Class II product: is it safe to measure high voltage between SELV output and earth?
Class II product: Is it safe to measure voltage? The answer to this is technical. Electrical appliances using mains voltage must at least provide two levels of protection to the user of the product. The protection ensures that if one of the two layers of protection fails, the backup layer will still be in place.
A Class I product only have a single layer of insulation, and therefore require a connection to earth. A Class II product does not require a connection to earth. The basic requirement is that no single failure can result in dangerous voltage becoming exposed so that it might cause an electric shock, even without relying on an earthed metal casing. This is achieved by having two layers of insulating material between live parts and the user.
Do notice that “Class II” should not be confused with “Class 2”, as the latter is unrelated to insulation.
If you connect a high impedance voltmeter to the output of a Class II product (power supplies, charger, etc. without connection to earth) you may still measure a voltage of 200-30 VAC, or 80-100 VAC if you measure to earth. This voltage may in some cases be felt like a "tickle" if touched by humans and may be visible as a small spark when connecting the output to other equipment. But there is an explanation behind it, even with the two layers of protection.
The reason is that there is a small capacitance connected between the output side (low voltage output) and the input side (mains) of a switch mode power supply. This allows for a small current to flow back to the mains or earth, making a reference to the measured voltage.
The main contributor to this capacitance is the safety approved Y-capacitors connected between the input circuit and the output circuit. There is also a small capacitive coupling between the primary- and secondary windings in the transformer.
Y-capacitors are designed to enhance electrical and mechanical reliability standards. Capacitance values are also limited to reduce the current passing through the capacitor when AC voltage is applied, and reduce the energy stored to a safe limit when DC voltage is applied. However, this is normal, and not the cause of a faulty product.
The Y-capacitors are used in all switch mode power supplies and chargers for EMC-purposes (Electromagnetic Compatibility). The ratings of these components are regulated in international safety standards by setting a maximum allowed limit for the leakage current (also called touch current). Maximum leakage current is 0,25 mA. This applies to medical systems, and low earth leakage current is required.
Safe to use If the voltage is loaded, a small leakage current will flow through the capacitance. But if loaded more, the voltage will drop (as the circuit is floating it will not sustain the voltage when loaded). The limits for this leakage current is internationally set to safe values for humans. The values are 100 uA for medical equipment and 250 uA for other equipment. Both are measured through a defined network (2 kohms). It is therefore regarded safe to use, and measured voltage is to be expected.
To eliminate voltage, one must either isolate user accessible metal parts from electrical parts or isolate the user from access to metal parts connected to electrical parts. Alternatively, you must use a Class I (safety earthed) power supply/charger and connect accessible metal parts to safety Earth.