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Joined: Feb 2013
Posts: 264
Potseal Offline OP
Member
Okay, just to make sure I understand and that I am providing the right information I dug up a diagram of how an OR theatre should be wired according to CSA's Z32 code book regarding health care facilities. I have done some work in an OR theatre as far as taking the cover off the panel and inspecting the guts so some of this looks familiar:

[Linked Image from i58.tinypic.com]

If I understand you correctly, a DMM would measure a percentage of supply voltage from L1 to grnd and N to grnd because of the ground detection device (Line Isolation Monitor?) but if it were out of the circuit then there would be zero voltage measured?

BTW, the measurements I took on my isolation transformer were done with a Fluke 117 with 5 Mohm R and the Weston has 11.535 Kohm R at 150 V setting.


A malfunction at the junction
--------------------------------------
Dwayne
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Joined: Nov 2005
Posts: 827
Likes: 1
J
Member
"If I understand you correctly, a DMM would measure a percentage of supply voltage from L1 to grnd and N to grnd because of the ground detection device (Line Isolation Monitor?) but if it were out of the circuit then there would be zero voltage measured?"

That's right, your measured voltages should be close to zero. Also, with the 2-pole breaker for the ground detector off, I'm guessing that you would measure a resistance through the detector that is substantially lower than your meter input impedance.
Joe

Joined: Feb 2013
Posts: 264
Potseal Offline OP
Member
Alright, I follow that.

Here's a diagram of my Hammond isolation transformer from the website:

[Linked Image from i62.tinypic.com]

http://www.hammondmfg.com/171.htm

In my opinion it is similar to the diagram of the isolated system above minus the ground detection device. The above system has a grounded shield and my transformer, according to the website information, has an electrostatic shield (Faraday shield?) that is grounded.

My DMM measurements on the secondary show voltage where ideally their shouldn't be. The Weston shows no voltage where, according to what you're saying, there shouldn't be voltage.

So here's the million dollar question - is it possible to suggest that the DMM is simply the wrong meter for measuring voltage on the secondary of an isolated system/isolation transformer. I think more than one of you may have already suggested that earlier with me being slow to catch on.


A malfunction at the junction
--------------------------------------
Dwayne
Joined: Nov 2005
Posts: 827
Likes: 1
J
Member
Originally Posted by Potseal
Alright, I follow that.

My DMM measurements on the secondary show voltage where ideally their shouldn't be. The Weston shows no voltage where, according to what you're saying, there shouldn't be voltage.

So here's the million dollar question - is it possible to suggest that the DMM is simply the wrong meter for measuring voltage on the secondary of an isolated system/isolation transformer. I think more than one of you may have already suggested that earlier with me being slow to catch on.


I'm just trying to nudge you along towards a deeper understanding of things. They mostly fall under the "nice to know", rather than "need to know" heading. I would never suggest that you give up a hi-z DMM for a VOM or a Wiggy. The reason is that you can always make a hi-z lower, while making a lo-z higher, results in much lower readings. You can always toss a power resistor with leads into your meter pouch if you want to load down your measurement. A 50K, 2W resistor across your DMM will yield readings more like a Simpson or Weston, and the difference in the readings gives clues to the source impedance that you're looking into. Just always consider power dissipation in cases like that. A 2W resistor would be fine for a 240 volt system, but not 480V.
Joe

Joined: Feb 2013
Posts: 264
Potseal Offline OP
Member
Here we go again Joe. I'm trying to wrap my head around this so hopefully I'm not wearing your patience thin...

The DMM has a high input impedance and therefore should have minimum impact (meter loading?) on the measurements I was taking on the secondary of the Hammond Isolation transformer - right? Therefore the result was the voltage readings of 72 V and 20 V.

The analog meter with it's lower input resistance would have an impact on the measurements and act as a load - right? Thus I got the 0 volts reading.

After reading the information below am I on the right track? Without a substantial load on the secondary of the transformer the voltage readings I am getting with the DMM are a result of, and I quote, "under lightly loaded conditions, the reactive drop may not provide sufficient voltage reduction, or attenuation"???

Quote
Bulletin No. 7400PD9202
April, 1992
Milwaukee, WI, U.S.A.

Subject: The Performance Of
Shielded Isolation Transformers

It is the primary purpose of a transformer to transfer power, as faithfully as possible, from the primary to the secondary. Unfortunately, this often means coupling into the load input transients and other troublesome deviations from the ideal, clean power wave. A transformer couples power most efficiently at its design frequency, (usually 60HZ). Since the transformer reactive impedance increases with frequency, the higher order harmonics and high frequency waveform spikes are greatly attenuated with the transformer under load. This is simply because they are
dissipated across the XL component of the transformer impedance. However, under lightly loaded conditions, the reactive drop may not provide sufficient voltage reduction, or attenuation.

Last edited by Potseal; 07/20/15 05:01 AM.

A malfunction at the junction
--------------------------------------
Dwayne
Joined: Feb 2013
Posts: 264
Potseal Offline OP
Member
Forgot to mention that the above quoted information was put out by Square D.

Last edited by Potseal; 07/20/15 05:00 AM.

A malfunction at the junction
--------------------------------------
Dwayne
Joined: Apr 2004
Posts: 349
Member
I think Joe is saying (in part) that to try to measure the “difference in potential” between two electrically random points that have no electrical connection to one another (until you connect a meter) is sorta nonsensical in a way, meaning that any reading you get means next to nothing. And taking two different (& rather random) voltage readings between two sets on electrically unrelated points – I would not expect the sum of the two readings to add up to anything significant.

I think the higher the meter impedance, the wilder the readings you may expect due to a reduced impact on the circuit under test. The lower the meter impedance, the closer to 0V you will likely read. Remember you can connect (i.e. SHORT) any ONE point of an ungrounded (or isolated) system you wish directly to earth ground without ill-effect. That’s why Navy shipboard electrical systems are intentionally UN-grounded. Tactical reliability.

However, caution is needed, as Joe has mentioned, due to various forms of coupling, the actual effective EMF to ground of a given point in an isolated system can be dangerously high, if left to float completely independently (no anchor). If that point is touchable, it can be dangerous.


There are 10 types of people. Those who know binary, and those who don't.
Joined: Feb 2013
Posts: 264
Potseal Offline OP
Member
Originally Posted by Radar
I think Joe is saying (in part) that to try to measure the “difference in potential” between two electrically random points that have no electrical connection to one another (until you connect a meter) is sorta nonsensical in a way, meaning that any reading you get means next to nothing. And taking two different (& rather random) voltage readings between two sets on electrically unrelated points – I would not expect the sum of the two readings to add up to anything significant.

I think the higher the meter impedance, the wilder the readings you may expect due to a reduced impact on the circuit under test. The lower the meter impedance, the closer to 0V you will likely read. Remember you can connect (i.e. SHORT) any ONE point of an ungrounded (or isolated) system you wish directly to earth ground without ill-effect. That’s why Navy shipboard electrical systems are intentionally UN-grounded. Tactical reliability.

However, caution is needed, as Joe has mentioned, due to various forms of coupling, the actual effective EMF to ground of a given point in an isolated system can be dangerously high, if left to float completely independently (no anchor). If that point is touchable, it can be dangerous.


I'm not sure about any of this but I do know that connecting a meter across the secondary of an isolation transformer or isolated system will indicate supply voltage. Measuring voltage from either conductor to ground is where it seems to get wiggy.

BTW, what you stated about a floating/isolated system and it's built in safety aspect as far touching one conductor and ground is something I tested with my Hammond isolation transformer. I connected one conductor directly to ground and flipped the power on to see if it would function as it should. As advertised, nothing happened and no measurable current found with my Fluke clamp-on meter (with the conductor wrapped around it several times).


A malfunction at the junction
--------------------------------------
Dwayne
Joined: Apr 2004
Posts: 349
Member
Navy shipboard AC electrical systems are ungrounded for that very reason. You can inadvertently ground any one point with no mishap whatsoever. But one point only.

Those systems have ground detection circuits built in, to alert the crew of an inadvertent ground, so it can be cleared before some other point becomes inadvertently grounded.

However - there is some coupling between the floating electrical system and ship's ground that occurs because of the ground detection circuitry, and additional coupling that occurs due to capacitance. One can receive a severe shock, fatal even, by coming in contact with an ungrounded system "Hot" and the ships hull (which is all around, by the way).

But that hazard aside, while the electrical system floats, you can still ground any single point you like in the electrical system and nothing will happen.


There are 10 types of people. Those who know binary, and those who don't.
Joined: Nov 2005
Posts: 827
Likes: 1
J
Member
Originally Posted by Potseal
Here we go again Joe. I'm trying to wrap my head around this so hopefully I'm not wearing your patience thin...

The DMM has a high input impedance and therefore should have minimum impact (meter loading?) on the measurements I was taking on the secondary of the Hammond Isolation transformer - right? Therefore the result was the voltage readings of 72 V and 20 V.

The analog meter with it's lower input resistance would have an impact on the measurements and act as a load - right? Thus I got the 0 volts reading.

After reading the information below am I on the right track? Without a substantial load on the secondary of the transformer the voltage readings I am getting with the DMM are a result of, and I quote, "under lightly loaded conditions, the reactive drop may not provide sufficient voltage reduction, or attenuation"???


I have lots of patience for people who give a darn and are willing to take the time to figure things out!
That said, your last attachment is leading you down the wrong path. That material is letting you know that harmonics, noise, and transients, that wouldn't typically have much amplitude in a normal load impedance, can create problems with a high load impedance. Again, the ratio of impedances comes into play. Your source 50 or 60 Hz power should have an extremely low source impedance, while the harmonics and noise should be a hi-z source. But if your load z is high, you might see close to the same amplitude on the secondary.

You're dealing with more like what we consider "parasitics" in electronics. You showed a description of a shielded isolation transformer. Think of a twisted, shielded cable Vs a non-shielded twisted-pair cable. The pF/foot is much higher on the shielded cable so although you have more noise immunity, you suffer more high frequency attenuation. Your transformer probably has a greater amount of capacitive coupling between the secondary Neutral and the grounded case. You should draw a resistor, a cap, and an inductor, between each leg and ground. Then draw a voltmeter symbol to represent your DMM, and another for your Weston. Then write down the input resistance and capacitance for each. All of the Rs and Cs and Ls make complex voltage dividers that cause readings like the ones that you're getting.

A ground detector is there to detect grounds, not to protect from getting shocked. It actually creates a path that may or may not limit currents to a safe level.
Joe

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