Choosing a plug fuse

Introduction

A selection of BS1362 fuses

If you live in the UK, or another country that has adopted the UK style 13A plug and socket design, It is likely that most of your mains powered appliances will sport a BS1363 13A Mains plug on the end of the flex. This plug is highly regarded for its safety (the ability to reliably carry high currents for long periods without overheating, the standard inclusion of shutters to make them "child safe" etc), but it also has another uncommon feature, the inclusion of an internal Fuse.

This article will explain how to choose which rating fuse to use in a UK mains plug.

Background

In electrical wiring systems, fuses and circuit breakers are used to protect both the users and the systems themselves when something goes wrong. They are a deliberate "weak link" that will cut the power, and so greatly reduce the risk of fire and serious electrical shock.

The inclusion of this fuse allows UK socket circuits to be protected by a much higher current circuit breaker. So the circuit as a whole can provide power to many sockets and appliances since the circuit breaker for the whole circuit no longer needs to have a low enough trip threshold to the protect the flex of every appliance on the circuit as well as the circuit cables themselves.

What fuse should I use?

The plugs accept standard BS1362 fuses, which are commonly available in 3A, 5A, and 13A ratings. (other capacities are also available, but not as easy to find in normal shops).

With most modern electrical appliances (there are exceptions - see below), the fuse in the plug has one purpose and one purpose only - to protect the flex between the plug and the appliance from the effects of the massive "fault current" that could flow in the flex should it be badly damaged or even cut.

The term "fault current" has a very specific meaning when talking about electrical circuits. It is what you get when current follows an abnormal path through or around the load - usually directly from live to neutral, or from live to earth.

In normal operation the appliance itself limits the current flowing, but under fault current conditions there is nothing to limit the current flow other than the tiny resistance of the copper cables themselves. This can result in huge currents that will typically be 100s of amps, or in some cases 1000s of amps.

Unsurprisingly, if you stick 1000 amp through a flex that only has a maximum continuous current rating of 3A or 13A, it is going to get very very hot very quickly. Its insulation will melt and char, it may burst into flames, and in some cases could even "explode" (a very dangerous event known as an "arc flash").

The key to stopping this from happening, is to interrupt the current flow very quickly, This needs to happen in a fraction of a second (i.e. a 10th of a second or less).

It may seem counter intuitive, but since fault currents are so large, even a 13A fuse will protect a 3A flex from the effects of a fault current in the vast majority of cases.

Good Practice

So you might wonder, if a 13A fuse can do the job, why would you need any of the other values? There are some specific cases we will cover shortly where a 13A fuse can't be used, but while no longer strictly necessary, many still consider it good practice to pick a fuse that is more closely matched to the expected load of the appliance. If nothing else you might get fewer sparks when you accidentally cut the mains lead, and a lower risk of a melted flex if you manage to damage it in such a was as it only has a partial short circuit on it.

In most cases the decision will be :

Is the appliance power consumption less than 720 watt?,

• If yes, fit a 3A fuse,
• otherwise fit a 13A fuse.

(The 720W number being 3A x 240V since the power of an appliance is typically the product of its voltage and current draw).

A slightly more nuanced approach would be to divide the power rating by 240V and pick the next fuse size larger than that.

So for a 1kW appliance, the sum would be 1000 / 240 = 4.16A. So fit a 5A fuse or higher.

Typical low power appliances where a 3A fuse would be appropriate

Appliance	Notes
Table Lamp	Most lamps are limited to 60W loads or lower. With modern replacement LED lamps the actualy load may be significantly lower.
Alarm clock	Or any mains powered clock
Flat screen TV or monitor	Modern LCD and OLED screens
Radio	and other small electronic devices
Laptop computer

However there are some appliances that while they have normal operating currents less than 3A, they may experience high "inrush" currents at startup. These may result in a 3A fuse blowing when on the face of it, it looks like it would be ok.

Some low power appliances that may need a larger fuse

Appliance	Notes
Old style CRT Television	Colour TVs have a powerful "degauss" feature that will briefly draw a high current
Appliances with larger transformers	Like some high power HiFi amplifiers. Microwave ovens. (Also the input power requirement of audio amplifiers and microwave ovens is higher than the output power rating. A 700 watt (output) microwave might consume 1100 watts (input - fuse rating).)
Some computer systems	Those with high power CPUs and Graphics cards (often "gaming" PCs)
Laser Printer	During normal operation most laser printers draw relatively little power, however they do have a heater in their fuser unit that may draw a high current for a short time when powering up or coming out of standby.
Appliances with induction motors.	Induction motors swallow a big gulp of current while getting up to speed. You will often find these in devices with "quiet" motors like fans, fridges, freezers, pumps etc.

Special Cases

Modern UK appliances are designed to meet harmonised standards that will allow them to be sold anywhere in the European Union and frequently worldwide. (Even though the UK is no longer part of the EU, all equipment made here will still meet the same standards to allow export to other countries). Since most countries don't have fused plugs, the equipment design can't take advantage of the presence of the plug fuse in the UK for any purpose other than for protection of the flex - since sold in most countries, it won't be there!

Old appliances

With a modern appliance design, if it it known that it needs some kind of overload protection (say because there a way it can be used or abused, or there is a known mechanism that could cause it to draw more current than it should), then it must include this internally. While the plug fuse might also be able to protect the appliance in this circumstance, the design must not rely on it.

Old appliances are one special case. If you go back far enough in time (30 - 40 years or more) you will find some appliances which were designed specifically for the UK market and not intended for export. Here the designers could and did rely on the plug fuse to also protect the internals of the device as well as the flex. The plug for these must be fitted with the fuse specified by the instructions, or if these are not available, by calculating the size as described above.

Alas there will not necessarily be an obvious indication when you are dealing with appliances of this nature. So with older appliances it is often better to fit the "correctly" sized fuse.

Moulded Plugs

A moulded plug showing its maximum fuse rating.

Many modern appliances, and most detachable mains leads will come with "moulded on" plugs. Usually the plug itself (and often a label fixed to the flex) will state what fuse is required. You must never fit a fuse with a higher capacity than specified.

Extension Leads

Extension leads, and in particular extension leads with multiple sockets on the far end, may often specify the maximum capacity of the plug fuse. This may be a less common rating like 10A.

This is an example of an "appliance" (if that is not stretching the definition a bit!), where the plug fuse is also being relied on to provide some overload protection, and not just protection from fault currents. If the lead has several sockets on the end, it would be possible to connect a load of significantly more than 13A. Better quality leads will have a fuse, or a thermal cutout, in the socket end to stop this overload being "felt" by the lead, but many don't

Many longer extension leads are also of a "marginal" design - using slightly undersized cable to keep weight and cost down (with the anticipation that the loads on the end will likely be shorter term loads (power tools etc), and hopefully the user has read the instructions about not using the lead while still coiled!).

Many multi-way power strips are built to a price and quality which makes them unsuitable for higher loads anyway, and it is often advisable to fit a 10A fuse to these, even if they come pre-fitted with a 13A fuse.

Modified Appliances (extended cables)

If an appliance has had its mains lead changed for a longer one, it may need more closely matched fuse protection. Often when manufactures fit mains leads with a thin cross section (say 0.5mm², rated for ~3A continuous load), they will also only fit a relatively short length. This is to ensure that the overall cable resistance remains low enough to blow a 13A fuse or open a 16A circuit breaker should a fault occur at the far end of the lead.

If you extend the lead and use the same gauge of flex, this may no longer be true, and you may also need to fit a lower capacity fuse to ensure it is adequately protected.

(there is a way of calculating when this becomes necessary by using the adiabatic check formula described here).

Non compliant and fake / counterfeit equipment

An unknown quantity of electrical goods that are of very low quality and don't meet expected safety standards also make it into the market. Frequently through third party sellers using Amazon or eBay, and also direct from foreign manufacturers via international shipping web sites. Much of this kit is built to a very low price and corners are cut. Some of it might be borderline, and fitting a closely matched fuse might make it safer, however without detailed examination and testing, this can be impossible to asses.

There are some tell tale signs to look out for:

• Plugs with partially sleeved earth pins
• Very small moulded plugs with no fuse at all
• Flexes with dangerously undersized conductors.
• Appliances which should be earthed, but where the earth pin of the plug is not actually connected to the appliance at all

Please see the Fatally Flawed web site for more examples of how to spot dangerous plugs and leads.

Here is a tear down on a set of dangerous fairy lights that illustrates many of the common problems

Fake BS1362 Fuses

Fake BS1362 fuses also sometimes make it to the market. Fake fuses can be particularly dangerous since they may fail to disconnect the power at all in the event of a fault or an overload therefore dramatically increasing the risk of fire and electrocution.

Many will explode when subjected to fault currents, and in adverse conditions, some may even trigger a dangerous arc flash explosion.

There are some good guidelines on spotting fake fuses here.

See also

• Flex
• Cables
• Fuse
• MCB