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Down to earth in Germany
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.
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
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.
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.
do not distinguish by the eye but by the mind which is the proper judge of
Seneca. 8 B.C.- 65 A.D.
"The future of television is the
Internet" Mike 2000 A.D.
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