Your fuse box or Consumer Unit (CU) does the very important job of providing protection in the event of an electrical fault. Just for larks, let’s take a look at how...
Do you still have an old rewirable BS3036 fuse box like that pictured below? These things were standard fare for about three decades so there are still many of them out there although their numbers are dwindling year-on-year as people upgrade to new consumer units.
BS3036 is still a relevant standard and, despite some flaws, if you do still have one of these old boys then rest assured it will continue to provide the level of protection it was designed for so long as it hasn’t been altered, damaged or compromised in any way.
If you installed a modern consumer unit loaded with Miniature Circuit Breakers (MCBs), it would do exactly the same job as the rewirable fuse box, except that when a breaker blows it can be reset by flicking a switch which is far more convenient than having to find a screwdriver and the right size of fusewire when having to perform the fiddly repair on a blown rewirable fuse.
Ask most people, and they’ll tell you they believe a fuse or breaker is to protect them from being electrocuted, but actually they're installed to protect the wiring. The thicker a cable, the more electric current it can carry, so a domestic lighting cable may only be 1mm2 thick as lighting around the home draws relatively little current, whereas an electric shower may have a 6mm2 or 10mm2 cable as it draws a much higher current. If you wired a shower using a 1mm2 cable, that cable would burst into flames almost as soon as you stripped off and gripped your loofah. To protect a 1mm2 cable, only a 5 Amp fuse or 6 Amp breaker would usually be used, whereas a 6mm2 shower feed may perhaps have a 40A fuse/breaker suited to the kind of load our example shower would pull.
It's important to use the right size fuse/breaker for the circuit. One that is rated too highly will mean the possibility of the cable becoming overloaded and burning out, one that is too low will see nuisance tripping under normal load conditions.
Although the job of the fuse/breaker is to protect the cable rather than people, it does protect life indirectly by decreasing the chance of an electrical fire in the event of an overload or short circuit. Under certain fault conditions however, it is entirely possible for you to be receiving a fatal electric shock while your fuse/breaker happily does nothing at all to cut the power simply because the current being drawn, while more than enough to injure or kill you, is not enough to risk damaging the cable.
Instead there are other ways your life is directly protected from a fizzing electrical demise every time you use that dodgy toaster held together with gaffer tape.
Like water flowing down a hill, electrical current courses from a source of high potential to a point of low potential through the easiest route possible. If it can avoid something in the way resisting its flow then it will do so. Electrically speaking, the ground we stand on, i.e., the Earth, is deemed to have a potential of zero volts. Your electrical devices, be they sockets, lights, showers or whatever have a potential of about 230V in the UK, and the electric current flowing through your gadgets and gizmos wants to get from the higher potential and flow to the lower potential. If your toaster is faulty and the metal casing becomes live, when you touch it your body creates a path which allows that current to flow through you and into the ground you’re standing on.
Obviously, this isn’t going to do you much good, and the fuse/breaker will only trip if the current flowing through you is high enough to actuate that protective device, but as it only takes a few milliamps of current to kill you, waiting for the 13A fuse in the toaster’s plug to pop probably means it’ll be you who gets toasted... and long before either the fuse or those crumpets you were hoping to lightly brown for breakfast.
To avoid such mishaps, we rely on there being good earthing throughout the electrical installation. The toaster’s metalwork will be connected to the earth pin of the plug which mates with the earth outlet on the socket which goes back to the earth bar at the fuse box or CU and which is connected to physical Earth via the supply provider or an earth rod (literally a long length of copper sunk into the ground). This earth wiring should always have a lower resistance than your body which, although containing current-friendly salty wet bits, will nonetheless usually have a resistance in the kiloOhm to MegaOhm range. As electric current wants to take the easiest path to earth, a copper cable with an overall resistance of, say, 0.5 Ohm will always be a more preferable path to our example human who perhaps happens to have a body resistance at the time of 500,000 Ohms.
Those of you with rewirable fuse boxes are especially reliant on that earth wiring being good throughout the installation as any breaks or high resistance faults on this protective wiring could mean your body becomes the preferable path for current to traverse if a fault occurs. Although the cost of upgrading to a modern consumer unit may be hundreds of pounds, not upgrading means there may come the day when that faulty toaster ensures the last thought going through your mind is about how cost effective that upgrade might have been, especially now you know you’ll not be leaving a good looking corpse considering the current ripping through your body and the contents now being involuntarily deposited into those Sooty & Sweep pyjama bottoms you assumed nobody would ever see.
Modern consumer units have an advantage over old rewirable fuse boxes because they provide additional protection in the form of a Residual Current Device (RCD). Unlike the fuse/breaker which is there to protect your cabling, an RCD is there to directly protect your life.
An RCD is an electromechanical device which is "in balance" so long as the current going out down the line wire matches the current coming back through the neutral. If we plug our faulty toaster into an RCD protected socket outlet, as the metal casing becomes live some current starts leaking to earth, but now the current going out down the line wire doesn’t match the current returning in the neutral wire and so the RCD mechanically trips and cuts the power. For an RCD providing domestic additional protection, this should happen when the current leak is below 0.03A (30mA) and within 300 milliseconds to ensure the metal casing doesn’t have a chance to reach a potential higher than fifty volts. So long as it cuts off within these thresholds, even if you were touching the toaster when you plugged it in, then you won’t even realise how close you came to meeting a messy end and instead you’ll just be annoyed that you’ve got to go to the effort of resetting the RCD and having cornflakes for breakfast instead of muffins.
An RCD may protect multiple circuits in a consumer unit. It will have a TEST button which you are advised to push periodically to ensure correct mechanical operation of the device, but doing so will kill power at all circuits connected to that RCD, so it’s best to ensure nobody is in the shower or using the computer at the time!
An RCD (right) protecting four circuits to its left, two with 32A breakers, another rated at 16A, and one at 6A. If this RCD were to
trip, thenn the wiring, fixtures and appliances on these four circuits would come under scrutiny in order to locate the cause.
Instead of a consumer unit containing an RCD protecting several circuit breakers, it is increasingly common to find another device: the RCBO. The Residual Current Breaker with Overprotection is a two-in-one device which contains an RCD and breaker thus protecting a single circuit from both current leakage and short circuits. These are usually more expensive to install, but they do have the advantage that a fault on one circuit doesn’t affect any others in the event of a trip. Although, at a glance, an RCBO may look like an ordinary circuit breaker, it will have a built-in TEST button for you to check its operation.
Our three protective devices, from left to right, (orange arrow), a Miniature Circuit Breaker (MCB), in this case rated at 40A for a shower and protecting a 6mm2 cable. An RCD (blue arrow), twice the width of the MCB and with a yellow test button for quarterly checks. This RCD protects the five circuits to the left, the shower being circuit 1. Finally we have an RCBO (black arrow), in this case rated at 6A to protect a 1.5mm2 cable feeding the smoke alarms. In this configuration, a fault on any other circuit causing an RCD or MCB to trip won't affect the smoke alarms. Although the RCBO looks much like an ordinary breaker, it has a test button so that the RCD component can be checked, ideally two to four times a year.
So, now we know what a fuse/breaker, RCD and RCBO do, let’s look at why they might trip.
If you’ve lost power to one or more circuits, then you need to look at the fuse box/consumer unit to locate the blown/tripped protective device and you need to have an idea of why the protective device has operated as it’s no use going to the trouble of replacing a fuse if it’s going to immediately blow again. If the sockets have gone off, what was plugged in at the time? Were you knocking nails in walls where you might have caught a cable? Is there a water leak near any wiring? Did it trip when you plugged in or switched on a particular appliance?
If you cannot reset a fuse/breaker/RCD then whatever caused the fault is still there. In the case of a fuse/breaker you need to be looking for an overload – an appliance pulling more power than the circuit can allow, or a short circuit through DIY miswiring or a damaged appliance or cable. Often the dying act of a filament light bulb is to briefly short circuit which will blow the fuse/breaker but will be cleared when you restore power to the circuit. You can recognise if this has happened by spotting a dead bulb when the power goes back on, and you can mitigate against this by replacing filament lamps with good quality energy saving bulbs.
If an RCD won’t reset, switch off all circuits connected to that RCD, then put the RCD back on. Next, switch on each circuit in turn. The circuit which pops the RCD is possibly (but not always) the one to take a closer look at. The current from that circuit is escaping and it’s often through water getting in to where it shouldn’t. Have you got any outside lights or sockets that may be damaged, especially if it was raining when the trip occurred? Were you operating a water-using appliance at the time such as a kettle or coffee machine? Is there any debris in your toaster shorting the heating element to the metal casing? Can you isolate or remove any appliances on that circuit by pulling out the plugs to see if stability improves? If the fault is on a common neutral bar between circuits, then disassemby and testing is required using specialist equipment.
Intermittent trips can occur, especially with RCD’s, and by their nature they are tricky to track down. You can try and look for a pattern such as if there is a particular time of day when it happens and you can try to isolate certain circuits or appliances to eliminate them as the cause.
Things can also get tricky if there is a cumulative effect. You may have individual circuits with poor insulation resistance which would happily sit on an RCD on their own without any problem, but when added together bring the overall insulation resistance down to a point which causes trouble. Similarly there may be small earth leakages on individual circuits which are well below the 30mA trip of a domestic RCD, but when added together these leak currents get close to, or exceed, the trip threshold if those circuits all share an RCD upstream.
With any of the above, only look into the cause of a protective device trip as far as your comfort and competence limits allow. Some faults such as low insulation resistance can cause tripping without you ever being able to locate it unless you have the specialist equipment and expertise to do so. If the cause is not obvious or the remedy not simple, then I would advise against pulling out the screwdrivers as you may only make things worse. It's quicker and easier for the likes of me to find a fault if called in while the original issue is present rather than after things have been changed. Call in a registered sparky so they can faultfind, fix and certify the circuit.
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Let's take a look at some troubleshooting...
Blown rewirable fuse
Turn off the fuse box and locate the fuse that protects the circuit which has failed. You may not know which fuse does what as, more often than not, these old boxes have lost their covers which labels what’s going where. Generally, lighting will be the 5A fuses marked with white spots, storage/immersion heaters will be 15A and marked with blue spots, cookers & sockets will be 30A and marked with red spots, and showers may also be 30A (red) or may be 45A (green). There are also rarer 20A (yallow) variants to keep an eye out for which may be serving sockets or outbuildings generally. If, for example, the lights aren’t working, look closely at the 5A white fuses.
The fuse may be extracted by pulling it, but the main switch must be moved to the OFF position before doing so. You can’t put the fuse back in the wrong place as the colour coded inserts behind each won’t allow it. By looking into the hole at the rear of the fuse, you will be able to see if it has blown or if the wire still looks intact. Some of these old boxes may have cartridge fuses instead of rewirables in which case you need to test the fuse using a continuity tester or Ohmmeter if you have such.
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Left, rewirable fuses, right a cartridge fuse with holder
Either way, whether inserting new fusewire or replacing the cartridge fuse, you must ensure the same rating of fuse/fusewire is used. If you’re out of 5A fusewire, don’t be tempted to use some 30A fusewire, and never bypass a fuse with something conductive such as a nail or screw. That fuse blew for a bloody good reason and if you don’t identify or correct that reason while maintaining the same level of protection, an electrical fire is almost a certainty.
In the longer term, it’s worth considering upgrading to circuit breakers. Your existing fuse box can be changed from rewirable/cartridge fuses to circuit breakers for easy resetting of a blown circuit, although I would advise a whole new RCD equipped consumer unit if budget allows so you have the benefit of shock protection against current leakage.
A BS3036 board where one fuseway has been upgraded from rewirable to a circuit breaker
Probable cause of blown fuses
Short circuit (line and neutral conductors coming into contact with each other directly or via a conductive medium).
Faulty appliance or fitting drawing more current than the circuit was designed for.
Dodgy DIY, i.e., a high current appliance being spurred/connected to a lower current circuit, e.g., trying to run a 45A shower off a 30A socket circuit.
Damage to cabling, i.e., drill or nail through a cable hidden in the wall, crush damage to cabling, fire damage, etc.
Water ingress at outside accessories or internal cable decaying from being used outdoors.
Failed heating element (in shower, kettle, immersion heater, oven, etc.)
Tripped circuit breaker
Easier than messing with rewirable or cartridge fuses, simply identify the tripped circuit and flick it back on. If it won’t come back on then the fault remains and needs to be tracked down. Don’t try to bypass the breaker or to force it into position mechanically with a screw or suchlike unless you really want to burn down the house and end up in prison for being an idiot who endangers life and property.
Probable cause of tripped breakers
Short circuit (line and neutral conductors coming into contact with each other directly or via a conductive medium).
Faulty appliance or fitting drawing more current than the circuit was designed for.
Dodgy DIY, i.e., a high current appliance being spurred/connected to a lower current circuit, e.g., trying to run a 45A shower off a 32A socket circuit.
Damage to cabling, i.e., drill or nail through a cable hidden in the wall, crush damage to cabling, fire damage, etc.
Water ingress at outside accessories or internal cable decaying from being used outdoors.
Failed heating element (in shower, kettle, immersion heater, oven, etc.)
Tripped RCD
I’ve already mentioned how to atempt identification of which circuit may be causing an RCD to trip. As an RCD will trip when there is a current imbalance, you need to find where the current is leaking to. Usually it's a leak between the line/neutral conductor and their associated earth conductor, but that current could be escaping in all sorts of ways including a leak to the neutral on a different circuit, into earthed metalwork or water pipes in contact with a damaged cable/appliance, or even directly into the Earth via a damaged cable buried underground. I should also mention that some RCD’s flick into a middle position when tripped and require being pushed down into the OFF position before you can switch them back up into the ON position.
Probable cause of tripped RCD's
Faulty Class I appliance. Any appliance that uses an earth is Class I, i.e., toaster, kettle, extension lead, power tools, etc. Class II appliances such as mobile phone and laptop chargers don’t have an earth and cannot cause an RCD to trip unless they happen to be obviously damaged and in contact with earthed metalwork or water. Look at the earth pin on the appliance’s plug – if it’s all plastic then it’s Class II).
Water ingress, i.e., extension lead or electrical equipment left out in the rain, leak inside kettle or coffee machine, damaged accessories outdoors, leaking roof or pipes near electrical junctions, etc.
Bad insulation resistance, i.e., the insulation within a cable breaking down allowing current to ‘jump’ between the conductors. This is a result of wiring which is old, exposed to the elements, strained, over-bent, crushed or poorly cut/terminated.
Cross-connected or 'borrowed' neutral, i.e., two circuits on different RCD's but wired into the same neutral (sometimes found on landing/stairs lights). This would only occur if you've just had work done on your consumer unit, or a replacement consumer unit, and the installer cocked it up.
Current leaking into non-electrical metalwork such as water pipes, or into the Earth directly at a damaged outdoor cable or accessory.
Current leaking between circuits.
Tripped RCBO
Because this is a two-in-one device, it may be difficult to tell whether it’s tripped because of an overload or a current leakage fault. If it goes bang when you try to switch it back on, that’s definitely an overload and the fault's still there! If there's no bang, it may be tripping because of a current leak. As with the RCD above, you'd need to find where the current is leaking to. Usually it's a leak between the line/neutral conductor and their associated earth conductor, but that current could be escaping in all sorts of ways including a leak to the neutral on a different circuit, into earthed metalwork or water pipes in contact with a damaged cable/appliance or even directly into the Earth via a damaged cable buried underground.
Probable cause of tripped RCBO's
Faulty Class I appliance. Any appliance that uses an earth is Class I, i.e., toaster, kettle, extension lead, power tools, etc. Class II appliances such as mobile phone and laptop chargers don’t have an earth and cannot cause an RCD to trip unless they happen to be obviously damaged and in contact with earthed metalwork or water. Look at the earth pin on the appliance’s plug – if it’s all plastic then it’s Class II).
Water ingress, i.e., extension lead or electrical equipment left out in the rain, leak inside kettle or coffee machine, damaged accessories outdoors, leaking roof or pipes near electrical junctions, etc.
Bad insulation resistance, i.e., the insulation within a cable breaking down allowing current to ‘jump’ between the conductors. This is a result of wiring which is old, exposed to the elements, strained, over-bent, crushed or poorly cut/terminated.
Cross-connected or 'borrowed' neutral, i.e., two circuits on different RCD's but wired into the same neutral (sometimes found on landing/stairs lights). This would only occur if you've just had work done on your consumer unit, or a replacement consumer unit, and the installer cocked it up.
Current leaking into non-electrical metalwork such as water pipes, or into the Earth directly at a damaged outdoor cable or accessory.
Current leaking between circuits.
Short circuit (line and neutral conductors coming into contact with each other directly or via a conductive medium).
Faulty appliance or fitting drawing more current than the circuit was designed for.
Dodgy DIY, i.e., a high current appliance being spurred/connected to a lower current circuit, e.g. trying to run a 40A shower off a 30A socket circuit.
Damage to cabling, i.e., drill or nail through a cable hidden in the wall, crush damage, fire damage, etc.
Again, with any of the above, only look into the cause of a protective device trip as far as your comfort and competence limits allow. If the cause is not obvious or the remedy not simple then call in a registered sparky so they can faultfind, fix and certify the circuit.
The probable causes listed above are just a pointer and don't cover all scenarios. For the pedants out there, I will also point out that there are other protective devices not listed here, but this is written as a basic guide for what you're likely to come across in the domestic environment.
Remember that the protective device is an important line of defence protecting your life and property. Never override, alter or compromise these safety systems. If trips or fuse failures are occuring, then get the fault properly remedied and the circuit certified as safe for continued operation. It always horrifies me that B&Q sell consumer units as though they are DIY installable; they're not. Replacing a consumer unit has to be done hand-in-hand with full inspection and testing to ensure complaince with the wiring regulations and that there won't be any nusiance tripping afterwards. If your home didn't have an RCD before, how would you know the wiring is sound for use with an RCD now if you haven't tested it? How do you know you're using the right size breaker for the circuit? What are you going to do in the event of a borrowed neutral? How do you know your earthing and bonding is up to par? Consumer unit changes are also required to be notified to your Local Authority under Part-P of the Building Regulations.
The picture below shows what can happen if you don't hire a professional. This was taken by one of my partner firms and shows an installation where the homeowner, an elderly gent, had forked out £680 for a non-electrician to change his fuse box to a dual RCD consumer unit. Needless to say, there was no inspection, testing or certification and the installer took the cash and ran. He installed the unit incorrectly so couldn't afterwards get the cover on, hence the use of gaffer tape. There are holes left in the front exposing the live busbar, there are no labels, and despite being a dual-RCD board, all circuits are wired into the breakers on just the right side RCD because of borrowed neutral issues causing tripping which he didn't have the skillset or equipment to identify and rectify.
Further to this, because the circuits were jammed into one side of the board, single breakers were operating multiple circuits and no consideration had been made to the size of the breaker and the size of the wiring, so 1mm2 lighting cables were on 32A breakers which would never have protected them. He didn't even check the circuits for correct operation afterwards, and the reason my partner firm was called in was because the cooker circuit was tripping the RCD and taking out all circuits through a line to earth fault the installer had left at the cooker isolator.
Incorrect installation of the very devices which are there to directly protect you and your family from meeting an unpleasant end through electric shock or fire is never worth trusting to DIY, well meaning friends, cash-in-hand cowboys or any non professional. Had there been a shock or fire incident in this case, then it would be the homeowner who is prosecuted for breaching the wiring & building regulations because he hired an unqualified, unskilled idiot. The old boy could have faced a fine or imprisonment besides any injury to himself or his neighbours, or he could have avoided the legal red tape altogether had he got himself killed.
The irony is that he paid the cowboy £680 cash for this, yet my partner firm would have done it properly in the first place for about £400. Now the old fellow is having to pay to get the mess rectified.
Electricity is like driving a car. It's dangerous, accidents can happen, but the odds are that unless you do something silly then you're going to get though your day using it without incident. With that analogy, your electrical protective devices are like your vehicle's brakes and you shouldn't trust maintenance, upgrades or replacements to any non-professional third party, no matter how well meaning they may be, who offers to perform the work for cash with no guarantees or acceptance of liability.
After all, they won't be the ones who ultimately rely on these things working on the one day when the odds aren't in your favour and you really need them.
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