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Author Topic: Power and Ground - more than you want to know - in 3 parts  (Read 7220 times)

John Klett

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Power and Ground - more than you want to know - in 3 parts
« on: April 23, 2004, 01:26:28 PM »

Okay...  THIS IS A MONSTER!!!!             IN THREE PARTS

Here is a topical thread...  one of several...  from the recpit.  I pulled it out and gave it a light edit to present here.  I'll do a more through review, boil out some of the repetition and condense this into an article...  MAYBE (because time is a limited commodity in my life as it is with most of you I am sure).

As with all my posts…  the things I suggest are grounded on simple practical empirical experience backed with as much real physics and technical grounding as I can muster.  Now – the world is not perfect.  Everything in my experience will follow the “laws” of physics but theory and practice do not always line right up…  why?   Because in practice there are lots of variables, things that are coming in from outside your theoretical model that you are trying to apply in a real world implementation… for example…  it is really REALLY difficult to avoid all ground loops in a real world studio system – my solution is minimize them and make all the ground connections – including remaining loops as solid, as low impedance and as small in cross sectional area as practical.  Another item…  there is no perfect ground…  there is no wire we can lay our under funded hands on that is superconducting at room temperature.  So…  we have to avoid the use of ground as a landfill for noise, stray line current leakage from partially blown MOV’s in surge protectors that protect one time and then, often, leak to ground, audio currents driven to ground when active balance outputs are connected to unbalanced inputs (transformers on outputs or made available on patch as needed are a good thing in my book)…  and so on…  all this can be discussed in this R/E/P forum as time goes by.

Give this a read - it may not have a damned thing to do with your particular noise, power, ground problem but as power will certainly come up...  this does cover a lot, it will answer someone's questions and inspire many more…  and may cause some debate, which, if kept civil and to the point, are fine.  Find two techs who agree on everything and you have found an anomaly.

NOTE: the following is centered around permanent installed power systems for music and post studios.  Portable Road systems have other sets of issues and I don't deal with those day to day.

NOTE:  IMPORTANT – POWER SYSTEMS HAVE VOLTAGES THAT CAN REALLY HURT OR KILL YOU SO TAKE ALL OF THIS AS INFORMATIONAL – THE FOLLOWING INFORMATION WILL NOT MAKE YOU QUALIFIED TO DO ALL THIS YOURSELF…  YOU MUST PROCEED WITH AT YOUR OWN RISK AND WITH EVERY REASONABLE PRECAUTION TO PROTECT YOURSELF AND KEEP YOUR SYSTEMS SAFE…  HIRE A LICENSED ELECTRICIAN IF YOU CAN AFFORD TO DO SO.

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shelton
Here Often

Post subject: Studio Electrical System Questions

I recently came upon your article in Mix magazine about studio electrical systems.  In it you recomend running the system off batteries and then through an isolation transformer. Can you provide me with a link to a manufacturer of this type of battery system? The only kind I can find are APC UPS types.

Thanks,

Shelton

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klett
Has No Life

Post subject: Studio Electrical System Questions

Actually, this unit is an Uninterruptible Power Supply (UPS).  There are different kinds and many manufacturers.  What you want ideally is one that is always "on line" and does not actually switch into a standby mode

so you do an "and" search on Google with these words

        double conversion UPS single phase KVA

Double conversion UPS systems are always online so there is no changeover - you run on the regulated and controlled UPS output at all times instead of only when power fails.

Double conversion means it is converting to dc and converting back to AC simultaneously

Here are links

http://www.stacoenergy.com/singlephase.html

http://www.engineeringdynamics.com/single_phase_ups.htm

maybe this link will work for MGE

http://www.mgeups.com/products/pdt120/1ph/cometex/help/resul t.php

If you are in an area where 3 phase “208Y” service is the norm you can do a three phase UPS and distribution (which works just fine in larger studios if don’t correctly and I can expand on that elsewhere) OR you can have a single phase UPS set to take 208VAC on the input and output 120VAC or 240VAC on the output.  When you do an install where the UPS is doing a phase and/or voltage conversion (from two 117VAC hot legs spread 120 degrees apart to two 115 to 120VAC hot legs 180 degrees apart (or single phase 120VAC, or balanced power with two 60VAC hot legs 180 degrees apart) you can’t bypass the UPS unless it’s with a transformer…  that takes us elsewhere so…

and... by the way...  someone was asking about transformer (autotransformer actually) lighting dimmers…

http://www.stacoenergy.com/variable_transformers.htm#Panel%2 0Mount

_________________
Klett
www.technicalaudio.com/techmecca/


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speers
Should Get Out More


Post subject: Studio Electrical System Questions

One clarification:

I looked at a few of the double-conversion units. None actually run off the batteries [as the article stated incorrectly] . If they did, battery life would be drastically shortened.  They use large capacitors, with the batteries used only for outages, as with "simpler" UPS's.

This is still, as John says, always converting to DC then back to AC, but the "battery" thing is misleading.

Perhaps there IS a model that actually uses the batteries all the time, but I haven't seen one. I would think that the capacitor thang achieves the same isolation and cleanliness.


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klett
Has No Life

Post subject: Studio Electrical System Questions

I always run the risk of oversimplifying or jamming out too much information - so...

In double conversion supplies you are not really running off the batteries all the time - the batteries are being held at full charge until needed... ...AC power comes in to UPS - gets turned to high voltage DC, filtered like a mofo (big caps), used to charge the batteries AND feed an inverter which takes it back to AC at your selected line voltage and frequency.  If the UPS does not get enough power at it's input to hold up the output then the batteries drain into the inverter and keep you going. THAT is also an oversimplification... but no matter how you describe it the desirable UPS is one that is always “on line” and does not need to "change over".  Thus you want an “on-line double conversion UPS”.

_________________
Klett
www.technicalaudio.com/techmecca/


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thedug
Should Get Out More

Post subject: Power

Doest the UPS have any effect on the "quality" of the line?

I always thought of a UPS as more of insurance to avoid intermittency.

How does this compare to ... balanced power?

Thanks.

d./

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klett
Has No Life

Post subject: Power

thedug: "Doest the UPS have any effect on the "quality" of the line?"

Not the incoming line but on the output that your equipment sees you get controlled and regulated power.  You really should view a UPS as we are applying it and a power and voltage regulator with the added feature of a battery back up that buy’s you time to save your files/projects and shut down everything in an orderly manor.  One badly timed blackout or line fluctuation can cost you more than a UPS will so it does pay for itself – It’s not really a luxury.

NOTE - you do need the look at the incoming power/neutral/ground because, as with most things, you should start with the foundation and build up from there...  if your service and technical grounds blow you can only get so far.  Those really come before the UPS.

The UPS holds the line at a constant voltage with a reasonably (for power) low distortion sine wave. When you have brown outs and other off-voltage conditions the bias in just about ALL your tube gear will change... so guitar amps will sound good one day and not the next because of line voltage changes (Joe Perry powers his back line off a large UPS for this reason).  In low voltage situations pre-regulation supply voltages drop so filtering and regulation in solid state gear can be degraded... etc.  you want stable supply voltage to your gear.  

In some older and stressed out parts of the NYC power distribution we see incoming line voltage running down to 95 volts on a hot day when ALL the air conditioners are going full blast and then pop up to 130 volts at night when it cools down and offices clear out.  It helps...  assuming you buy a decent, properly-sized, UPS of the right type.  I talked about that in some detail in the article.



HERE is some sort of fun thing... if you have an enviable budget...

This is a "carrying it all the way through" power system - more than 95% of people are willing to do.

If...if...    if a client demands solid and consistent power in a big SSL/Neve Vseries kind of studio and they want to stay up seamlessly 24/7 even if there is a severe ice storm and some guy spins off the road and knocks down a pole a mile down the road that carries the power feed to your place... and it takes five days for the utility to get out to you to fix it.... ...like that.

block description...

INCOMING LINE TO METER - prefer 240VAC single phase 200A (min) service which will be the assumed for this list... suburbs.  Three phase systems are nearly the same but there are some other things to look for that we can get in to...

MAIN SERVICE CUTOFF

MASTER BREAKER/RELAY/CONTACTOR - has a trip control input

OVER-VOLTAGE CLAMPING – giant MOV’s that will trip the main breaker if you get a big transient

MASTER GROUND STAKES – a whole topic on it’s own

CHANGEOVER FOR GENERATOR (automatic once up to speed) – An MG set of some sort feeds this – there are “on line” versions of those too that I can tell you about.

MAIN HOUSE PANEL – feeds all convenience outlets, lights, air conditional… everything not within your studio systems

Here is where house power and tech power part company

TECH BREAKER - single 2 pole breaker with alarm contacts to kill power in the event of a fire

LINE CONDITIONING / FILTERING ahead of UPS – optional – helps protect UPS

UPS - Double Conversion On Line 18-25KVA 30mins @12-15KVA load with hard bypass switch for servicing (see note above about bypass)

K-20 (high K factor or primary to secondary isolation)ISOLATION TRANSFORMER - typ 15-18KVA
208VAC/60Hz or 240VAC/60Hz primary: primary is where dirty "mechanical" ground stops
230 single phase split 115/115 secondary (which is balanced 230VAC and what most suburban house get off the pole): secondary is where clean isolated ground starts

MAIN TECH PANEL - single and 2 pole breakers

Large items like consoles, workstation racks, big analog machines and other stuff that does not move much... large power amps... they all get what is effectively balanced 230VAC via twist-lock outlets.

Outboard gear and small format machines get unbalanced 120VAC.  Outboard gear racks don't draw a lot so they are 120VAC with 0V neutral…  load the two sides of the 230 volts equally.

Optional – on some installs we’ve added some European style 220(230)VAC outlets on producer desk/racks in control rooms – this is balanced power and some Euro gear won’t like is (most will).  Contrary to what some people think – 220 is not balanced in most of Europe – not that I have seen.  I’ve done power systems there.  Anyway – some of those English stove plug strips (with fuses on both neutral and hot if there are fuses at all – those are often inside the plug) and round pin Euro outlet strips are cool to have if someone comes in with a Bel delay they just brought over and have not converted.

Optional - on two installs we added an EquiTech Wall mount system fed from a two-pole breaker in Main Tech Panel and distributed balanced 120 as well...

I won't go in to the grounding details because that has been or will be covered in other topics I’ll bring forward from the ‘pit


thedug: "I always thought of a UPS as more of insurance to avoid intermittence How does this compare to ... balanced power?"

120VAC (60/60) Balanced Power is not a big increment up from a well-done unbalanced power distribution.  It can sometimes be more trouble than it’s worth…  you have to make accommodations for gear that hates balanced power and once you connected, say a guitar amp to the standard power before your EquiTech (or whatever) you have blown your isolation.  This is why end up with the two transformer in the over the top illustration above.

Anyway… There are certainly positive aspects.  Balanced Power is not a bad thing.  It’s just not a panacea.  In addition, it does not fix incoming line problems.  I have not seen an off the shelf ferro-resonant isolation transformer that does 120 split 60/60 on the secondary (these transformers self regulate output voltage within a narrow range).  



This whole approach I just went through evolved out of the need to keep sessions from loosing a data if power drops. You have to figure that in a 9K with Recall and Ultimation (or Neve V88R w Encore - whatever) studio there is a lot of info that could be lost if the timing sucks... and the ProTools system... ANY computer or workstation, and all the other stuff that needs to save before shutting off... There is a lot of potential for data loss and more coming all the time.

Once you look at backing up your CONSOLE you may as well go the whole nine yards and do the whole thing... better anyway from a distribution point of view...

Ho-Kay?

_________________
Klett
www.technicalaudio.com/techmecca/


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thedug
Should Get Out More

Post subject: Small Install

I am currently converting my garage to a control room. I don't have the money to do all these big things.

But I would love input on the inexpensive things I can do to avoid costing a lot later to upgrade.

For instance, If I have want to run balanced power I should prolly get the outlets that don't connect neural to ground.

I am thinking that I'll install those outlets and run each one straight back to the panel so I can star ground them.

Any tips?  Any things I can't live without?

Thanks,

d./

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End of part one of three – go to next R/E/P post…………

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John Klett / Tech Mecca
http://www.technicalaudio.com

John Klett

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Re: Power and Ground - part 2
« Reply #1 on: April 23, 2004, 01:27:29 PM »


Second of three parts………….  
An R/E/P revisit of a power and ground topic from klett’s recpit forum


klett
Has No Life

Post subject: Studio Electrical System Questions

thedug: “I don't have the money to do all these big things.”

No DooDoo – it’s an illustration of all the things one can do

Most people cannot afford the full out electrical system I outlined... but I've seen bigger and even more beefy systems at facilities around...

Start with basics.

Dedicated load center (breaker panel), isolated ground buss bar, three insulated wires to every outlet box, no shared neutrals, no shared grounds, no conductor smaller than 12 AWG , stranded THHN wire

Once that is in place you can install a clean isolated ground. In the meantime you would bond the isolated ground buss bar to the panel ground. If you are on a budget use standard decent quality 20 Amp outputs (they grip a little better) that costs a buck and a half at home depot... buy the isolated outlets for six times the price later... as long as you have the three conductor wire out to the outlets (and by that I mean three insulated wire and perhaps a bare copper ground to ground the box itself) you can retrofit the IG (Isolated Ground) outlets - Make tight connections everywhere....

Unbalanced vs. Balanced...  This is not always the first move... but for that you do need isolated ground outlets... not standard outlets that tie the ground pin to the tabs.  Balanced power depends on a clean isolated ground return to the center tap of the transformer secondary... which is both an isolated ground AND what amounts to a neutral that rides on the third ground pin.   Standard outlets don’t tie the standard neutral to ground by the way.  The shared function (IG and Neutral) of the ground pin in balanced power systems confuses people.  It’s a center tap – it’s both.
_________________
Klett
www.technicalaudio.com/techmecca/


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thedug
Should Get Out More

Post subject: Studio Electrical System Questions

On 01 Mar 2004 18:11 klett wrote:

klett: “…three insulated wires to every outlet box…”

3 insulated? Do they make romex with the ground insulated?

So.. just to repeat.. the neutral would go straight to the box’s ground bar and the ground wire would go to the new ground bar. Then the new ground bar gets connected to the boxes ground bar.

When you say 20 Amp outputs, are you talking breakers or outlets?

[…clip…]

d./


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klett
Has No Life

Post subject: Studio Electrical System Questions

The ground wire has to have it's own insulation - not just in a Romex jacket but after ground is out of the outside jacket and in to the box. This would be a green insulated wire in most cases.  You can a three wire (plus ground) Romex that has a black, red, white and just wrap green tape around the red wire to re-designate it as ground... green with yellow stripe really - to designate a different ground than mechanical "dirty ground".  

I hate Romex.  Romex works but I’ve had several rodent experiences. I hate problems like this so I use BX or MC cable.  I really like a metallic enclosure around all wire so steel boxes, MC cable with three insulated 12AWG stranded THHN (white, black, green) and one solid bare copper conductor (I tie mechanical box ground down) and isolated ground (IG) outlets…

Neutral is a current carrying conductor EXCEPT in balanced power applications (see above).  In balanced power systems, the common neutral, IG, center tap conductor should be carrying no current at all (in a perfect world).  In standard power distributions neutrals in branch circuits carry current and go to their own buss.  That neutral buss bar has to be isolated from the tech power panel enclosure.  

Service entrance load centers (basically the main breaker box in your house for example) often have a common non-isolated neutral/ground buss that is connected to the panel enclosure.  This is a no no for tech power done right because you will want to install, at some point, an isolation transformer and make a new clean isolated ground and bond the tech neutral to that…  home run to a really good earth ground (and THAT is where you have to connect service ground and clean ground.. ultimately you MUST have that connection for safety reasons).

The technical panel or load center has to have an isolated neutral buss bar, an isolate IG buss bar for your clean tech power ground and a mechanical ground bus bar that is bonded to the box.  That uninsulated buss is where the uninsulated ground wires found in Romex and the MC cable I use should end up.  In fact – when you do conventional sub panels in non-technical power systems you should use an isolated (bondable) neutral buss because neutral in standard distributions really should bond to ground ONLY at the service entrance…  but codes do vary on this point.

Outlets… your standard outlet takes a three pin "Edison" plug (in the US.  Outlets should offer three pins (contacts, poles) in most world standards.  If you look at the face of a US standard “Edison” outlet the two flat slots are hot and neutral. The shorter of the two slots is the HOT.  The longer one is NEUTRAL.  The round-ish or "U" shaped hole is ground.  In standard outlets, the ground pin is connected to the metal mounting tabs on the outlet.  This takes the output to the box which, if it is a steel box fed though metal conduit, EMT, BX, MC etc. is ground, OR a ground wire takes the box and outlet to ground, as through ROMEX... which is what that bare copper wire is for.  In standard non-isolated systems the ground wire can go to the box or a green screw on the mounting tab.  That ground is called mechanical ground.

I use 20 Amp outlets fed through 12 AWG wire from breakers sized to the load.  You can use 10, 15 and 20 Amp breakers but the wiring and the outlets should be done for 20A to keep impedances lower.  A big console running on 120VAC wants at least #10 wire, a 30Amp breaker and a 30Amp twist lock outlet.  If you don’t do an isolated ground right away – then use Specification Grade or Heavy Duty 20Amp outlets with a standard pin configuration (ones with a sideways “T” shaped slot that will take standard and air conditioning plugs are fine).  These grip the plug better and make a lower impedance connection.

With the wiring and outlets as above you are set up for IG outlets and you’ll be a little cleaner anyway because of the dedicated ground return

IG Outlets have no connection from the round-ish or "U" shaped hole.  It has it’s own screw that you would connect your insulated clean ground wire to.  The mechanical ground wire then goes to the box.  You have two ground systems.  MG is mechanical or “dirty” ground and IG is isolated or “clean” ground.

Back to the panel… You can have a large panel with 40 20 Amp breakers in it  (or 42 in three phase) and yet have a master panel breaker rated at 200 Amps no problem... typically breakers are there simply to break if there is a hot to ground or hot to neutral short on a particular branch circuit. If you want them to break sooner then install a smaller breaker but you don't have to do this.  Don’t add up all the breakers and say – wow I have 40 20Amp breakers – that’s 800 Amps…  you are generally running only a few Amps (if that) down each circuit.






Isolated Ground and "IG" outlets... more detail

We want the audio gear... all of it... to see a different "cleaner" ground so an Isolated Ground IG outlet brings the Ground Pin (via round or "U" shaped hole) to it's own isolated terminal... generally through a green screw terminal. Now you have two grounds at the outlet box but Isolated Ground is the only ground the gear should ever see". The Box, conduit etc. are all connected to Mechanical Ground. Metal Racks etc. that any gear mounts in has to be isolated from mechanical ground so you have to have ground breaks to keep racks "floated" from Mechanical Ground while being "referenced" to Isolated or "Clean" Ground.  There are a number of techniques for this.

The idea is that, if you do all this right (... and PLEASE don't stick your hands inside a live electrical panel and kill or injure yourself - because you CAN... ! ...) , you would be able to FIRST! de-energize your panel totally AND THEN disconnect the main Isolated Ground feed that takes the IG buss bar to the ground stake. You should measure high impedance from the IG buss bar to the Mechanical Panel Ground (the box itself...) and high impedance from the IG buss bar to the Neutral (White Wire) buss bar. DO NOT ENERGIZE YOUR MAIN TECH PANEL UNTIL the main isolated ground wire is connected to ground and to the IG Ground Buss bar.

You master ground should, preferably, be a nice ground rod system near the main electrical service entrance but any decent ground will do just to make the system safe... a strap to the panel ground - for Safety.  Safety First.

If you don't know the basics of electrical wiring then buy a Home Depot book and ask one of the older guys in the electrical aisle for advice you have a 50/50 chance...  better yet hire a licensed electrician.

_________________
Klett
www.technicalaudio.com/techmecca/


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thedug
Should Get Out More

Post subject: Studio Electrical System Questions

klett: “The ground wire has to have it's own insulation - not just in the Romex jacket but after ground is out of the outside jacket and in to the box. This would be a green insulated wire in most cases…”

Is there still a "dirty ground?" I.E. Is the Romex 3 conductor or 4 conductor?

[…clipped…]

There is an existing box that I'll be adding breakers too. There is also an existing rod ground. So I think I am ok there. I just need to have a look at the ground bar and how to strap to the panel ground. What gauge should I use for this?


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klett
Has No Life

Post subject: Studio Electrical System Questions

If you are simply taking the IG buss to the box for a safety ground you are using a 10AWG wire to connect the IG buss bar to the MG buss bar…  If you have your ground stake done and are running a dedicated clean ground to that I size that the same as the hot and neutral feeds to the box.  Really – the service ground and master clean ground stake system should be the same.  THAT is the lowest impedance to ground.  In NYC we can’t do ground stakes so we have to go to building steel (not my first choice) or to the main cold water pipe right as it comes in to the building.  This is most often where the utility bonds to ground.  In city buildings you have to look at what the utility is bonding to.

The reason the service ground and the Isolated clean ground stake and bonds have to be the same or have to be joined is for safety.  I learned this by seeing the aftermath of a lightning hit on a studio in a building where the service entrance ground was at one end of the building and the ground rods were pretty far from that…  no hard wire connection.  When lightening hits nearby you can have a impulse potential running 10,000 to 100,000 volts per meter through the earth.  In this case I guess there was over 100,000 volts present between mechanical ground and the isolated ground…  not for long but that was enough to blow up just about everything in the place.  If you have two different grounds for service and for IG then a nice solid (non-stranded) bare copper wire buried in the ground outside or running around the foundation line should be installed (outside is good – inside I am not so comfortable with).  This wire can be 10AWG or heavier…  #6…

Most of the Romex people buy for home use is really two insulated conductors plus a copper ground - there are three wires but one is a bare ground wire... in electrician talk... yes it is a conductor - it's a piece of wire - but no it is not a conductor because the ground wire is not conducting current unless there is a fault and then a breaker should trip - so it is not conducting.

If you are wiring for isolated ground you need three insulated wires so you buy three conductor Romex plastic.  This would be a red, a black and a white – you tape the red over with green and use that for ground…  whatever you use a wire for you have to keep the colors right or down the road it will all become a big confusing mess so have a roll of green tape and re-designate function by changing the color of the wire such that people say…  hey – this red wire is now a green ground wire – I get it.


End of part two of three – go to next R/E/P post…………

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http://www.technicalaudio.com

John Klett

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Re: Power and Ground - part 3
« Reply #2 on: April 23, 2004, 01:28:31 PM »

Third and, thankfully, last of three parts………….  
An R/E/P revisit of a power and ground topic from klett’s recpit forum



REPEATING THE IG THEME (sorry – but I repeat things in an attempt to say it as many ways and enough times to burn it in)

You will use one of the insulated conductors to carry isolated ground.  The bare wire ground takes the box and faceplate of the outlet to ground - mechanical ground...  The Insulated Isolated Ground is treated exactly as if it were a conductor in that it has to be isolated from mechanical ground and neutral and hot and stay that way all the way to the main ground stake where it does hit ground same as mechanical but only at that point alone.

There are two ground connections on an IG outlet. ONE connects to the mechanical ground via the mounting tabs on the outlet and the OTHER connects to the ground pin in the outlet. The mechanical ground does not appear on any of the blade connections in the outlet - just the bracket... so gear never see's it.

Metal racks and furniture that holds equipment on rack rails etc., can not connect to mechanical ground... it will be grounded eventually but only via the clean ground path... so yes, racks and equipment chassis have to be kept from making any metallic contact with mechanical ground connected things like faceplates of electrical outlets, baseboard heating, air conditioning ducts, electrical conduits, water pipes and so on.... including metal racks sitting right on a concrete floor... you need a wooden riser or rubber feet.

The isolation test assures you have no stray connections between mechanical "dirty" ground and isolated "clean" ground... you'll still have ground loops in your clean audio system simply because 99% of the studios that exist today are not 100% transformer balanced i/o and almost always have some piece of unbalanced gear somewhere... but what ground loops do exist should be all low impedance to earth ground, fairly small in cross section, be low current and have very low voltage drops around them because everything is connected via a fairly solid home runs to a lower impedance ground than any stray grounds will have... In larger studios with really quiet consoles like 9K's we add dedicated chassis grounds to gear and tie them together within racks and tie the racks to IG.

For an existing box you need to have enough room to have an isolated ground buss bar... If the box you intend to put breakers into is a long way from the place where the various branch outlets are going to go you'd be much better off doing a sub panel that you organize your technical power into rather than mixing technical and non-technical in one box... for one thing you have to have a dedicated panel if you are ever going to do balanced power or an isolated ground

Generally the smallest wire we use for primary ground bonding is #6 but, again, for larger facilities with many interconnection and very low noise floor large format consoles we use heavier wire to make the impedance as low as possible so #00 is not uncommon. #6 is fine for small studios or size it to match the feed to you tech panel if those are larger

If you need to know more... READ...

buy this aes issue... it's $15 to non aes members and well worth it.

SHIELDS AND GROUNDS: SAFETY, POWER MAINS, STUDIO, CABLE AND EQUIPMENT, (special excerpt) The June 1995 issue of the Journal was a definitive and comprehensive collection of information on this important topic. The seven papers by Neil Muncy and other experts in the field have been reprinted into a convenient guide for designers and practitioners.

http://www.aes.org/publications/journal_issues.cfm

_________________
Klett
www.technicalaudio.com/techmecca/


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oudplayer
Has No Life

Post subject: Studio Electrical System Questions

Thanks so much for your thorough and detailed posts, John. I for one really appreciate your commitment towards teaching those of use who... have not quite got the ins and out of professional studio electrical yet!

My (hopefully simple) small follow-up question involves pieces of gear, which are balanced yet use wall-wart power supplies (the Grace 101 preamp, for example) that are 2-prong, more specifically gear that doesn't have a ground lug on the back. How should (or should?) these units get connected to the isolated ground you describe? Do these fall under the category of non-balanced gear you mentioned in the previous post?

Thanks in advance,

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thedug
Should Get Out More

Post subject: Studio Electrical System Questions

I think I know this one!

They are grounded through the chassis. Which is why you don't want the rack connected to mechanical ground. They will get clean ground through the other gear via the chassis and rack.

d./

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klett
Has No Life

Post subject: Studio Electrical System Questions

That’s it – that’s the general idea

And - forgive any lack of clarity - I have not completed my first quad shot mocha latte´ and I have to rush this out so I can get back to fixing BA boards (just about the least fun thing I can imagine doing outside of wiring). The following may or may not make sense... let's see how it goes...

Since we are talking about grounding and electrical, opening up gear and so on... something I should have said up front [did but it is worth repeating].  You have to proceed at your own risk... If you break something or hurt yourself because you acted on something you read here it's on you... these commentaries are by necessity generalized and are not to be considered as sufficient training to take someone who doesn't already have some grip on electricity, electronic theory and practice and make them 'qualified' to poke around in live gear... take all this as an article in theory. You are on you own as far as practical application. YOU HAVE BEEN WARNED!



In the situation you mentioned you don't treat the signal i/o differently.

You are right…   In most cases the chassis of a piece of gear with a two prong (no ground pin) power cord or a piece running off a wall wart will still see ground when it is screwed into a rack with other grounded gear or if a sleeve or "pin 1" connection is made to ground (this would be via a shield and not what I would recommend). Sometimes the external case or chassis is all plastic or otherwise completely isolated from what is going on inside... there are always special cases.

We try to standardize grounding references within, and to, all the gear.  It's a bit of a process because gear is varied.  We have to handle each individual piece.  Generally there are some isolation tests that you can do to see how XLR pin one and 1/4" TRS sleeve connections are dealt with relative to chassis, the ground pin on a power cord and other connections audio i/o and electrical power hot and neutral. This is an automatic when we do balanced power systems.

In hard-core installations where we are working toward getting the lowest system noise floor we throw a chassis ground lug on to the back of every piece of gear that does not already have one.  To that we pull the signal zero, chassis ground and green wire to electrical plug ground pin... well over 90% of the gear we find in studios do not have a ground lug like this.  Signal zero, chassis ground and the electrical plug ground pin have to hit ground sometime and they all have to end up at the same ground potential.  We make sure they all really do that via the ground lug.  Even if the lug is not taken anywhere we have at least made sure that audio zero and chassis are not different from each other and that they get a connection to (isolated) ground via the power cord (at least) assuming there is a ground pin on it.

We often take older vintage gear that do not have three wire electrical cords and knock a hole in the chassis for an IEC inlet so a three-wire power cord can be used.

There are a few items that cannot be handled this way (ground lugged as above) - and within a control room, these are few.  Anything with a "hot" chassis has to be treated differently.  Hot chassis gear is gear that has the chassis connected to Neutral (or Hot if the plug is flipped over) either from design or because it is broken somehow OR has the chassis as a power common at 120VAC,  some old tape machines for example. There are only a couple things that you might expect to see this in - some old guitar amps for one... some old Ampex decks have issues related to this (OLD! like 300's etc.).

You can't hard connect neutral to your isolated ground anywhere in your system... They do eventually come together but this is not supposed to happen at multiple places inadvertently around the studio – one place – where neutral and IG come together at the main tech box or at the technical earth stakes.   This is one reason why non-isolating D.I. boxes can be very problematic - like a metal box with non-isolated 1/4" thru jacks on the input side and an XLR with pin 1 hard tied to the chassis... or even if there is a pin 1 ground lift switch and you have a mic cable that has a jumper in the female XLR between pin 1 and the XLR chassis (the old Gotham cables we used to get with Neumann mics are like this - good for making sure the case of the mic is taken to ground but BAD for use with some direct boxes)... If you have a hot chassis item you have to set that aside and deal with it as a special case latter - I won't get in to any further here.

We always measure impedances from hot and neutral to ground... there are a number of "buzz" tests one can do... that AES Journal I mentioned has an article on this very thing and we do pretty much the same things plus some... . You'll find hot chassis connections and other problems this way.

Within a rack the ground lugs on the individual gear are taken to a small ground buss bar bolted to the inside of the rack. A ground wire for each rack goes to a central ground buss or plate that we often mount near the patch since all the audio wiring goes there anyway.  If the rack is metal, it is grounded to IG. The rack and all the gear in it are treated as a single piece of equipment this way. The central ground plate is taken to the isolated ground buss bar in the local tech distribution panel and from there to the main tech panel (multi-room facilities have local panels next to each control room, often in the machine area that are feed from facility power distribution panel boards or panels)

This system (lugs and ground wires to a ground plate) is a redundant low impedance star-ground that sits right on top of the third pin isolated grounding system provided by the electrical system. Yes - you get multiple connections to ground so there are theoretically ground loops. No you do not ground-lift all the electrical plugs to clear those loops. With or without that redundant ground strapping system you will have ground loops... mainly you want to avoid incorporating stray connections to dirty mechanical ground (see previous posts) since this will dump lots of stray currents in to your clean system.

Ground loops are not a problem unless you have circulating currents.  A loop, just by existing, is not going to have significant circulating current on it unless you live next to an electrical sub-station that puts your gear and system within a significant alternating magnetic field.  Beyond that there has to be something going on to make circulating currents happen.  Some gear or outlet strip with a blown out surge protector is dumping crap onto ground.

In the systems we have done like this circulating currents are essentially shunted to earth directly since every piece of gear has a very solid low impedance connection to a central ground that itself is the lowest impedance "earth" ground within the system.

You want to minimize stray currents that can get onto ground via leaky MOV's (Metal Oxide Varistors or surge protectors) in outlet strips and gear and or via other fault conditions that would leak currents to ground.  When we do ground lugs, the gear is open already so we find those and clip them out - preferring instead to use a master over-voltage surge suppression for the whole system.

Whatever small leak currents do exist after you clean everything up will tend to flow to the lowest impedance point in the system... so if there is a clean path straight there that is where it goes... not all over the place through rack rails and five other pieces of gear all with impedances where small leakage currents can turn in to voltages that can be amplified and contaminate your audio.

Generally when we are all done - we do the isolated ground system isolation test (I mentioned this up a couple posts) and then measure chassis and pin 1 connections from any gear to any gear connected within the system and expect to find well under 1/10th ohm dc resistance anywhere to anywhere.  With an energized system, we want to find near-zero mV AC potential between chassis and pin 1 connections from any gear to any gear anywhere...


Keep in mind that 775 milli-volts is 0dBu…  775 micro-Volts RMS is -60dBu...  77.5 micro-Volts is -80dBu...  Take 1mA of leak current and drop that through 1/10th of an ohm - you'll have a voltage drop of 100 micro-Volts... stick that somewhere around where a mic preamp or buss amplifier gets it's zero reference and that WILL become audible.



okay... I'm beginning to poop out on this topic. There are some good resources. That AES Journal issue I mentioned is well worth the $15. The Equitech site has good information about isolation but this is mixed with some stuff I don't agree with in practical application and will probably contradict some of what I have been saying. There are other resources - various books and paper.

_________________
Klett
www.technicalaudio.com/techmecca/


  -----------------------------------------------------------T HE END

POST PROCESSING

There other ways to do isolated grounding systems and some of these – while incredibly difficult to accomplish in a large studio – can actually sound a little better.  They may not measure better.  We’ve gotten system noise floors well below the noise floor of most consoles made so that the console unconnected with all faders up and assigned measured nearly the same as with everything connected and with all faders up and assigned.  When we ran into our first 9K install we saw where the rigor really paid off.  That console is quiet and really does not mask much.

What we do is certain not being presented as the only way to go and is not “right” while all others are “wrong”…  it just works well and can be replicated – we know what to expect…  that’s all.


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Johnny B

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Re: Power and Ground - more than you want to know - in 3 parts
« Reply #3 on: April 23, 2004, 07:31:25 PM »

A little light reading, thanks.

We may have to rethink the way we're
using our iso X-frmers.

Always room for little improvements...Aye


Smile
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"As far as the laws of mathematics refer to reality,
they are not certain; as far as they are certain,
they do not refer to reality."
---Albert Einstein---

I'm also uncertain about everything.

John Klett

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Re: Power and Ground - more than you want to know - in 3 parts
« Reply #4 on: April 26, 2004, 10:33:03 AM »

Yes well

all of that topical thread can be cut down...  and then you'll note that there are missed details  - also nothing at all about signal wiring.

This should all be developed and posted here.  I can mine some of that out of the recpit though it appears that recpit is back on line for reading only

http://recpit.prosoundweb.com/index.php

and searching

http://recpit.prosoundweb.com/search.php
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