When checking the safety of an electrical appliance in the UK, a PAT Tester will be required to check that the fuse in the plug is of the correct value.
If the fuse fitted is too big, it won’t offer the protection necessary and may lead to overheating or a fire if the appliance develops a fault.
If the fuse is too small, it might blow at an inopportune time, leading to inconvenience, or the temptation to fit a bigger fuse (or even worse a nail!)
The main purpose of the plug fuse is not, as most people believe, to protect the appliance, but to protect the mains cable on the appliance. The appliance will have its own fuse if it needs one, but the cable may overheat in the event of a fault, and so is protected by the plug fuse.
The traditional method of selecting the fuse involves checking the POWER rating on the appliance – this should always be marked (in Watts) on the rating plate on the appliance. From the power rating, we can work out the CURRENT draw of the appliance. Although it is possible to calculate the current accurately using a formula (power equals volts times amps), it is sufficient in this case to use the ‘rule of thumb’ that 1000 Watts is equal to about 4 Amps. In other words, a 500W drill will draw about 2 Amps and a 3000 Watt heater will draw approx 12 Amps.
The cable should be capable of carrying the current to the appliance – in other words, it must be thick enough.
The fuse in the plug should, therefore, protect the cable – so a 13 Amp cable would be protected by a fuse of 13A or less, and a 6 Amp cable would be protected by a fuse of 6 Amps or less. (a 6 Amp fuse is not available so a 5 Amp would be used.)
More recently the manufacturers have attempted to simplify fuses, to make the system easier for the public to understand. There are now only two values recognized by the British Standard, 3A for small appliances and 13A for larger (high powered) appliances. This method is very simple, but by combining the two methods we can establish a simple but effective
The recommended method currently for checking the fuse in appliances is now as follows:-
- If the appliance is fitted with a 3A or 5 Am fuse then leave it at it is – no further action is necessary.
- If it is fitted with a 13A fuse – check the cable on the appliance. If it is thick (compare with the cable on other appliances) then it’s probably ok -if it’s thin there may be a problem.
- Check the power rating on the appliance, and use these figures as a guide.
- less than 500W – fit a 3 Amp fuse.
- more than 500W but less 1000W – fit a 5 Amp fuse
- more than 1000W – fit a 13 A fuse, but check that the cable is thick enough.
IEC cables on computers and kettles etc may be fitted with 5A or 13A, but the smaller cloverleaf leads found on some laptops should be fitted with 3A fuse. The ‘small figure of 8’ leads should always be fitted with 3A or 5A fuses.
There is, of course, no substitute for professional training such as that offered on a PAT Testing Course but hopefully, these simple rules will be useful.