ptay wrote:black taj,
I was just trying to figure out what you meant by neutral currents flowing on the ground wires. The green wire should have no AC current flowing on it, right? It's just there to trip the breaker. Right?
Pure L wrote:Lots of food for thought up in here. Thanks for the replies.
So my detached garage (~25 from my house) has its own power box (containing six 20 amp circuits) running from the service panel in my house.Just make sure that the neutral is not bonded to ground in the garage panel.
I'm wondering about something though.......
Since both the house and the garage power boxes are grounded to their own respective grounding rods, will this be a problem? No. NEC requires the use of a second ground rod at any outbuilding containing anything larger than a single 20 A circuit. A ground rod's only function is dissipation of lightning strikes on the service mast.
I remember reading somewhere that this (the double grounding) could indeed introduce EMI problems. Double grounding is when you bond the neutral in more than one place, giving the ground parallel paths
Any truth to that? Is this required by code? (It was an electrician who did this.)
Also, is it advisable to do IGs for all of my outlets? Or just the ones I'll be using for audio-type stuff? As stated before, I believe in a non metallic raceway, isolated grounds are pointless. You could install them for your audio equipment and be perfectly fine. A suggestion was made earlier to run a star topology ground. There is a standard grounding system in commercial high rises that might make this installation more elegant. If you'd like, PM me and I can tell you how to install a Hogan ground
How about lights? Anything I should think about there?Electrically, the loudest thing there is are HID lighting and fluorescent ballasts. If it's not a pain, separate the lighting circuits from your power and you'll be fine.
There is no return current that matters because both hots are both return and supply. With no neutral, every 240V outlet regardless of load, distributes the current flow evenly between the two hot legs.ptay wrote:But i would argue that return currents are the same whether there on a grounded conductor or a second hot.
ktone wrote:Return current becomes an issue is if there is a neutral that is shared by more than one outlet. This is where the resistance of the neutral wire creates a voltage drop that is reflected as difference in potential between the neutral connections of different outlets on the same neutral. I'm not sure if I understand this. Are you saying that the neutral is losing voltage because it's shared with another outlet? Where are you measuring the drop from? Hot to neutral? This is especially a problem with electrically noisy gear where the noise is also reflected on the neutral only to be picked up by other sensitive gear sharing that neutral.
Return current also becomes an issue by having a ground and neutral connected at one point (as they should be). Under heavy current use, the resistance and resulting voltage drop on the neutral causes a difference in potential between the neutral and ground at an outlet that has a heavy load. Again, I'm not sure if I get this. The voltage doesn't drop on the hot in a resistive circuit? The neutral is gaining a voltage potential relative to ground?This then appears at every outlet sharing that neutral.There is no return current that matters because both hots are both return and supply. With no neutral, every 240V outlet regardless of load, distributes the current flow evenly between the two hot legs.ptay wrote:But i would argue that return currents are the same whether there on a grounded conductor or a second hot.
There can even be a difference of potential between two outlets sharing a pair of hot legs but it has no effect because it occurs on the hot legs equally. Any noise on the line, whether internal or external, is also reflected equally and oppositely on both hots and cancels itself out as well. The ground always sits exactly between the two hot legs and never encounters the stress of having a different potential than the neutral. The neutral sits between the hots as well. The potential is the same as ground because of the electrode being bonded to the neutral. It's the reason that a balanced neutral carries the difference of opposite phases rather than the sum of the currents.
The star topology makes the balanced system just that more effective.
I wasn't proposing having a bunch of transformers for stepping down. I was proposing everything running on 240V and if you do have a 120V piece of gear, using a (non-autotransformer) transformer retains the balanced effect for the 120 V gear.
This will not work here in Hong Kong or the UK (HK follows the UK system) because they use a neutral and suffer the same problems. This would only work in the US/Canada because balanced 240 is available everywhere.
As far as noise from stray magnetic fields from transformers, this is usually not an issue. We all already know how to manage this from having used tube guitar amps and other gear with big AC transformers (like just keeping your mic cables away from them). Toroid transformers are better regarding stray magnetic fields - use those.
I'm sure you have all seen the lights in a room dim when someone has a big stereo blasting in their house. If that sag (voltage drop) exists along the 120V hot it also exists on the neutral and will raise the neutral above the ground as much as the hot line sags. Now consider some newbie wiring 10KW of amps on four circuits off the same 120V leg of a distro. 2.5KW is on each circuit hot but they may all return on one neutral that now has to carry 10KW. I've measured 10 volts or more at the mixing desk when measuring from neutral to ground from amps drawing large amount of current on big high volume surges like a kick drum. I've had problems with this doing large reinforcement systems (of 50KW of amps and much greater) so I'm a little touchy about this sort of thing.black taj wrote:Again, I'm not sure if I get this. The voltage doesn't drop on the hot in a resistive circuit? The neutral is gaining a voltage potential relative to ground?
Exactly, because if you consider this nasty pulse occurring in the above 120V scenario then that noise travels along the neutral to another circuit that suffers the noise even though it has a separate and quieter hot.ptay wrote:So say one dirty little box has a switching psu, or a failed TVS, and once every cycle pulls in a brief but high current pulse on the load side of the transformer. The primary has to supply it, so you have a pulse of current and volatage fluctuation on the two wires connected to the primary. It is actually happening all the time ang because this pulse is Normal Mode it gets into other loads even if they're on different circuits.
You are correct to mention that the pulse, being common mode, stays away from the facility ground and of course has nothing to do with any neutrals. This is a big advantage.ptay wrote:The only thing to recommend this from how I understand it would be that the fields radiated by the two conductors would be the same. Perhaps our dirty little pulse is not put onto the facility ground eith, which seems good. I just dont think it will lower radiation in the studio with noisy loads or has anthying to do with shared neutrals.
ktone wrote:I took this to mean that for an instant the load increased for a brief period of time. Increased current means the two legs are closer together in potential as the current "pull" sags both lines equally and oppositely and in common mode.
ktone wrote:If the sloppy PSU was tossing back some weird harmonic EMF, I could certainly believe that may be normal mode.
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