The story of Electricity Generation from pipelines to pylons
Part 6: Getting down to Earth; Electrical Safety considerations.
We've now shown how a 230V a.c. supply is derived from a 3-phase 415V transformer, noting also that a consumer's neutral wire connects to the transformer's star/ neutral point further away. A separate "protective" earth (the steel armour of the underground cable) runs between the consumer's earth terminal and the (earthed) star point of the transformer as well.
In the final part of this feature, we examine the need for electrical "earthing" (or "grounding" in the USA/ Canada).
It's widely understood that the need for "earthing" is a safety measure designed to prevent electric shock from wiring faults or insulation breakdowns. More accurately, earthing is used to ensure that no open or exposed metalwork can accidentally become live should an internal insulation fault arise: instead a fuse will melt rapidly and disconnect the supply.
This principle is used in everything from a steel-cased electric heater right up to the infrastructure of an entire power station. This aspect is now examined in greater detail, and it's useful to start by seeing what happens in our adopted power station, Killingholme "A", before looking at a domestic situation.
Since the power station generator's output is a 15.75kV three phase supply, the first question to ask is, "15.75kV with respect to what?". Voltages exist between two points as a potential difference. Readers will know that electronic circuits contain signals and voltages which are almost always measured with respect to the 0V rail. When we say that something is "at +5V" or "-12V", it's implied that this is with respect to (w.r.t.) the 0V rail unless otherwise stated. (See my feature How To Read Circuit Diagrams for more explanation.)
In the field of electricity generation, though, any voltages expressed are always understood to be between phases rather than with respect to earth or "ground". A 415V three-phase supply has 415V between phases, not between a phase and ground (between which, 230V a.c. exists). Recall how a 9.1kV generator coil produces 15.75kV between phases, and it is this latter voltage which everyone talks about.
However the ground or earth also plays a fundamental safety-related role in a power station. Remembering that the power generator's output takes the form of a star connection, the star point has a net voltage and current of zero.
^ A generator's star point - an overhead box with three phases entering. Nothing much exciting to look at here!
In practice, the generator's star point is connected to earth through a "Neutral" Resistor.
^ A Neutral Resistor housed in a metal cabinet is used on the power station's generators.
In the generators I saw at Killingholme "A", this resistor filled a large metal cabinet, but in older plants the resistor can actually be in liquid form, made from a tankful of potash and capable of handling kilo-amps of current. However remember that the only currents which ever flow to earth through the neutral resistor are fault currents. This aspect is explained below.
^ Every girder, metal platform and structure in the power station is heavily bonded with copper bus-bars and ground straps.
All exposed metal work, instrument racking, chassis, walkways, handrails, steel cabinets and even the metal girders of the buildings themselves are heavily interconnected with straps and bonding wires and also physically connected to the earth. This "ring of steel" ties all of the open metalwork together to form an escape route for fault currents to flow. It means also that all earthed parts are at the same potential - zero volts or very nearly so. It's the power station's equivalent to the equipotential bonding used in your home.
^ This temporary earth clamp was seen on an open cabinet during maintenance
If any fault develops in the generator - e.g. an insulation failure - then if any part of the exposed metal work becomes "live" there'll be an immediate short to earth, because the star point is connected to earth as well. The purpose of the Neutral Resistor is tol limit the fault current and prevent a serious failure. Although a fault current will now flow, this will be detected by earth fault relays, current transformers, circuit breakers or other devices. In the case of a major generator fault, the neutral resistor is capable of withstanding many tens of amperes for five seconds, at a potential of some 10kV or more.
|Fig. 17 How the generator's Neutral Resistor limits the current caused by a fault, until the power station's circuit breakers can trip.|
In Fig. 17 above, a generator's Red phase has shorted to an imaginary steel girder, one end of which is connected to earth using copper bonding straps (photo above). The Neutral limiting resistor is now the load for the 'red' phase and a fault current will flow down to earth and through the resistor, which limits the current.
In my several days on-site at Killingholme "A", I saw countless examples of all kinds of earth straps, bus-bars and earth leads which ensure that high tension fault currents find their way directly to earth. It clearly makes sense to make it easy for fault currents to flow through a massive conductor - the ground - and trip a circuit breaker in the process to disconnect the supply. A similar form of protection is used in the home. (I discuss domestic electrical safety next.)
Remembering that the power station needs electricity itself for its own systems, then even if the worst happened and power was lost entirely, a Battery Room is installed that contains several very large banks of lead-acid accumulators which offer 48V, 110V and 220V d.c., sufficient to power auxiliary equipment for a considerable period. I had never seen so many Varta lead-acid batteries strapped together in one place but safety regulations (danger caused by sparks) prevented any photography in the Battery Room.
Welcome, Earth links: domestic electrical earthing and safety
Finally, we turn to the domestic need for earthing and look at what it offers the end-user.
As previously mentioned, the "neutral" wire of a 230V supply (UK) is provided by the star/ neutral point of the transformer and is therefore always close to zero volts with respect to the incoming "live". The star/ neutral point is also directly earthed as shown in Fig. 14b and Fig. 15 in Part 6. A separate good quality metal earth connection usually runs between the installation's earth circuit back to the transformer (e.g. to its metal body, which is also earthed).
The domestic arrangement is virtually identical to the one used by the power station generators to isolate failures in the insulation or other faults: if every piece of exposed non-live metal is earthed, then it will be very easy for "escaping" fault current to flow to earth as well: it will strive to complete the circuit back to the earthed star point of the 415V transformer. In so doing, a massive current will flow which will melt an in-line fuse or operate a circuit breaker. (A separate section "Fuses and the race to protect" gives a little more background to domestic circuit breakers and fuses.)
Let's take the example of an ordinary electric heater (see Fig. 18), its metal casing is connected to the earth terminal of the mains plug. Alternating current flows between the live and neutral wires when the heater functions normally.
|Fig. 18 How earthing (grounding) ensures that any insulation breakdowns will cause a fuse to melt, thereby disconnecting the supply. When e.g. a loose wire in an electric heater shorts the 230V live wire to earth via the metal case, a large fault current flows to earth. The fuse therefore melts, and/ or the supply's RCD will trip.|
If a live wire comes adrift inside the heater and touches the metal cabinet, then a short circuit is formed with the 415V transformer's earthed star / neutral point and a heavy current flows to earth which melts the fuse in the mains plug, thereby disconnecting the supply. This prevents the user from being able to touch a "live" metal cabinet and acquire a mains potential, because otherwise he or she could be fatally injured by the fault current flowing from the live metalwork and through the human body en route to earth.
(These days, an RCD / ELCB (Residual Current Device/ Earth Leakage Circuit Breaker) provides life-saving protection against such potentially lethal faults. They detect the imbalance of input current and output, so when a fault current flows to earth, the device trips within milliseconds. RCDs are a life-saver and should be used on all outdoor electrical devices, gardening tools etc. ARW.)
There's a counter-argument which says that if no earth were installed, then even a human being did accidentally touch a live terminal, no ill effects could arise because there would be no reason for current to flow through the body to earth. The human body would merely be "floating" at the live voltage but because no potential difference existed across him or her, then no harm would be done.
However, standards of electrical insulation are sometimes poor, and leakage currents can easily occur. This means that the earth (or, say, the chassis of an apparatus) could still not be completely isolated from the supply — even if the supply has no apparent connection to earth and is supposed to be "floating". Poor insulation allows a leakage current to flow given a suitable opportunity.
There are more subtle ways in which the earth can form a return path for fault currents, even if the supply is supposed to be "floating", thereby causing a shock. As (UK) IEE guidelines state, electrical insulation can itself be thought of as a capacitor dielectric, with the live wiring forming one "plate" and the earth forming the other.
|Fig. 19 How a "floating" (unearthed) a.c. supply can still cause electrocution due to stray capacitance or poor insulation completing a path back to neutral. Electrical insulation itself can act as a capacitor dielectric. (Adapted from Guide to IEE Wiring Regulations (15th Edition) - J.F. Whitfield)|
This stray capacitance could form an adequate route for an a.c. fault current to pass, should a human body accidentally contact a live wire (see Fig. 19 above). There's thus the risk of receiving an electric shock from earth fault currents, even if it's thought that the mains supply is floating and supposedly "unearthed".
Still on the subject of accidental electrocution, the human body, being full of water, is the walking equivalent of a 3 kilohm resistor as shown, and it only takes a current of 20mA to pass through a muscle to cause uncontrollable spasms and render the body unable to release a live wire. A separate section I entitled "Heartfelt Shock" explains some basic facts about the human body and its reaction to electric shock, and there is also some suggested First Aid advice to help with cases of suspected electrocution.
There are various earthing configurations permitted under the UK wiring regulations, but how earthing is achieved in an installation depends on whether the incoming supply is via underground cables or from an overhead supply. Due consideration is also paid by installers to the method by which earthing has been implemented in the locality by the supply authorities.
It should be again emphasised that the domestic electricity supply should never be interfered with except under expert guidance. It's worth pointing out that one should never totally rely for any reason on the neutral being at zero volts: the author has witnessed occasions where the electrical supply to a wall socket had been incorrectly wired by an electrician, with the result that the "neutral" pin was actually live at 230V with respect to earth. Various simple electrical testers are available which will reveal such faults, but in all cases a competent qualified electrician should be called if any doubts exist, and no form of measurement or interference with the mains supply should be undertaken except by experienced and knowledgeable persons.
That concludes this special feature. I described the entire UK power generation and distribution system starting with gas that fuels our adopted power station, all the way through to the sub-stations and transformers that ultimately deliver 230V safely to our homes. We looked at the principles of power generation and how a typical CCGT power station works. The UK distribution network was also outlined, showing three-phase electricity being delivered to industry and residential sites. Then the principles of safety-earthing and other safety-related aspects were described.
I found the whole project fascinating and I had great fun writing this article back in 1999. National Power (as it was then) was generous with their hospitality and their proof-reading. I examined every aspect of the entire power station and I can now admit that I did panic a bit (lots) when confronted with a vertical ladder to an overhead gantry (one of those open-mesh steel platforms - I'm afraid of heights!), but risking my life is how I got the overhead images of the gas burners and turbines under maintenance.
- Electricity Pylons: an introduction
- Leaps & Volts: Safe Working Distances
- First Aid Guidance
- Fuses and Circuit Breakers
My thanks to all those who helped at the time, Station Manager Keith Ulyett who later went to the USA, staff at National Power and especially to Richard Power who facilitated my in-depth tour of Killingholme "A".
The power station subsequently changed hands and at the time of writing (Feb. 2011) is now operated by Centrica Energy.
Some diagrams adapted from ABB (Asea Brown Boveri) originals, IEE Regulations and National Power kindly provided some illustrations and training materials to support the article.
This feature first appeared in August and September 1999 Everyday Practical Electronics with ETI. This web-enabled reprint is Copyscape Protected and it may not be reproduced in any form without prior permission. Royalty-Free uncropped images of many photos are available for non-commercial use direct from the author - please enquire.
Copyright © Alan Winstanley 1999-2011; © Wimborne Publishing Ltd 1999.