balanced system will often appear to show about 60V to ground, but not necessarily. The two-pole breakers from the isolation panel will supply the 120V (line-to-line).

Guess I was a bit wrong, thanks everyone for your input... learn something new every day.

 

[Do You Like Ham]

Note: typically OR isolation panels are ungrounded. There is no solid reference to ground or a "neutral" tie to ground. A balanced system will often appear to show about 60V to ground, but not necessarily. The two-pole breakers from the isolation panel will supply the 120V (line-to-line).

Right, this is an Isolation panel for an OR... as in http://static.schneider-electric.us...ucts/Hospital Isolation Panels/4800CT1201.pdf
From what I see in the diagram, as on page 8, there is no neutral tie to ground as you say.

I think you're wrong on this one here. If you're referring to an isolation panel like the ones used in operating rooms, there is most definitely a neutral, otherwise how do they supply 120V single phase loads in the OR? The neutral/ground is "floating" as it is not connected to the main grounding system, nor is it separately derived. Correct me if I'm wrong...

What do you mean by "floating"?

It may be semantics, but I always thought that a Neutral by definition was connected to ground.

Why wouldn't the secondary of the transformer be considered separately derived?

Finally, why would 120V 'most definately' have a neutral? Aside from what the NEC probably says, what would physically prevent transforming 480V line to line to 120V (using a 4:1 winding ratio) to feed a receptacle?

 

[roger]

What do you mean by "floating"?

There is not a floating neutral in these Hospital systems, there is no neutral at all, just two 60 volt legs.

Why wouldn't the secondary of the transformer be considered separately derived?

It is an SDS in the most pure sense IMO

Finally, why would 120V 'most definately' have a neutral?

In this case there isn't a neutral and since this is an "ungrounded" system it is monitored for leakage and ground coupling by a Line Isolation Monitor.


Roger

 

[ActionDave]

What do you mean by "floating"?

It may be semantics, but I always thought that a Neutral by definition was connected to ground.

A neutral is usually required to be connected to ground, but until it is it is just a neutral. Once connected to ground it then becomes grounded.

A delta service with a corner ground would be an example of a grounded conductor that is not a neutral.

 

[Pharon]

There is not a floating neutral in these Hospital systems, there is no neutral at all, just two 60 volt legs.

Correct. If anything, it's a floating ground, which is monitored and only makes an audible alarm when there's a fault.

 

[kwired]

In a typical 208v to 120v isolation transformer installation, where the 208v is comming from a 3-phase service, is the 120v secondary still 120 deg out of phase line-to-line?
As I understand it (trying to digest http://stevenengineering.com/tech_support/PDFs/45HIPS.pdf), there is no neutral (e.g., where the two conductors on the 120v secondary connect to a receptacle), these are actually two hot/line conductors... which is why they have special color coding (page 8) is that right?

I'm trying to digest the calculations in http://www.pglifelink.com/pdf/isolated-power-explanation.pdf

On page 3 after diagram 6, the value 1201000 is in the denominator... where did this number come from?

Related post I found: http://forums.mikeholt.com/showthread.php?t=51993
In that 277 to 120 case, it says "59.5 Volts on a "phase"", I'm curious what was meant by that. What would be the voltage on a "phase" be in my 208 to 120 case? Is it 69.3 (120 / sqrt(3))? My first reaction would have been that the voltage would have been indeterminate since it is isolated (thus, nearly infinite impedence?)

Thanks, much... I've never worked with one of these scenarios before.

More clarification of exactly what you are asking may be necessary.

If you are talking about a single phase 208 primary to 120 volt secondary (208 being supplied by two phases conductors of a three phase system) your two secondary conductors are 180 degrees apart. When line 1 is at its negative peak line 2 is at its positive peak.

Remember most utility power is generated and distributed as three phase, and for single phase services or sections of distribution they just send two wires of that system out. All systems connected to it are single phase with 180 degree phase difference in the voltage of each point of any separately derived systems. Now if a single phase winding has a center tap it would be 90 degrees difference to the outer ends, with it being at zero when each outer end is at opposing peaks.

Not sure if I am hitting what you are asking about here but maybe it will help some even if it is not exactly what you are after.

 

[mbrooke]

I think you're wrong on this one here. If you're referring to an isolation panel like the ones used in operating rooms, there is most definitely a neutral, otherwise how do they supply 120V single phase loads in the OR? The neutral/ground is "floating" as it is not connected to the main grounding system, nor is it separately derived. Correct me if I'm wrong...

There is no neutral, and no grounded leg either. Just 2 AC power legs left floating. 120 volt loads don't need a neutral to function, as most don't care about a ground reference. In an OR or mission critical system the last thing you want is a current carrying conductor to have a ground reference.

In what is called a technical power system, the supply comes from a center tapped 120V winding with the center tap solidly grounded. The receptacle "hot" and "neutral" contacts are each at 60V to ground in opposite phase.
If it were used or even just brought to the outlet, the center tap would be a true neutral. But it is not. The two ungrounded conductors st the 120V outlet are thus both hot. Just like a 240V only outlet in a 120/240 3 wire system.

Tapatalk!

No. Its an ungrounded system, there is no center point grounding nor any corner grounding. The whole purpose is to limit fault current to a few milliamps and have the first fault on any phase trip no breakers. Supply colors are brown and orange to identify it. yellow for the 3rd phase in 3 phase. A line isolation monitor (glorified ground detector) makes sure the system is floating and alarms when not. During wiring XHHW or a cable with a high insulation resistance must be used and runs must be kept as short as possible to limit the fault current to a few milliamps.

These systems are used a an alternative to GFCIs, where faults cant cause major sparks but the biggest reason by far today for the US of ungrounded systems is service continuity. You don't want a conductor becoming grounded to trip a circuit.

 

[mbrooke]

Right, this is an Isolation panel for an OR... as in http://static.schneider-electric.us...ucts/Hospital Isolation Panels/4800CT1201.pdf
From what I see in the diagram, as on page 8, there is no neutral tie to ground as you say.



What do you mean by "floating"?

It may be semantics, but I always thought that a Neutral by definition was connected to ground.

Why wouldn't the secondary of the transformer be considered separately derived?

Finally, why would 120V 'most definately' have a neutral? Aside from what the NEC probably says, what would physically prevent transforming 480V line to line to 120V (using a 4:1 winding ratio) to feed a receptacle?

Just to add usually these isolation transformers have an electrostatic shield to filter noise as well as preventing a primary winding from crossing into the secondary.

 

[Do You Like Ham]

There is not a floating neutral in these Hospital systems, there is no neutral at all, just two 60 volt legs.

Thanks, Roger... how would one know that these are 60 volt legs? Diagramatically, I'm thinking of it this way:



There doesn't seem to be any reference point that you could measure 60V. If you measure L1 (or L2) to G, there is no potential to ground, if I'm to believe what is stated on page 12 of http://stevenengineering.com/tech_support/PDFs/45HIPS.pdf "No current flows if either secondary conductor of the transformer touches the plumbing (ground)."... so, what would one be measuring between to find 60V?

 

[GoldDigger]

I think what is being referred to is that if all of the connected loads have balanced stray capacitance to ground, the resulting capacitor network will tend to reference the neutral point in the system to ground.
A similar effect is seen even in an ungrounded delta system.
This is a phantom voltage, and a low input impedance meter or a wiggy would not read 60V to ground on either ungrounded wire.


Tapatalk!

 

[ActionDave]

Thanks, Roger... how would one know that these are 60 volt legs? Diagramatically, I'm thinking of it this way:

View attachment 10272

There doesn't seem to be any reference point that you could measure 60V. If you measure L1 (or L2) to G, there is no potential to ground, if I'm to believe what is stated on page 12 of http://stevenengineering.com/tech_support/PDFs/45HIPS.pdf "No current flows if either secondary conductor of the transformer touches the plumbing (ground)."... so, what would one be measuring between to find 60V?

You would be measuring to a centre tap on the secondary winding between L1 and L2 to get 60V. The only way to get a 60V reference to ground is if that centre tap is connected to ground.

 

[Do You Like Ham]

If Billy Idol or John Denver is on your Ipod go and re-evaluate your life.

Nice .

 

[roger]

I think what is being referred to is that if all of the connected loads have balanced stray capacitance to ground, the resulting capacitor network will tend to reference the neutral point in the system to ground.
A similar effect is seen even in an ungrounded delta system.
This is a phantom voltage, and a low input impedance meter or a wiggy would not read 60V to ground on either ungrounded wire.


Tapatalk!

Agreed.

Roger

 

[steve66]

I'm trying to digest the calculations in http://www.pglifelink.com/pdf/isolated-power-explanation.pdf

On page 3 after diagram 6, the value 1201000 is in the denominator... where did this number come from?

That looks like the "product over sum" formula which is used to find the total resistance of 2 resistors in parallel. So the bottom number is the impedance of the capacitor (1,200,000 ohms) plus the resistor (1000 ohms).

I don't think its 100% correct to use it in this case since they have a capacitor in parallel with a resistor.

But since the resistance of the resistor is so much less than the capacitive reactance, the formula gives a pretty close answer. (The current through the capacitor will be much less than the current through the resistor.)

So their conclusion (which is valid) is that we can basically ignore the one capacitor. You can see in the next diagram (#7) that is what they did to simplify the circuit.