IEC protection classes for power supplies

The International Electrotechnical Commission (IEC) has set out three safety classes for power supplies — otherwise known as appliance classes or protection classes. These are used to identify and implement methods that prevent power supply users from potential electric hazards.

While these risks are divided into three clear categories, they are not always easy to follow, which is why YHY power will break down the specific meaning of each class to clear up any confusion surrounding the topic.

IEC PROTECTION CLASSES

Power supplies fall into one of three protection classes, based on the need (or not) for a protective earth connection — otherwise known as ‘earthing’. This works by providing a path for a faulty electrical current to flow to the ground, shielding users from shocks when equipment insulation fails.
 

Class I - where user protection from electric shock is achieved through a combination of insulation and a protective earthing/ground.
 

Class II - where user protection from electric shock is achieved through two levels of insulation (either double or reinforced), without the need for earthing.
 

Class III - where the input is connected to a safety extra-low voltage (SELV) circuit meaning no further protection, such as earthing, is required.



CLASS I POWER SUPPLIES
 

Class I power supplies protect the user through at least one layer of basic insulation first. It then uses a ground wire chassis — a grounding connection usually on the casing of the source — that grounds the hazardous voltage before it reaches the user should the basic insulation fail. A safety ground is a requirement for all Class I devices.

CLASS II POWER SUPPLIES
 

A Class II component, or open frame power supply, needs no earth connection for safe operation. The min distance from any live part to the enclosure must be observed whether it is conductive or not in order to maintain the two levels of protection needed against a single failure in the system. The use of a Class II external power supply is straightforward, with the main difference compared to a Class I product being that it requires only 2-core main leads for secure operation. Protection is maintained by double or reinforced insulation, rather than relying on both protective earthing and basic insulation. In this way, IEC Class II power supplies are not mandated to have a safety ground conductor brought to the power supply.
 

However, there is often confusion between the IEC Class II power supply and an NEC Class 2 power supply. The former describes the insulation protection to shield users from an electric shock, while the latter concerns the installation of an electrical system. NEC Class 2 is used to assess wiring requirements between supply output and load input, considering factors like wire size, installation, and deration (the amount of current that wires carry).  

While a Class II power supply does not require a protective earth, some lower power Class II products find applications in Class I systems, and Class II applications often use a functional earth in the system. The Class II supply is designed to meet EMC requirements for emissions and immunity. However, if the output of the supply is connected to a safety earth or a functional ground, it will create a low impedance path for noise, altering the power supply’s characteristic performance. Therefore, it is likely that extra filter components will need to be fitted external to the power supply for emissions compliance.

CLASS III POWER SUPPLIES

For Class III equipment, the power source is not dangerous, meaning the user does not require protection from it. This is defined within IEC62368-1 under the heading "classes of equipment with respect to protection from electric shock" and is defined as equipment in which protection from electric shock relies upon supply from an ES1, or Class 1 energy source, and in which ES3, or class 3 energy source, is not generated. ES1 defines an energy source that Is safe to touch by an ordinary person.