WHAT IS LEAKAGE CURRENTS?

In any electrical installation, some current will flow through the protective ground conductor to ground (Protective Conductor Current).
In the absence of a grounding connection, it is the current that could flow from any conductive part or the surface of non-conductive parts to ground if a conductive path was available, such as a human body (Touch Current).
Leakage current most commonly flows in the insulation surrounding conductors and in the filters protecting electronic equipment around the home or office. So what’s the problem? On circuits protected by GFCIs (Ground Fault Current Interrupters), leakage current can cause unnecessary and intermittent tripping. In extreme cases, it can cause a rise in voltage on accessible conductive parts.

WHAT CAUSES LEAKAGE CURRENT?

Leakage current is caused by a parallel combination of capacitance and dc resistance between a voltage source (ac line) and the grounded conductive parts of the equipment. The leakage caused by the dc resistance usually is insignificant compared to the ac impedance of various parallel capacitances. The capacitance may be intentional (such as in EMI filter capacitors) or unintentional. Some examples of unintentional capacitances are spacings on printed wiring boards, insulations between semiconductors and grounded heatsinks, and the primary-to-secondary capacitance of isolating transformers within the power supply.

HOW TO MEASURE PROTECTIVE CONDUCTOR CURRENT?

Test according to standard 60990.
The Protective Conductor Current shall be measured by inserting an ammeter of negligible impedance (e.g. 0,5 ohm) in series with the protective conductor. Measurement of Protective Conductor Current is made with the equipment and power distribution system running in all normal operating modes.
This test will give a result composed of a 50Hz (or 60Hz) contribution and the contribution of high frequencies.

Test according to 60335-1.
First Protective Impedance is disconnected from live parts before testing. The test is carried out at room temperature and the DUT (Device Under Test) not connected to supply mains.
A test voltage equal to 1.06 times rated voltage, for single-phase appliances, and 1.06 times rated voltage divided by √3, for three phase appliances, is used. The test voltage is applied between live parts and and accesessible metal parts that are connected to metal foil having an area not larger then 200mm x 100mm in contact with accessible surfaces of insulation materials.
The current is measured within 5 seconds after the application of the test voltage.
This test will give a result of  only a 50Hz (or 60Hz) contribution.

HOW TO MEASURE TOUCH CURRENT?

Test according to 60335-1, that makes a reference to 60990.
First Protective Impedance (e.x. PE) and radio interference filters (e.x. Y2-Capacitors) is disconnected from live parts before testing. The use of a test transformer for isolation is optional.  (if used make sure not to connect PE to N after the isolating transformer).
The current is measured by means of the measuring circuit described in figure 4 in 60990 between any pole of the supply and accessible metal parts connected to metal foil having an area not exceeding 200mm x 100mm, witch is in contact with accessible surfaces of insulating materials.
E.x. of setup of DUT (Devise Under Test):

Here is a example of a Measuring Circuit:

$$I_{leakage}=frac{U_{2}}{500} (peak value)$$

This is one way to measure leakage current. It can be outdated and I strongly advise reading the current active standard that you test against (like the standard 60990 or 60335-1).