Another take on ground hum problems and grounding practices

[J. Larry Hendry]

> From: Harry Bissell <harrybissell at netscape.net>
> ) This is a very delicate subject... I would have the Chassis connected
to earth
> ground through a capacitor here (like Fender amps)...  It is common for
most tube
> giutar amps to have a switch to allow "floating" chassis, or ground to
one side of
> the line or the other... This is a setup for a nasty shock. With the
capacitor in
> series, you are current limited (you will get knocked on your @ss but not
killed)
> without it you will be very unhappy. Those amps that have the chassis
hardwired to
> "neutral" have no place in my studio or stage use. I insisted my bass
player replace
> a Traynor he had with a hot chassis... "before the fvcking thing kills
me..."

Harry, 
	You make an excellent point here that in my opinion is one of the most
frustrating things to encounter when trying to solve these problems
(neutral connections to chassis ground).  I have found this on USA
equipment outfitted with 2 prong plugs prior to the popularity of the
separate earth ground connection.  Fortunately, much of this equipment had
a polarized plug assuring that the chassis connection always got connected
to neutral rather than hot.  However, not all of it did.  And, I have seen
these plugs replaced with non polarized ones (musicians, no doubt).

We have heard a lot of good suggestions for grounding and eliminating hum
here.  Thanks to everyone for sharing.  I would like to add one of my own
from more of a "set-up" procedure that will eliminate or reduce hum
significantly and prevent many from having to take these other measures.

It seems to me that most hum of this nature can be attributed to current
flows in the shield connections of our interconnecting audio cables.  And,
as many have pointed out, breaking one of the ground connections to the
offending device will often cure that specific problem.  However, if you
look at your studio set up from a global view and ask "what can I do to
keep the multiple ground paths from causing me trouble?", you might think
about this:

First, I must offer a disclaimer for NON-USA readers of this message.  As I
am not altogether familiar with the earth-neutral-hot leg relationships of
overseas power mains, this observation is aimed at a US audience only. 
Those of you familiar with overseas mains may add to or correct me as
necessary as this may not apply in your case.

Neutral return currents through ground can be virtually eliminated without
breaking grounds if you plan your studio AC distribution to virtually
eliminate neutral current from the conductor between your main supply panel
and star distribution point in your studio.  The easiest way to do this is
to run a separate 240 VAC supply to your studio from your main panel.  In
the US, each 120 volt leg is 180 out of phase with the other 120 volt leg
in reference to neutral.  If each leg of a 240 load is exactly equal, no
current flows in the neutral between the main supply and the studio
distribution.  If no current flows in this main neutral, no voltage drop
occurs and the potential for neutral currents to seek ground paths is all
but eliminated by the fact that under normal conditions there is virtually
no difference between earth voltage and the neutral voltage.

So, when I wired my studio, I ran one new 5 conductor cable from the main
to my studio distribution point (4 would have done).  I installed a new 240
VAC breaker in the main panel for the studio.  The conductors are used as
follows:
# 1 hot leg 1
# 2 hot leg 2
# 3 ground
# 4 neutral
# 5 neutral

The 5th neutral was overkill I admit, but I could get 5 conductor # 10 for
free, so, I doubled the neutral to further eliminate any voltage drop.

When I planned out the loads in my studio, I was careful to balance the
loads between the opposite hot legs.  Of course, only the power amps are
the big consumers, but plan a general sharing of the other loads (does not
need to be exact) between hot legs.  Although I have two 30 amp capable 120
volts circuits (180 degrees out of phase) with all my equipment working, I
have virtually NO current in the parallel neutral connection.  Therefore, I
have very little neutral current seeking ground returns.

Does this eliminate the need to solve some individual ground loop problems?
 No.  Does it keep you from having many of these problems?  Yes.  Also,
keep the power amp AC cables from the "star" ground point you have selected
in your studio to the amp short and big wire.

I was really upset by an article in keyboard mag a few years ago that was
supposedly an "expert" telling people how to do AC distribution in their
studios.  The said the hot legs of a 240 volt system were 120 degrees
apart. (No, that's only true on 3 phase 120/208).  And, they recommended
placing ALL loads on the same leg and to be sure NOT to get different hot
legs.  That was the worst possible advice (although you can be successful
with only a 120 volt service to your studio).

While I am on the subject, here are two more things about grounding and
supply that will help prevent lightning damage to your gear.

1.  For about $35 US, you can buy a 30 amp three-phase (I'll explain that
later) 240 volt safety switch.  These are designed as a disconnection point
for servicing equipment and such.  My cable from my studio to the mains
runs through this switch.  I used a three phase instead of single phase as
it has three blades in the switch.   I use one for each hot leg and one for
neutral.  The ground in not ever interrupted.  When I am using my studio, I
can "plug in" everything at once with this switch.  If I am not using my
studio, it is turned off.  This is the same idea as unplugging everything,
but more practical if you have 100 things plugging in.  If the switch is
OFF, lightning damage is virtually not possible (maybe a direct hit would
still get you).

2. If you have a computer in your studio with a phone line connected and
the PC is connected to your studio equipment you have another potential
problem.  The most frequent cause of equipment damage and serious injury
from lightning is from multiple ground connections.  Your house should have
only ONE ground.  That ground should be at or near the electrical service
entrance.  The trouble is that in the US, the damn phone company is famous
for grabbing the closest ground they can find during an installation. 
Sometimes they even use an incoming water pipe.  During a lighting strike
nearby, the voltage of the pipe and proper ground rod can be significantly
different.  If you have these two dissimilar grounds meeting at your PC,
you have trouble.  Make sure your phone grounds are tied to your proper
ground and not to any separate ground connection.

Larry Hendry