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Tuesday, 25 March 2014

What do fuses protect?

Selection of UK plugs showing fusesFrom an earlier post, I indicated I would post another question that was, "what do fuses protect?"  I hope to make an attempt to clearly answer this question, which rarely enters our minds. Understanding the logic behind my answer may help to identify anomalies and also explain some odd choices of fuse you may come across on a day today basis.
Always comply with any regulation or law that applies in your country or region. If unsure, consult a qualified electrician.


What do fuses protect?

So the question remains, what do fuses protect?

Actually the conductors in a 3A cable could quite easily carry 15A so why is cable only rated at 3A?

So the essential answer to the question of: "what do fuses protect?"

Are you sure?

The Exception to the rule.

What do fuses protect?

It would appear in the USA they have a breaker panel while in the UK we have a consumer unit, which fulfil the same purpose of a central point where power is distributed throughout the property.  The breaker panel or consumer unit provide a housing for the various forms of protection available for those circuits.  Breaker boxes are fitted with various forms of Circuit Interrupters (CI) and in the UK consumer units are fitted with various forms of Circuit Breakers (CB) but essentially they are different names for the same devices performing the same function.  These are fitted with devices that detect and protect against distribution faults.

In this post I will refer to a fuse as any device which disconnects the power when there is an over current condition.  This condition may be because of a short circuit or any other reason that the circuit is drawing a current greater than it was designed to take. So I will include the various forms of resettable fuse (MCB, CB) as well as fuse wire. Other devices that can be found in a breaker panel or consumer unit such as a GFCI, RCCB, AFCI, RCBO or isolator switch, these are not strictly fuses so will not be discussed in this post.

The oldest protective device was the fuse which was one of the most basic of over current protective devices. The early fuses used fuse wire, which heated and melted so needed to be replaced after each fuse event.  Fuse wire was replaced by the more convenient resettable fuse in its various forms, which there are many. The name suggests its function of being able to be reset as opposed to having to be refilled or replaced. Ie it is not consumed in the process of protecting against a fault.

But what does a fuse protect? The most obvious answer to the question is, "you from harm." Thinking about it harder questions this logic because it becomes apparent that a 400A fuse fitted at a substation isn't going to protect "you" from anything.  Equally the fuse in your radio isn't going to protect the nearest substation station either.

So the question remains, what do fuses protect?

Essentially the fuse is designed as the weakest point in the circuit and will blow or trip to save the circuit.  So the next question is what, or where, is the circuit the fuse protects?  The circuit the fuse protects is nearly always "down wind" from the source. The fuse nearly always protects the appliance side of the circuit rather than the power source side. It would have difficulty protecting the source side when it has blown or disconnected, because there is nothing further it can do.  The source will still be supplying power and so cannot be stopped from causing damage to the circuit before it has reached the protective device. The fuse effectively protects the part of the circuit up to the next fuse. For example in the UK:

  • The 400A fuse at the substation only protects the line "from the fuse" to your home. 

  • The 100A fuse fitted by your distribution company only protects "from the fuse" to the electricity meter and the tails to your consumer unit or breaker panel.

  • The 32A fuse (MCB) in a consumer unit only protects "from the fuse" to the socket. 

  • In the UK where we have fused plugs the fuse in the plug only protects from the fuse to the end of the cable.

    • Yes that's correct, the fuse in the plug only protects the cable from the socket to the appliance. "Not the appliance!"

    • The Fuse in the plug is designed to protect the cable and not the appliance, although it would offer some protection to the appliance. A small mains powered clock radio consuming 10 watts (0.042A) would be toast long before a 3A fuse would have blown.
    • In the USA the 15A or 20A breaker in the breaker panel protects the cable to the receptacle. This effectively means that the appliance cable plugged into the receptacle is also protected by the same breaker. That is why regulations insist on AFCI in domestic environments.

Actually the conductors in a 3A cable could quite easily carry 15A so why is cable only rated at 3A?

The conductor may be able to carry 15A but it would heat the cable up greatly through transmission losses during the process of conducting that level of current.  The reason that the cable is rated at 3A is because the heat generated by the current travelling through it, at a constant 3A, would be radiated safely preserving the integrity of the cable.  At a sustained 15A the insulator could melt or be dangerously compromised. Without insulation there is a real danger of a short or electrocution.

  •  Transmission loss due to excessive current would also cause an unacceptable voltage drop robbing the appliance of  its full designed mains supply voltage.

That is why if you bury a cable in insulation, the current rating of the cable is reduced.  Not because the copper is no longer capable of carrying current but because the insulation cannot be guaranteed to remain safe due to its inability to radiate heat through its normal cooling processes.

  • This negative consideration is known as, derating. The same goes for a cable running over something that is hot such as a central heating pipe.  This is why when using power tools with a reel extension cord, it is advised that the cord is completely unwound from the reel to ensure the heat generated is safely dissipated so preserving the integrity of the insulation.

  • The heating process is known as "I2R" heating.  That is, the square of the current multiplied by the resistance.  So doubling the current will generate four times the heat for any given cable. The resistance of the cable is governed by its cross sectional area of the conductor.  The higher the resistance not only the greater the heating but the greater the distribution loss through heating as the heat is lost energy. Different insulators allow for different cable ratings more so than the size of the conductor. The choice of insulator is governed by desired properties of the material as well as its cost. 

So the essential answer to the question of: "what do fuses protect?"

The answer is:
"The integrity of the insulation for the section of the circuit it's protecting."

If you have jumped straight to the answer and you're perplexed, then please re-read the post and it may all become clear.

A simple example is one I've come across in the automotive world.  One expensive and time-consuming part of the car is the wiring loom in the car.  This is responsible for transporting energy and control signals around the car and consists of a tightly packed bunch of cables. I have seen occasions where the wire has got so hot that not only has it melted its insulation it has also melted the insulation of the other cables surrounding it in the loom. The car still worked but it had lost functionality and become intermittent. Thus necessitating replacing the loom.  It would have been much cheaper had the wire been protected by a fuse giving an indication of a fault, as well as a chance to fix it before expensive and extensive damage occurred.

In a house it is better that you only lose a short piece of cable than have to re-thread a new circuit and potentially other circuits which just so happened to be in close enough proximity to be damaged. Not having your house burn down is another clear advantage.

So what of the appliance? Well the same applies there, the fuse in the plug protects the cable from the plug to the appliance.

Are you sure?

Well yes, I have noted that the general advice is that the fuse protects the appliance.  Though technically not incorrect, it is ambiguous advice.  In the case of a cable extension, the appliance is the cable not what item is finally plugged into it.  The rating of the cable should always be observed and this takes priority, not the rating of the appliance it supplies.

  • For example if you are using an appliance which requires 4A but the only lead you have available is a 3A then you do not change the 3A fuse in the cable for a 5A.  The course of action that needs to be taken is to get another cable correctly rated at 5A or above.  It is acceptable to put a 3A fuse in a 5A extension but this is really poor practice.  If you decide to use the extension elsewhere, it is likely you may blow the fuse unexpectedly.  In the UK they have tried to address this by using moulded plugs which are fitted by the manufacturer for the specifically chosen rated cable.  There are two advantages with this method.

    1. The plug is wired properly and the properties of the plug such as the strain relief are fitted as designed

    2. The Plug is printed with the rating of the fuse intended for that plug and cable. 

    3. In the past, and still today, all plugs are printed with the rating of the plug which is 13A, as they cannot predict what cable is going to be wired into it.  This is a problem as aftermarket fitted plugs (ie. DIY installations) do not indicate the correct fuse rating and so is an opportunity for error.

Any well designed appliance will have a number of internal protective devices to protect the appliance itself. Take the example of a 60w lamp.  It will usually have a 3A cable supplying the lamp socket and the physical apparatus to hold the lamp where you want it, ie the body of the lamp.  The 3A fuse, in the plug, will allow a maximum of 720w to the lamp socket but the body may not be able to take the heat of a 720w incandescent bulb.  The bulb fitted will be 60w and in a fault condition resulting in element failure, the glass envelope of the bulb is designed to remain intact protecting the user from internal live terminals and the body of the lamp from damage.  So the protection of the appliance is inherent in the blub and not the fuse fitted to the lamp plug. If any smaller fuses are required then these will be designed into the appliance by the manufacturer at the point of manufacture.

The Exception to the rule.

In the UK, there is one exception to this that I can think of. It is legal but it's a frowned upon practise of adding a spur (unfused single cable extension) to a ring main.  Essentially, a ring main using 26A cable would be protected by a 32A fuse but it could be argued that it has a current handling of 52A for any socket on the ring.

The exception occurs when a spur is added which would usually serve a single, double gang socket with a 26A rated cable. The socket is rated at 2 x 13A outlets and the fuse in the plug limits the current drawn by an appliance to 2 x 13A.  Thus the two 13A plugs (one for each appliance) are effectively protecting the short piece of 26A cable of the spur which would otherwise be protected by a 32A MCB. So there is a potential scenario that a faulty socket could draw current greater than 26A and less than 32A meaning the wire used for the unfused spur will be operating outside its specifications. Again, a very unlikely event but not impossible!

The RCD, and the North American equivalent the GFCI, will also help detect conditions from where they are installed all the way to the appliance and not just locally, with regard to the circuit.  RCD will protect more than only the insulation.

Always comply with any regulation or laws that applies in your country or region. You should always consult a qualified electrician where there are any questions about wiring or safety.

For further reading and references: please see my resource page