AC POWER DISTRIBUTION
By Marty McCann
Copyright 1994
When you are specifying equipment for large installations such
as a church, nightclub, or recording studio, you should also
include recommendations for a proper electrical power distribution
system for that installation. If you ignore the electrical
system, it is very likely you will be spending more time in
the future trying to solve ground loop and EMI interference
problems. More often the time spent running down these gremlins
is beyond that which you estimated as labor time for the installation.
If everything is taken care of up front, then everyone benefits.
I started working with electronics as a means of making a living
thirty-two years ago. I have been involved in audio for sound
reinforcement for twenty-six years with six years in communications
electronics and process control electronics before that. I
have come to learn many tricks (which aren't really tricks
at all) to overcome ground loops and other interference problems
in audio systems.
You can put the audio system on a dedicated, isolated electrical
service with its own power transformer. You can even establish
a "Star" ground, an AWG #000 ground buss, or even a separate
technical ground on the electrical service system to the audio
equipment. However, doing any or all of these will only solve
a few of the problems that can prevent your customer's absolute
satisfaction with the installation.
I have known for some time that if you can absolutely balance
the power distribution system, you can eliminate the source
of your problems. It is impossible to control what other electrical
equipment is placed on the rest of the building's electrical
power distribution service. Even if you could control this
situation, you would not eliminate all of the source's possible
contributions to hum and noise problems. Read on to learn more.
The conventional method of power distribution in the U.S.A.,
according to the NEC (National Electrical Code) is just not
the best that it can be for our increasingly sophisticated
audio applications. The standard electrical system is bases
on a 240-volt AC feed from the transformer on the electrical
power pole outside into the building. At the building's main
electrical service box, the 240-volt service is split into
two circuit paths of 120 volts each.
This is accomplished because there are three wires feeding
the main electrical service panel from the transformer on the
power pole, the third wire being a center tap of the 240 volt
secondary of the power transformer. At the building's electrical
service panel, this third wire center tap from the 240 volt
secondary of the power transformer on the pole is bonded to
the neutral wire and the building's electrical system's service
ground in the building's main electrical service panel. We
distribute two legs (circuits) of 120 volts each throughout
the building (It is not correct to refer to this as a two-phase
system, because it is actually two legs of the same electrical
phase).
These two legs are L1 & L2 (Leg 1 and Leg 2). L1 Hot (Black)
and Neutral wire (White) make up one leg, and L2 Hot (Red)
and the Neutral make up the second electrical power leg. Note
that both 120-volt circuit paths share the common Neutral.
If we do have a 240-volt appliance such as an electrical stove,
hot water heater, clothes dryer, etc., the 240 volts is obtained
by using the two hot leads L1 (Black) and L2 (Red).
In an ideal world there would never be more current drawn by
one leg of the electrical distribution system than current
drawn by the other circuit leg. Since we are not in an ideal
world and have no control of what is plugged into which power
leg throughout the building, we have no way to insure that
the distribution of power between L1 & L2 is balanced. Some
of our problems can be, and are, caused by compressors, motors,
and other equipment in the building that have different current
demands over time.
But in installations where we are using lots of audio equipment
and drawing significant amounts of current, it is the power
line filter capacitors and grounding schemes used in the audio
equipment itself that lead to problems. There are also things
to do in theory, because sometimes all of the best tricks can't
totally eliminate the noise problem (See the article on Hum & Pin
#1). We can minimize the problems at the time of installation,
but what happens six months down the road when we are not there
and the customer decides to hook up some newly acquired piece
of gear?
The best thing we can do for the customer is to specify a balanced
AC electrical service system. I prefer to call this a balanced
electrical service because that is what it actually is, not
a "two-phase" system, as some call it. The way in which this
is accomplished is that instead of a hot lead with 120 volts
referenced to a grounded neutral, we actually provide two hot
leads of 60 volts each. How do we do this? By specifying an
electrical power transformer that has a 120 volt secondary
with a center transformer tap.
If the customer's electrical contractor can install one of
these available center tapped power transformers and connect
the center tap to the normal ground buss, we then end up with
a transformer isolated balanced service. This service can provide
a center tapped 120 volt secondary, so we will then have 60
volts on each of two hot AC legs.
What was previously known as the Neutral wire (White) is now
the other 60-volt Hot wire. What was previously known as the
ground wire (Green) is now the system common. This common return
to the new transformer is connected to the center tap of the
120-volt secondary and is still connected to bonded ground
at the audio system's electrical service panel. Once again,
both wires are hot with 60 volts of potential each or 120 volts
between them (60 + 60 = 120). Of course now each hot leg must
include a circuit breaker. You must use double pole circuit
breakers in the service panel for each circuit, because you
now have two hot voltage legs. If the entire 120-volt circuit
draws 20 amperes of current, then each hot leg (L1 & L2) will
each have a 20-amp circuit breaker. The system is still grounded
in the conventional manner, but this is via a separate technical
ground from the normal main electrical service.
Power transformers are rated in KVA (Kilo-Volt-Amp). Electrical
power equals the electrical potential, or pressure, measured
in volts times the rate of electrical current flow measured
in amperes. In audio we measure power in watts: 2000 watts
would have a KVA rating of 2 KVA.
I recommend that the electrical contractor obtain the orange
iso-system duplex AC outlets like the ones used in hospitals
to identify those outlets that are on the isolated balanced
electrical distribution system.
In the past this approach to electrical power distribution
was not covered in the NEC code, if installed correctly, it
did not go against or violate the NEC code. It just wasn't
covered, and it may have taken some explaining to a local inspector
as to why all of this is necessary.
Since I first wrote this article, this method for a balanced
power distribution system has been written into the 1996 National
Electrical Code (NEC) under Article 530, Part "G."
So there is no excuse for not employing this method when circumstances
require absolute clean power. There are companies that now
offer turnkey systems consisting of the transformers, electrical
panel, and circuit breakers.
The beauty of this approach is that everything (audio or video
wise) you plug into this isolated circuit will be balanced
as far as the current between chassis ground and the two 60
volt hot leads. There will be no voltages between the chassis
or any two pieces of equipment plugged into this system. You
have eliminated the cause of your troubles at the source. You
have transformer isolation between your audio electrical system
and any other electrical equipment in the building.
Check out this information. Give me a call if you have any
questions or comments. For more information, check out the
article in the June '91 issue of RE/P magazine by Martin Glasbland.
Mr. Glasband's article is very good, even if he does call this
a "2 phase" system. An even better article on the subject is
in the Nov '94 issue of Mix magazine. I hope the above information
was of help to you. |
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