“Hey man, there’s something wrong with your power! You've got voltage between neutral and ground!”
Ok, here's the explanation for a phenomenon that some people get really worked up over that is generally no cause for alarm or an indication there is an AC "power problem" .
Since we know that if;
1) No current should flow in the ground.
and
2) Current flows in the neutral in an active AC circuit.
and
3) Current flowing through a resistance produces a voltage (of course you knew that)
and
4) Cable has a specific resistance stated as ohms per 1000 feet of cable; (of course you knew that too)
Then: It should really come as no surprise when we measure voltage between neutral and ground because of the current flowing in the neutral wire that is not flowing in the ground even though ultimately they are bonded together back the source.Whenever current flows through a resistance it produces a voltage across the resistance!
Which in this case is the length of the neutral wire. It's basic Ohm's law. Voltage equals current times resistance. V=I*RAll cables have resistance. In many instances the distance between your portable AC system’s circuit breaker panels and the point where the main service neutral-to-ground bond takes place can be hundreds of feet. For example, 2/0 power cable is specified as having a resistance of roughly .1 ohms per 1000 feet of cable. Let's assume you are in an sports arena for a show and have completely setup a PA system and it's at idle. Your breaker panel is a total of 250' from the venue transformer where the ground to neutral bond takes place. If there is 30 amps flowing in the neutral we can roughly calculate the ground to neutral voltage by using Ohm's Law (V=I*R). This is very roughly calculated as this is essentially a DC calculation and does not include power factor or frequency!. The formula is written as V=30*(250*(.1/1000)). The result is .75 AC volts measured between neutral and ground at your breaker panel. This is not an uncommon reading. The farther an AC power distro is from the actual neutral/ground service bond, the more neutral to ground voltage will develop when it is under load. The only time there SHOULD be no neutral to ground voltage is if the system is on a separate transformer shared with no one, the ground to neutral bond is good and not too far away, and there is no power being drawn through your AC distro. Under those conditions if you are reading neutral to ground voltage this might be an indication that there is a poor neutral to ground bond from the venue's source as portable AC systems should maintain completely discrete ground and neutral paths throughout the AC system.
Having voltage between neutral and ground is not necessarily an indication that there is a power problem but you should absolutely examine your AC system connections to make sure it is not something more sinister than being far away from the source. Often, excessive ground to neutral voltage is an indication that the source ground to neutral bond is poorly executed. However, if you were using a "balanced AC power transformer" you'd be seeing 60 volts between neutral and ground! And that would be correct. (More on that later in "Balanced AC Power" )
[ Ground ] [ AC Ground ] [ Neutral ] [ N>G voltage ] [ 3ø AC Power ] [ Chassis Gnd ] [ Audio Ground ] [ Referencing ]
