Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums > AC Power and Grounding
Power from multiple sub panels
Lyle Williams:
--- Quote from: Steve M Smith on December 04, 2013, 02:57:16 AM ---The ground connections will all be connected together at some point in the building so I wouldn't worry about that.
--- End quote ---
Then there is nothing to worry about. All grounds are perfect and all grounds are equal.
Steve M Smith:
--- Quote from: Lyle Williams on December 04, 2013, 05:40:20 AM ---Then there is nothing to worry about. All grounds are perfect and all grounds are equal.
--- End quote ---
The problems start when you start to add additional grounds such as the shields in link leads causing ground loop problems.
Steve.
TJ (Tom) Cornish:
--- Quote from: Lyle Williams on December 03, 2013, 10:25:13 PM ---Induced noise due to different ground potentials would be my fear. Fears don't always turn into reality.
I'd manage the problem by keeping clear demarcations on what is powered from what feed. I definitely wouldn't run a desk and an effects rack from different power! (Too many unbalanced insert connections)
Every signal that crosses from one power system to another should be balanced and may need a xlr ground lift or better still a transformer.
--- End quote ---
This is good practice. Unless there are severe electrical problems, the current-carrying conductors in both the supply side and load side will be separate, so it makes no difference where the power comes from. Any potential issues would be from currents on ground conductors (which are always there, even if all circuits are sourced from the same panel) affecting poorly-designed equipment with "pin-1" problems.
This can be mitigated to a large degree with the "Poor Man's Distro" concept: bring all of the power together via some means - i.e. multiple rack-mount power strips in the same rack, and then ground them together.
This can be as simple as scraping the paint off the rack rails and the backs of the rack-mount power strips so the strips are grounded together through the rack rails. This is perfectly legal, safe, and other than scraping some paint off, doesn't require modifying any equipment.
Another way to do it is to build a project like this:
http://www.padrick.net/LiveSound/BarDistro/BarDistro.htm
Ground loops are frequently misunderstood. Here are some things to keep in mind:
- Ground loops are everywhere, and cannot/should not be totally eliminated.
- Ground loops are generally a harmless result of correct grounding practices - most devices need to be grounded with their electrical supply connections, and many devices that are metallic need to be grounded together to ensure voltage drops don't build up between different pieces of equipment.
- The cause of trouble is current on the ground conductors, caused either by leakage through a device, or more commonly induced from the current-carrying conductors near the ground conductors.
- The magnitude of this ground loop current depends on the length and size of the grounding path, the amount of device leakage, and the amount of current flowing near the current-carrying conductors. While it may generally be the case that ground loop issues are reduced if circuits come from the same panel, this is not axiomatic. Say I have a sub panel at location A with heavy-gauge feeders to the main distribution, and another sub panel at location B near the stage, 200' from the distribution, also with heavy feeders. Most of my audio circuits come from panel A. If I run a 20A circuit from panel A 200' to the stage through 12-ga wire, it's actually possible that I will have more current on the ground of this long circuit from panel A than if I ran a circuit from panel B which is close by, since the size of the grounding conductors from panel B back to distribution is so much larger than the ground conductor of the long branch circuit from panel A.
- It is never OK to lift the equipment ground - i.e. do not remove the ground pin of any power plug. This may appear to solve the ground loop problem, but you've now created a huge safety issue, as the primary function of the equipment ground is to conduct fault currents away. If the equipment ground is missing, this fault current will now go through the signal grounds, and/or the person touching the signal ground - the vocalist, guitar player, etc.
Having a few DIs with ground lift functionality and a few isolation devices like these: http://www.radialeng.com/r2011/twiniso.php should solve most problems. If you still have trouble, it's time to look at repairing or replacing equipment with pin-1 problems.
Frank DeWitt:
--- Quote from: TJ (Tom) Cornish on December 04, 2013, 08:31:31 AM ---This is good practice.
Having a few DIs with ground lift functionality and a few isolation devices like these: http://www.radialeng.com/r2011/twiniso.php should solve most problems. If you still have trouble, it's time to look at repairing or replacing equipment with pin-1 problems.
--- End quote ---
+4 on the whole post.
Lyle Williams:
Man those radials are expensive! I think I'll stick to my cheapie ART boxes. Not as good but at least I can afford to have enough on hand.
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