Audio/Visual Systems Power Supply

[rcallen]

I have had several AV companies request that their systems be supplied from a single phase "to reduce noise". When I ask them for technical backup for such a request they can only say that this is common practice in their industry, or they refer me to papers that discuss single-point-grounding.

Has anyone else come across this "one phase" design criterion? Do you have any comments regarding it, where it came from and why? The NEC (Article 647.3) refers to a special transformer with a single phase secondary rated 120 volt line-to-line and 60 volts line to neutral (i.e., center-tapped 120 volt coil) but I can find no one who manufactures this as a standard. Cutler-Hammer is willing to make it as a special at 2-3 times the cost of a normal transformer, so it obviously is not being used very often.

I am coming to the conclusion that we are really seeing an issue with garden variety electrical noise due to multiple bonding of neutral to earth within a building, but I would like to know if anyone else has faced this issue and what your comments and experience might suggest to me.

 

[ron]

I've heard this from CCTV vendors when they have 120V cameras. It seems that the line to low voltage conversion is either still AC or uses a fluctuating DC and then does synchronization at the switcher with that.

 

[dereckbc]

Not sure where you get the term "Single Phase" from as that applies to common single phase electrical service supplied to just about every household in the US.

I believe you meant Balanced Power which there are many manufacures like http://www.equitech.com/ or a dedicated circuit. You quote of 647 is inaccurate 120/60 volt system has no neutral, just two phase conductors and an equipment ground. Equipment does not know the difference since there is 120 volts line-to-line. Only problem you might encounter using Balance Power is some equipment manufactures install line conditioning like MOV's between N-G on the input for FCC purposes.

I have worked with this quite a bit if you have a specific question. Equi-Tech has quite a bit of background, I suggest you start there.

 

[edamico11]

rcallen

rcallen

I work primarily with portable, broadcast, Audio and Video gear. Go across the country doing shoots, concerts, TV shows etc. My uppermost concern is that I have no strange hum, fuzz, waves etc on my projection screens and that my record, audio, program is spot on, no buzz low frequency hum etc. I can tell you this: dereckbc is correct what he says about a balanced signal, and in a studio, home and theater installations this would be a good thing to research. However if you are doing the garden variety home, hotel or small business my advice to you is simple. Make sure that all circuits dedicated for any A/V system have 0volts between neutral and ground at full load. This is the most important thing (IMO). If I have, even 1volt, between neutral and ground there can, and usually are, problems. Now, insuring this can be difficult, careful design is a must but if you are in charge of design, planning for it right from the beginning should not be terrible. You started hitting the nail on the head with issues: multiple bonding of neutral to earth, you can add in,, lighting circuits mixed with receptacle circuits, loose neutrals, incorrect wiring methods, poor grounding at panels, etc, etc.

Bottom line: 0v between neutral and ground at full load = no issues.

My 2cents.........

 

[don_resqcapt19]

Make sure that all circuits dedicated for any A/V system have 0volts between neutral and ground at full load. This is the most important thing (IMO). If I have, even 1volt, between neutral and ground there can, and usually are, problems.

The only place there is "zero" volts between the grounded conductor and the grounding conductor is at the service panel where the main bonding jumper is installed. At all other locations there will be a voltage between neutral and the grounding conductor that is equal to the voltage drop on the neutral. There is no way to make this voltage go away.
Don

 

[jtester]

I have had several AV companies request that their systems be supplied from a single phase "to reduce noise". When I ask them for technical backup for such a request they can only say that this is common practice in their industry, or they refer me to papers that discuss single-point-grounding.

Has anyone else come across this "one phase" design criterion? Do you have any comments regarding it, where it came from and why? .

I agree, it is a commonly asked for item from audio guys. I have designed a couple of churches that have extensive audio systems by two or three local audio companies. They always ask that the stage outlets and the booth outlets come off the same phase, e.g. Phase A in a 120/208 system.

I don't know why it works, but it makes these guys happy, and if I didn't specify it that way and there was a problem I'd be blamed whether or not it really works.

Jim T

 

[dereckbc]

Bottom line: 0v between neutral and ground at full load = no issues.

That is impossible to achieve, becuase of the I*R drop along the length of the neutral conductor. Even the tighest desongs allow for 2 volts, and that can only be done by either very short runs, or over-sizing the circuit conductors

 

[edamico11]

In defense of

In defense of

0volts between ground and neutral I submit this:

1) I work in a unique situation agreed. However I can tell you this. When I do work from Venue to Venue, the generator truck gives lighting it's transformer and me (Audio,Video) my transformer. When show goes into rehearsal, lights go, camera's, LED walls, Plasma's, video projection record etc. I read all phases and Neutral to Ground. I have received 0Volts no question. There are times I do not,, but it has been achieved. 2) Recently I built a home, during the normal course of the day, (washing machine on, kids on computer, lights full up and read 0 Volts between ground and neutral, it can be done. Conversely, I just did a re-design of a CATV system for a private hospital. The hospital provided the power and we could not achieve 0Volts, for the reasons I believe you guys are saying. But derecbc even you say it can be achieved, (in a commercial setting) by short runs or over-sizing. I think if we were to start from scratch, (even just power wise) it could be achieved, certainly not unrealistic. I guess you could also just isolate system with line isolators, point is you do not need a "single phase secondary rated 120 volt line-to-line and 60 volts line to neutral (i.e., center-tapped 120 volt coil".

 

[tom baker]

The technical power systems virtually eliminate hum and noise. They have been allowed in the NEC since the 1999 version, and were developed in LA with input from the inspectors. However, the 2002 NEC restricted their use to certain locations.

 

[brian john]

EDAM:

I trouble shoot noise, hum issues all the time for major broadcasters, high end churches, Imax and on and on one.

If you have 0 volts between neutral and ground at the load, you have a problem i.e. ground short on the neutral at the load, your meter is broke or you missing something.

Think about it this way, if you have a 20 amp 2-wire circuit (phase/hot and neutral/grounded conductor and Equipment Ground Conductor EGC) a 100 feet long, there is 10 amps on the phase conductor, there has to be 10 amps on the neutral. Due to the length of the conductors and the current on the circuit, there is voltage drop in the phase conductor; correspondingly there is voltage drop in the neutral conductor. Both voltage drops can be calculated and measured with a digital multimeter. Measure voltage at the circuit breaker and measure voltage at the load then use simple math or measured across the conductors, from line to load. So if the voltage drop across the neutral conductor is 1.5 volts (I did not do the math so bear with me), and the circuit is properly wired per the NEC, the EGC will not have this voltage drop, there is no load on the EGC. So at the service or separately derived source the neutral and ground are connected and the measured voltage should be “0” (depending on where you measure, amount of current and resistance of connections). But at the load end there is the voltage drop to account for, so the measured voltage between the neutral and EGC will be close to the same as the voltage drop in the neutral conductor, assuming little or no leakage current.

So can you read zero volts between the neutral and ECG, no but you cam minimize the voltage, change the circuit impedance. Shorter conductors between the service and separately derived source and/or larger conductors. I have read were using single phase branch circuits will neutral ground voltage issues, but if you think about it with a multi-wire branch circuits if the phase currents are close to balanced the neutral current should be minimal, resulting in less voltage drop between the neutral and EGC. I have never tried to prove or disprove this, just a thought.


I have seen specs for all loads on the same phase, and when the systems were installed correctly per the NEC (circuits on multiple phases) with no wiring errors the systems worked fine. But, the wiring was changed to meet the manufactures specifications, which complied with the NEC.

The fact is IMO most of the hoodoo voodoo many manufactures spec. work, but in many cases it works or was designed to get around errors in the wiring system. If the wiring had been correctly installed this hoodoo voodoo would not be required. Just my experience and opinion.

 

[don_resqcapt19]

edamico11,

Recently I built a home, during the normal course of the day, (washing machine on, kids on computer, lights full up and read 0 Volts between ground and neutral, it can be done.

Where exacty do you read this "zero" volts?
Don

 

[brian john]

http://www.fluke.com/download/library/1989076_w.pdf

 

[Pierre C Belarge]

Brian
Your post (#10) is very well written and gets the point across for all to understand.

You and Dereck always help to make these topics easier to digest.

Thanks!!!

 

[dbuckley]

I have had several AV companies request that their systems be supplied from a single phase "to reduce noise".

Is this for a fixed installation?

There are AV sites that are wired single phase, most of the older BBC facilities in the UK, for example, the logic being that video signals are unbalanced and thus subject to hum pickup from currents in the screens of the cables, so if the installation is single phase and the TV signal is syncronised to the video, as it was once, then you dont get moving or multiple hum bars.

 

[edamico11]

don_resqcapt19

don_resqcapt19

Where exactly do you read this "zero" volts

.

At home I can read O volts between neutral and ground at any receptacle. Instrument used WaveTek DM23XT. Set for AC voltage and set for 2 volts. Should I be using another method?

Brian:

I agree with your post, however something is still not making sense to me. I am setting up a show in my shop today. 10 dedicated 20 amp 110volt circuits and 5 240 volt circuits. These circuits are coming from a PD 3 phase 5 wire. I will have mixed components, (some 110volt some 240 volt) I am going to read neutral to ground back at the PD with the system under full load. I am also going to look at the audio wave through a spectrum analyzer, out of the record decks. The PD run to the equipment will be 100'. I will attempt to induce hum, buzz etc after I attain a "clean" signal. Will let you know results.

ed

 

[don_resqcapt19]

Ed,

At home I can read O volts between neutral and ground at any receptacle. Instrument used WaveTek DM23XT. Set for AC voltage and set for 2 volts. Should I be using another method?

If you are reading "0" volts at the receptacle between the grounded and grounding conductor, then there is no load current flowing on the grounded conductor between the receptacle and the main bonding jumper. If there is any current, then your meter will read the voltage drop on the grounded conductor. If you are reading "0" volts with a load, then you have a "bootleg" ground. That is where the grounded conductor is connected to both the green and white screws at the receptacle. There is no possible way to have "0" volts if there is any load. The voltage drop should be small, but there will be one.

I am going to read neutral to ground back at the PD with the system under full load.

Assuming that the PD is a seperatly derived system, then there will be a system bonding jumper at the PD and there should be no voltage between the grounded and grounding conductors at this point, because there is no voltage drop at this point. You will read voltage between the grounded and grounding conductors at the load 100' away. Again, if you don't you have some type of bootleg ground which is a safety hazard and code violation as well as one of the major causes of the problems that you are trying to avoid.
don

 

[dereckbc]

.

At home I can read O volts between neutral and ground at any receptacle. Instrument used WaveTek DM23XT. Set for AC voltage and set for 2 volts. Should I be using another method?

Agree 101% with Don. If you are reading 0 then there is either no load or an improperly wired circuit. There is simply no way around having some sort of voltage drop between N-G on a properly wired circuit with load. Try the same test except put something high wattage on the receptacle like an iron or blow dryer. Then you should anywhere from 1 to 5 volts.

For example lets take a code compliant 20-amp circuit, using # 12 AWG copper wire, 100-feet in lengrh, with 10-amps of load. You would measure 2-volts RMS between N-G at the receptacle, and 116-volts between L-N assuming 120-volt supply at the panel.

If you decide to upsize the copper wire to say a #10 AWG, then you would have 1.25-volts between N-G, and 117 between L-N.

You are dealing with Ohm's Law and there is no way to violate the law of I*R...

 

[edamico11]

To my friend Mr.OHM

To my friend Mr.OHM

I am somewhat pressed for time right now, since on am on a show site. Howerver I just wanted to respond in humble appreciation of all the posts. dereckbc and don_resqcapt19 are absolutely correct. I have turned my attention fully to the test at hand, and when we set up the proper perameters, walla voltage down the neutral. Another mystery solved..



Thanks again

ed

 

[brian john]

Several years ago I read an article (Durn I wish I could put my hands on it, I usually keep stuff like this). A test was set up that resulted in 15 volts between neutral and ground, as I remember several types of electronic equipment were utilized as part of the test. With a properly installed branch circuit (with exception of the neutral to ground voltage) all equipment tested operated without error.

 

[mxslick]

For cinema sound racks..

For cinema sound racks..

I have always run power from more than one phase or leg if you prefer and never had any problems, as long as a few conditions were observed:

• Run dedicated neutrals for each circuit used for the audio equipment;
• Connect ALL of the "source" (before the power amps) equipment to the same phase/leg;
• Try to distribute the amplifier loads equally amongst the phases/legs; and
• Observe good audio wiring techniques, especially use of "telescoping" shields.

Isolated grounds are not necessary in most applications as long as you use good audio wiring practice and the EGC wiring is properly sized and bonded as per Code.

The dedicated neutrals are a must. I know that some will say they're not necessary, but 20+ years of experience and over 700 screens wired tell me otherwise. I will not try to argue the physics of why, other than to state that it avoids any possibility, no matter how slight, of harmonics getting into the rack.

Telescoping shields means that the shield of the audio line is connected at one end only. In my specific application, that means from the Cinema Processor (source) to the power amps, the shields are connected at the amps end only. Most of the lines into the Cinema processor are connected shields at the Cinema processor end only.

For most other audio installations, try connecting the shields at the switcher or processor end only first. You may have to reverse and connect at the source equipment end only, or sometimes even at both ends.

I think it was already mentioned here, but bears repeating:

Do not, ever, lift the equipment's power cord/line ground!!

You're not solving the hum problem that way, just creating a potentially fatal hazard!