This issue came up as a topic to be addressed in a session about electrical safety to which I was invited to discuss.

The answer at first sight would be logical: all products must comply with the regulations issued by the legal entities of each country, in the case of products sold in European Union countries enacted by the same to be marked with the “CE” marking. As these standards are very specific and strict, specially in terms of safety criteria and are constantly evolving, the probability of this happening under normal use conditions will be practically null.

In summary, these rules for electrical equipments state that they must have additional means of protection for the user in the event of a failure. For equipment with metal case, an internal failure in the electrical insulation could put the user in danger of electric shock when touching its metal surfaces. In order to prevent that, an earth connection of the electrical installation is added as an additional means of protection, to which this energy will be directed and thus protecting the user. This, in turn, will be detected by the residual current circuit breaker of the electrical installation and automatically turned off.

For class II devices no protective earth is required, however the additional safety level must be ensured by means of double or reinforced insulation according to EN60950, or equivalent depending on the country. Double insulation are two independent insulation layers that, in the event of a failure in one level the protection remains ensured by the second one. Reinforced insulation, on the other hand, is just an insulation layer but with the same level of protection as double insulation, such as increased distance between electrically active parts.

The chargers for electronic equipments that do not have earth protection are classified as class II, so in it construction is required double or reinforced insulation between the mais supply connected section and the electrical parts accessible by the user like the metal tab of the connector which can extend also to other connected devices. For that case the distance to assure the reinforced insulation usually is greater than 6.4mm, depending on the operating conditions and specific regulamentations.

So how can we guarantee the isolation between the mains supply and the user accessible parts?

To answer this question we need to look to the internal construction of the adaptors. The most common are high frequency AC / DC flyback converters, which uses a transformer as a mean of energy transfer from the primary (mains supply side) to the secondary (user side). Besides the transformer, other typical components that touch the primary and secondary side are the optocoupler used for output voltage feedback and a bypass capacitor. All these components must be safety rated with double or reinforced insulation.


The transformer is the main component of the converter and its basic construction consists of one wire winding for the primary and other for the secondary over a ferrite core. It work principle is in switched mode, that is, in the first stage of the cycle it stores energy in the ferrite core from the mains network through the primary and in the next phase it releases that energy to the load through the secondary winding.

How the two windings are overlapped, their insulation becomes critical in terms of safety. To overcome that, in special in very compact designs is used a wire with triple insulation (TIW, or triple insulation wire) to guarantee the required protecion level. In addition to that, how the ferrite core material is conductive is also required to guarantee it safety distance.

Image 1: Transformer of a flyback converter

This transformer is constructed with TIW and it distance from the core to the secondary side is 8.7mm, ensuring the required level of security

Primary – secondary capacitor:

The fast transients of the switching voltage in the transformer causes high frequency noise in the secondary that can conducted or radiated by the cable, causing interferences with nearby devices. To reduce that effect and comply with the electromagnetic compability standards a capacitor is added between the secondary and primary to conduct that noise back.

As the failure of this capacitor can result in fatal electric shock, it must be classified as type Y1 according to EN 60384-14. This means that, compared to normal capacitors, they have greater electrical and mechanical reliability, have double or reinforced insulation class and withstand transient voltage up to 8kV.

Image 2: Primary-secondary capacitor

In this capacitor it is possible to see it type (X1Y1) and it certification bodies


To regulate the output voltage and / or current of a converter it is necessary to send back that information to the control stage in the primary. The most common technique uses an optocoupler to feedback that information. Like the previous components, the optocoupler requires also double or reinforced insulation, and as such it must comply with the EN60747 standard.

Some controllers obtains this feedback through an auxiliary winding in the transformer. The advantage of that is not requiring the optocoupler circuit, but at the cost of an increased complexity in the control stage and worst regulation.

Image 3: Optocoupler in a power converter

Additional optocouplers may also be used for overvoltage protection or for independent voltage and current regulation

Other components

In addition to these components, reinforced insulation will also be required in the printed circuit board and between any other component involving the primary and secondary. The purpose of these requirements is to guarantee the safety in any situation, making the answer to the question at the beginning clear: no, it is not possible.

However, by doing a brief search on the internet about this subject the results were surprising: several news were immediately found in reliable sources that reported several cases and in different places of people who were electrocuted by electric chargers of mobile phones or tablets.

So, how that can happen?

To answer this question, some adapters were carefully analyzed, being specially intriguing the compatible adapters sold at very low prices. The results resume is in the images below:

Image 4: Not enough clearance

The 1.6mm of distance between primary and secondary is much below the minimum required to guarantee the safety

Image 5: Distance between the transformer pins and ferrite core

Not enough distance between the transformer primary or secondary and ferrite core neither triple insulated wire

Image 6: Primary-secondary capacitor

Non Y1 safety rated capacitor between the primary and secondary

In addition to these examples, there are others that become more difficult to detect, such as the certification of optocouplers and in some cases the insulation of the transformers.

These results were clear: many were unable to comply with the requirements of safety standards. Even more serious, several security flaws were detected on the same adapter. If using these adapters in a normal conditions the risk is high, in cases of moist environment or transients in the electrical network, this equipment can become fatal. Many of the cases occurred when people put the cables in their mouths or used the appliances in bathrooms. In other words, the problem is not with safety standards, but with non-compliance.

When disassembling the original adapters provided with the equipments, even though they are very compact all of them seems to meet the safety requirements.

What can we do?

The conclusion that we can draw is that we should always use the original adapters or provided from reliable manufacturers, since they have an increased responsibility. In case of doubt or if you feel any unusual electrical discomfort with the equipment, do not use the adapter. We must also avoid using the equipment while charging, especially in damp environments such as bathrooms and paying special attention to children so that never put the adapter plug or the equipment in the mouth.

Finally, it is important that the electrical installation is protected by a low residual current detector (30mA or less) for greater safety and maintained in good condition. With these examples it has been shown that a low-priced converter can become very dangerous.