Leakage is where an electrical current is passes down a path it is not intended to travel. This could be in an electrical installation or an appliance.

 Some appliances are designed to have a certain level of leakage current to earth (usually in the order of 2 or 3 milli-amps maximum i.e. 2 or 3 thousandths of an amp or 0.002A) – these are usually power supplies to electronic equipment.

 If however a fault or damage to a cable causes a component to become live then there is  a potential for leakage current to be pass through a person or animal.

 So to eliminate the possibility of dangerous leakage currents causing electrical shocks there are a number of safety features that should be incorporated into all installations.

 Before looking at these it is important to look at the effect of electrical current passing through the human body.

  Current flowing through the body is detectable at a current of about 0.5 mA (ie 5 ten thousandths of an amp.

• At 1 mA a slight tingling is felt.
• When the current is about 5 mA a slight shock will be felt, it is not dangerous as a current but will have a startling effect on the body potentially causing a reaction accident.
• Above 6 mA the shock becomes more painful with increasing current. At 20 mA breathing difficulties are experienced and it becomes difficult to “let go” of the equipment that is causing the shock. As the current increases heart rhythm is affected and cardiac arrest is possible although the effects are reversible up to about 40 mA.
• Above 40 mA there is a risk of non-reversible disturbances to the cardiac cycle (ventricular fibrillation). Burns are now possible as is full cardiac arrest.

 The above relates to a healthy person and the effects above are also exposure-time critical. The young and elderly are more seriously affected as are people in poor health.

 The factors affecting the current and thus the level of the shock are dependent on a number of environmental and health issues:

 Damp skin conducts electricity more readily than dry skin and will increase the potential shock. Cuts and bruises will also increase the shock level as will the path of the current through the body. Assuming the path to earth is through the feet then the insulating properties of shoes and the floor construction will also have a bearing on the level of current that will pass. Wet leather soled shoes whilst stood in the garden will lead to a significantly higher potential shock current than rubber soled shoes on the floor of a typical dry carpeted room.

 So the above indicates how small a current has to be to cause a significant threat to life. 40 mA (that is 40 milli amps or 40 thousandths of an amp) is a serious shock – the smallest fuse or circuit breaker that is likely to be present in a fuse board or consumer unit is 5 A (5 amps or 5 thousand milli amps) – so at 125 times the shock level you can see that the 5 A fuse is not going to protect you by itself.

 The next article will look at the systems and components used to prevent electrical shock

Overload – Protective Devices

There are a number of devices that are used in fuse boards, distribution boards and consumer units to prevent circuit overload. These are:

1: Rewireable fuses
2: Cartridge type fuses
3: MCB’s or miniaturised circuit breakers

It must be remembered that these devices are used to protect the wiring in the installation from an overload (and to help protect the building from a potential fire) – they do not necessarily protect people from receiving an electrical shock.

1 Rewireable fuses

These are historically very common and made up the vast majority of domestic and light industrial circuit protective devices in the 60’s and 70’s. They have the advantage of being relatively low cost to produce are technically straightforward and tend to be very reliable in use.

The disadvantage of these fuses are that when a fault occurs (or in the case of a lighting circuit, a lamp fails) and the fusible element “blows” disconnecting the power to the circuit then the fuse, by its nature, has to be manually rewired. Of course it usually happens when it is dark, or failing that the fuseboard is sited in a dark under stairs cupboard making the process time consuming, awkward and potentially dangerous.

It is also important to determine that the cause of the original fuse failure has been rectified otherwise the rewiring process will again be necessary.

Another disadvantage is that the correct fuse wire to replace the “blown” wire is often not available or the person replacing the fuse wire is unaware of the different grades of fuse wire and thus puts in the incorrect grade with potentially disastrous results.

An example of rewireable fuses and the type of fuse board in which they are generally found are shown below.

On some occasions we have found the fuse carrier to have been rewired with a section of copper wire which would not “blow” until the cable to which it was attached over-heated and set fire to the building. We carried out electrical inspection and testing on one property in Taunton and found that the cable had become so hot, due to a fault, that the PVC insulation was charred and dripping from the cable – we are not sure how the house did not catch fire – all because someone had used a section of copper wire instead of the correct fuse wire.

Based on our experience we would recommend that fuse boards containing rewireable fuses are replaced with modern MCB types for safety reasons especially if the property is let – we have found that many tenants do not understand the need to replace fuse wire with the correct grade!!

2 Cartridge type fuses

These are very similar in performance to rewireable fuses in that they are very straightforward and are very reliable in use. However they do suffer from the same shortcomings as the rewireable types requiring the user to replace the fusible element. They are generally faster to replace than the rewireable fuses but again it relies on the user having spare fuses to hand (which is rarely the case).

The fuses themselves are all of dissimilar sizes (in domestic applications) and therefore eliminate the potential of replacing with the incorrect size of fuse.

Examples shown below:

3 MCB’s or miniaturised circuit breakers

These are now the preferred choice in the majority of new domestic and commercial installations. They are relatively low cost and can be reset, although in a short circuit situation they can be overloaded and damaged. It is essential to ensure that the potential short circuit current at the board does not exceed the rated short circuit capacity of the MCB.

There are different types of MCB and modern ones to BS EN 60898 are of type B, C or D:

Type B MCB’s are generally for normal use where the loads attached do not have high switching currents on start up and domestic type circuits generally fall into this category.

Type C MCB’s are used in commercial or industrial circuits where switching surges can be quite high, for example banks of fluorescent lighting or equipment with motors.

Type D MCB’s are not suitable for general use and are generally limited to equipment like X-ray machines or industrial welders.

An example of a consumer unit and some MCB’s are shown below.

To sum up – all of the devices above are designed to protect the wiring in the installation to prevent overheating and potential fires.

They are not there to protect appliances or their wiring (that’s why plugs have fuses!).

It is important to ensure that the correct size of fuse or MCB is chosen for the wiring it is supplying. Lighting circuits are usually 1 to 1.5 mm2 in cross section and are generally protected by up to a 6 A MCB or fuse. Radial power circuits are generally 2.5 mm2 and have up to a 20 A protective device and ring circuits in 2.5 mm2 a 32 A device.

Cables that run through or within insulation may have to be down-rated to prevent overheating if they are unable to dissipate any resistive heating effects.

Two fundamental electrical faults that require protective measures within any installation are:

• Overloads or shorts

• Electrical leaks or faults that can lead to electrical shock

There are, however, devices that will take appropriate action for other faults such as over-voltages or surges, overheating equipment, smoke or fires or low voltages for example.

Overloads or shorts

This is where cables are running beyond their maximum design capacity.

These situations will occur if:

• A piece of equipment develops a fault and draws excessive current
• If a phase (live) to neutral or earth fault occurs due to damage etc (ie a short circuit)
• Where a multi outlet circuit (eg socket circuit) is loaded beyond its rated capability.

The result of a cable drawing too much current is that resistive heating will occur within the cable. All cables have an internal resistance. The smaller the cross sectional area of the cable then the greater the resistance.

To take this to an extreme picture the old style electric bar heaters. A 1 kW heater produces this heat from only about 4 amps of current flow – this current is sufficient to cause the wire running around the ceramic core of the heater to glow red hot and emit 1kW of heat energy. Most lighting circuits have fuse ratings of 6 amps and socket ring circuits are generally rated at 32 amps. The reason the bar heater glows red hot is because the resistance of the element is sufficiently high (due to its very small cross sectional area and material used) to cause this heating effect.

Excessive current beyond a cables design current will cause similar resistive heating which the cable has to dissipate; if the cable cannot shed the heat faster than it is being generated by the resistive heating within the cable it will overheat, potentially causing a fire.

So it is quite clear that to protect the cables within an installation some form of protective device is required to ensure that the current within a cable does not exceed (for a significant period of time) the design capability of the cable by disconnecting the supply to the cable.

This protection is usually carried out at one or more central points within an installation and was historically called a fuse board where cartridge fuses or rewireable wire fuses were used. These have in the main been replaced with miniature circuit breakers (MCB’s) and the boards are generally referred to as distribution boards (DB’s) or in the case of domestic installations as consumer units (CU’s).

The three main types of overload protection (domestic and light industrial use) are:

1 Rewireable fuses (to BS3036)
2 Cartridge type fuses (to BS1361/2 or BS88)
3 MCB’s (miniature circuit breaker) to BSEN60898 or BS3871

The next article will look at each of the above methods of overload protection and discuss the merits and disadvantages of each.

Electricity travels easily through a conductor – metals are generally good conductors and copper is especially so – hence its use in domestic and commercial cables.

Electricity does not travel well through insulation – ceramics and plastics are good insulators and are widely used for this purpose. Ceramics don’t tend to bend or flex too well so plasticised PVC is generally used as an insulator around basic cables.

Electricity will only travel in a circuit – if you break the circuit the electricity will cease to flow – useful phenomena which the light switch takes full advantage of.

Electricity prefers to take the path of least resistance.

In a conventional UK 230V supply electricity will always try to find a path to the ground (or earth). This is a feature of the way electricity is generated and supplied in the UK – more on this later.

Power generation

When electricity is generated at power stations in the UK linked to the National Grid it is produced in the form of an alternating current supply.What this means is that the voltage on the phase (or live) cable alternates between a positive value and a negative value compared to the neutral cable in a sine wave format as below, Fig 1.

Three outputs are produced simultaneously with each supply 120˚ out of phase with each other, hence three phase supplies as shown in Fig 2 with each of the phases superimposed on each other. Single phase supplies are simply one of the three phases. In a local area adjacent properties are likely to be supplied from different phases so that the loads on the three phases are kept in balance.

Electricity produced by a battery is in the form of a non alternating voltage (known as direct current or dc). The two cables are generally marked up as +ve and –ve.

When electricity first became generally available to towns and cities in the UK it was on a local basis, supplied as an alternating current (ac) and there was no standardisation of voltage or frequency of the alternating current produced. There were frequent power cuts and the quality of the supply was at best variable. In 1920 in London alone there were 50 different generating systems with 24 different voltages and 10 different frequencies. Imagine what was involved in moving home if you were wealthy enough to own some electrical appliances!

Very few of the power stations were interconnected until late in 1920’s when the Central Electricity Board was created to build a grid of high voltage interconnections between the major power stations which were then standardised at a frequency of 50Hz. These were originally run as seven separate systems until imbalances in the generating power between the North and South lead to them being interconnected at the end of the 1930’s. In the mid 40’s the generating industry was reorganised and became first the British Electricity Authority but unfortunately confusion of the initials with British European Airways lead to the new name of the Central Electricity Authority and evolved in the late 50’s into the Central Electricity Generating Board, CEGB, which brought on-line bigger power stations. Demand for electricity at this stage was doubling every 10 years. The National Grid was created when the CEGB was privatised in 1990.

This all means that we now have a standardised electrical supply system with numerous generating companies feeding “The Grid”. For practical reasons a power station has to be able to supply electricity to the grid that is precisely in phase with the alternating current that the other power stations are producing and (to reduce the heating losses) does so at a voltage well in excess of the generated voltage, up to more than 400,000V.

So it has to be transformed-up at the point of generation and transformed-down at the point of use At the power stations, for practical reasons, the neutral cable is connected to the earth (or ground). This is why 230V mains electricity will always be trying to find a
path to the ground.

This means that anyone stood on the ground, or a structure that is electrically connected to the ground is already half way towards getting a shock as they are effectively stood on the neutral terminal of a 230V electrical circuit – makes you think doesn’t it.

In contrast, if you grab hold of either one of the two terminals of a high voltage standalone battery you will not get a shock since you are not completing the circuit.

So that’s it for the first part of the series of articles I hope this makes sense and explains why you only have to touch the phase wire of a 230V standard grid supplied mains to get an electric shock. The next instalment will start to explain about the two fundamental electrical safety protection methods.

If you have any questions or feedback on the content of this article please let me know.

Note: References to CEGB and historical points taken from “The CEGB Story” published by the CEGB, London EC1A 7AU.

Over the past few months we have been making a number of electrical safety presentations to professional bodies allied to property and the people who use them.

 As a result we thought that the subject matter that we have been discussing might be useful to others in the form of an on-going series of articles. 

We are proposing a number of articles which will cover the following subjects: 

1. The basics of electricity (230/400V ac)
2. Electrical safety – Protection methods
3. Overload – Protective Devices
4. Electrical leaks or faults that can lead to electrical shock
5. Devices and systems to protect against electric shock
6. Recent changes that have taken place to the wiring regulations
7. A series of topics based on common faults that are frequently found in properties.

 If anyone has any subject matter they would like to see please contact us and we will try our best to cover it.