Fusing and Current Protection

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Forums and Instruction: Fusing and Current Protection

There are many forums across the globe covering protection techniques used in house wiring, car audio and general electronic projects. The one thing we should all be very wary of is that many of the calls for assistance are coming from people that openly display their ignorance.  Some of the replies are also sometimes bordering on the dangerous and ludicrous.  There is a circuit for a hot-wire cutter where the designed advocates using an auto-transformer to push up the juice. The description of the circuit is simple – the auto transformer is explained and there is an illustration showing blatantly that the entire circuit will be live. Many a student ios going to want more ‘juice’ because more is better, right? If a 10A fuse blows replace it with a 30A fuse. There is a forum where a contributor needed assistance – putting in two fuses in series is better than one because of the increased high voltage safety. This article needs to impress the importance of good safety practices and in doing so, maybe save lives as well. This may seem a bit overboard but it’s not – some of the solutions to circuit protection is downright dangerous.


Ohm’s Law

First of all know Ohm’s Law which is I = V/R or current = voltage divided by resistance. It’s a simple formula to remember but most learners get it wrong because they don’t necessarily understand the differences between voltage and current – this is clearly seen when ‘people’ talk about voltage not killing but the current. Voltage does kill – you need voltage to get the current. You need to get a high enough voltage to produce the current through a given resistance i.e. through your internal resistance to kill, normally hand to hand is one of the deadliest. So don’t make the mistake of thinking it is not voltage that kills but current. Connect your hands across a 440V bus and you’ll know what I am talking about.

Many experienced light current enthusiasts get in wrong when it comes to high current. Electronic buffs usually work with low voltage low current equipment whilst electrical engineers make their living out of high voltage and high current. Our natural crossing path is around our mains voltage supply – in the USA this is around 110V and in the UK and Australia this would be around 220V. In South Africa this voltage is sometimes around 238V. This is not irregular. Anything connected to mains voltage has to be done professionally and should include an isolation circuit which consists of a transformer or heavy use of plastics as insulation. All equipment connected to the mains in ANY country needs to abide by a specific code, this is usually your board or bureau of standards.  Mains voltages require proper insulation and therefore low voltage wires may NOT be used to connect to the mains power. E.g. security alarm cable. All cabling carries an insulation specification and maximum current rating. The vendor can tell you this or should be able to tell you this. If you don’t get it, don’t buy it.

110V and 220V mains both carry their own set of rules – the main one is P = VI or I squared R (I * I * R).  For a given wattage in the USA the current draw will be higher than in a 220V country. This means the cabling needs to be thicker i.e. thicker conductors, whereas with 220V although the copper conductor (or aluminium) may be of smaller gauge the insulation needs to be thicker. I don’t know the American wiring code but it’s just as strict as any other country carrying 220V mains supplies. (or stricter).

AC and DC

Is the voltage AC or DC? As the AC component flows from peak through zero volts breaker contacts do not need to be as hardy as for DC. You will note often on small relays (and bigger) that the maximum current handling of the contacts is higher for AC than DC.  You will see this in the arc pattern – Google it!

Theoretically an electronics project should never be connected to the mains path without the assistance and sign off by a person with a wiring license. In some countries you may not be allowed to make up an extension cable for longer than two meters without the sign off by a qualified electrician. Sound stupid? Not really. See below.


Ampacity is a term many electronics buffs have never heard of – do read up on this. I am not going to re-invent the wheel, there are many, many ampacity charts on the internet. Know what wiring to use and where. Coiling your cable and running 15A through it is not a good idea. Electricians will always tell you this.  Now on to protecting yourself and your project….

Whilst a mains fuse is ALWAYS essential it is often a very misunderstood part to your design. The rule of thumb is simple: the fuse must blow before any cabling melts or the foil on your PCB burns. Whist we see that mains fusing is essential there is often a bigger crises with car audio – in most designs the manufacturer cannot control what cabling the installer uses. In both mains and DC car audio the fuse should be as close as possible to the source e.g. distribution board or battery. Most dwellings nowadays have a distribution board – what is essential to know is the current handling capacity of the cable from the DB to the appliance.  Here is a typical case scenario – never use an extension lead which is rated smaller in current handling capacity than the breaker – usually 20A in a 220V country. Most extension leads can only carry upto 5A, some 8A, some 10A and very infrequently 15A. Cheap extension cords are dangerous – they are cheap because copper is expensive (cabling is expensive). Coiling them reduces the cooling and the current handling capacity of the cable, in effect reducing the ampacity, therefore the safe loading that can be done – it will always be lower than the manufactured spec. I use an extension lead which is 20m in length – it cost 90U$. It is designed to handle 15A OVER 20M. At the local supermarket I can pick up an extension lead without a manufacturer current handling certificate or specification for 20U$.  Many people use this to feed their 2kW electrical heater.

Fuses: The simplest protection

Fuses: the essence of protection. Your source must be protected i.e. the DB board or at the battery. The circuit must be protected, often with fuses. The fuse prevents catastrophic failure but more so, the likelihood of starting a fire. When replacing a fuse or more importantly when equipping your project with a fuse know what type to use. If you thought that fuse manufacturers are a dumb lot that exploit the user by supplying only marginal safety devices then think again. It is a highly technical area of electrical and electronic engineering. You cannot use an automotive fuse in an AC line of 110V or 220V – it is designed strictly for DC and at a maximum bus voltage of 32V. You cannot use a glass fuse alone in an area where there may be flammable gas.  Fuses are manufactured to every specification required to mostly every known industry. It is a highly technical area and especially when it comes to high voltage, high currents can ONLY be replaced or serviced by properly qualified maintenance personnel. Getting down to a simple device like your 5A fuse used at home can often be no different – there are very slow blow fuses often marked with a TT, slow blow with a T. There are special fuses designed for microwave oven usage. Do not use a standard glass fuse in a microwave – my own experience is even for testing if a door safety interlock is faulty it will create a dead short across the mains blowing the glass shrapnel in every conceivable direction – it will NOT just make the fuse blow with the standard noticeable black burn against the side of the glass showing a catastrophic failure – this is a very, very high energy discharge – think in the order of 10kW. Microwaves use an anti-rupturing fuse. Replace with the proper device and NOT what you see in the socket. Ensure it is the correct fuse – service technicians may not have had the proper fuse and replaced with a standard glass fuse.

Time delay fuses are often marked with a T, showing that there is a time constant on the current rating. The T stands for Träge (German for Slow). These T denoted fuses are mainly used in line with a transformer which has a high inrush current, electrical motors and circuits which may use large filter capacitors. Fuses must ALWAYS be rated a smaller current handling than the wiring rating and OFTEN the transformer manufacturer VA specification. (I use the word often here because design engineers always under-rate their equipment – so using a fuse that will not blow on inrush current alone is enough for a safety margin – they don’t design power supplies to be overloaded, in fact they look at worse case scenarios which makes some equipment prohibitively expensive but very safe from every angle. Going higher means the risk of a fire.

High Current circuits – Car Audio

In car audio many budding DIYers rate their equipment protection at about 60A. This means your wiring needs to be of the same gauge as that used in welding equipment. Car fusing is a science – manufacturers use special wire which is NOT cheap which means a) it has been designed for a special requirement e.g. insulation and heat. Heat causes standard insulation to become brittle, you would not use standard two core flex close to an engine sensor b) very low resistance which means lots of copper to have the lowest voltage drop possible at the rated current. Voltage drops, resistance cause heat. I squared R. My own feeling is that high powered car audio should have a fuse at the battery, at the amplifier(s) and an isolator. Out of ten high powered systems I have come across in the last five years NONE had an isolator. I do not know how many fires have been created by car audio enthusiasts wiring their sound system with inadequate protection but I do know that the web is full of specifically American based installers that do it correctly.


To recap – your fuse should be rated ALWAYS at lower amperage than the wiring installation. Transformers must have fusing on their primary circuit and of course, often are Time Delay fuses.  Your standard extension lead is not designed to carry high current loads like heaters – it may be ideal for a drilling machine only. Check the earth. Make up your own extension lead with proper 20A current handling cable.

Last but not Least

Last but not least but unfortunately the one that most people have trouble getting their heads around. A.C. power,VA, D.C. power and Watts.  A transformer may be rated at watts or VA but like any alternating current source the load plays a very important factor in just how much power the transformer can really dissipate.  The same applies to the mains as a source. Look up power factor and correction to P.F. In essence the lower the load P.F. rating the more current is needed to run it. The ideal load is a purely resistive one like a composite carbon film resistor. This is not usually the case though – loads are either inductive or capacitive in nature.  In a purely resistive load the current and voltage are in phase, in capacitive load the current leads the voltage and inductive load the voltage leads the current. (Remember this: CIVIL).  When there is phase difference there is a loss of power somewhere. Read up on resonance, another beastie in circuit design, especially with SMPS but NOT all of them. As you can see with A.C. circuits things start becoming a little more tricky – this is where your friendly electrical engineer should be of assistance, normally for a fee of course. I wrote this paragraph more to hold the casual reader in awe – mains electrical supplies are VERY sophisticated once they have been broken down into the components of voltage, current, power and phase.  For a 220VA transformer with a 220V mains input voltage a 1A anti-surge or T type fuse should be sufficient, right? It won’t be – it will blow, especially with a toroid. Use double the current rating because the transformer primary winding should handle the 2A safely – transformers are very resilient and can handle loads much larger than what they are designed for before dying. A 1kVA transformer at 220V input is a very heavy device and the primary winding will easily be able to handle more than 4A (1000/220). An 8A slow blow will work. Often high powered transformer based power supplies have circuits built in the primary stage to limit inrush current. If this is the case then stick to the VA rating and manufacturer specified fuse rating which may be around 5A T. A transformer with a burnt winding on the primary side will blow the fuse, a burnt secondary winding will also blow the fuse. Overheated transformers smell like popcorn. In my experience the most common transformers to fail are those in microwave ovens – listen for a loud hum. No, they don’t always blow fuses – they get very hot but your hearing and smell sensors will tell you very quickly that something is amiss. In my experience often it’s the bigger transformers, designed for longer duty cycle that succumb to breakdown, mostly in areas of high humidity or close to the sea. Go figure!

Unfortunately, power supply protection is not always as simple as what it is made out to be especially for the novice designer – always stick to logic and ensure that the wiring is properly spec’ced for a given current, stick to Ohm’s Law and power equals I squared x R or VI.  Novices to electronic circuits should stick to battery operated circuits and with low power dissipation. Stay clear of car batteries and mains voltages until you have a clear head on your shoulders and know what you are doing. I built a power supply circuit with a valve rectifier once and blew the tube apart because there was a dead short on it. I was 13 years old. If I learnt anything from that experience it was not just understanding the circuit before building it but be very patient with the wiring. Understand the operation of active and passive components before playing with it and connecting it to anything that resembles a lethal voltage. Mains power is awesome, but like anything that is awesome from jet turbines, steam motors and big engines comes its deadliness.

In forums always look at the credentials of the people trying to assist you – there are some exceptional people out there and no matter how clever you think you are there’s always going to be someone much wiser and more clever. Anyone having had extensive experience in industrial electronics, military or marine radar, telecommunications, switch gear, generators and control circuits should be able to give you advice. When it comes to your own or others safety always bear one thing in mind – don’t take short cuts.

Reading matter

Some great reading matter:

  • www.littlefuse.com
  • http://sound.westhost.com/
  • www.allaboutcircuits.com
  • www.edaboard.com





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