Mains Voltage

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Introduction

Mains Voltage in the UK has been unified at 240v ac for over half a century. The formal definition of the supply is given as 230V +10% / -6% (though the target voltage is still 240v). 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 usage, the term Low Voltage is used in the formal wiring regulations documents to refer to our normal domestic mains voltage of 240V. This 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). 240V mains fatally electrocutes around 20 people a year in Britain, and shocks over a million.

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 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. 110v site supplies are characterised by the use of yellow-coloured industrial plugs, cable and transformers.

Live

Traditional usage in the UK has always referred to the Live and Neutral wires of a circuit. The "live" being the "driven" wire at a voltage around 240V AC, and the neutral being relatively close to earth or 0V. While in common use, this is insufficiently precise for use in some of the formal documentation because it glosses over an implementation reality that means that the Neutral must also be treated as a "live" wire in some circumstances (see below for explanation). Hence when specifying wires unambiguously, the tradition in the UK has been to refer to the "live" wire as "Phase". This also ties in the with the fact that power generation and distribution in the UK is done using the three phase system.

Other countries more traditionally used the word "Line" in place of "Phase", and with the coming of the 17th edition of the wiring regs in the UK, the same practice has now been adopted here. So you will begin to hear talk of "Line and Neutral" or the "Line Wire". (remembering that Neutral itself remains a "live" wire!)

Why is Neutral a Live wire?

Neutral is considered a "live" wire because under normal conditions, a current flows in the neutral wire. Also a neutral wire can reach near 240v in some fault conditions, so needs to be touchproofed. You may consider the earth wire "dead" in comparison because under normal conditions, no current (or an extremely small amount) flows in the earth wire.

With the TN-S earthing system used in many UK homes, the neutral at the sub station is connected to Earth, in a perfect world, one should be able to assume that the Neutral wire will always be at earth potential wherever you measure it. However a small amount of voltage "drop" in the supply cables can mean that when measured far from the substation during periods of high load, the "live" (i.e. line) conductor voltage may tend to fall a little, and the Neutral will rise a little, giving a lower nett supply voltage to some houses. It also means that if one were to make an accidental connection between Neutral and Earth in these circumstances, a significant current could flow between Neutral and Earth, and the earth connection between your property and the substation now becomes an alternative Neutral conductor for the neighbourhood!

Can I get a shock from a Neutral wire?

Usually no, but in some situations neutrals become live enough to shock or even kill.

The main causes of neutral shocks are circuit faults & wiring mistakes. If the neutral connection back to the CU is interrupted by a fault or a switch, the neutral wire becomes live when an appliance is connected. If live & neutral are swapped, what appears to be neutral is live.

During a short circuit fault, thousands of amps can flow down neutral wiring, resulting in significant voltage rise on the cable. Howevr this is not sufficient to kill under ordinary circumstances (unlike earth voltage rise on TT systems, which in similar condictions can raise earth wiring to nearly 240v).

"Borrowed" neutrals are also a shock risk. The live conductors of each final circuit should be separate from those of other circuits, but '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.

Finally, although the Neutral voltage rise described in the previous section won't shock you, the current flow if conected to earth can be quite high, and needs treating with some respect.

See Also