Electrical Safety in product design

If your product is going to be powered by batteries or use an external plug top supply, then this subject is not a priority for you. However a well rounded electronics engineer should have an appreciation of electrical safety in a product. If your product is going to use mains voltage, then it is vital that you have a good understanding of this.

(This is an extract from the book "From Prototype to Product - A Practical Guide for Electronic Engineers" available from Amazon as a print and e-book)

Low Voltage Directive

All electrical appliances supplied within the EU must comply with the Low Voltage Directive (LVD). This ensures that equipment provides a high level of protection to users. Any voltage greater than 30V AC or 60V DC is regarded as hazardous and users should be protected from direct contact with this without the use of a tool. A single fault should not result in users coming into contact with these voltages.

Class of construction

All electrical appliances using mains voltage have to provide at least 2 levels of protection to the user. This is to ensure that if one of the protection layers were to fail, there is the back-up of the second layer still in place. This makes electrical equipment very safe to use.

Depending on how exactly the protection is provided, electrical appliances are mainly of Class I or Class II construction.

Class I

Here the protection is provided by a combination of insulation and use of the mains Earth. It is best shown by referring to the photograph of an electric fire that has been taken apart.

In the plug the three wires connect to the Live, Neutral and Earth pins. Inside the fire, the brown Live wire and the blue Neutral wire connect to a plastic connector. The green/yellow Earth wire connects to the metal case of the fire.

The user is protected from electric shock by the plastic insulation of the connector. This holds the Live and Neutral wires in place and prevents them from touching the metal case of this electric fire. This plastic insulation of the connector is known as basic insulation.

If this basic insulation were to fail, say due to excessive movement of the cable and the Live wire touches the metal case then the user of the fire can receive an electric shock if not for the fact that the Earth wire is present.

By connecting to the metal case of the electric fire, the Earth wire keeps all this metal at Earth potential. What this means is that it is impossible to get an electric shock even when the metal case of the fire is connected directly to the Live voltage. In practice a fuse would blow either in the plug or the main fuse box to protect the user.

 

In summary, in Class I appliances the user is protected by a combination of basic insulation and the provision of an Earth connection, thus providing two levels of protection.

Class II

In a Class II appliance, the user is protected by at least two layers of insulation. For this reason, Class II appliances are also known as ‘double insulated’. They do not require an Earth connection.

This is a Class II electric drill which has been opened up. Inside one can see that as well as the plastic connector providing basic insulation, there is additional insulation provided by the plastic enclosure of the drill.

The user is therefore protected by two separate layers of insulation. Class II appliances are always indicated by the double box symbol on the rating plate.

Levels of insulation

Engineers can use different levels of insulation to achieve the required safety in products that are powered by mains voltage. These are generally known as basic, supplementary and reinforced insulation. This is explained below.

Basic insulation

This is insulation that is used in conjunction with another method to protect the user from electric shock.

For example, in the electric fire we looked at earlier, the main protection was provided by the use of an earth connection. However the Live and Neutral wires are prevented from touching the metal case by the use of the plastic connector which provides basic insulation. Most Class I appliances use basic insulation along with the earth connection to provide 2 layers of protection.

Supplementary insulation

This is insulation that is provided in addition to basic insulation to protect the user. For example, in the drill we looked at earlier, the plastic enclosure can be considered to be providing supplementary insulation.

Reinforced insulation

When an insulating material is such that basic and supplementary insulation are provided together then this is known as reinforced insulation. If this insulation is provided by two separate layers, then this is known as double insulation. This is the type of insulation that is found in the drill we looked at earlier.

Creepage and Clearance

These refer to the safe spacing within products when you depend on air as an insulator. If you are designing a product that uses mains voltage then an understanding of this is very important.

Creepage

This is the spacing between live parts and low voltage parts along the surface of insulation. For example, when you have live tracks and low voltage tracks on a PCB, this is the safe spacing between the two.

The specification of what is acceptable depends on which level of insulation you are trying to achieve, the type of environment and many other factors and is too complex to go into here. As a rule of thumb, when I am having PCBs laid out and am trying to achieve reinforced insulation I allow 5 mm gap between live and low voltage tracks. 

Clearance

This is the spacing through air between live parts and low voltage parts that the user can touch. For example, in the electric fire, this is the spacing between the heating element and the enclosure.

The specification of acceptable clearance distances again depends on a number of factors. In the electric fire where one is trying to achieve basic insulation, the required distance is 2mm.

Rating Plate

All electrical appliances must have a rating plate showing at least the following.

• Voltage of operation
• Power (or current consumption)
• Double Insulation mark if it is applicable
• Any special warnings (such as indoor use only)
• Appropriate approval markings (such as CE)
• Wheelie bin symbol to show users not to discard to landfill

Here are some examples of rating plates from a variety of appliances.

  

Summary

Electrical safety is an important consideration for anyone involved in the design of equipment that uses mains voltage. This chapter introduces the engineer to some of the concepts. Further study and experience will be necessary before one can become competent in this field.