"Leakage Current" is current that flows in alternative paths,
Welcome to the world's first full definition of Leakage Current. I know this as I was at a conference when someone asked what leakage was; Everyone could explain it, but even a chap who had memorized every electrical specification of note could not define it. There will be others who will claim ownership to this, but you read it here first!
For those who wonder what the heck the statement's on about, it is simple. Leakage is often determined through the difference in current between the Phase(s) and Neutral i.e. how much current leaves on the Phase and does not return on the Neutral (the Phase(s) and Neutral being current carrying conductors). So, if it hasn't returned on the Neutral it must have, well, leaked to another path somewhere.
This path is typically between the current carrying conductors and the frame of the device and, with the frame almost invariably connected to Earth, the leakage continues, but not exclusively, via the protective and/or functional Earth conductor. Other paths open to leakage currents are conductors of signal cables coupled in some way to the frame of the device, especially if such signal cables are connected to other devices offering a direct path to Earth.
Some portable equipment (such as some laptops) have no direct Earth connection (2-pin plugs) and any leakage current will therefore occur solely via any signal cables connected to the portable appliance where the other end of the signal cable is attached in some way to the earthing system.
So it's all very well making sure extraneous energy is returned to the source through a defined path. But, the fact that some bright spark (or group of them) decided that mains wiring should be fed as a Live and a Neutral, with Neutral to have the same potential as Earth, has left us with a small issue - Earth Leakage. It is a falacy that it is current that "just disappears" as current cannot flow without a complete circuit.
As an aside; There is a huge move to disassociate the word "Earth" with "Leakage" with a preference being given to "Protective Conductor Current". This is terrible move as a high proportion of leakage could exist on data cables as opposed to the Earth cable (let's call it by its real name!). Disconnecting the Earth cable on a system with an earthed signal cable will reduce the Earth cable current, but not rid the device of earth leakage!
Back to the subject..
Ask any audio specialist and you'll be told that feeding a signal in an unbalanced format is nuts! But, we're stuck with it. And, as a result, so too with leakage. The primary leakage is simply the wiring. The fact that Live wriggles around at 50 oscillations a second, reaching peak voltages of 325V, all that's now needed is a little capacitance (which all wires have) and the stage is set for leakage to occur.
Any RF nutter should be able to confirm that the typical capacitance of a piece of wire suspended in free air is a minimum of 30pF per metre. At 230VAC we need 13.7nF to cause 1mA of current to flow. At 30pF/m we need less than 500 metres of wire and the leakage is up to 1mA.
Now we did say suspended in free air. In a typical installation the wiring is far from suspended and the mere fact it is closer to earthed objects (such as conduits) the capacitance is greatly increased meaning the amount of wiring required to produce the same leakage is dramatically reduced. Even 3-core mains cable will be hard pushed to be less than 100pF/metre.
Take a look around the office. It only takes 140 metres of such cable and 1mA leakage is present. Most cables may be 3m in length, but that means we need 50 such cables. Now don't forget the kettle, the photo copiers, telephone switch boards, lamps,....... It doesn't take long! And we haven't even started with the actual building wiring yet!
Then comes technology with all the mains input filters, and pales wiring leakage into insignificance! We deal with this later in the section on "Technology" but suffice to say at present that it takes, on average, only 3 pieces of standard modern technology to cause 1mA of earth leakage at 230VAC - it does not take a lot and many amps can be drawn.
Leakage, as a rule of thumb, should not exceed 0.5% of total current drawn. However, this is only a rule of thumb! When a device is turned off then the leakage current will be a lot higher in ratio to the total current draw than when operating at full whack. Don't be misled by such rules. It is probably more correct to actually specify the ratio versus operating current.
However, the issue with leakage is not that it exists - this being through the way mains is fed - but the fact that the return paths are often not defined or controlled. When the return path is defined then the design can cater for the currents involved. When not, there are two areas of concern.
Firstly, the currents may find their way through wiring that cannot cope with the leakage current and heat up, which could have dramatic consequences. Secondly, the return path may be through the building structures and as this is not the same path as the main current the magnetic fields cannot cancel. With VDUs being almost common place in areas where high currents are drawn (offices) the chances of visual disturbances are extremely high.
But uncontrolled leakage paths don't only affect VDUs. There are many devices that could malfunction with such a high magnetic field. Anything that uses a transformer for anything but the mains is open to disturbance. Modems and other telephone circuits. Magnetic microphones (as opposed to condenser types), guitar pick-ups, and other audio equipment and especially if Public Address. Even computer magnetic tape storage is prone to such magnetic fields as these will slowly erase the tapes. An annoying issue will be automatic earing aids that switch to loop mode in the presence of magnetic fields.
There is one aspect to leakage not often known amongst power quality investigators being that it is highly prone to having a DC component. Some pieces of modern technology are direct rectified and the negative tied to the chassis (actually, this started when TV sets followed this fashion). Although this does not in itself cause leakage (or at least anything to consider worthwhile), it's when such devices are coupled in some way back to earth. A good example is the antenna connection on a TV set. This is fully explained in the section on leakage in modern technology.
Again, the issue is not with the fact leakage exists, it's with whether the path it takes can carry it safely, and without disturbance to other devices.