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 hope fully 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 portable power tools. Often derived from the normal mains LV (i.e. 240V supply) via a site transformer.
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 "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?
If you look at the following picture:
(diagram of substation to property supply here)
It shows a common way in which power is supplied to many urban UK homes from a substation (setups like this are known a TN-S earthing systems). The grid supply comes in the form of three phases (or lines) and no neutral. The Neutral is created at the substation using a star configuration transformer attached to each of the phases. The secondary windings of the transformer generate the local Low Voltage (i.e. 240V) supply take to each house. Usually the loads are equally distributed between the three phases which helps ensure that the nett current flowing in the neutral at the sub station should be relatively low (with a perfect load match across all phases it would be zero).
As the neutral at the sub station is connected to Earth, in a perfect world, one should be able to assume that the potential on the Neutral wire will always be 0V (with respect to earth) wherever you measure it. Alas however this will not always be the case. In the first instance, the local potential of "Earth" at a distance from the substation may not be the same (often caused by currents flowing to earth, changing the local potential slightly). The second and far more significant cause, is simply a result of the supply cables to each property not being electrically "perfect" and instead having a small resistance. A group of houses may draw a significant combined current from the supply. This will result in a small amount of voltage "drop" in the supply cables. Hence measured far from the substation, the live conductor voltage may have fallen to say 230V, and the Neutral may have risen to 10V, giving a nett supply of 220V to some houses during times of high loading. 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!
Hence, although the voltage on the Neutral is very unlikely to rise enough to pose a direct shock risk if you touch it, the current flow to earth could be significant, and is also unlikely to be interrupted by circuit protective devices like fuses or MCBs since these are not usually placed in the neutral. (this is why turning off the whole supply at the CU with a double pole switch before working on the system is of particular importance on TN-S installations).