# Proper grounding for an Isolation Transformer



## LightJockey (Mar 29, 2015)

My company does audio visual services. When we use a single 120/208 VAC Three-phase "Y" power source for lighting, audio and video we get noise caused by the lighting dimmers injected into the audio and video. We purchased a 30 KVA Isolation transformer. We tie-in power and run it first to the lighting dimmers then put the isolation transformer in line before powering up the audio and video equipment. It eliminates buzz in the audio and interference lines in the video. The transformer has three hot and one ground connections on the input side (no neutral on the input side) and on the output side a ground, neutral and three hots. Is it correct for me to wire the input ground to the transformer case and wire the output ground to the output neutral and not connect the output neutral and ground to the case or input ground?


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## HawkShock (Nov 27, 2012)

Do you understand how the transformer works? It creates it's own neutral, so you don't need to bring one to it. So it needs bonded on the other end, somewhere. It is an isolation transformer, so there should be no real mechanical connection between sides except for the bonding.


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## don_resqcapt19 (Jul 18, 2010)

LightJockey said:


> ... Is it correct for me to wire the input ground to the transformer case and wire the output ground to the output neutral and not connect the output neutral and ground to the case or input ground?


That would be a code violation. The secondary neutral must be connected to the transformer frame and must be connected to a grounding electrode.


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## RIVETER (Sep 26, 2009)

Transformers don't "create" neutrals.


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## btharmy (Jan 17, 2009)

RIVETER said:


> Transformers don't "create" neutrals.


Well, it is a separately derived system which has its own neutral apart from the one associated with its feeders. 3 phase without a neutral input to 3 phase with a neutral output. So, it kinda does.


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## RIVETER (Sep 26, 2009)

btharmy said:


> Well, it is a separately derived system which has its own neutral apart from the one associated with its feeders. 3 phase without a neutral input to 3 phase with a neutral output. So, it kinda does.


Terminology is everything.


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## ponyboy (Nov 18, 2012)

RIVETER said:


> Terminology is everything.



Maybe you could explain, in your own words, the essence of the secondary neutral point in a delta wye transformer. Being vague and uppity only gets you so far when you never actually contribute with actual knowledge. 


Sent from my iPad using Tapatalk


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## wildleg (Apr 12, 2009)

the transformer is delta wye, therefore the neutral is derived (ie. created) at the secondary. period.



> Article 100. Definitions
> 
> Separately Derived System. A premises wiring system
> whose power is derived from a source of electric energy or
> ...


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## Bad Electrician (May 20, 2014)

LightJockey said:


> My company does audio visual services. When we use a single 120/208 VAC Three-phase "Y" power source for lighting, audio and video we get noise caused by the lighting dimmers injected into the audio and video. We purchased a 30 KVA Isolation transformer. We tie-in power and run it first to the lighting dimmers then put the isolation transformer in line before powering up the audio and video equipment. It eliminates buzz in the audio and interference lines in the video. The transformer has three hot and one ground connections on the input side (no neutral on the input side) and on the output side a ground, neutral and three hots. Is it correct for me to wire the input ground to the transformer case and wire the output ground to the output neutral and not connect the output neutral and ground to the case or input ground?


I have done many audio, visual projects (TV, recording, telco, Data Centers, large control centers..... (I trouble shoot for grounding issues)
If properly wired and installed per the NEC 99.9 0/0 of your noise issues are eliminated. And all the rest is smoke and mirrors generated by folks trying to sell product or they do not understand what they are doing.


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## HawkShock (Nov 27, 2012)

RIVETER said:


> Terminology is everything.


So explain where a secondary side neutral comes from, using your terminology. You haven't yet. 
If there wasn't a completely separate neutral point on the secondary side, an isolation transformer would be rather useless. 
The transformer doesn't just pull a neutral out of the air.


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## LightJockey (Mar 29, 2015)

Thanks for the replies. I do know how transformers work, thank you. The transformer is mounted on wheels. There are Cam-Lock E-1016 single pole connectors on the input and output sides. It is used in concert arenas and hotel ballrooms to isolate the power that feeds audio and video from the lighting dimmers. A Silicon Controlled Rectifier dimming system creates spurious noise due to the saw-tooth waveform and the over-shoot and under-shoot dimming curve. And venues may have "dirty" power, i.e., non-smooth sinusoidal wave-forms. 
The input is a three-wire plus ground DELTA and the output is a four wire plus ground WYE, so a neutral is "created."
The transformer is moved from job site to job site and may get its power from a "company switch" or disconnect in the back of a theater, convention center, hotel ballroom or from a portable generator. Therefore, it is not required to follow the same rules for grounding that a permanent installation does.
I am asking for input about the safest way to wire the transformer. When I acquired the unit the input ground went to a bolt through the steel case of the unit. The output ground is also attached to the same bolt. There is also a bond from the neutral on the output side of the transformer to that bolt. Therefore, the output of the transformer is not truly isolated from the input. The George R. Brown Convention Center in Houston is notorious for "noisy power." Audio and video often have buzzes and interference bars. I assume that these come from reactive lighting and H VAC loads and that all of the buildings many power transformers and sub systems share a common ground. 
So, what are the safety issues that I should consider with removing the bond between the input ground and the output neutral and ground? Creating a "floating" neutral and ground on the output side of the transformer.
Thank you.


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## cuba_pete (Dec 8, 2011)

LightJockey said:


> So, what are the safety issues that I should consider with _*removing the bond between the input ground and the output neutral and ground*_? Creating a _*"floating" neutral*_ and ground on the output side of the transformer.
> Thank you.


*That* is what you cannot do.:no:

You're trying to apply a Band-Aid to a system which has likely not been implemented correctly to begin with.

Unfortunately for you, you would likely have to step it back to the power source and determine its health (PQA). The troubleshooting will likely lead to _that_ power's source.


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## saedlund (Mar 30, 2015)

I just installed a UPS System using an Isolated XFMR 480 delta primary and 120/208Y secondary. On secondary take XO to ground and also t get your neutral from XO.


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## saedlund (Mar 30, 2015)

saedlund said:


> I just installed a UPS System using an Isolated XFMR 480 delta primary and 120/208Y secondary. On secondary take XO to ground and also :thumbsup: get your neutral from XO.


Corrected


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## saedlund (Mar 30, 2015)

Isolated XFMRS are basically built to keep the noise between the windings minimal , to isolate the noise from your equiptment you need to isolate the equiptment using a triad grounding system apart from the existing ground rod your using(Per DCS proper grounding )


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## don_resqcapt19 (Jul 18, 2010)

LightJockey said:


> ...
> The transformer is moved from job site to job site and may get its power from a "company switch" or disconnect in the back of a theater, convention center, hotel ballroom or from a portable generator. *Therefore, it is not required to follow the same rules for grounding that a permanent installation does.*
> ...


Can you cite the code section that says that?


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## don_resqcapt19 (Jul 18, 2010)

saedlund said:


> Isolated XFMRS are basically built to keep the noise between the windings minimal , to isolate the noise from your equiptment *you need to isolate the equiptment using a triad grounding system apart from the existing ground rod* your using(Per DCS proper grounding )


The code does not permit that.


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## saedlund (Mar 30, 2015)

https://www.isa.org/standards-and-publications/isa-publications/intech/2008/march/networking-best-practices-for-process-instrumentation-cabling/


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## saedlund (Mar 30, 2015)

I was not implying a code violation nor a requirement only some ways to help him get the noise out of his system.


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## don_resqcapt19 (Jul 18, 2010)

saedlund said:


> https://www.isa.org/standards-and-publications/isa-publications/intech/2008/march/networking-best-practices-for-process-instrumentation-cabling/


And that paper very clearly shows that the instrument "triad" grounding array is connected to the plant grounding grid as required by the NEC. Your previous statement implied otherwise.

Also there is zero evidence that installing the ground rods in a "triad" is any more effective than installing them in any other arrangement.


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## LightJockey (Mar 29, 2015)

don_resqcapt19 said:


> Can you cite the code section that says that?


NEC sections 520 through 530 apply to stage, portable event and motion picture settings and set out exceptions to wiring that would be allowed in permanent residential or commercial settings. 

Please see NEC 525.31 "The grounded circuit conductor shall not be connected to the equipment grounding conductor on the load side of the service disconnecting means or on the load side of a separately derived system disconnecting means."


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## saedlund (Mar 30, 2015)

Hey Lightjockey ,
Good work!


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## Bad Electrician (May 20, 2014)

saedlund said:


> Isolated XFMRS are basically built to keep the noise between the windings minimal , to isolate the noise from your equiptment you need to isolate the equiptment using a triad grounding system apart from the existing ground rod your using(Per DCS proper grounding )


You are violating the NEC. As for a triad if you believe this buys you anything over any other driven electrode with the same number of electrodes and copper, you really need to go back to and do some home work.

The vast majority of transformers we (all of us) utilize are isolation transformers. I assume you mean shielded isolation transformer?

Even a buck boost transformer is an isolation transformer by design, it is only in the connections that this transformer has the primary connected to the secondary, and is no longer isolation.

Lastly for the record we have proven several times that true isolation of grounding systems is next to impossible, in commercial establishments, with rebar, metal studs, water piping, electrical systems. There was a site where the specs called for a totally isolated "ground". After the job was completed we came in and did testing for the EC. There were numerous points where the ground was violated.

A properly installed and tested electrical distribution system will be a cost effective and works.


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## RIVETER (Sep 26, 2009)

wildleg said:


> the transformer is delta wye, therefore the neutral is derived (ie. created) at the secondary. period.


Good job. DERIVED was what I wanted. I was going to say that until I read your post. Besides that, I'm too UPPITY, anyway.:thumbsup:


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## markbrady (Jun 2, 2014)

*transformer talk aside*

Just did an audio/visual system for a major university and to cut out the noise we used only one phase of a 3 phase system. In other words we used A phase and not B or C phase for the system. We put other items not related to the audio/visual on those phases. There was no noise and the system sounds great. I am no expert in audio/visual but learned that it was designed the way I indicated because it cuts out interference.


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## wildleg (Apr 12, 2009)

LightJockey said:


> NEC sections 520 through 530 apply to stage, portable event and motion picture settings and set out exceptions to wiring that would be allowed in permanent residential or commercial settings.
> 
> Please see NEC 525.31 "The grounded circuit conductor shall not be connected to the equipment grounding conductor on the load side of the service disconnecting means or on the load side of a separately derived system disconnecting means."


you need to understand what that means, it is true for all places on a premesis. The grounded circuit conductor is the neutral. It can only be connected at the main bonding jumper, or the bonding jumper for an SDS. There still has to be a bonding jumper at the SDS and at the main, but it is not allowed downstream. this has nothing to do with your earlier contention regarding grounding.

this is the same thing you posted that is repeated several places in the code. here it is in 250.24


> (5) Load-Side Grounding Connections. A grounded conductor
> shall not be connected to normally non–currentcarrying
> metal parts of equipment, to equipment grounding
> conductor(s), or be reconnected to ground on the load side
> ...


here are some definitions you need to understand:



> Article 100 Definitions
> Bonding Jumper, Main. The connection between the
> grounded circuit conductor and the equipment grounding
> conductor at the service.
> ...





> 210.9 Circuits Derived from Autotransformers. Branch
> circuits shall not be derived from autotransformers unless
> the circuit supplied has a grounded conductor that is electrically
> connected to a grounded





> 250.2 Definitions.
> Bonding Jumper, Supply-Side. A conductor installed on
> the supply side of a service or within a service equipment
> enclosure(s), or for a separately derived system, that ensures
> ...


and this is something you need to understand that you need to bond the transformer (it is repeated somewhere in 450 as well)


> 250.30 Grounding Separately Derived Alternating-
> Current Systems. In addition to complying with 250.30(A)
> for grounded systems, or as provided in 250.30(B) for ungrounded
> systems, separately derived systems shall comply
> ...


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## don_resqcapt19 (Jul 18, 2010)

LightJockey said:


> NEC sections 520 through 530 apply to stage, portable event and motion picture settings and set out exceptions to wiring that would be allowed in permanent residential or commercial settings.
> 
> Please see NEC 525.31 "The grounded circuit conductor shall not be connected to the equipment grounding conductor on the load side of the service disconnecting means or on the load side of a separately derived system disconnecting means."


That section does not say you don't need a grounding electrode conductor on the transformer secondary. It says you can't have on on the load side of the transformer disconnecting means. Not even sure why that section even exists in 525 as 250.24 says that for the load side of the service disconnecting means and 250.30(A) says the same for the secondary of a transformer.

There is nothing in those articles that say that 250.20 does not apply. 

Remember that everything in Chapters 1-4 apply to Chapter 5 
installations unless the is a specific modification in the Chapter 5 article. There is no specific modification that says you don't need a grounding electrode for the secondary of your transformer.


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## saedlund (Mar 30, 2015)

It was a pleasure corrisponding with you Don , you seem to be strong in the NEC Code, most of my 35 years has been industrial ive been an inspector the past year, I look forword to future solutions. Thanks !


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## Bad Electrician (May 20, 2014)

markbrady said:


> Just did an audio/visual system for a major university and to cut out the noise we used only one phase of a 3 phase system. In other words we used A phase and not B or C phase for the system. We put other items not related to the audio/visual on those phases. There was no noise and the system sounds great. I am no expert in audio/visual but learned that it was designed the way I indicated because it cuts out interference.


That is good who-doo voodoo electrical work. Leaving the power off has the same effect. And shows the same level of understanding.


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## saedlund (Mar 30, 2015)

Ive never installed one I assume everything is 120v ? Just seems like a lot of work and material not to utilies Band C phases . Just sayn.


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## saedlund (Mar 30, 2015)

My bad didnt finish reading the Quote! you are using B and C ! Okay.


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## cabletie (Feb 12, 2011)

Bad Electrician said:


> markbrady said:
> 
> 
> > Just did an audio/visual system for a major university and to cut out the noise we used only one phase of a 3 phase system. In other words we used A phase and not B or C phase for the system. We put other items not related to the audio/visual on those phases. There was no noise and the system sounds great. I am no expert in audio/visual but learned that it was designed the way I indicated because it cuts out interference.
> ...


We also did the same thing at a theatrical theater as per design on the drawings. The specialty panel only had one phase for the 12 or 24 circuits. 

I asked about it here awhile ago, and whether or not it eliminated noise. Mxslick explained the same thing could be accomplished with proper grounding. 

Also, it has always been my understanding tha an isolation transformer has the same input and output voltage. Other than that it is wired no different than a normal step down, grounding and all.


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## Bad Electrician (May 20, 2014)

cabletie said:


> We also did the same thing at a theatrical theater as per design on the drawings. The specialty panel only had one phase for the 12 or 24 circuits.
> 
> I asked about it here awhile ago, and whether or not it eliminated noise. Mxslick explained the same thing could be accomplished with proper grounding.
> 
> Also, it has always been my understanding tha an isolation transformer has the same input and output voltage. Other than that it is wired no different than a normal step down, grounding and all.


A standard dry type transformer is by design an isolation the primary has no connection (other than ground) to the secondary.

A lot of black magic goes into these designs, with little or no theory behind it.

1. That is how we have always done it.
2. Well it worked to fix the last problem (band aid)
3. It is in the Specs.
4. In lieu of doing it right lets do something fancy.

And the number of electricians and engineers that subscribe to triad grounding thinking there is magic in the triangle is laughable.


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## cuba_pete (Dec 8, 2011)

*still not right...*



LightJockey said:


> NEC sections 520 through 530 apply to stage, portable event and motion picture settings and set out exceptions to wiring that would be allowed in permanent residential or commercial settings.
> 
> Please see NEC 525.31 "The grounded circuit conductor shall not be connected to the equipment grounding conductor on the _*load side of the service disconnecting means*_ or on the load side of a separately derived system _*disconnecting means*_."


Removing the bond at the transformer would be _*before*_ the service disconnecting means..._*not after*_.

Pay attention to _*Disconnecting Means*_


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## markbrady (Jun 2, 2014)

sooooo


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## markbrady (Jun 2, 2014)

Bad Electrician said:


> That is good who-doo voodoo electrical work. Leaving the power off has the same effect. And shows the same level of understanding.


So are you saying that motors which may control screens, shades or other equipment, lighting ballasts and dimmer switches don't introduce noise into the ac power system? 
I was told that from an engineering perspective to isolate the audio on one phase with some visual on the same circuit but that motors, ballasts and dimmers be on separate phases; load balance the phases to stop noise current from one phase going down the common neutral and then being able to enter the phase the audio is on.


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## guest (Feb 21, 2009)

markbrady said:


> *So are you saying that motors which may control screens, shades or other equipment, lighting ballasts and dimmer switches don't introduce noise into the ac power system? *


I don't think that was his intent, as yes those items can introduce noise intothe AC power system.




markbrady said:


> I was told that from an engineering perspective to isolate the audio on one phase with some visual on the same circuit but that motors, ballasts and dimmers be on separate phases; *load balance the phases to stop noise current from one phase going down the common neutral and then being able to enter the phase the audio is on*.


That was once the school of thought, but the past 20 years I have spent doing hundreds of cinemas, screening rooms and two University screening/classrooms has disproved the need to isolate all of the audio gear to one phase.

On the power side, the only spec I insist on is that each _*circuit have a dedicated neutral back to the panel. (No MWBC's)

*_All of the installations I mentioned have dedicated neutrals, the "front end" (or line level) audio gear on the same phase when possible (but not always) and most importantly, proper grounding on the AC power side and proper use of shielding on the audio side. _* (telescoping or single ended sheilds).
*_ 


cabletie said:


> We also did the same thing at a theatrical theater as per design on the drawings. The specialty panel only had one phase for the 12 or 24 circuits.
> 
> *I asked about it here awhile ago, and whether or not it eliminated noise. Mxslick explained the same thing could be accomplished with proper grounding. *
> 
> Also, it has always been my understanding tha an isolation transformer has the same input and output voltage. Other than that it is wired no different than a normal step down, grounding and all.


Absolutely and thank you for bringing it up.  Proper installation practices on the audio side (and sadly, far too many "professional" A/V installers do not have a clue about what to do) is far more effective at ensuring a quiet, noise-free installation. :thumbsup:


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## markbrady (Jun 2, 2014)

20 years ago? Then they must be teaching these people the same thing and not evolving because the audio/visual guy was about 30 years old and I doubt he learned his info at around 10 years old and just ran with it


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## Roger (Jul 7, 2007)

markbrady said:


> 20 years ago? Then they must be teaching these people the same thing and not evolving because the audio/visual guy was about 30 years old and I doubt he learned his info at around 10 years old and just ran with it


Unfortunately yes, some are still teaching antiquated ways of doing things because, "it's the way they have always done it" 

Heck, some engineers still design the earlier mentioned three ground rods driven in a triangle configuration with the thought that somehow the design has a magical quality, but in reality they don't do anything more than a single rod

Roger


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## Bad Electrician (May 20, 2014)

markbrady said:


> So are you saying that motors which may control screens, shades or other equipment, lighting ballasts and dimmer switches don't introduce noise into the ac power system?
> I was told that from an engineering perspective to isolate the audio on one phase with some visual on the same circuit but that motors, ballasts and dimmers be on separate phases; load balance the phases to stop noise current from one phase going down the common neutral and then being able to enter the phase the audio is on.


No I am not saying that, I am saying for your sensitive equipment in my experience and I have been doing for some time, utilizing all phases has never caused a problem UNLESS there was an equipment issue. Which we have had and when the equipment was replaced the issue was resolved.


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## Bad Electrician (May 20, 2014)

markbrady said:


> 20 years ago? Then they must be teaching these people the same thing and not evolving because the audio/visual guy was about 30 years old and I doubt he learned his info at around 10 years old and just ran with it


We do quite a bit of government work and often the specs are copy and pasted. Read a spec that referenced a product that has not been manufactured since the 1950's.

In their defense if it worked in the past why monkey with something. Should there be issues with a new job and it is found out someone dared try something different in regards to the installation it could be a CLM. 

CLM= Career Limiting Move.


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## guest (Feb 21, 2009)

markbrady said:


> 20 years ago? Then they must be teaching these people the same thing and not evolving because the audio/visual guy was about 30 years old and I doubt he learned his info at around 10 years old and just ran with it


The 20 years I referred to was the span of the installations I have done (actually more like 30 years now that I think about it...) I was taught the right ways of doing the grounding and audio connections by an old-school tech who believed as I do, that the so-called "proper" way of one phase for all audio and isolated/star grounding was BS. His work over the decades (and mine) has proven him right.


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## wildleg (Apr 12, 2009)

I got a kick out of the triangular counterpoise grounding comments. I still see that on plans as late as last year.


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## cuba_pete (Dec 8, 2011)

*Triad?*



wildleg said:


> I got a kick out of the triangular counterpoise grounding comments. I still see that on plans as late as last year.


Now that y'all mention it, the ground for my facility is two triads side by side.

I never really thought about it too much, just assumed it was for saving space. We have another ground field in which the grounds are just in a row.

I haven't come across the reasoning for, or a suggestion to use a "triad" to improve grounding.

Without wasting my time Googling around and coming up with a witchcraft hocus-pocus web site (like those recommending going barefoot at work under your desk, and "grounding" your mattress), can anyone provide a text/manual which recommends a triad configuration of ground rods (with maths to support)?


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## Bad Electrician (May 20, 2014)

cuba_pete said:


> Now that y'all mention it, the ground for my facility is two triads side by side.
> 
> I never really thought about it too much, just assumed it was for saving space. We have another ground field in which the grounds are just in a row.
> 
> ...


But if it saves time, money or space that is logical. For some to assume it there are magical qualities is different.

I think it part of the connection beliefs is the conductor is continuos?


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## cuba_pete (Dec 8, 2011)

Bad Electrician said:


> I think it part of the connection beliefs is the conductor is continuos?


Is that a question? If it is then, no, not really continuous. But, the bonds are cadwelded (permanent).

I wonder if there is some "ancient" text out there which delineates the magical properties of "Pyramid" or "Triad" grounding scheme or some such nonsense.:001_huh:

I have an original set of Hawkins' Electrical Guides at work...I'll have to check those tomorrow.


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## Jlarson (Jun 28, 2009)

No one can support that triangle ground rod voodoo. It's always... I'm an engineer with a degree from blah and blah years experience... That's when I walk away. :laughing:


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## chewy (May 9, 2010)

Just doing a building refurb and havent located a main ground yet, specs are calling for 3 earth stakes in a delta pattern 600mm apart in a basement 4ft high aswell as wricon earthing so we have to expose some rebar somewhere, going to be ****ing fun.


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## glen1971 (Oct 10, 2012)

I had to add a new ground grid a few years ago for a radio tower in a gas plant.. The spec was 3 - 60' rods, done in a triangle configuration, connected in a "Y" to avoid circulating ground currents, with all connections to be braised or cadwelded, using 750 mcm... 
With only being able to sink the rods into 5' of undisturbed soil they passed the engineer's resistance test.. We gave up on braising, as it was only weakening the connection, so they were fine with a "normal" c-tap, glyptal and tape.. We wound up using the extra rods on additional jobs for that client..


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## Bad Electrician (May 20, 2014)

cuba_pete said:


> Now that y'all mention it, the ground for my facility is two triads side by side.
> 
> I never really thought about it too much, just assumed it was for saving space. We have another ground field in which the grounds are just in a row.
> 
> ...


I'd bet there is an "ANCIENT SPEC" out there somewhere.

There is nothing wrong with doing this and we see it all the time, I just think it is funny some swear by it. what about a quad.


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## Fibes (Feb 18, 2010)

Bad Electrician said:


> what about a quad.


Nope, "Tetra" isn't nearly as cool of a word as Delta or Triad is. 

Delta and Triad sound important, Tetra (four sides) sounds like a game or a shot for a disease. :thumbup:


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## Bad Electrician (May 20, 2014)

Fibes said:


> Nope, "Tetra" isn't nearly as cool of a word as Delta or Triad is.
> 
> Delta and Triad sound important, Tetra (four sides) sounds like a game or a shot for a disease. :thumbup:


What about Quadrophenia - The Who
https://www.youtube.com/watch?v=nyN7WUKRicw&list=PL436BFCEB7848BC00


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## Fibes (Feb 18, 2010)

Bad Electrician said:


> What about Quadrophenia - The Who
> https://www.youtube.com/watch?v=nyN7WUKRicw&list=PL436BFCEB7848BC00


 Now you might be on to something. :thumbsup:


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## cuba_pete (Dec 8, 2011)

*Who*



Bad Electrician said:


> What about Quadrophenia - The Who
> https://www.youtube.com/watch?v=nyN7WUKRicw&list=PL436BFCEB7848BC00


I have tickets for September...should be fun.


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