All electrical appliances using mains voltage must provide not less than 2 levels of protection to the user. Here’s in order for if one of many protection layers were to fail, there’s the back-up of the second one layer still in place. This makes electrical equipment very safe to make use of.
Depending on how precisely the protection is equipped, electrical appliance are put into 5 Classes of apparatus construction which can be Class I, II, III, 0, 01. Of those the major are Class I & II. For completeness the entire Classes are described below.
CLASS I
Here the security is equipped by a mixture of insulation and use of the mains Earth. It’s best shown by pertaining to an electrical fire that was taken apart.
In the open plug the 3 wires connecting to the LIVE, NEUTRAL and EARTH pins. Contained in the fire, the brown LIVE wire and the blue NEUTRAL wire hook up with a plastic connector. The fairway/yellow Earth wire connects to the metal case of the hearth.
The user is protected against electric shock by the plastic insulation of the connector. This holds the LIVE and NEUTRAL wires in place and forestalls them from touching the metal case of this electric fire. This plastic insulation of the connector is called basic insulation.
If this basic insulation were to fail, say because of excessive movement of the cable where it touches the metal case then the user of the fireplace can receive an electrical shock if not for the truth that the EARTH wire is present.
By connecting to the metal case of the electrical fire, the EARTH wire keeps all this metal at EARTH potential. What this implies is that it really is impossible to get an electrical shock even if the metal case of the hearth is hooked up on to the LIVE voltage. In practice a fuse would blow either within the plug or the key fuse box to guard the user.
In summary, in school I appliances the user is protected by a mixture of basic insulation and the supply of an EARTH connection, thus providing two levels of protection.
CLASS II
In a category II appliance, the user is protected by a minimum of two layers of insulation. For that reason, Class II appliances also are referred to as Double Insulated. They don’t require an Earth connection.
This is healthier shown by looking inside a category II electric drill which was unfolded. Inside you will see that in addition to the plastic connector providing basic insulation, there’s 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 at the rating plate.
CLASS III
Equipment built to Class III standard are designed to be supplied from a unique safety isolating transformer whose output is called Safety Extra-Low Voltage or SELV. This must never exceed 50 V AC and is generally is below 24V or 12V. All Class III appliances are marked by a distinct symbol. There is not any use of an Earth at school III appliance
The electrical safety of sophistication III appliances are sorted within the safety isolating transformer design where the separation between the windings is akin to double insulation. The transformer is marked as being suitable to be used with Class III appliances.
CLASS 0 & 01
This form of equipment isn’t for standard use in business or residential environments. It’s only presented here for completeness.
Class 0 appliances depend only on basic insulation for cover from electric shock. For that reason, they don’t have 2 levels of protection in-built and aren’t allowed on the market. The brass lamp shown this is an example of a two wire, metal cased appliance with only basic insulation. There isn’t any provision for connection of an earth to the bulb holder.
In Class 01 appliances, there’s provision for an Earth connection, nonetheless it is wired with either twin core cable or only has a 2-pin plug, so an Earth can not be connected. AS at school 0 equipment, one relies only on basic insulation for cover from electric shock. As they simply have 1 level of protection, Class 01 appliances usually are not allowed on the market.
IDENTIFYING CLASS I & CLASS II APPLIANCE
As the PAT testingcarried on Class I and sophistication II appliances differ, it is very important identify one from the opposite. There’s no other area of PAT testingthat causes more confusion than this and there are lots of myths surrounding this. It’ll be informative to list these kind of.
If there’s a fuse within the plug, then it should be Class I.
It is made up of metal so it ought to be Class I
The case is plastic so it ought to be Class II
It has a 3 core cable so it need to be Class I
The plug has a metal Earth pin so it should be Class I
None of the above statements is a fool-proof approach to identify Class I and sophistication II appliances and a few are quite misleading.
The easiest rule to use is the single below.
If the rating plate has a double box then the application is Class II. If it doesn’t then it’s Class I.
Example – Kettle
The rating plate in this kettle clearly has no “double-box” symbol, so using our rule, it has to be Class I. The Earth connection from the plug is terminated at the outside metal casing of the heating element.
Example – Plug-top power supply
The rating plate in this Plug-top transformer clearly shows the “double box” symbol, so this can be a Class II appliance. Note that it has a plastic Earth pin, as this isn’t required for sophistication II. (Not all Class II appliances have a plastic earth pin)
Example – Mains extension
The rating plate in this extension is moulded inside the plastic. It clearly doesn’t have a “double-box” symbol, so it should be a category I. (Note that here is an example of a plastic Class I appliance)
Example – Table lamp
The rating plate in this table lamp clearly shows the “double-box” so it’s a Class II appliance. (Note that this can be a Class II appliance it is largely in a metal enclosure). The bulb holder is fabricated from plastic and offers the necessary double insulation.
Example – Desk fan
The rating plate for this fan not just doesn’t have a “double-box” symbol, it also says that the applying ought to be earthed. So it is clearly a category I appliance. Note that it doesn’t have any user accessible metal.
Example – Metal Lamp
If this metal lamp had a rating plate, then it might be a category I appliance because it has an earth point at the lamp holder. However, because the rating plate is missing, this will likely need to be failed.
Are Class I and sophistication II appliances just as safe-
As both have 2 levels of protection inbuilt, they’re both safe for general use.
However with Class I an appliance, one of many layers of safety is equipped by the earth connection. For this to be effective, the wiring within the building needs to be inspected regularly to ascertain that the Earth within the mains socket is properly taken to the local earth potential. This can be usually picked off the Earth sheathing of the mains cable getting into the premises, or by driving an area stake into the floor. So Class I appliances rely upon the external wiring within the building to totally provide the two levels of protection.
Class II appliances however always provide 2 levels of protection regardless of the status of the wiring installation. Both layers of protection are built into the design making Class II appliances are a whole lot safer than Class I appliances.