Lighting Phase Cancelations & Amps

[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.

You are right about delta. In my fervor to correct errors about neutral overcurrent, I made a mis-statement about delta voltages int he US.

240V delta is a valid configuration, and is the one that is used for a center-tapped or "high leg" delta.

However, it is almost never found in the theatre or arena.

I should have said "240V delta is rarely found in the theatre" rather than "typical delta phase-to phase voltage in the US is 208V or 480V, never 240V"

Apologies.

ST

 

[STEVETERRY]

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!

Cool!

I'm glad the experiment worked!

ST

 

[SteveB]

Unless there is a load on the system, there will be no voltage difference. )

There's almost always some sort of load on a system, be it exit lights, HVAC systems, computers in an office, sump pumps, electric hot water heaters, etc... they all add up and may be inducing voltage on the neutral that gets carried back up the ground in panels with improper internal bonding.

This is why it's a ***** to troubleshoot. Best method, if you work in a building, is to attempt to be aware of any changes/add-ons to a system so as to watch for developing problems. Next best thing is to find the person that has worked the building for 20 years and ask questions.

SB

 

[STEVETERRY]

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.)

Some of my most interesting road experiences involved neutral-to-ground shorts within my own touring system. They're a ***** to find, you have to disconnect the feeders, apply a continuity tester between the neutral and ground inputs to the rack, and then start disconnecting branch circuits or multicables until the short goes away. It could be anywhere, or there could be more than one!

In the 1980's, a favorite point of such a short was an inadvertently rotated flag lug in a Union 2P&G connector.

If the noise boys complain when you bring up dimmers, this is a good thing to start investigating.

ST

 

[SHARYNF]

Deferring to the experts for additional comment, but one of the points that is important regarding the discussion on neutral amp rating, etc. is that you typically don't have any breaker of fuse on the neutral, especially since if for instance the neutral fuse or breaker were to go and the hot legs remained hot, you could have a real disaster, and the rating for the fuse or breaker would need to be higher than the legs per the previous discussion, SO if you look at the conventional wisdom that the breakers or fuses are there to really protect the wiring from a dangerous situation due to overload, and at this point we don't AFAIK have any effective way to safely protect the neutral, there is a real danger with undersized neutrals, both from the typical overheating, melting etc, but even if the neutral connection were simply to fail, the resulting situation where you had the hots connected and the neutral disconnected would be very dangerous

Sharyn

 

[STEVETERRY]

Deferring to the experts for additional comment, but one of the points that is important regarding the discussion on neutral amp rating, etc. is that you typically don't have any breaker of fuse on the neutral, especially since if for instance the neutral fuse or breaker were to go and the hot legs remained hot, you could have a real disaster, and the rating for the fuse or breaker would need to be higher than the legs per the previous discussion, SO if you look at the conventional wisdom that the breakers or fuses are there to really protect the wiring from a dangerous situation due to overload, and at this point we don't AFAIK have any effective way to safely protect the neutral, there is a real danger with undersized neutrals, both from the typical overheating, melting etc, but even if the neutral connection were simply to fail, the resulting situation where you had the hots connected and the neutral disconnected would be very dangerous
Sharyn

You are absolutely correct. Failed transformers and burned out neutral conductors are both artifacts of non-linear loads. An open neutral on a 3-phase four wire feed is indeed a dangerous situation.

That's why the NEC explicitly states that neutrals shall be considered current-carrying conductors for the purpose of conduit fill and ampacity de-rating when they are feeding non-lindear loads like SCR dimmers. It also states that the design of the electrical system must take into account the effects of non-linear loads, if they are present.

ST