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Author Topic: Star ground.  (Read 5209 times)

Jerome Malsack

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Jerome Malsack

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Re: Star ground.
« Reply #1 on: March 22, 2017, 08:02:16 pm »

My gut feeling is this is not a recommended answer.  Using multiple Generators or Power from several buildings ?  No not good answer.
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Frank DeWitt

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Re: Star ground.
« Reply #2 on: March 22, 2017, 08:22:30 pm »

5 outputs depend on one of the 5 input cables having a good ground.   The ground return is 1/8 the size of the conductors for the total load.  The one golden grounded input is not labeled.

Be afraid Be very afraid.

 
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Rob Spence

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Re: Star ground.
« Reply #3 on: March 22, 2017, 09:57:40 pm »

How is this any different than the "poor mans distro" ?


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rob at lynxaudioservices dot com

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Mike Sokol

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Re: Star ground.
« Reply #4 on: March 22, 2017, 10:37:15 pm »

The problem with this kind of gadget is that if the single ground fails, then all chassis in the sound system are tied together and a line-to-chassis fault on one circuit will result in a reflected voltage on everything else plugged into it. I'm not so worried about the EGC bus being too small since it's unlikely you'll have multiple line-to-chassis faults at the same time. But this is all grounding eggs in one basket, so that better be a perfect single ground or there will be serious trouble.
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Erik Jerde

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Re: Star ground.
« Reply #5 on: March 22, 2017, 10:48:40 pm »

Saw this on Ebay,  any expert review and comment welcome. 
http://www.ebay.com/itm/Hubbell-Star-Grounding-Five-20A-Input-to-Five-20A-Output-Power-Distro-/282230566364?hash=item41b640addc:g:otQAAOSwA3dYDY5c

No good.  My interpretation of the description is that all output grounds are tied to input 1 ground.  In a PMD all grounds are tied together.  In this case the ground conductor on input one would need to be rated for 100A, not the 20A rated 12ga wife that it's going to be connected to.  The device would also need to be rated to 100a which it's not.
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Rob Spence

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Re: Star ground.
« Reply #6 on: March 23, 2017, 12:08:28 am »

No good.  My interpretation of the description is that all output grounds are tied to input 1 ground.  In a PMD all grounds are tied together.  In this case the ground conductor on input one would need to be rated for 100A, not the 20A rated 12ga wife that it's going to be connected to.  The device would also need to be rated to 100a which it's not.

Ok.


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Keith Broughton

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Re: Star ground.
« Reply #7 on: March 23, 2017, 06:16:06 am »

SOLUTION: Use only one ground source for the entire system.
NO!
The solution is to use transformer isolation between offending components.
+1 on the single ground wire gauge being too small.
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Geoff Doane

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Re: Star ground.
« Reply #8 on: March 23, 2017, 09:42:41 am »

The problem with this kind of gadget is that if the single ground fails, then all chassis in the sound system are tied together and a line-to-chassis fault on one circuit will result in a reflected voltage on everything else plugged into it. I'm not so worried about the EGC bus being too small since it's unlikely you'll have multiple line-to-chassis faults at the same time. But this is all grounding eggs in one basket, so that better be a perfect single ground or there will be serious trouble.

Simply adding ground wires from the other four input flanges to the ground buss would make the device safer and probably more useful.  Although because the original builder pop riveted the flanges in place, it might be difficult to get at those particular terminals.

You don't really care if there are ground currents on the line side of the poor man's distro.  As long as you have your act together with your gear, there shouldn't be anything significant flowing on the load side.  Assuming that the bus bar is good for 100A, connecting all the line grounds at one end, and all the loads at the other should keep the loads at the same potential no matter what happens.

GTD
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frank kayser

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Re: Star ground.
« Reply #9 on: March 23, 2017, 10:06:27 am »

Simply adding ground wires from the other four input flanges to the ground buss would make the device safer and probably more useful.  Although because the original builder pop riveted the flanges in place, it might be difficult to get at those particular terminals.

You don't really care if there are ground currents on the line side of the poor man's distro.  As long as you have your act together with your gear, there shouldn't be anything significant flowing on the load side.  Assuming that the bus bar is good for 100A, connecting all the line grounds at one end, and all the loads at the other should keep the loads at the same potential no matter what happens.

GTD
The box itself is grounded via bus bar, no?  ...and therefore the rivets/flanges.  Why pull rivets to attach ground wires?
Add ground wires from additional inlets ground terminals to the bus bar would effectively turn it into a PMD, so ground wire size would be "handled" and there would be no reliance on that individual input ground system.   So, basically, all this "distro" would do is to get all the inlets and outlets at the same ground potential, correct?
frank
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Matthew Knischewsky

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Re: Star ground.
« Reply #10 on: March 23, 2017, 11:25:43 am »

The box itself is grounded via bus bar, no?  ...and therefore the rivets/flanges.  Why pull rivets to attach ground wires?
Add ground wires from additional inlets ground terminals to the bus bar would effectively turn it into a PMD, so ground wire size would be "handled" and there would be no reliance on that individual input ground system.   So, basically, all this "distro" would do is to get all the inlets and outlets at the same ground potential, correct?
frank

The flanged inlet/outlet connectors are non metallic, so they're not grounded. Even with connectors that have bonded metallic parts (duplex receptacle) there should still be a wire attached to the device's ground terminal.

Geoff's suggestion that it could be difficult to add the ground wires is a good one. Using pop rivets during assembly is convenient at the time but can make servicing a pain. Screws and nuts take longer to assemble but are easier to repair when the time comes. Eventually those terminals will need to be tightened up...

If the ground wires were installed to all of the inlet connectors it would basically be a twist lock PMD.
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Stephen Swaffer

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Re: Star ground.
« Reply #11 on: March 23, 2017, 12:38:50 pm »

I would agree that adding the grounds to all inlets would make this a reasonable solution.  Wired as is, it is a major safety hazard and should not be used-as others pointed out you are relying on one ground that may or may not get connected-and Murphy still lurks in my parts anyway.

The size of the EGC (grounds) is determined by the breaker supplying the system-so assuming that all of the 20 amp inlets are fed from circuits protected at 20 amps as they should be, then a #12 ground is sufficient.  Multiplying the grounds to get to 100 amps doesn't make any sense and is not found anywhere in the NEC.

Perhaps the biggest hazard is the one present with any PMD-if the ground potential of 2 sources is different, there could be significant current flowing in the ground wires which could present a problem.

The basic premise of grounding is that anything on the same premises should be bonded together-so some provision need to be made somehow to bond everything together if multiple sources are used.  Of course, arranging to use one source solves a host of problems and should always be the preferred method.
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Steve Swaffer

Jean-Pierre Coetzee

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Re: Star ground.
« Reply #12 on: March 23, 2017, 01:02:27 pm »

If I understand correctly they are saying you need 100A because all 5 connectors are linked to 1 ground connector. That means that should there be some significant failure there could be 100A shunted to ground. If that was only a 20A rated cable it could catch fire quite easily.

A lightning hit and some cheap badly designed surge protectors can very easily cause that situation if not worse...
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Stephen Swaffer

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Re: Star ground.
« Reply #13 on: March 24, 2017, 12:48:34 pm »

Wire ampacity is primarily determined by insulation and the effect heat has on it.  If I run a 100 amp circuit, I am required to use an 8 AWG ground which using the same temperature rating as that which gives #12 a 20 amp rating only gives #8 a 40 amp rating-and dumping 100 amps fault current onto the ground circuit with a 100 amp circuit is many thousands of times more likely than having 5 20 amp circuits simultaneously dump 20 amps to ground to add up to 100 amps-and even so, it would only have to carry that fault current for a few miliiseconds before the breakers started tripping.

FWIW, check out the ampacity of #12 TGGT wire-you will find it is rated at 55 amps-the copper is not the limitation on how many amps can flow through a given wire-ohms law will determine the maximum ampacity.  The only other limitation is temperature-TGGT at maximum ampacity will run 482 deg F, the copper won't melt until 1985 deg F.

I can run multiple circuits in a conduit-and share an EGC.  The size of the EGC is determined solely by the size of the largest breaker supplying those circuits-regardless of the number of circuits. 
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TJ (Tom) Cornish

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Re: Star ground.
« Reply #14 on: March 24, 2017, 02:48:42 pm »

Wire ampacity is primarily determined by insulation and the effect heat has on it.  If I run a 100 amp circuit, I am required to use an 8 AWG ground which using the same temperature rating as that which gives #12 a 20 amp rating only gives #8 a 40 amp rating-and dumping 100 amps fault current onto the ground circuit with a 100 amp circuit is many thousands of times more likely than having 5 20 amp circuits simultaneously dump 20 amps to ground to add up to 100 amps-and even so, it would only have to carry that fault current for a few miliiseconds before the breakers started tripping.

FWIW, check out the ampacity of #12 TGGT wire-you will find it is rated at 55 amps-the copper is not the limitation on how many amps can flow through a given wire-ohms law will determine the maximum ampacity.  The only other limitation is temperature-TGGT at maximum ampacity will run 482 deg F, the copper won't melt until 1985 deg F.

I can run multiple circuits in a conduit-and share an EGC.  The size of the EGC is determined solely by the size of the largest breaker supplying those circuits-regardless of the number of circuits.
This is not directly related to the thread and not challenging your post in the least, but I think it's worth pointing out that the NEC is published by the NFPA - the National Fire Protection Association.  As such, they are concerned primarily with life safety - both fire and shock, and are much less concerned with functional best practices, such as how much power you lose heating up your wire to 75C or the 482F in your example above.  :)

I'm always amazed at how much money and effort people spend trying to get another couple dB out of their system (a 3dB increase in SPL requires doubling your electrical input, all else equal), but then hang their system off of undersized/overlength cords or other inadequate wiring.  QSC PLX series amps have been criticized because they shutdown at about 90 volts.  Seriously?  Maybe it's the 90 volts that's the problem, not the amp.
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Stephen Swaffer

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Re: Star ground.
« Reply #15 on: March 24, 2017, 05:27:03 pm »

I agree-and the number they should be looking at is the resistance of the wire, which is directly related to the voltage drop.  "Ampacity" is really only related to the fire hazard-which other than the distraction of a cord going up in smoke is minimal in many outdoor situations!  If you do the math, voltage drop is usually far more limiting than you think it is. It only takes 135 feet of #12 cord to equate to a 30 volt drop at 20 amps.

Inspectors and instructors often remind electricians that the code is a minimum safety standard, NOT an engineering guide-though far too often it is used that way.
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Steve Swaffer

David Buckley

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Re: Star ground.
« Reply #16 on: March 26, 2017, 04:51:47 pm »

How is this any different than the "poor mans distro" ?

If you go back in time far enough, this was exactly the way the original PMD worked.

It was subsequently updated to the way it is today with every ground wire being commoned.
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Re: Star ground.
« Reply #16 on: March 26, 2017, 04:51:47 pm »


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