Mains Voltage
Introduction
Mains Voltage in the UK is traditionally thought of as being 240V AC. The formal definition of the supply is given as 230V +10% / -6%. This is a fairly loose specification that is designed to encompass the range of supply voltages used around Europe, as well as deal with the natural variations in supply voltage that will result from compromises in the design of the distribution network.
If you read through various documents such as the wiring regs (BS7671) or its companion the On Site Guide, you will encounter a number of less familiar terms used to describe mains and other voltages. This article will hopefully clarify some of these terms and their usage.
Low Voltage or LV
Contrary to popular expectation, the term Low Voltage is used in the formal wiring regulations documents to refer to our normal domestic mains voltage of 240V. In fact the definition of Low Voltage includes any voltage up to 1000V AC, or 1500V DC.
It is important to understand here that the term "Low" should not in any way be associated with "safe". It is low only relative to the very much higher voltages used in the national grid distribution system (typically 11,000V and higher - rising to in excess of 100,000 V in many cases). A 240V supply can and and will kill you given a chance.
Extra Low Voltage or ELV
ELV is typically used to describe voltages less than 50V AC or 120V DC. Frequently when people talk about Low Voltage lighting, they are in reality referring to one of a number of ELV systems (frequently based on 12V AC supplies).
To add a bit of extra complication there are a few variations on the ELV term:
- SELV (separated extra-low voltage). An extra-low voltage system which is electrically separated from Earth and from other systems in such a way that a single fault cannot give rise to the risk of electric shock. This is the most common system used for ELV lighting and other systems where safety is critical such as in bathrooms.
- PELV (protective extra-low voltage). An extra-low voltage system which is not electrically separated from earth, but which otherwise satisfies all the requirements for SELV. Less commonly encountered in domestic use.
- Functional extra-low voltage (FELV). An extra-low voltage system in which not all of the protective measures required for SELV or PELV have been applied. Even less common!
Reduced low voltage system
A system in which the nominal phase to phase voltage does not exceed 110 volts and the nominal phase to earth voltage does not exceed 63.5 volts. This is a system you will frequently find on building sites, for supply to (110V rated) portable power tools and lighting equipment. Site supplies are characterised by the use of yellow-coloured industrial plugs, cable and transformers.
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Can I get a shock from a Neutral wire?
The Neutral voltage rise described above is very unlikely to rise enough to pose a direct shock risk if you touch it, but the current flow to earth could be significant, and hence needs treating with some respect.
One situation where a neutral can pose a significant shock risk is from a "borrowed" neutral. Although the live conductors of each final circuit should be separate from those of other circuits, 'borrowed' neutrals are sometimes encountered - with the attendant shock risk from a disconnected neutral when working (for example) on a lighting circuit you thought you had isolated.