Why An Open Neutral Kills 120V Devices

[gordon mcgregor]

Re: Why An Open Neutral Kills 120V Devices

Hill Audio wardrobes! in my youth I well remember those monsters there was a music venue in Glasgow with a 16ft high stage try getting those things up onto there, much hilarity and strong words etc.
Mike UK earthing I suspect isn't much different than what you do in the States, voltages and currents will be different but the basic idea of path of least resistance still applies.
These days we tend to use Cee plugs for most heavy current stuff thankfully the days of wiring into panels are starting to go away and most regular venues and also generators have plugs available, still meter them though and still use the sacrificial PAR can as well. G

 

[Mike Sokol]

Re: Why An Open Neutral Kills 120V Devices

Hill Audio wardrobes! in my youth I well remember those monsters there was a music venue in Glasgow with a 16ft high stage try getting those things up onto there, much hilarity and strong words etc.
Mike UK earthing I suspect isn't much different than what you do in the States, voltages and currents will be different but the basic idea of path of least resistance still applies.
These days we tend to use Cee plugs for most heavy current stuff thankfully the days of wiring into panels are starting to go away and most regular venues and also generators have plugs available, still meter them though and still use the sacrificial PAR can as well. G

Yeah, those Malcolm Hill M6 cabinets were seriously heavy. I originally used them with a bunch of Chameleon Amps, all wired for 240 volts. We used a big step-up transformer on 120-volt cam-locks to turn it into 240 volts, then distro'ed it using C-Form connectors.

Good times....:lol:

 

[brian maddox]

Re: Why An Open Neutral Kills 120V Devices

Yeah, Scarfe is mostly Martin now. But his original AC/DC M6 rig with all those Tannoy cones staring at you was a pretty awesome site to behold. The more cabinets you stacked up, the more the bass coupled together so there were no subwoofers required even though it was all 10" (correction, not 8") drivers. And the mix of 240 volt amp racks and 120 volt drive racks was interesting, to say the least. But we could use that system for anything from a symphony to a hip-hop rave, and it always got the job done. Did I mention it was LOUD!

Continuing the swerve...

I was with another company providing second FOH and monitor drive on an earth day festival about a Hundred years ago when I first encountered those cabinets. And yeah, there're 10" drivers. But there are eight of them per cabinet IIRC? Anyway, show was at merriwhether post (a typical shed) and mike put up a giant pile of those things. I want to say. we were pushing close to a hundred cabinets by the time you added in out fills and the like.

Anyway, when Collective Soul hit the stage and the kick drum hit, I think I had a Spiritual Awakening right then and there. So much delicious low end from nothing bigger than a ten and no subs. Awesome!

I do recall very well the large transformers used to run the rig. And some patient MHA guy explaining a bit of it to me. In retrospect, mike, it could have even been you. But with a mix of 120 and 240 power, it was pretty important to stay on point with all that stuff, including making sure we had a good solid neutral at all times....

And we're back on topic.

 

[Mike Sokol]

Re: Why An Open Neutral Kills 120V Devices

Back OT....

Sometimes the neutral can open up as the result of harmonic Triplen currents building up on 3-phase transformers used with Triac dimmers. See the thread at http://soundforums.net/lighting-ele...m-causes-buzz-hum-circuit-breaker-panels.html

Triplen currents are odd order harmonics that add rather than subtract on the neutral bus between the feed transformers themselves. So if the neutral bus wasn't oversized properly when first installed, it can overheat and open up in the middle of a gig. I've not personally seen this at a concert, but did live through it in an industrial plant where it overheated and opened the neutral on the transformers feeding a 3-phase WYE 480/277 volt, 600 amp panel. That did all sorts of electrical damage as well as shutting down most of the plant for a day or two. We ran around with an infrared camera looking for overheated bus connections after that. Unbalanced heater loads and variable speed motors with Triac control (think light dimmers) were most likely the cause.

Sometimes you don't have a choice, but I really dislike tapping power from the lighting panel for just that reason, too many harmonics on the Neutral bus.

Neutral is probably the most important bus of all...

 

[John Roberts]

Re: Why An Open Neutral Kills 120V Devices

bus ? or was that a buss fuse that opened up?

JR

 

[Mike Sokol]

Re: Why An Open Neutral Kills 120V Devices

There was no fuse, it was the 4/0 (or even larger) bus wire (sorry my bad, only one "s" in bus) itself that failed. The clamping bolt and the busbar had heated up enough to oxidize the copper wire, and once that begins to happen it's basically an avalanche failure since heating begets more oxidation, oxidation begets more resistance, and resistance begets more heating, etc.... (First time I've been able to use "beget" in a post).

This was 35 years ago so I don't remember all the details, but the power company came in and gave us a stern warning about unbalanced loads and Triplen currents in 3-phase systems. These were really big pad-mounted transformers the size of a car since they powered a million square foot warehouse with 100K sq ft factory floor. The production manager was not happy when we blew up his plant!!! Lucky for all the 3-phase motors, they don't care if the neutral is disconnected. Florescent lights are a different story.

 

[Kevin Graf]

Re: Why An Open Neutral Kills 120V Devices

****
Mike UK earthing I suspect isn't much different than what you do in the States, voltages and currents will be different but the basic idea of path of least resistance still applies.
*******G

The British power system is very different from the type in North America (after all they drive on the wrong side of the road).

Jim Brown writes:

In the United Kingdom, the bond between neutral and earth is made only at the power utility.
Inside the building, Neutral and PE (Equipment Ground) are carried separately to all equipment and outlets.

That's from pages 39 to 41 of his excellent paper:

Power and Grounding for Audio and Video Systems
A White Paper for the Real World – International Version

http://www.audiosystemsgroup.com/SurgeXPowerGround.pdf

 

[Kevin Graf]

Re: Why An Open Neutral Kills 120V Devices

Here's the REALLY scary thing. Even if you had corner and tongue jack stands sitting on the dirt under your genny or production trailer, they offer such a high impedance path to earth that it's unlikely they'll have any grounding effect at all. A standard 8 ft ground rod buried in the earth can have an impedance as high as 25 ohms and still be within code. A great ground rod impedance for electronic assembly plants is maybe 5 ohms, and requires wet soil with salts added for conductivity on a regular basis. So 120 volts connected to a ground rod without benefit of a G-N bond will only draw perhaps 5 or 6 amps of current, (120 volts / 20 ohms = 6 amps), not enough to trip a 20 amp circuit breaker, but certainly enough to trip a GFCI if it was in the circuit.
***********
You should NEVER feel a shock on stage or standing at a mixer or lighting console. If you do, there's something seriously wrong with the grounding/bond somewhere in the power distro system, and it should be corrected immediately before somebody gets killed.

In real life a single 8 foot ground rod could have an impedance of 100 Ohms or more!
In the NEC rule, if a single ground rod has a measured impedance of under 25 Ohms then it's permitted otherwise a second ground rod must be driven. But it requires a special meter that cost's about $2500 and takes a skilled technician about 1/2 hour to complete the test. So almost all residential and small business electricians skip the test and just drive a second ground rod.

 

[Mike Sokol]

Re: Why An Open Neutral Kills 120V Devices

In real life a single 8 foot ground rod could have an impedance of 100 Ohms or more!
In the NEC rule, if a single ground rod has a measured impedance of under 25 Ohms then it's permitted otherwise a second ground rod must be driven. But it requires a special meter that cost's about $2500 and takes a skilled technician about 1/2 hour to complete the test. So almost all residential and small business electricians skip the test and just drive a second ground rod.

I agree that a single ground rod can have way more than the code required max of 25 ohms, but modern "Fall of Potential" ground resistance testers have come down in price a lot over the last few years. I have a B&K Model 309 which list for $195 and takes about 10 minutes to do an earth resistance test. Plus it uses a 400 Hz test tone and a high-pass filter to ignore any 60 Hz currents flowing in the ground rod already. So you don't have to disconnect the ground rod from the building to do a test like in the old days.

See Model 309, Digital Earth Resistance Meter - B&K Precision

Here's a pretty good tutorial from Fluke about the Fall-of-Potential test procedure. Fall-of-potential test method

Most electricians will still just drive a second ground rod, but I feel that ground rods should be tested for earth resistance after any lightning strike on the building. I saw one last year that melted the sandy soil around the ground rod into a glassy popsicle with very high resistance. I talked to my local inspector and power company engineer, who both confirmed there was no requirement for testing ground rod resistance after a lightning strike.

 

[Richard Pyne]

Re: Why An Open Neutral Kills 120V Devices

Back OT....

Sometimes the neutral can open up as the result of harmonic Triplen currents building up on 3-phase transformers used with Triac dimmers. See the thread at http://soundforums.net/lighting-ele...m-causes-buzz-hum-circuit-breaker-panels.html

Triplen currents are odd order harmonics that add rather than subtract on the neutral bus between the feed transformers themselves. So if the neutral bus wasn't oversized properly when first installed, it can overheat and open up in the middle of a gig. I've not personally seen this at a concert, but did live through it in an industrial plant where it overheated and opened the neutral on the transformers feeding a 3-phase WYE 480/277 volt, 600 amp panel. That did all sorts of electrical damage as well as shutting down most of the plant for a day or two. We ran around with an infrared camera looking for overheated bus connections after that. Unbalanced heater loads and variable speed motors with Triac control (think light dimmers) were most likely the cause.

Sometimes you don't have a choice, but I really dislike tapping power from the lighting panel for just that reason, too many harmonics on the Neutral bus.

Neutral is probably the most important bus of all...

I have seen the same neutral over-current effect in office buildings full of switching power supplies on computers.

 

[gordon mcgregor]

Re: Why An Open Neutral Kills 120V Devices

"In the United Kingdom, the bond between neutral and earth is made only at the power utility.
Inside the building, Neutral and PE (Equipment Ground) are carried separately to all equipment and outlet"

That is not quite correct most modern premises now have the earth and neutral bonded at the service entry, andt then the conductors are taken to the power points either by ring circuits or by spurs
Electrical wiring in the United Kingdom - Wikipedia, the free encyclopedia gives basic info follow the links at the bottom if you want mor info G

PS YOU drive on the wrong side of the road as evryone drove on the left at first)

 

[Mike Sokol]

Re: Why An Open Neutral Kills 120V Devices

I have seen the same neutral over-current effect in office buildings full of switching power supplies on computers.

That's wild!!! I never thought about computer power supplies creating Triplen currents, but that's certainly a possibility....

How about switch mode power supplies in class-D power amps. I wonder if they can create Triplen currents in 3-phase circuits? That would be something really interesting. I'm going to think on this a bit.

 

[John Roberts]

Re: Why An Open Neutral Kills 120V Devices

The output side technology of a power amp had no influence on what the mains distribution sees, other than a load variation based on efficiency.

Amp power supplies range from conventional transformer, to simple DC to Dc switchers (that rectify the mains voltage), to power factor correct, power supplies that spread the current draw out over the full waveform.

Door #2 could introduce larger current spikes from charging the mains side capacitors, but only incrementally vs, secondary side capacitors with the additional transformer winding impedance in series.

JR

 

[Chris Davis]

Re: Why An Open Neutral Kills 120V Devices

Back OT....


Neutral is probably the most important bus of all...

Bumper sticker of the day.

 

[Mike Sokol]

Re: Why An Open Neutral Kills 120V Devices

A sound buddy of mine actually has a bumper sticker that says "Friends Don't Let Friends Wrap Cables" and a T-Shirt that says "Yes, I know what all the knobs do".

 

[David Buckley]

Re: Why An Open Neutral Kills 120V Devices

That's wild!!! I never thought about computer power supplies creating Triplen currents, but that's certainly a possibility....

You bet they do.

In large buildings with lots of IT equipment, the neutral current often exceeds the phase currents. It also happens badly in data centres. I wish I could find the picture I have somewhere of a multi hundred amp switch which has a missing neutral link; it melted through overload due to excessive triplen harmonics. That neutral letting go caused nearly two hundred thousand pounds worth of damage to power supplies of equipment. None of the equipment itself was damaged, just power supplies.


On a slightly different tack, to all intents and purposes the USA and UK (and most other countries for that matter) power supply arrangements are the same once you are inside a premise. The power all generally looks (for a single phase supply) like this:

World Term	USA Term	UK Wiring Colour	Europe Colour	USA Colour
Phase, or more commonly but incorrectly, Live	Hot	Red	Brown	Black
Neutral	Neutral (rarely!) or Grounded Conductor	Black	Blue	White
Earth	Ground or Grounding Conductor	Bare or Green / Yellow	Green / Yellow	Bare or Green

The arrangement of the power in the building is usually (but not always) T-N-CS (in civilised countries) and it is mandated to be so by the NEC in the USA, only they don't call it that as far as I can recall. Description of the magic letters can be found on WikiPedia in the article entitled Earthing Systems

The big difference in supply arrangements is that in the UK the neutral/earth bond is provided by the utility company, and the utility thus hands you two separate conductors, a neutral and an earth. In the USA (and New Zealand!) the utility hands you just the neutral conductor, and the contractor is responsible for providing the neutral / earth bond, and the grounding rods(s) as required.

European electrical folk will raise their hands in horror at a neutral being described with a word like "grounded", as they will recognize the neutral conductor as to be treated as live, one of the big differnces in attitude between the NEC way and the rest of the world way...

 

[Tim McCulloch]

Re: Why An Open Neutral Kills 120V Devices

European electrical folk will raise their hands in horror at a neutral being described with a word like "grounded", as they will recognize the neutral conductor as to be treated as live, one of the big differnces in attitude between the NEC way and the rest of the world way...

Actually, the US NEC defines the "neutral" as the "grounded, current-carrying conductor."

Also, for "single" (or "split") phase 120/240v service in the USA, you will have 2 phase conductors, color coded black and red, although 120v devices utilize only one of the phase legs.

 

[Mike Sokol]

Re: Why An Open Neutral Kills 120V Devices

Actually, the US NEC defines the "neutral" as the "grounded, current-carrying conductor."

Also, for "single" (or "split") phase 120/240v service in the USA, you will have 2 phase conductors, color coded black and red, although 120v devices utilize only one of the phase legs.

And to show you just how crazy this can be to think about, here's a basic schematic I drew up for my No~Shock~Zone RV classes where I discuss voltage drops in power distro systems for campgrounds. And no, it doesn't even include the phase-cancelling current paths that occur in a 240 volt center-tapped USA system. No wonder we all get a little confused at times. :lol:

 

[David Buckley]

Re: Why An Open Neutral Kills 120V Devices

In that diagram would there not be a link between the utility neutral to the secondary neutral as per standard distribution practice?

 

[David Buckley]

Re: Why An Open Neutral Kills 120V Devices

Actually, the US NEC defines the "neutral" as the "grounded, current-carrying conductor."

Indeed but you are missing my point. The very use of the word "grounded" implies "like ground", suggesting that it isn't a "live" conductor.

European regulations require that you think of the neutral the same way you think of a hot; potentially, it could shock you, its not at all like ground.

Hence my observation European electrical folk will raise their hands in horror at a neutral being described with a word like "grounded". It is contrary to European regulations. But exactly in accordance with American regulations, and American custom and practice.