PAT TESTING FUSES
One aspect of electrical appliances that tends to cause a lot of confusion is the question "What fuse should this appliance have?" In this article, we look at fuses in detail and give guidelines on how to work out the correct fuse for the appliance.
What are fuses?
A fuse is an electrical component that is designed to prevent the flow of current that is higher than the value of the fuse. Eg a 3A fuse will "blow" if the current through it exceeds 3A.
Fuses are used in circuits to prevent excessive currents flowing under fault conditions and causing a fire. All mains plugs used in the UK are fitted with 3A, 5A or 13A fuses. The primary purpose of the fuse is to protect the mains cable and prevent it melting and/or catching fire under fault conditions.
The value of fuse used is based on the power of the appliance. A higher powered appliance, such as an electric heater will take more current and will typically require a 13A fuse along with a cable that can carry this level of current.
Fuses are colour coded for easy identification as shown below.
3A - Red
5A - Black
13A - Brown
Fuses in moulded mains plugs
As these plugs are moulded to the cable the fuse in these can be fixed by the manufacturer. They know what the maximum current that the cable can carry and the fuse is selected to protect the cable and "blow" if the current exceeds the capacity of the cable.
The value of the fuse that is fitted inside the moulded plug is marked on the outside. This makes the job of checking the fuse in a moulded plug very easy. Just lever out the fuse and check that the value is the same as that marked on the plug.
Fuses in rewireable mains plugs
These are plugs which the user can wire. These plugs can handle a maximum of 13A of current and are marked as such. Many people assume that this means a 13A fuse should be used in this appliance which is a mistake.
The fuse used in rewireable mains plug is sometimes marked on a paper label fixed to the plug or is referred to in the user manual. But in many cases, the person inspecting the appliance has to work out the value of the correct fuse.
First of all one needs to find out what the power consumption of the appliance is. This is usually indicated in Watts on the rating plate.
An iron is fitted with a 13A fuse and the power consumption of the appliance is 1900W. As this is greater than 700W, this is the correct fuse for the appliance.
If the cable on this iron is being changed, then it is important to replace it with an identical type, capable of carrying 13A.
The advice on the rating plate of this lamp says that the maximum lamp that can be fitted is a 40W one. The plug is fitted with a 13A fuse.
As the power rating is so low, we can quickly see that the fuse is too high and has to be changed to a 3A one.
This sandwich toaster has a plug fitted with a 13A fuse and according to its rating plate has a power consumption of 700W. As this is right on the decision threshold, what should we do?
In general, heating appliances are fitted with a 13A fuse. If in doubt, check the cable used for this sandwich toaster. If it is capable of taking 13A, then a 13A fuse is the correct choice.
What is the relationship between Watts and the fuse rating?
There is a simple calculation to determine this. Just divide the Power (in Watts) by 230V (mains voltage in the UK) to work out the current (in Amps) taken by an appliance.
In Example 1 above, this would be 1900W/230V which gives us a current of 8.3A. One can see that a 3A or a 5A fuse would blow constantly if used in the plug for this iron leading us to choose a 13A fuse.
This is how we get to our 700W decision threshold. An appliance with 700W power consumption takes a current of 700W/230V which is 3A. Below this we can use a 3A fuse and above this we use a 13A one.
A travel kettle for use around the world on either 110V or 230V has a 13A fuse fitted and is rated at 600W. The cable used is capable of taking 13A and the kettle has a switch to select 110V or 230V operation. Should we change the fuse to 3A as this is below our 700W threshold?
If we use the calculation above, we can see that the current taken by the kettle is only 2.6A (600W/230V). So a 3A fuse should be acceptable.
However, when the kettle is used, say in the US, the switch would be set to the 110V position. When the kettle is in use, it will take 5.5A (600W/110V). So a 3A fuse would keep blowing.
On this travel kettle, as it is designed for 230V/110V use and has a 13A cable fitted, the use of a 13A fuse is the correct choice.
This drill has a power rating of 570W but is fitted with a 5A fuse. Using our calculation it only takes a current of 2.5A (570W/230V). Why is it not fitted with a 3A fuse?
Some appliances, like drills take a surge of current when switched on, which can exceed it's normal operating current of 2.5A. If a 3A fuse is fitted, one would find that this would frequently blow in operation.
In this case, the 5A fuse that is fitted is the correct choice. Remember: If the fuse fitted is 3A or 5A do not change it.
Once a plug has been checked for the correct value of fuse, it is good practice to "seal" the plug. If on future inspection this seal is intact, then this is a good indication that no one has tampered with the wiring or changed the fuse.
How can you tell if a cable can take 13A?
The size of a cable is indicative of its current carrying capacity. So the easiest way to check a cable's current rating is to check its diameter. If the outer diameter of a cable is around 7.5mm or more then it is very likely to be rated for at least 13A. Each conductor also has 40 wires in it.
Remember: If the fuse fitted is 3A or 5A do not change it just because the cable rating is 13A. Only check the cable rating is correct if the fuse is 13A.