Wiring Matters 52 - Autumn 2014
Protection against electric shock
BS EN 61140 tells us that hazardous live parts shall not be accessible and that accessible conductive parts shall not be hazardous live, when in use without a fault or in a single fault condition. In this article, Richard Townsend, Senior Engineer at the IET, looks at the fundamental rules governing the requirements for protection against electric shock.
Put simply, protection from shock under normal conditions is provided by basic protective provisions referred to as ‘basic protection’, such as insulation. When the installation is in a single fault condition the shock protection is provided by fault protective provisions referred to as ‘fault protection’, which normally involves an automatic disconnection of supply (ADS) using overcurrent protective devices. In this way, shock protection is provided whilst the installation is in use, whether without a fault (basic protection) or with a single fault condition (fault protection).
This is in keeping with Regulation 410.3.2, which requires a protective measure to have the following characteristics:
410.3.2 A protective measure shall consist of:
(i) an appropriate combination of a provision for basic protection and an independent provision for fault protection, or
(ii) an enhanced protective provision which provides both basic protection and fault protection.
Additional protection is specified as part of a protective measure under certain conditions of external influence and in certain special locations (see the corresponding Sections of Part 7).
NOTE 1: For special applications, protective measures which do not follow this concept are permitted (see Regulations 410.3.5 and 410.3.6).
NOTE 2: An example of an enhanced protective measure is reinforced insulation.
Fundamentally, BS 7671 only provides guidance and protection against single fault conditions. In a healthy and properly designed installation, we apply one or more protective measures, taking account of external influences that may adversely affect it. The protective measures that we generally permit in BS 7671 are:
- double or reinforced insulation;
- electrical separation for the supply to one item of current-using equipment; and
- extra-low voltage (SELV and PELV).
Automatic disconnection of supply
ADS is the most widely used protective measure. Its purpose is to limit the magnitude and duration of the voltage between exposed conductive parts of a circuit and other exposed conductive or extraneous conductive parts so as to prevent danger.
ADS is a protective measure that includes both basic and fault protection. These two forms of protection are described in BS 7671 as follows:
(i) basic protection is provided by basic insulation of live parts or by barriers or enclosures, in accordance with Section 416, and
(ii) fault protection is provided by protective earthing, protective equipotential bonding and automatic disconnection in case of a fault, in accordance with Regulations 411.3 to 6.
All electrical equipment is required to comply with one provision for basic protection as described in Section 416 of BS 7671. These measures can include:
- basic insulation;
- enclosures; or, in certain circumstances,
- placing the electrical equipment out of reach.
Fault protection is provided by:
- protective earthing;
- protective bonding; and
- automatic disconnection in the event of a fault.
Protective earthing ensures that the circuit protective device will disconnect the supply in the event of a fault and limit the rise in potential, above Earth potential, of any exposed conductive parts during the fault.
Protective bonding is required to minimise any potential difference between exposed conductive parts and extraneous conductive parts during a fault. The outcome of bonding these two parts together is to equalise potential and not to carry fault current; however, in some cases, bonding conductors may carry fault current where they form a parallel earth return path to the source of supply during the loss of a neutral conductor in a PME supply.
This highlights that the term ‘Earth bonding’ should not be used when applying earthing or protective bonding.
Examples of extraneous conductive parts that may require bonding include:
- water installation pipes;
- gas installation pipes;
- other installation pipework and ducting;
- central heating and air-conditioning services; and
- exposed metal structural parts of a building.
Automatic disconnection in the event of a fault
ADS in the event of a fault is achieved when the circuit protective device operates within the required time period for a given supply system. This is dependent on circuit design parameters and limits, which are not the focus of this article. In TN and TT systems an overcurrent protective device or an RCD may be used to provide circuit fault protection. There are specific requirements when using RCDs for fault protection that should be adhered to. Reference to RCDs is made in Regulation 411.4.4 for TN systems and TT systems are referenced in Regulations 411.5.2 /411.5.3.
In certain circumstances, RCDs are also used for additional protection where more onerous situations are present, see Regulation 411.3.3. Relevant sections of Part 7 also stipulate the use of RCDs, when required, in special locations. Understanding the different types of RCD is now important as new inclusion to BS 7671, Section 722, includes specific reference to RCDs of types ‘A’ and ‘B’.
Once all the basic and fault protection requirements for ADS have been met we can say that the electrical circuit/installation/system has shock protection.
Double or reinforced insulation
This type of shock protection is not very common and requires that only equipment that is of a class II construction, which has double insulation or reinforced insulation, is to be used in the installation. To that end only equipment with no exposed conductive parts that may become live in the event of a fault can be used.
Double insulation uses basic protection provided by basic insulation and fault protection is provided by supplementary insulation.
Reinforced insulation is where the protection is provided by single or basic insulation but has the same protective properties as double insulation.
In both cases, the installation does not require a protective conductor and must therefore have effective supervision in normal use. This type of protection must not be used to supply socket outlet or luminaire couplers, which could allow the accidental inclusion of a class I piece of equipment.
In some cases, manufacturers will apply reinforced insulation to a piece of equipment for use in a standard installation. In these cases, the installation will not provide shock protection via double or reinforced insulation; moreover, the equipment is protected in this way and would therefore not require effective supervision whilst in normal use. An example of this is where some manufacturers of recessed down lights use two single insulated cables to connect the lamp holder to the circuit connector outside of its enclosure. The cable used for the connection in this way has single insulation but its properties are that of double insulation.
Allowable wiring systems for use with double or reinforced insulation systems are also tightly referenced; they must be enclosed in either a non-metallic sheath or other non-metallic conduit or containment system and have a rated voltage greater than the nominal voltage of the system and at least 300/500 V.
As a general rule, equipment must be clearly marked with the double square symbol, indicating it is class II, see below:
Double square symbol
Circle symbol: no earth present
Likewise, equipment having un-insulated live parts must have basic and supplementary insulation provided. If this cannot be achieved it is allowable to use reinforced insulation as long as in doing so a degree of protection not less than class II is provided. Again, a symbol to indicate no earth present must be clearly displayed on the equipment.
Electrical separation for the supply to one item of current-using equipment
Shock protection, via electrical separation, for a supply to one item of current using equipment, is provided when fault protection utilizes simple separation of the circuit from the main circuit and all of its associated circuits and Earth. Basic protection is provided by insulation or barriers.
When the basic protection fails in this type of installation the fault protection is afforded by simple separation from the main circuit because it has no fault path to Earth and therefore no electric shock. The downside of this is that with the loss of basic protection the fault is generally not cleared and goes undetected until there is a second fault, which may prove to be hazardous.
A generator commonly utilizes this type of simple separation, which is isolated from the main installation and not connected to Earth or any other earthed circuit. It is vital when using this type of protection method not to allow any exposed conductive parts of the equipment to be connected to the protective conductor or exposed conductive parts of other circuits. Another very common example of this type of protection is a shaver socket. See below for diagram of a generator supplying one item of current using equipment:
Diagram of a generator supplying one item of current using equipment
The protective bonding conductors associated with the
MUST NOT BE CONNECTED TO EARTH
Extra-low voltage (SELV and PELV)
Shock protection by separated extra low voltage (SELV) and protected extra low voltage (PELV) are the same. This is arguably the safest measure as there are three protective provisions in play:
- the limitation of voltage (limited to a voltage not exceeding 50V a.c or 120V ripple free d.c);
- the protective separation; and
- basic insulation.
By its definition SELV is electrically separated from Earth and so a single fault cannot result in an electric shock.
Protection from contact with normally live parts is provided by basic protection in the form of enclosures, barriers or basic insulation. If the nominal voltage of a SELV system or device does not exceed 25Va.c or 60Vd.c, protection from contact with normally live parts is negligible and may depend on the design use of the equipment. For example, some SELV lighting track systems will have the track section live, whilst in use, allowing the user to move the luminaires freely along the length of the track. The touch voltage in this case is not considered to be hazardous in general use.
SELV and PELV supplies are those supplies that are specified as safety isolating transformers in BS EN 61558-2-6. They are almost identical – in so much as both have a primary and secondary winding. The main difference is that the SELV transformer has no connection between Earth and its secondary winding whereas the PELV transformer has a connection to Earth from the secondary winding. Examples of these types of supplies are shown below:
SELV and PELV supplies
It is for this reason that where a PELV transformer is used and the nominal voltage does not exceed 25V a.c or 60V d.c, in a normally dry location, the exposed conductive parts must be connected by a protective conductor to the main earthing terminal for the installation, where protection against contact with live parts may be omitted.
Fault protection in a PELV system is provided by the primary circuit protection, given the secondary winding’s connection to the installation earth terminal. One downfall of a PELV system is that faults elsewhere in the installation are likely to induce voltages on the PELV system through its protective conductor.
The downside of a SELV system is that it must be totally separated from any LV circuits as it does not utilise any protective conductors. To this end, the overcurrent protection in an unearthed SELV system dictates that, where required, devices are present in both live conductors. The need to ensure total separation in a SELV system and the lack of protective conductors requires the use of socket outlets that are not interchangeable with LV or other ELV systems present in the same property. This will further reduce any likelihood that a connection to Earth can be made in error.
Further information on shock protection can be found in the IET publication Guidance Note 5: Protection Against Electric Shock.
In this issue
- Protection against electric shock
- Counterfeit alert
- Near disaster on a railway caused by rats
- Spotlight: David Latimer
- City & Guilds courses: an introduction
- Forthcoming events
- The effect of mechanical cooling on lamp colour and efficiency
- Energy efficiency: achieving sustainability
- Electrical Energy Storage and the Smart Electrical Installation
- London Fire Brigade Interview: changes to BS 7671 in 2015
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