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 Post subject: Star grounding distance
PostPosted: Tue Oct 18, 2011 5:00 pm 
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Joined: Fri Oct 22, 2010 11:02 pm
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Location: Brisbane, Australia
Hey

Basically just wondering how far you'd have to run the ground cable before it wouldn't be worth it to star ground. Where I am it's about 25 metres from the furthest receptacle back to the circuit breaker box. I read somewhere that if that distance is too high it may not be worth it and to just run normal daisy chain ground. Is this true, or is it better to star ground no matter what? I'm happy to run everything off of one duplex and power strips but obviously it'd be ideal to be able to plug in anywhere.

Any help is appreciated.

Thanks.


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PostPosted: Tue Oct 18, 2011 10:21 pm 
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... better to star ground no matter what?
Yup! :)

Actually, it's backwards from what you read, in terms of distance: there's a MINIMUM distance where start grounding isn't necessary, but no maximum. For example, if you have two sockets a half inch apart, then just daisy-chain them together, no problem. The tiny resistance over such a short distance is negligible, and there won't be any real voltage drop. But as distance rises, so too resistance increases. Therefore the potential difference can also increase, thus "ground loop" current also increases (Ohms law).

So what if your sockets are one inch apart, not half an inch? Still no problem. But if they are ten feet apart, now you have a potential problem (excuse the pun!), and star grounding is definitely needed.

So there is a point where the distance between sockets is sufficiently great that the resistance of the grounding wire could cause a problem. The question is; what is that distance? It depends on may things, mostly the type of wire you are using, and the possible voltages that could appear across the two sockets, and the acceptable ground loop current flow. It even depends on things like the supply voltage: 110v, 220v, 380v, single phase or three phase, etc. All bring different issues.

So there is no exact answer that says "anything beyond precisely 11.3752 inches is too much". The rule of thumb is basically if you have dual or triple sockets in the same single unit (same electrical box in the wall), then just wire the ground pins together with thick grounding wire and make a single run to the star grounding point, but as soon as you get to separate boxes, then go with a separate grounding run for each box.

Others might disagree, but that's the way I've been doing it for years and never had a problem.


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PostPosted: Wed Oct 19, 2011 6:02 pm 
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Joined: Fri Oct 22, 2010 11:02 pm
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Location: Brisbane, Australia
Thanks Stuart! Yeah that makes heaps of sense. What I'll end up doing is star grounding all the individual boxes in the walls except one, which will remain daisy-chained to the rest of the building. Considering at the moment that the rest of the building involves the bedroom and ensuite, I don't anticipate that anything there should have cause to give me grief. If it does I can always just not use that receptacle.

Cheers
Cel.


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PostPosted: Wed Oct 19, 2011 10:13 pm 
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If you do that, be absolutely sure to mark that one receptacle clearly, so that no audio equipment is ever plugged into it! As soon as you plug in something that has any type of connection whatever to the rest of your audio gear, you will very likely inject hum into the system, since you will be creating a ground loop like that. You should always use some method of indicating which receptacles are on the clean technical system, and which are on the dirty service system.

What I normally do when I build post production facilities, is to use different colored face-plates on the receptacles. For example, green ones for technical power, blue for technical power that is on the UPS, and red ones or plain white ones for ordinary unprotected "dirty" service power. That way, everyone knows what can be plugged in where. And be certain to let everyone else in the building know what your color scheme means, so that nobody comes along and plugs in a coffee machine to the UPS power, or a vacuum cleaner to the technical power! All heavy consumption and non-technical consumption only goes on the service circuits, audio and video equipment only on technical, and the UPS circuits are reserved only for critical equipment.

In other words, make it very clear, both visually and verbally, that there are different types of power, and that they cannot be mixed. You can even label your color-coded face-plates with text, such as "UPS ONLY" or "AUDIO EQUIP. ONLY". As soon as you start mixing things up, plugging anything in in anywhere, that's where your problems will start. And be warned: hum and noise in audio systems is a real bitch to trace and fix! Once it gets in, it appears everywhere. Believe me, from plenty of first had experience,. prevention is MUCH better than cure here... Anything you can do to prevent it from ever happening now, will save you hours and days of time later in trying to troubleshoot what went wrong.


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PostPosted: Wed Oct 19, 2011 11:11 pm 
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Thanks for the good advice. I'll know which one it is; all the power points are along the back wall except this one, which will be right next to the drum kit. When I say this power point will be daisy-chained to the rest of the house, if the rest of the house isn't actually connected (audio wise) to the studio system, doesn't that count as a separate run to ground anyway?


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PostPosted: Thu Oct 20, 2011 2:21 pm 
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if the rest of the house isn't actually connected (audio wise) to the studio system, doesn't that count as a separate run to ground anyway?
Well... yes and no! It is a separate run to ground, yes, but it is going to a DIFFERENT ground, and hence the very high likelihood that it will NOT be at the same potential as the technical ground, at the center of your "star".

- Stuart -

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PostPosted: Thu Oct 20, 2011 4:39 pm 
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Ah! I see your point. But we're actually running all our star grounds to the main circuit board, which is also where this odd-one-out receptacle is grounded, to comply with code. So in that case, they should all see the same potential as ground. I think.

Cel.


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PostPosted: Thu Jun 14, 2012 12:35 pm 
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So there is a point where the distance between sockets is sufficiently great that the resistance of the grounding wire could cause a problem. The question is; what is that distance? It depends on may things, mostly the type of wire you are using, and the possible voltages that could appear across the two sockets, and the acceptable ground loop current flow. It even depends on things like the supply voltage: 110v, 220v, 380v, single phase or three phase, etc. All bring different issues.


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