Lighting Phase Cancelations & Amps

[JD]

For 30 years, my standard protocol on setups was to turn everything on (as well as throw a few questionable scenes up) and throw an amprobe on the feeds to make sure there was reasonable balance. Never did I find a higher current on the neutral then was available from any of the hots. A little understanding of 3 phase geometry makes it clear that it would be impossible. First some terms:

WYE = three windings with their low end tied to neutral and their high end providing the hots. Usually 120 volts to N and 208 volts between hots.
DELTA = three windings with their ends tied to each other in the shape of a triangle. Usually 240 volts between hots.

A transformer (source) will be one of the two. A load may be either or a combination of both. Watch what happens in the following three load cases based on equal loads on a WYE source:

One leg loaded: 100% on the affected hot, 100% on the Neutral
Two legs loaded: 100% on both affected hots, 33% on the Neutral
Three legs loaded: 100% on each affected hot, 0% on the Neutral

What happens here is that our load changes from Wye to Delta as it balances out. The current finds its return feed through the other operating hots. A similar thing happens on single phase circuits using a 120-0-120 source. If the two legs are balances, the neutral does very little. In a three phase system, each phase leg lags the prior leg by 120 degrees as compared to 180 degrees in the single phase application.
Back to the real world. No such thing as a perfectly balanced load. During the course of scene and intensity changes the neutral will do a lot of work, but never, I repeat never, over 100% of the work that any given leg will handle.

One last note: What holds true on (our) load end does not hold true in the transformer box itself! Even though our load may be pusdo-delta, the transformer is not! Current between windings ALL flows through the neutral buss, and it may see currents of 300% of the rated load as each winding seeks its backflow through the buss. The transformer box however, is none of our business!

 

[STEVETERRY]

John, I'm sorry to flame, but your statements are just plain wrong.

In the world of linear loads, you would be correct.

In 2007, in the world of non-linear loads, neutral overcurrent (greater than the maximum phase current) is a well-understood fact of life.

There has been a lot written on this subject.

I suggest you look at some of the articles available online.

Alternatively, you could get yourself an SCR dimmer pack and a true-RMS ammeter and conduct a short science experiment. With the ammeter on the neutral, take the handles of three dimmers on the three phases in your hand. Move them around, and let us know how long it takes for you measure neutral current greater than the maximum phase current.

This would be especially important given the audience on CB--young people that rely on authoritative statements from people who have experience. It's incumbent on us to make those statements correct.

Finally, typical delta phase-to phase voltage in the US is 208V or 480V, never 240V.


Cheers

ST

 

[STEVETERRY]

http://srforums.prosoundweb.com/index.php/m/223248/24440/0///15773/#msg_223248
http://srforums.prosoundweb.com/index.php/m/223663/24481/0///15773/#msg_223663
Here is a link re the very confusing situation where you have a distro, two 120 volt legs and are trying to determine the amp load, down the thread, is the explanation that if say you have 40 amps on one leg and 3 amps thru the other you will have 37 amps on the neutral
http://srforums.prosoundweb.com/index.php/m/200335/22030/0///15773/#msg_200335
the links are to the first post in the thread, Lee P is usually the electrical guy
Sharyn

Thanks for the links, Sharyn.

I must say that the last link is amazing. There are a lot of people on that thread that do not, unfortunately, have even a basic understanding of AC power distribution. I'm not sure where to start to correct their many misconceptions and mistakes, so I won't.

Cheers


ST

 

[David Ashton]

Yes John is wrong, totally wrong and as this subject is a major source of problem it must be understood.I deal with neutral burnouts on a monthly basis and always have problems explaining this to electricians.I have show them the neutral current to make them understand.Do the experiment with the dimmers, please.

 

[Logos]

Hi David: Do you wanta pop over to the new member thread and put on a message introducing yourself. It'll give you an opportunity to tell us all about you.
Have we run into each other on Theatre Australia?
Gafftaper: I did not invent David either.
David I am glad to have a new supporter in the mighty metric war.

 

[JD]

National Electric Code:
http://www.mikeholt.com/mojonewsarchive/EC-HTML/HTML/WyeTransformerCalculations~20030916.htm
Y is 208/208/208 with 120 to neutral, Center tapped Delta is 240/240/240 with 120/120/208 to neutral.
Nuff said!
And to the ***** who suggested I get an scr dimmer and try it out, What the **** do you think I was running these shows with for 30 years!!
Geeezzz, these young ones will be the death of me!
To the rest of you, there is a big difference between cut-phase loading and power factor loading which is what upgauging is about. On a separate return system, you would have to upgauge the neutral, but not on a dedicated system or power distro. HOWEVER: Neutral burn is a whole other topic brought on by the nature of the chopped waveform used in all standard dimmers. It basically attacks connectors like an electrical jackhammer. The result can be destructive as the whole wave average current appears as a short duration over-current pulse between the cut-off and cut-on of different dimmer channels. Still, Rule #1 is not broken, the average current carried by the neutral does not exceed the highest leg current.

 

[JD]

Originally Posted by SHARYNF
http://srforums.prosoundweb.com/inde...73/#msg_223248
http://srforums.prosoundweb.com/inde...73/#msg_223663
Here is a link re the very confusing situation where you have a distro, two 120 volt legs and are trying to determine the amp load, down the thread, is the explanation that if say you have 40 amps on one leg and 3 amps thru the other you will have 37 amps on the neutral
http://srforums.prosoundweb.com/inde...73/#msg_200335
the links are to the first post in the thread, Lee P is usually the electrical guy

********
The distros pictured can be run phase independent in the 3 phase mode, that is why there are individual neutral returns. In other words, in a worse case scenario, if only 120 volts are needed, you could tie everything to a single 120 volt winding and everything would be within code with no overheating.

 

[David Ashton]

Thank you John for so clearly demonstrating the problems of neutral overload, I have been in this business before the days of thyristor dimmers when it was not a problem and I have been with highly qualified engineers who have been in denial about the high neutral currents even when they see them in situ and on a meter, "it must be a problem with the meter" or "The meter is reading wrong because of the 150hz"{Australia}But the meter is not wrong and the current in the neutral always exceeds the phase currents in wave chopping dimmers{when the dimmers are between approx 30% -60%} and your jackhammer theory,while novel is wrong.Now please go and put a meter on your neutral and see for yourself.A cheap moving iron meter is still a valid way to measure the 'confused" current flowing down a neutral and a very expensive rms digital meter is not necessary, but will still work accurately.I am sorry to go on about this subject but it is vital that all lighting techs are aware of the problem and take steps to avoid it.

 

[SHARYNF]

In my experience this is a topic that is important, and at the same time is subject to major discussion.
My understanding is that on most transformers here in the US, theside from the "pole" is a delta connection, and the side to the building is either delta or wye.

A lot of the interest and discussion re distro's is the situation where people do need to connect to a variety of services in a touring situation, and it is easy to look at the two legs of a three phase as being the
"same" as your typical home single phase.
Sharyn

 

[STEVETERRY]

In my experience this is a topic that is important, and at the same time is subject to major discussion.
My understanding is that on most transformers here in the US, theside from the "pole" is a delta connection, and the side to the building is either delta or wye.
A lot of the interest and discussion re distro's is the situation where people do need to connect to a variety of services in a touring situation, and it is easy to look at the two legs of a three phase as being the
"same" as your typical home single phase.
Sharyn

You are right about the typical transformer configuration in a building--the primary is typcally delta, and the secondary is typically wye or delta. A delta secondary is typically used in a situation where motor loads do not use a neutral. Another delta situation is for an intermediate transformer--say 12,270V to 480V delta, that is then further reduced to 208Y/120 with local transformers.

A rarer case is "high-leg delta" or center-tapped delta where a small amount of 120V is required in a large amount of 208V delta loads. This is almost never found outside a factory environment, and is must be identified as such since two of the legs are at 120V with respect to neutral, and the third leg is at about 180V with respect to neutral. The code requires that this leg be identified (usually orange or brown). The high leg cannot be used by theatrical 208Y/120 equipment--the service must be treated as a 120/208V three-wire plus ground service. If you connect to it as 208Y/120, you will have blown up lamps and possibly dimmers in your near future.

ST

 

[STEVETERRY]

National Electric Code:
http://www.mikeholt.com/mojonewsarchive/EC-HTML/HTML/WyeTransformerCalculations~20030916.htm
Y is 208/208/208 with 120 to neutral, Center tapped Delta is 240/240/240 with 120/120/208 to neutral.
Nuff said!
And to the ***** who suggested I get an scr dimmer and try it out, What the **** do you think I was running these shows with for 30 years!!
Geeezzz, these young ones will be the death of me!
To the rest of you, there is a big difference between cut-phase loading and power factor loading which is what upgauging is about. On a separate return system, you would have to upgauge the neutral, but not on a dedicated system or power distro. HOWEVER: Neutral burn is a whole other topic brought on by the nature of the chopped waveform used in all standard dimmers. It basically attacks connectors like an electrical jackhammer. The result can be destructive as the whole wave average current appears as a short duration over-current pulse between the cut-off and cut-on of different dimmer channels. Still, Rule #1 is not broken, the average current carried by the neutral does not exceed the highest leg current.

A few points:

1. Center-tapped delta is almost never found in a theatre or arena. See my post to Sharyn.

2. You are completely, totally, indescribably wrong about neutral current never exceeding phase current. On an SCR dimming system, the RMS neutral current will regularly exceed the phase current depending on the setting of the dimmers. Your "jackhammer theory" and "Rule #1" is nonsense and has no engineering basis. While the current waveform of an SCR dimmer system is non-sinusoidal and does contain triplen harmonics, it is RMS overcurrent in the neutral we are discussing here.You should really, really do the experiment we have suggested before you put your foot any further into your mouth. In your 30 years of running shows, you probably did not have an ammeter on each phase and the neutral during the shows, else you would not be so sure of totally incorrect facts.

3. As to "these young ones", I am 54 years old. I've been in this business 36 years. I am a member of National Electrical Code Panel 15 (the guys that write the theatre-related articles of the NEC), and I founded the USITT National Electrical Code Committee. I am also a member of the ESTA Electrical Power Working Group. But please, don't take my word for it. Google "neutral overcurrent" and see what you find. Or, just do the experiment suggested.

ST

 

[SteveB]

And to the ***** who suggested I get an scr dimmer and try it out, What the **** do you think I was running these shows with for 30 years!!
Geeezzz, these young ones will be the death of me!.

In Steve's defence,

Steve Terry was the person suggesting the dimmer pack demo. He's correct in the issue and description about SCR dimming systems causing harmonic distortions and overload currents in the neutrals conductors. This really is very well documented.

I'm also surprised that you would question someone who is about as authoritative on the subject as Steve is (well.... always question the answer, but respect the source). He was really one of the first folks in the industry to understand the issue, back in the days when very large dimmer-per-circuit lighting systems were becoming common place. That disqualifies him, BTW as being one of those "these young ones will be the death of me!".

Steve subsequently wrote many an article about the subject. It was a wake up call for the industry as well as forcing (if memory serves) the writers of the National Electrical Code to change the code for portable dimming systems.

FWIW, Steve is a driving force (advisory / alternate ?, I can't remember) on the Nat'l Electrical Code Panel for the entertainment/theater sections and has been instrumental in getting a lot of the code pertaining to theatres, changed to be more rationally based, rather then simply following rules that pertain to the electrical industry as a whole, regardless of whether they have any bearing on our industry - use and specifications of single conductor feeder cables, use of SJO cable-ing in certain situations, how much feeder capacity to provide for DPC systems, etc....

Now that kind or resume doesn't necessarily always make him right (You did sell me 2 Colortran Prestiges's after all ! (grin), though on this particular issue, I would be doing a lot more research before I would question someone who's been studying this problem, as well as developing solutions for them, as long has Steve has.

Steve Bailey
Brooklyn College

 

[JD]

Glad to hear about your credentials, it allows me to digest your statements in the context of an educated person, as compared to a flamer or spammer. (I am an engineering grad myself in my 50's) Regarding the road testing I did, I used an Amprobe, which is a magnetic meter that can be slapped over any conductor of a feeder cable without cutting into it. I stand by those results.

You did however cost me some sleep last night and I ended up on the bench at 4am trying some experiments.

The test rig was as follows: Source- 3 phase Wye, 208/120 Dimmer - 3 old EDI scrimmerets each wired on one phase leg. Load- three 1k par lamps. Test- a 1 ohm resistor in series with the neutral, fed into a tectonics scope via a diode bridge and a 10 uf capacitor, drained by a 100 ohm resistor, as well as an amprobe around the neutral. I should explain the jig- by converting the voltage drop to a dc voltage on a scope, the "average" current can be measured. The bypass resistor was selected by test using know loads until a one-to-one ratio was obtained. (one amp rms producing 1 volt on the scope input.) This also agreed with the reading on the amprobe.

Test #1: Loading on phase leg by bringing up one dimmer. As expected the load current peaked at 8.3 amps. the only surprise was that the crest was non-liner on the scope with the highest rise in current draw occurring before 33%.

Test #2: Loading on all three phase legs, bringing up the dimmers at the same time. The peak neutral load was 8 amps at 33%, with the level dropping to near 0 by full brightness. So far, no surprises.

Test #3 I added a second jig to one of the phase feeds so it's level could be compared to the neutral. On this test I concede one of my points! At 33%, the "hot" load measured at 4 amps, while the neutral load measured in at 8 amps!

By removing the filter, I could see the pathology of the neutral. As all dimmers were drawing their current in the last 1/3 of the waveform, they literally filled in each others gaps. So, I concede to you that neutral current can exceed the current draw on the hot feeds. I tried several other tests trying to find any spots where I actually exceeded the total max draw of 8.3 amps, but could not find any. I could however see that if this distribution was to be used to feed low power factored loads, this could occur. As the current draw drops as a light is dimmed, and filament lamps do not introduce phase delayed current, I do not see this happening with standard lighting. However, as the use of magnetic ballasts for HMI and other lights due introduce PF errors, I could see this happening in the stage environment.

Now, on to the "Electric Jackhammer." I did not coin a new phrase here, it comes from a 1969 General Electric Labs report on the damaging effect of electronic motor control systems on connectors. Basically, these were the early days of chopped waveforms on a large current scale. Motor Control. (Same SCR systems that became dimmers.) What is not well understood is that when you plug a plug into a socket, you have not made an electrical connection yet! A think layer of oxygen remains between the contacts. Once some power flows (very little is needed) the dielectric strength of this layer is ruptured, and current flows. (Usually, a lot smaller of an area then you would think!) What can happen is that a current pulse can open-circuit this path like a fuse blowing. Immediately, another area breaks down and the current flow continues. In applications such as motor control, this breakdown can occur so often that a visible degeneration can occur. Left unchecked, the connector may even fail. As the power is being switched on and off 120 times a second in mid waveform, the damage level is much higher than an application where motor control is not used, therefore, connectors were failing even though they were within their rated ampacity. My observation is that in three phase distribution, this effect is going to be felt three-fold on the neutral connector.

Now, I'm going home and getting some sleep. My apologies if I have hurt anybodies feelings.

Goodnight!

 

[David Ashton]

I'm glad we cleared that up, but could I suggest that you skewed the result with your 1 ohm resistor in the neutral line.I have a 4 meter set which I use to show students neutral flow in dimmers and I have never had a situation where I could not demonstrate a 100+% neutral current, so I think that resistor acted as a current limiter.

 

[JD]

I'm glad we cleared that up, but could I suggest that you skewed the result with your 1 ohm resistor in the neutral line.

Sleep is overrated anyway!

I think you are correct about the jig. At 8 amps, the jig would be dropping 8 volts during a time when the load was seeing a "rms equivalent" of 40 volts. That represents 20% being taken out of the equation. (hey, it was 4 am) Also, the cap circuit was calibrated using a straight load. The more complex waveform may be "thermally" correct, but may have introduced its own error. What I failed to say above, which is interesting, is that when the scope was telling me there was an 8 amp flow, the amprobe (Which is balanced for a true sign waveform) was reading 5 amps. As the waveform was anything but sinusoidal, I trust the scope read. As all my historic tests were done with the amprobe, what the meter was saying was different than what was happening.

One other factor could actually lead me to believe that neutral current could actually reach 300% in a very specific setting- Cold lamp inrush is much higher than operating current. (that's why most dimmers have a pre-heat setting) Here is the mental experiment (theory only): All lamps are cold. (no preheat) With blackout engaged, set all dimmer channels to 15%. Punch off the blackout. All dimmers will draw full inrush current but limited to the last 20% of their phase angle, meaning there is no overlap of waveforms. In this case, each phase will require the neutral while the others are off, so the neutral will be doing 300% of the work of any given hot, and for ever so brief a time, at a near full load capacities! (Not sure if that one would have any effect on the real world.)

Our industry is very small, so we don't get too much in the way of research papers! If you want to get a real eye-full of stuff that is relevant to our industry, you will find it in a very unlikely place- Diesel Locomotive Design! For those that don't know, a Diesel locomotive is actually a large diesel generator mounted on a flatbed rail car, with large electric motors powering the wheels. Guess what's between the two? ! They had to deal with all these issues long ago!

 

[David Ashton]

Oh dear you mentioned the pre-heat word, in my opinion the pre-heat is the biggest hoax perpetrated by the dimmer manufacturers on a gullible public.A lamp filament is designed to get very hot, very quickly and cool down very quickly, if it didn't do this it would be a pretty useless.Within a few seconds of being turned off its resistance is back to its cold level. I hear of elaborate pre-heat rituals taking up to half an hour.What does a pre-heat do?1 - it wastes a lot of power. 2 - it burns all the lamps below the quartz ion cycle causing blackening and reduced lamp life.In practice there is always a little leakage through the dimmers anyway, so a very mild heating is unavoidable, but pre-heating in general is a con.

 

[JD]

Oh dear you mentioned the pre-heat word, in my opinion the pre-heat is the biggest hoax perpetrated by the dimmer manufacturers on a gullible public.

I more or less agree with that point of view.
As quoted from:
http://www.answers.com/topic/lamp-inrush-current?cat=technology
"The initial surge of current when an incandescent filament lamp is turned on; may be as much as 50 times the rated current and may last several tenths of a second for high-wattage lamps."

Don't know if it is that high as I have never run the study, but a 5% preheat will save a lot of Triacs and SCR's. (that's why the manufacturers are SO big on it, Warranty!) Filament rupture also tends to happen at cold inrush for the same reason as a fuse blows during maximum current draw. Some very large lamps will downright blow out the first time you turn them on if no preheat precaution is taken. (see other threads on this forum.) We are talking 5k and 10k lamps. (Most often, the resistance of the feeder cable is enough to "soft start" them. If the above is to be believed, the inrush on a 10k, 120 volt lamp would be over 4,000 amps! (50 X 83 amps))

I think a huge amount of electricity is wasted by theater systems that are left idling. (And much more on architectural systems!) It is a hot topic with a lot of pros and cons. The bottom line is if you are not using the system, turn it off. In the old days with analog dimmers (EDI, TTI, Etc.), most idle sets were made from the board. Turn the board off and the system goes to 0. Now, with digital dimmers, most have a pre-heat setting right on the pack. Bad. Unless you cut your mains you lamps are still idling when you go home.

Overdoing the preheat is also useless. It is a simple waste of power, and visible preheat will indeed damage the lamps. Generally, a minute of preheat prior to using the system is all that is needed, and there are no additional benefits to doing it any longer. As it is kind of off topic, I will leave it at that. Just a personal choice.

 

[SHARYNF]

Since we have world class experts here, instead of starting another thread, can I raise another issue:

The issue is neutral and grounding, and the potential problems there. There is belief on the part of a lot of people that the standard, plug in tester with the leds will accurately tell you if you have a problem with grounding on a an outlet. In my experience, and I am open to correction, this is not the case, and I have seen situations were infact the neutral was connected to the ground connection in the outlet, and the ground was connected to the neutral, and also situations where the neutral and ground were sorted together, and situations where the neutral and ground were joined at the switch and NOT remaining isolated until they are tied together at the service point.

The other situation is when in an older facility like a bar where you run into the connection being an old Range 3 pin connection.

Any comments and corrections are more than welcome

Why I think all these discussions are excellent is that some of us work in a set facility, but many times run into a local electrical contractor that is NOT familiar with this stuff, OR soem of us also travel with road shows in Hotels and other "venues" and again run into all sorts of situations.

Sharyn

 

[SteveB]

Since we have world class experts here, instead of starting another thread, can I raise another issue:
The issue is neutral and grounding, and the potential problems there. There is belief on the part of a lot of people that the standard, plug in tester with the leds will accurately tell you if you have a problem with grounding on a an outlet. In my experience, and I am open to correction, this is not the case, and I have seen situations were infact the neutral was connected to the ground connection in the outlet, and the ground was connected to the neutral, and also situations where the neutral and ground were sorted together, and situations where the neutral and ground were joined at the switch and NOT remaining isolated until they are tied together at the service point.
The other situation is when in an older facility like a bar where you run into the connection being an old Range 3 pin connection.
Any comments and corrections are more than welcome
Why I think all these discussions are excellent is that some of us work in a set facility, but many times run into a local electrical contractor that is NOT familiar with this stuff, OR soem of us also travel with road shows in Hotels and other "venues" and again run into all sorts of situations.
Sharyn

You are correct in that an off-the-shelf circuit tester that actually and typically uses neon lamps, either for a standard Edison U ground receptacle, or the 2P&G versions (All USA stuff here) will not indicate when the neutral is bonded to ground at a location other then service entrance. No way such a simple device can.
The only method I am aware of is to measure voltage potential between neutral and ground to determine of there's stray current where there shouldn't be.
Then it's a matter of actually physically checking at each and every connection point to see if there's bonding when there shouldn't be. Not a great solution and perhaps others with more knowledge can chime in with a better method.
FWIW and to those reading who are wondering how a condition as Sharynf has described can occur, typically it's a case of a new circuit breaker panel being installed that becomes the new service entrance. Often it's an upgrade of total feeder capacity. Often, the original SE panel still exists, only now it's a distribution panel, not SE. It's not uncommon for the installer to forget to lift the bond of the neutral to casing at the old panel.
Another example is a new panel installed that can do dual use, distribution or SE. Often the neutral buss is bonded to casing (for SE use) and it's a simple matter of lifting the bonding wire, which I have also seen installers forget to do.
In any event, the effect is the same, with neutral no longer bonded solely at SE, any current in the neutral can find it's way back up the ground, rather then finding the path of least resistance, which would be earth ground at the SE, if it were connected correctly.
Steve B.

 

[JD]

Finding the bond is very problematic and a pocket tester with lights will not give you the answer. We all know that the only point bonding should occur is at the service distribution panel. A single point ground. From there, it is a "star" topology. No bond should occur at any sub panels, and surely not at the receptacle! (Range plugs come to mind.) The problem is how to test for it. As most of the conductors are of large gauge, a multimeter is not going to give you a good read. Unless there is a load on the system, there will be no voltage difference. If you connect a load to the system, some drop will occur across the neutral. Now with a load you might be picking up a volt or two on the meter. But are you reading a drop that indicates the lines are separate to the service panel? Or, is the drop only on the lines from the last distribution panel that may have an improper bonding? There is no easy way to figure this out. If you can find a local cold water pipe you may get an answer. In theory, service entrance bonding is done between earth, Neutral, telephone, plumbing and other services. So, you take your meter and look for a any voltage between your local plumbing ground source, and your local electrical ground. There should be nothing! BUT, if the building uses plastic supply components, that goes out the window. (Most commercial building use copper. Older building may use iron.) So, we end up back at square one. If the building has a steel structure, you may want to take a safety read there as well. That may however not give you a good read due to creep current or RF pickup. (Remember, a big loop made out of anything is also known as an antenna.)

My life was spent mainly on the road. I could tell you some pretty wild stories. When you walk into a venue, trust nothing. Let your eyes explore. Use the pocket tester as a starting point, but not the final word! I once ran into wall outlet where the neutral blade and ground were at hot potential and the hot blade was at neutral. The bar was wired with 12/2 no ground. The wires were swapped. By the way, the pocket checker gave it a clean bill of health!


(ac has no polarity. pocket testers only look for the potential between the three points.)