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Down to earth in Germany



Dear Phil.
 
This is from Sweden. Our power systems are, contrary to the UK and the US, quite alike.
 
I have,  in my past, designed and implemented many video and audio studio installations. The principle of getting it right is simple, the practical installation is not.
 
You may have to forget "regulations" as regulations or electrical installers dont know or care if you are running up tens of volts in your earth or neutral. If you do it right the earth protection will be as good as or better than "regulation"
 
1. Establish your own earth grounding point as part of a star network in you equipment rack or other convenient spot. Use a tapped copper bar and pull 2.5 sq.mm green/yellow earth cables to each device to a point I will decribe later down.
 
2. Remove the existing earth from each devices power socket. You can do this easiest by disconnecting the earth lug in the power distribution socket in your equipment rack. Put a label on this socket so others will know it has been modified. Thus the equipment does not need be modified.
 
3. Earth/Neutral link
Our system uses separate earth and neutral wires to the fusebox. Fuses are normally only on the live wire side. Normally, in your fuse box the earth and neutral have different bars. At this point only is it where the two have the same electrical potential by a wire link. Do not remove this link. Instead run a a 10 sq.mm yellow/green wire from the earth bar in the fuse box to your rack ground bus point. This now completes the electrical safety circuit while making sure that all your devices have the same grounding potential while staying away from neutral current mismatches.
 
4. No separate earth conductor
A lot of devices today are of floating type (the flat mains plug) Its not inferior as the chassis use a double insulation. This works fine for conumers products. If you have such devices you must create an earth point. If you dont do this there is a chance that the screens of the video or audio cables will carry an earth potential differerence and this is when your problems begin...
To modify you need to have some understanding of powersupplies inside your equipment. You hould locate the secondary null point and attach a cable to a from the chassis isolated grounding screw. Doing this is easy if you are use to having your hands inside equipment. With switched power supplies it is a bit more tricky but same principle applies.
 
5. One more thing
This above decribed procedure will stop all earth ground loops that may affect video. For audio you generally need to go one step further. Again the principle for an audio screen is to offer a lowest possible impedance to earth, such that the "noise" is shortcircuited before it reaches the innerconductor and ruins your day. But, as most audio cable screens are connected at both ends they always carry a small but most often disturbing AC voltage. The remedy is easy if you already have all your equipment ground referenced. Cut the screen at the receiving end of the plug. This applies fr both XLR, Phono, patchbays etc. But it only applies to equipment that is correctly groundreferenced as described.
 
6. A final word.
Electrical contractors seldom do what you tell them simply beacaues they dont understand why. They will think you are crazy wanting to pull 2.5sq. mm wires from a local bus to each device. Make sure that no other heavy loads are loading up your fuse boxes if you have to communicate with other parts of a building. When you need to exchange signals within studios all you need to do is to pull a heavy earth cable between the local earth bus bars in your rack equipment. Install earth fault detectors for second level of safety. Note that with digital signals all may seem fine until it fails in a big way so make sure its done right from start. You will have no problem getting well above 100dbV noise floor, which is better than your 16bit digital audio.
 
Good luck
 
Mike Reichel
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