What's Wrong with a Fuse?

Fuses, MCBs, RCBs and RCBOs explained

A fuse is a simple enough device. It’s a thin piece of wire running between two electrical contacts. As electrical current flows through the wire, it heats up. If too much current flows, it melts, breaking the circuit.


This provides basic protection against overload currents, or overcurrent protection. Why is this useful? Simply because if you didn’t have a fuse, and something went wrong resulting in a large current through an electrical circuit, the wiring would melt, and possibly set fire to something.


So fuses protect your wiring. Nothing else. Not you, generally – the amount of current needed to melt a fuse is way more than would kill you. Not really your appliances either – they may be damaged beyond repair by the time a fuse breaks.


If you have a fuse board, you may have a 30 amp rewireable fuse protecting your cooker. Let’s say your cooker element is damaged and all of a sudden 35 amps of current is flowing. The fuse will cut the circuit, right? Well, no, actually. The 17th Edition of the Wiring Regulations (BS7671) shows us that a 30A rewireable fuse made to BS3036 will only fuse when the current reaches 45 amps – and even then, it’ll take around 2 hours 45 minutes to melt. If you want it to melt within a second, you’ll need to put around 105 amps through it.


What do you do when a rewireable fuse in your fuse board breaks? Obviously the first thing you should do is find out why it fused, as it’s indicating a fault somewhere. But to replace it, you need to find the correct rating of fuse wire, pull the fuse, find a screwdriver, rewire the fuse, cut the wire using the wirecutters you have conveniently to hand – and if it’s your lights that have fused, you’ll have to do this by torchlight or candlelight! Assuming, of course, you have replacement fuse wire of the right rating conveniently to hand...


So fuses don’t protect you (only your wiring), they take some time to have any effect, and they’re a pain to rewire when they melt.


Enter the MCB, or Miniature Circuit Breaker, sometimes known colloquially as a trip switch. This little device cleverly uses an electromagnet to break two contacts apart if the current exceeds a certain threshold. It has two big advantages:

  1. It can be reset by flicking a small plastic switch. Exceedingly useful when a light bulb has just blown and tripped the MCB.
  2. It reacts reliably quickly. A correctly specified MCB should break the circuit within 0.2 to 5 seconds.

MCBs are sometimes available as direct replacements for fuses. For example, if you have a Wylex fuse board with rewireable fuses, these can be directly replaced by MCBs in a few seconds and at minimal cost.


If you have a traditional fuse board, I would strongly recommend replacing your rewireable fuses with MCBs, as they are inherently safer, and much more convenient.


This isn’t the whole story, however. When an appliance fails, or something goes wrong (such as putting a nail through a wire, or cutting through the cable of a lawnmower), an overcurrent doesn’t necessarily result. This means that your electrical system is perfectly capable of supplying enough current to cause serious injury or fatality without a fuse or an MCB breaking the circuit.


An RCD (residual current device) is designed to protect people, more than wiring. The RCD monitors the amount of electricity going down the live and neutral wires. If all is well, the amount of current down the live wire should be exactly the same as the amount of current coming back down the neutral wire, which means the current on the earth wire is zero.


However, if some of the current from the live wire leaks through to the earth wire, or escapes altogether, this indicates a fault, such as a nail or strimmer wire through a cable, or worse, a live current flowing through a person. In this eventuality, the RCD will detect a difference in the current between live and neutral wires and respond within a fraction of a second – typically a few milliseconds – and cut out the circuit altogether.


(Actually a small difference is permitted - usually 30mA - to take into account that some appliances will "leak" a small amount of current to earth even if there's not a fault condition.)


Conveniently, 50 milliseconds is a short enough time even with 230V electricity to considerably reduce the risk of fatality. So, with a correctly specified & fitted RCD in your circuit, properly tested, your electrical system instantly becomes far safer than any system relying just on a fuse or MCB.


The current building regulations and British Standards state that, when fitting a new circuit, an RCD should be fitted in almost every circumstance – for example, with any circuit near a bath or shower, any outside circuit, where a cable is buried less than 50mm in a wall without armoured sheathing, etc. There are in fact very few circumstances where a new circuit isn’t required to be RCD protected, and it’s all in the name of protecting human life.


So what should you do if you have an older system that isn’t using RCDs?


At the very least, you are strongly recommended to get RCDs fitted to certain key appliances, particularly power showers and any outside sockets. A power shower can become deadly if it fails, instantly making the water supply live; an RCD will solve this instantly and at minimal cost.


Alternatively, and more safely, you can get your old fuse board replaced by a modern Consumer Unit, which features both RCDs and MCBs protecting every circuit. As part of the installation, your installer is obliged to conduct many dozens of separate tests on all aspects of your electrical system, which will highlight a variety of safety issues you might have been unaware of.


Once installed, you can sleep safely knowing you have a modern, reliable electrical installation which will protect you from harm in every eventuality.


Oh, what about RCBOs? An RCBO is simply a device providing MCB and RCD functionality all in one. They're not plug-in replacements for fuses, because they need to monitor live and neutral (and sometimes earth) at the same time. Most UK installations use separate RCDs and MCBs for safety and convenience, but increasingly electricians are installing RCBOs as a neater, cleaner solution which also ensures only one circuit is affected in the event of a "trip".

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