Electrical Safety

Most modern electrical devices and installations are, if used correctly, very safe. However, if things go wrong, electricity poses a serious risk to human health, either through electrocution or fire.

This document only provides basic information about electrical safety.

Where necessary, you should always employ a qualified electrical engineer or electrician.


The following table shows the effects of an electric current flowing through the human body:-

Current (mA)Effect
1Mild sensation
50Muscular paralysis, extreme breathing difficulties
100Danger of death due to ventricular fibrillation
200Breathing stops, heart forcibly clamped
1000Severe burns

where 1000 mA (milliamps) corresponds to one A (ampere), roughly equal to the current taken by four 60 W (watt) lamps connected across the 230 V (volt) supply used in the United Kingdom.

The resistance of human skin is around 500,000 ohms (500 kΩ) when dry, but falls to only 500 Ω when wet, increasing the ‘mains shock’ current from 2 to 2000 mA respectively, although the latter falls after a prolonged flow of current. A fatal shock from just 42 V has been recorded.

Practical Advice

Taking the following steps can minimise the effects of someone receiving a shock:-

  1. An electrical engineer must never work alone. When only one engineer is available another person must be present whilst work proceeds on any live equipment.
  2. The engineer must isolate the source if mains power before working on equipment. High voltage (HV) circuits should also be connected to earth prior to working on them.
  3. An engineer should avoid working with wet hands or standing on a wet or metallic surface.

If you believe someone has been electrocuted, you should take the following steps:-

  1. Isolate the source of mains power.
  2. If the supply can’t be isolated (or you’re unsure of what to do), simply push the victim away from the ‘live’ equipment using an insulating (non metallic) object, such as a broom handle.
  3. Use resuscitation. Note that the absence of a pulse does not indicate death: you should proceed with the accepted methods of resuscitation.

Preventing Electric Shocks

The chances of receiving an electric shock can be minimised by using earthing, double insulation or by installing a residual current device (RCD), as described in the following sections.


All modern electrical installations incorporate an earth continuity conductor (ECC), also known as the earth (E) circuit, which accompanies all live (L) and neutral (N) circuits.

Any exposed metalwork contained in an electrical appliance conforming to the Class I standards must be connected to this ECC circuit. This extra circuit is conveyed, along with the mains supply that powers the device, via a 3-core cable and/or a 3-pole mains inlet connector.

Now, if any metalwork inadvertently comes into contact with the mains supply the associated fuse or equivalent device that protects the equipment should ‘blow’, preventing the user from receiving an electric shock and also disabling the device until a repair can be implemented.

Double Insulation

A Class II device doesn’t have an earth connection, and is usually fitted with a 2-core cable and/or a 2-pole mains inlet connector, which only conveys the live (L) and neutral (N) circuits. Such equipment relies on a very high standard of electrical insulation, involving the use of appropriate materials and positioning so as to isolate the mains supply from any exposed metalwork.

Although the protection provided by double insulation obviates the need for an earth conductor it requires a very high standard of engineering design. For example, all transformers must be bobbin-wound and should incorporate a thermal fuse to prevent damage to the insulation in the event of overheating. Most Class II equipment is also designed so as to restrict the mains circuitry to a small area, sometimes requiring, for example, a non-metallic linkage to actuate the mains switch.

Residual Current Devices

Under some circumstances, for example where electrical equipment is used outside or where exposure to damp or accidental severing of power cables is a possibility, neither earthing or double insulation can fully ensure the user’s safety. In such situations, the use of a residual current device (RCD), sometimes known as an earth leakage circuit breaker (ELCB) or earth trip, can considerably reduce the chance of death through electrocution. RCDs should also be used in workshops, where equipment is operated in a dismantled state in order to implement a repair.

An RCD ‘trips’ whenever there’s an imbalance in the flow of current between the live (L) and neutral (N) circuits of the protected circuit, the residual current usually flowing through the person whose receiving a shock or through a faulty piece of equipment. Usually, the device doesn’t ‘trip’ until this current reaches 30 mA, which means that the ‘victim’ still feels the shock. However, this doesn’t normally cause the recipient any harm, unless they happen to be suffering from a heart complaint.

Technically, it’s possible to use an RCD that limits the current to a lower level. However, this has two disadvantages. Firstly, such an RCD could easily be set off by a collection of equipment that has a rather high earth leakage current, even though the individual items comply to accepted standards. Secondly, anyone receiving a shock could fail to notice it, resulting in the fault being overlooked.

Fires and Electricity

Generally speaking, electrical devices don’t catch fire unless they’ve been incorrectly installed or used in a way contrary to the manufacturer’s recommendations. Normally, a faulty device causes the appropriate fuse (or alternative protection device) to blow, cutting the supply from the equipment.

However, if the fuse rating is incorrect, a high current can continue to flow, possibly causing parts of the equipment to overheat, and, in extreme circumstances, to catch fire.

Preventing Fire

Problems with overheating devices can be avoided by observing the following rules:-

  1. Always use fuses of the correct type and rating. If in doubt, consult an expert.
  2. Ensure that equipment is properly ventilated in accordance with the manufacturer’s advice.

As a rule, the fuses within an electrical system should be selective and hierarchical. For example, the fuse feeding a power ring main in the United Kingdom is usually rated at 30 amps (A), the fuse in the mains plug (confusingly known as a 13 A plug) may be rated at 3 A and that within the device itself may be set at only 1 A. Using different fuses in any of these locations can compromise safety.

In some instances, a fault may not generate enough current to blow any fuses. Instead the problem must be accommodated within the design of the equipment itself, often by means of fuseable links, fuseable resistors or thermal fuses, all of which can offer extra protection.

Electrical Installations

For the purpose of this document, an electrical installation is deemed to be that part of the wiring up to and including the power outlets or lamp fittings, all of which should constitute part of the fabric of a building. The cables making up an installation are often fitted onto or buried into the walls or hidden behind partitions and false ceilings.

Containing the Mains

Ideally, all wiring should be placed within some kind of trunking or conduit. Plastic conduits are perfectly acceptable for domestic situations, although in most professional installations metal is preferred, since this is much more durable and offers some degree of electrical screening.

Domestic installations are often wired in twin and earth cable, dispensing with the need for trunking or conduit. This kind of cable has two insulated wires, coloured red and black for live (L) and neutral (N) respectively, and an uninsulated wire for the E (earth) circuit, all contained in an overall plastic jacket. The earth wire should be covered by a green/yellow sleeve during installation.

In a professional establishment, single wires are often used in trunking or conduit. Each wire is covered with a single layer of red, black or green/yellow plastic. Although this allows a large amount of wiring to be condensed into a small space it’s much more difficult to trace the L, N and E wires belonging to a specific circuit. In addition, there’s a much greater chance of cables overheating.

Each socket outlet must be attached to a matching pattress box, either in the form of a deep box for flush mounting or with a shallow box for surface mounting. However, the latter method of fixing is best avoided as it increases the socket’s exposure to physical damage. Other electrical hardware, such as light switches and lamp fittings can be mounted in similar ways.

Selective Fusing

The division between an installation and a portable appliance powered by an installation is illustrated in the following diagram, in which each group of L, N and E wires are shown as a single line:-

This illustrates the principle of selective fusing, sometimes known as hierarchical fusing, involving the user of lower fuse ratings as you move from the source of electricity towards the final destination. This ensures the wiring, which also has a smaller gauge (thickness) nearer the appliance, isn’t overloaded and therefore won’t catch fire. In addition, it means that a faulty device should only cause its ‘local’ fuse to blow, so that other equipment and the lighting continues to operate.

In the example shown above, fuse A, known as the company fuse since it’s owned by the provider of the supply, is commonly rated at 60 A for a domestic installation. This provides protection for the electricity meter (also owned by the company) and the inlet of the consumer unit, often known as a fuseboard, even thought it may contain circuit breakers instead of conventional fuses.

The wiring to the socket outlets is protected by fuse B, one of several fuses in the consumer unit, typically rated at 30 to 45 A. The cable from the appliance plug to the inlet connector of the appliance (if it has one) is protected by means of fuse C, which us fitted in the plug itself and is usually rated at 3 or 13 A, although in some countries such a fuse is incorporated in the socket outlet instead. Finally, the device itself may have its own form of protection in the form of fuse D, although some equipment is designed, rather unwisely, to rely on the fuse in the plug.

Fuses and Circuit Breakers

A fuse is defined as ‘a device used to minimise electrical fault current and so reduce the risk of injury to personnel or damage to equipment’. A conventional cartridge fuse consists of a thin wire centrally-positioned within a ceramic cylinder, which is then filled with heat-resisting sand and fitted with metal end caps. If an excessive current flows through the device the wire overheats, eventually melting (or ‘fusing’) so as to break the circuit.

Cartridge fuses can be installed in a fuse carrier, which can then be inserted into a fuseboard. Similarly, some portable appliances incorporate a fuseholder, fitted externally or internally.

Modern fuseboards often contain circuit breakers, also known as overload trips, in place of conventional fuses. Unlike cartridge fuses, these allow you to restore a circuit without obtaining a replacement fuse. In addition, you can use the buttons on each circuit breaker to reconnect the supply, or to turn it off. Unfortunately, circuit breakers, which use thermal or electromagnetic technology, can be ‘tripped’ by the surge current that occurs with some motors and transformers.

Older fuseboards often contain rewireable fuses, usually constructed of ceramic or other heat-resisting material formed into a U-section and fitted with a suitable gauge of fuse wire. Although the use of such fuses is often deprecated they’re perfectly safe when loaded with the correct wire.

13 A Plug Fuses

The 13 A plug is universally used for power connections in the United Kingdom. Unfortunately, may people assume that it should always be fitted with a 13 A fuse. In fact, it should have a fuse that matches the current capability of the associated cable and appliance.

Such fuses are rated according to the normal current: hence a plug connected to device that usually takes 3 A should be fitted with a 3 A fuse. Originally, fuses with ratings of 2, 3, 5, 7, 10 and 13 A were employed, although only 3 A and 13 A fuses are normally used today. However, there’s nothing wrong in using the other ratings: indeed, a carefully selected fuse can only improve safety.

Radial Circuits

Traditionally, all electrical installations used radial circuits for the connections between a fuse in the consumer unit and the final outlets or light fittings. A radial circuit looks something like this:-

For power circuits, as shown, the fuse in the consumer unit is rated at 20 A and the wiring to the sockets made using 2.5 mm2 cable, equivalent to the old ‘imperial’ 7/.029 cable, which consisted of seven strands of 0.029 inch wire. Such a circuit is normally used for outlets in a single room, although sockets in two or more smaller areas can be run from such a supply.

A similar arrangement is used for lighting circuits, but this time using a 5 A fuse, with the wiring to the light fittings made using 1.5 mm2 cable, equivalent to the old ‘imperial’ 3/.029, which consisted of three strands of 0.029 inch wire. In the domestic environment a separate circuit is used for all the lighting on each floor of a building. However, in other establishments it’s normal practice to restrict a circuit to a single room, thereby avoiding excessive disruption in the event of a fuse blowing.

Ring Mains

In the United Kingdom, most power circuits are wired in the form of a ring main, in which the supply passes from outlet to outlet, eventually returning back to the consumer unit, as in this example:-

As you can see, additional outlets in the form of spurs can also be added to a ring, although certain limitations apply to the number that can be added. A fused spur box also allows wired-in devices to be connected to a ring, usually taking the form of a flex outlet or a hard-wired connection.

A ring main has some advantages, especially the way in which power is distributed across the cables that form the ring. However, there can also be problems. For example, it’s very difficult to determine if there’s a break in the L or N circuits of the ring, which can result in overheating of cables.

A ring main that encompasses an audio recording studio or video studio may have different radio frequency (RF) currents flowing in each direction, causing interference in some devices. This problem can be minimised by creating a ring main whose cables don’t encircle the area, although such installations are often best served by the use of radial circuits.

Testing an Installation

In the United Kingdom all new electrical work should be accompanied by an IEE Wiring Regulations certificate. At the very least a new installation should be tested with what is commonly known as a polarity tester. This is a form of 13 A plug that usually contains three neon indicators wired between the L, N and E circuits. Two of these should light, showing that the L circuit is connected to the correct pin and that the N and E circuits provide a valid return path.

Unfortunately, a polarity tester doesn’t indicate if there’s a reversal in the N and E wiring or whether the N and E circuits are actually connected together by mistake. To make these tests you must disconnect the appropriate wires at their source and check the circuits with a continuity tester.

Some recording studios employ a technical isolator, a special ‘master switch’ that controls all the technical sockets, which are usually connected to a programme safety earth (see below), but doesn’t control any general service (GS) sockets. The following points should be noted:-

  1. The technical isolator must be easily accessible and mustn’t show signs of arcing.
  2. All technical equipment must be controlled by the isolator.
  3. Technical devices shouldn’t be connected to GS outlets.

In any installation, the integrity of the earth connection is paramount. The earth loop impedance, as measured at the earth pins of 13 A sockets and all exposed metalwork, and measured by means of a suitable earth loop impedance meter, should be 1.09 Ω (ohms) or less.

Special Earthing Systems

Some recording studios use a special programme safety earth (PSE) for all electrical outlets that power technical devices, as distinct from the general services (GS) earth employed for metalwork and non-technical equipment, thereby ensuring that any electrical interference produced by the latter doesn’t upset any sensitive audio devices. Although this form of clean earth has safety implications, especially in terms of equipotential bonding, it’s still widely used.

The following points should be noted in regard to a PSE:-

Portable Appliances

Portable appliance are usually supplied with a mains plug or an appliance cable fitted with a suitable plug, allowing it to be used as required. Most modern equipment carry markings that indicate conformity to various safety standards, such as BEAB, BS, BTAB, CE, IEC, ISO and VDE. However, all equipment should still be checked by a qualified engineer to verify its safety.

13 A Mains Plugs and Cables

Unfortunately, not all mains plugs fully conform to the BS 1363 standard: those that do are usually clearly marked. Plugs that possess any of the following features should be avoided:-

Replacement leads must always have the same type of plug. For example, if what appears to be a 13 A plug actually bears the legend 3 A or 3 amp then the replacement must be identical, otherwise the plug, which could have been designed for 3 A operation, may be fitted with a 13 A fuse in error.

The actual cable should contain wires that confirm to the international wiring colours, which are brown for L, blue for N and green/yellow for E. The older ‘imperial’ standard employed in the United Kingdom used red for L, black for N and green for E (which actually made more sense), while older cables from the USA often use white for L, black for N and green/yellow for E.

Cables with any of the following features shouldn’t be used:-

In addition, all cables must pass both an earth bond and 500 V DC insulation test (see below)

Distribution Boards

Ideally, every device should be plugged into a standard power outlet. In practice, their aren’t usually enough sockets available and many people resort to a 13 A distribution board. Unfortunately, such devices often don’t comply with the appropriate British Standards and can overheat, especially when used with high-powered equipment, such as heaters and kettles.

Generally speaking, such boards are best avoided, although an IEC distribution board, fitted with 6 A 3-pole IEC outlets, can be useful for powering small items in a confined space.

The earth impedance between a 13 A socket and an appliance must be under 1 Ω, equivalent to 60 metres of good quality flexible cable. The ‘worst case’ loop impedance is 2.5 Ω, made up of less than 1.09 Ω at the socket itself, plus the 1 Ω for the cable (as well as any distribution board and the appliance cable), plus the loop impedance of the device itself.

Appliance Inlet Connectors

Many devices are fitted with an appliance inlet plug, allowing the user to detach the mains lead for transportation of the equipment or to replace the cable. The following connectors are used:-

3-pole IEC Plug (10 A)

This Class I connector is sometimes known as kettle connector, or as a hot IEC connector, in order to distinguish it from the similar 6 A variety (see below). It conveys a current of up to 10 A, corresponding to a maximum power rating of around 2.3 kW on a standard 230 V supply.

3-pole IEC Plug (6 A)

This is the internationally approved input connector for Class I electronic equipment, rated at 6 A.

2-pole IEC Plug (6 A)

This reversible connector, which is designed for Class II equipment and conveys up to 6A, is less common than the 3-pole type. In fact, it’s almost considered to be non-standard.

2-pole Miniature Plug (3 A)

This connector, available in reversible and non-reversible versions, is used on Class II equipment and carries a current of up to 3 A. There seem to be several different types, which means that you may have to check to see if an appliance cable is suitable for a particular device.

3-pole XLR LNE (5 A)

The LNE connector was originally introduced by the BBC and subsequently adopted by other broadcasting organisations. Basically, it’s a variation on the original design of XLR audio connector, with a special arrangement of gold-plated pins rated at 5 A. Close examination indicates that the connector is traditionally used the wrong way round, since the ‘socket’ is actually a plug. Fortunately, the pins on this plug are reasonably shrouded, although probably not to modern standards.

This connector isn’t approved by British Standards, and for good reasons. For example, it uses a latch, which means that pulling on the cable can damage the cable or connector: there’s also an increased risk of the cable becoming a trip hazard. Secondly, the small pins appear to get damaged over time and, finally, the metal casing and connection arrangements leave something to be desired.

Testing Portable Devices

All portable devices must be regularly checked using a portable appliance tester (PAT). This is simply inserted between the device and its power socket. In addition, you must connect a clip (for a Class I device) or probe (for a Class II device) to the exposed metalwork of the device to be tested. You then select the required test on the PAT and press the Test button.

Electrical tests should include:-

Class I Earth Bond

This test, which should only be applied to equipment that has a 3-core mains cable, checks the reliability of the earthing by applying a ‘fault current’ for 5 seconds, as shown in this table:-

Fuse Rating (A)Test Current (A)Earth Resistance (Ω)
Under 510Less than 0.5
5 to 1325Less than 0.2 •
Fixed equipment should give less than 0.1 Ω

The resistance between the framework and all conductive parts should be less than 0.5 Ω.

Electric Strength

This measures the quality of insulation between the mains and the body of the device. The test should also be applied with live and neutral reversed (or the wires can be linked during testing).

ClassAC Test Voltage (kV)Leakage Current (mA)
I (3-core cable)1Less than 3.5 •
II (2-core cable)2.5Less than 0.4
Fixed equipment should give less than 5 mA

During routine testing, a test voltage of only 500 V DC is often used, which should give a leakage current of under 50 µA, corresponding to an insulation resistance of 10 MΩ (megohms) for both Class I and Class II devices, although British Standards specify 2 MΩ and 7 MΩ respectively.

Mains Plugs and Cables

All cables should be examined and tested as part of the equipment to which they are attached. Also, the engineer should ensure that they remain in situ when the equipment is swapped or replaced. If other staff are allowed to connect equipment, a ‘pool’ of tested cables should be provided.

General Considerations

The following problems should be dealt with as shown:-

Black-out: a complete loss of the power or lighting supply. This usually causes computers and other computer-based devices to ‘reset’, usually without causing any harm. A black-out shouldn’t occur very often, since selective fusing should prevent it from happening. If it’s a regular event you should have your electrical system examined in detail. In addition, some form of emergency lighting system, usually powered from rechargeable batteries, should be used. This ensures that people, in attempting to move around or leave the building, don’t cause injury to themselves or others.

Brown-out: a temporary drop in supply voltage, which may or may not cause computer-based equipment to ‘reset’. Such ‘dips’ in the supply, which can cause more problems than a black-out, are often a consequence of inappropriate equipment sharing a common circuit or electrical ‘phase’, such as a computer using the same power feed as a photocopier, coffee machine or air-conditioning plant. This kind of problem, which can also be symptomatic of something far more serious, can sometimes be fixed by rewiring part of the electrical system.

Damaged plugs or cables: should be removed safely, if possible, and reported immediately. If damage occurs on a regular basis the cause should be ascertained and the problem fixed.

Flickering Lights: can indicate a fault on an electrical ‘phase’, although not necessarily that used for the lighting itself. However, similar problems can be produced by a faulty component in a fluorescent light fitting, including the tube, starter unit or choke. Any of these can generate electrical interference, which is often manifest as a crackling noise on nearby audio equipment. In some instances, flickering lights can indicate a serious problem in the wiring of the installation.

New or loaned equipment: such devices, complete with any appliance cable, must be checked for safety. Connecting such a device via a mains isolating transformer doesn’t ensure safe operation. The equipment must not be used until it has been tested and formerly approved for use.

Overnight power: unless required for ‘out of hours’ operation, all sockets must be switched off at the end of every working day, either at the wall or by means of an appropriate isolator switch. The switches provided on electronic equipment don’t properly isolate the supply of power.

Overheating plugs or cables: if there’s any sign of charring or heating, the power should be isolated and the offending item or items removed. Otherwise, the fault should be reported immediately to an engineer and a warning notice placed in the proximity of the problem.

Personal electrical equipment: should be removed from the premises, checked for safety or locked up securely. Otherwise, other people may use such devices with unfortunate consequences.

Portable equipment and mains leads: these should be checked at least annually (see above).

Switches: if arcing of contacts can be seen or heard, the switch may need to be replaced. If the switch has a neon indicator this should be illuminated when the supply is switched on.

©Ray White 2004.