To avoid a lengthy discussion of AC power generation & transmission I'll only deal with the most common AC service we normally encounter in professional touring; the 5 wire 120/208 three phase "Y" power we use in the US. It has three phases (hot legs), each 120 degrees apart & 208 volts between each phase, plus a neutral and a ground. 120/208 is translated as 120 volts phase to neutral (not degrees), and 208 volts phase to phase. FYI ... the phase to phase voltage in three phase "Y" systems is the square root of three, times the phase to neutral voltage.
To help you understand about why the phases are 120 degrees apart, first imagine the audio analogy most sound engineers are familiar with: Two identical signals summed together that are 180 degrees out of phase (phase reversed) cancel each other out (2*180=360 degrees of total phase). If you had three identical signals summed together each 120 degrees out of phase, they too would cancel each other out (3*120=360). Assuming three identical phase-to-neutral loads the theory is that in the current domain, the summation of AC current on the common neutral will be a net zero. This is because the load on each phase returns the same amount of current 120 degrees apart from the next. In our audio systems the phase to neutral load for each phase is always different and unpredictable considering the complex loads and program material we amplify - making each of the three loads continuously and independently variable. For that reason we should not have any hope of seeing zero neutral current in a three phase "Y" system powering a PA system. However, most three phase powered systems attempt to balance the phase loads as much as possible to keep the neutral current within the neutral cable's current capacity rating considering there are three hot legs and only one neutral to return the current from all three. If the three phase system loading is way out of balance, a condition can easily result where the current flow in the neutral is in excess of the neutral cable's rating but not on any single phase leg - overheating it and creating a potential electrical hazard. This can easily happen when you consider that the neutral cable is generally the same gauge as the other phase legs.
There seems to be at least two common practices for trying to balance three phase AC distribution for portable audio systems;
1) Phase X to stage left, Phase Y to stage right, and Phase Z for Backline, monitors, and front of house
2) Run all three phases everywhere and try to balance the loads locally in each rack or area.
The first method is better for overall hum & noise reduction because the backline musical equipment, monitor system, and the front of house electronics are all powered from the same phase. The down side of this is that the FOH (front of house mix position) AC cable run is often a single circuit and the FOH suffers a drop in AC line voltage because of the cable’s long length (typically 100 to 300 feet), inherent resistance, and high current draw of today’s larger complex FOH setups. Also the backline, monitors, and front of house combined may not draw as much current as one side of a PA system especially in larger systems resulting in a possible unbalanced three phase load.
The second method stands a better chance of balancing but is trickier to keep quiet. Careful phase selection of a single phase to power the monitor and front of house consoles seems to help considerably. It is now the view that the three phase power we use is also a major source of hum in our audio systems due to reactive currents reflected into the ground (8- Glasband "The Origin of Balanced Power"). Even if you are only using a single AC circuit it is more than likely it is one phase of a three phase service. For more detailed explanations of how AC and its transmission conspires against quiet audio systems please see the excellent collection of technical articles presented on Equi=Tech's web site. By the way, Equi=Tech also has an excellent chart called "World Power Standards" showing voltage, frequency and connector type for each country - valuable information for the globe trotting sound engineer.
Having a three phase service run to the FOH greatly reduces the voltage drop due to the long cable run by spreading the load out over three circuits of power. Cable resistance is why there is very often a drop in voltage measured at the FOH as compared to the stage. The length of cable (often a single 10-12 gauge circuit) has a resistance just like the 2/0 cable example only higher. As in the neutral-ground voltage explanation, the smaller the AC wire gauge, the higher the cable's resistance and the more voltage will drop across it under load - lamp cord is 18 gauge for instance. As the AC load at the FOH increases (from more gear being used), its collective AC load impedance decreases to the point where the resistance/impedance of the cable becomes significant in a series voltage divider fashion (more Ohm's law) in that the load impedance begins to approach the impedance of the AC cable itself. For clarification of this principle I'll use an extreme example: If a single circuit FOH AC load impedance was the same as the AC cable's impedance (resistance @60Hz) the voltage under load at the FOH would drop in half from 120 volts to 60 volts - 60 volts dropped on the actual equipment load and 60 volts dropped on the AC power cable's own impedance (like powering two speakers in series, each gets half the total voltage if they are both the same impedance). If the power at the stage is 120V and you are measuring 110V at the FOH ; 10 Volts of AC power are being used to "heat up" your FOH AC cable's inherent resistance! If you run a second FOH AC cable and split the load evenly, the drop in voltage on each line will be much less severe. For this reason I prefer at least two discrete AC circuits at the FOH. On some rock festivals for example, where there are often two or more complete FOH setups, I'll run two three phase FOH AC cables. Typically this results in six 20 amp circuits for a total of 120 amps of potential power, which of course I do not need unless I'm also powering some delay speakers there but it keeps the voltage up at the FOH very nicely. Some systems I've seen use an additional transformer or Variac at the FOH to kick the voltage back up due the cable loss. That's a cumbersome and inelegant solution. If you're losing more than 10% of the supply voltage at the FOH you can bet that your AC cable is getting hot and could be in danger of melting its contact housings. If the AC source is 120 volts and the FOH voltage has dropped below 115, I'll start looking for a solution - either load redistribution or a second AC cable run.
Three phase power distributed to every location in a system is not without its drawbacks. To check out one glaring potential problem see Overvoltage due to neutral loss in the three phase systems.
Coming directly from the secondary windings of a transformer, AC power is naturally "floating" until one leg of the secondary of the transformer is bonded to the local earth ground to create the "neutral". The US National Electrical Code (NEC) states (and most countries') that the neutral wire of the AC service transformer secondary be bonded (referenced) to the local earth ground. Ground is not bonded to the neutral in some remote areas of the world. This is called a "floating" neutral. - when you test the voltage you see the full line voltage across hot to neutral as normal, but you might see roughly half the line voltage between hot to ground and roughly half the line voltage between "neutral" to ground - two hots essentially although this type of power configuration is a rarity to encounter as bonding the neutral to the local earth ground has been adopted by most countries. Similar but very different is balanced AC power which is covered later.
[ Ground ] [ AC Ground ] [ Neutral ] [ N>G voltage ] [ 3ø AC Power ] [ Chassis Gnd ] [ Audio Ground ] [ Referencing ]
