电线和断路器选择指南

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简介:A guide to the selection of electrical cable and breakersPlease note that this article has not yet been updated in line with the latest (17th) edition of the IEE Wiring Regulations. I don't think very ...
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A guide to the selection of electrical cable and breakers

Please note that this article has not yet been updated in line with the latest (17th) edition of the IEE Wiring Regulations. I don't think very much has changed in the basic principles on which the article is based, but only a detailed scrutiny of the Regulations and, in particular, the extensive data tables will tell for sure. I don't have time to do this at present, so readers should be aware of potential non-compliances. Nothing in this article is intended to be a substitute for proper professional advice.

Overview

This article describes the selection of cables for 'difficult' domestic electrical installations. By 'difficult' I mean installations where cables are unusually long, currents abnormally high, or shock protection requirements especially rigorous. I have written this article for competent, sensible DIY enthusiasts who may already be doing electrical work, but lack the theoretical and regulatory knowledge to tackle these more difficult jobs. I have assumed that the reader has a basic understanding of electrical theory and is prepared to do some simple arithmetic. A brief introduction to electrical theory can be found here. Please note that if you're working on the kind of electrical installation that requires the kind of information this article provides, you're probably doing work that requires Building Control approval and inspection.

Cable selection is guided by two main principles. First, the cable should be able to carry the current load imposed on it without overheating. It should be able to do this in the most extreme conditions of temperature it will encounter during its working life. Second, it should offer sufficiently sound earthing to (i) limit the voltage to which people are exposed to a safe level and (ii) allow the fault current to trip the fuse or MCB in a short time.

To meet these requirements requires consideration of the circuit load current, the ambient temperature, installation technique, cable thickness and length, and the over-current protection device. In some cases you may need to consider factors that are outside your control, like the external earth loop impedance. Typical 'worst-case' values for these factors are given in the article.

Scope of this article

In most domestic wiring scenarios, the principles and techniques described in this article are simply not relevant. The materials and equipment currently available are designed to simplify installation, and common sense and the ability to read the manufacturer's instructions are all that is required. Ordinary domestic power and lighting circuits do not require any special skills or knowledge to install, beyond what you would find in a DIY handbook. This article covers the issues that DIY books steer clear of, like running long cables to outbuildings, installing supplies for electrical showers, and electrical wiring in bathrooms. It assumes that the reader has sufficient time and enthusiasm to get to grips with the theory, which can be rather technical in places.

This article is intended for readers in the UK, and in places where UK practices and regulations are followed.

How this article is organized

This article has three chapters and an appendix. Chapter 1 describes the theory of over-current protection, and discusses the properties of cables, fuses, MCBs and related devices. Chapter 2 describes principles of electrical shock protection and the effect of cable length and thickness on shock voltage and disconnection time. Chapter 3 describes a practical calculation based on the principles from the first two chapters. Finally, the appendix provides design tables for cable selection, based on the IEE Wiring Regulations and various manufacturer's product data sheets. Please don't use the design tables without reading the text; it will be easy to misinterpret the information if you do.

Warning and disclaimer

I would hate to think of anyone coming to harm as a result of reading this article. It describes procedures which, if not carried out competently, could lead to death or serious injury, or substantial damage to property. Please be careful. Always ensure that before starting to work on an electrical system, the relevant circuit has been isolated from the supply, and you have taken steps to ensure that it remains that way until you have finished work. Ensure that you understand the consequences and implications of any work you intend to carry out. While I have taken every effort to ensure that the information in this article is accurate, and will lead to a safe and reliable installation, I do not accept any responsibility for any adverse consequences arising from its use. Please note that the article is about modern domestic installations; the procedures, design tables, and calculations described may well be unsuitable for commercial or industrial premises and equipment. In particular, the article assumes the use of a single-phase supply, and predominantly resistive loads. If you don't know what these terms mean, I respectfully suggest you ought not to be reading this yet. In addition, this article does not describe any procedures for dealing with circuits protected by semi-enclosed (re-wirable) fuses. Although they are allowed with the terms of the IEE Wiring Regulations, they are obsolete and ought to be replaced.

Note on the text

The symbol '[IEE]' in the text indicates a guideline that should be followed to ensure conformance with the IEE Wiring Regulations, 16th edition. Where this symbol is followed by numbers, e.g., [IEE 528-01], this refers to a specific regulation in that document. Note that the IEE Wiring Regulationsis equivalent in practice to British Standard 7671 Requirements for Electrical Installations. To the best of my knowledge, this article complies strictly to theIEE Wiring Regulations.

1. Over-current protection

1.1 Overloads and short-circuits

Over-current is one of the two major safety hazards that must be controlled in a wiring system. The other, of course, is electric shock. Protecting against over-current provides a measure of protection against electric shock as well, as we shall see. Over-currents are dangerous because they lead to a risk of fire. In the UK every year about 50,000 fires are attributed to electrical faults. So it's worth paying a bit of attention to this issue.

This chapter provides a fair amount of technical detail, which you won't always need to know. For many applications, provided that you choose a fuse or MCB (see below) that has the same current rating as the cable to which it is connected, this will work nicely. For example, if the current rating of the circuit is 35 amps, say, then a 32 amp MCB (that's the nearest size available below 35 amps) should do the trick for most applications.

There will be situations, however, where this simple rule won't work. This chapter explains what they are, and what to do about them. It also explains why fuses sometimes blow when there's nothing wrong.

1.2 Types of over-current

An over-current is any increase in the current in the electrical system, above the level for which it is designed.

Electrical cables are intended to become warm in operation; heat is generated whenever a current flows in anything, and this is perfectly normal. However, the level of heat generated by electrical cables is only safe when it is kept within reasonable limits. Standard PVC-insulated cables are designed to run at temperatures up to 70 degrees Celsius; beyond this there is a risk of damage.

In practice, over-currents can be grouped into two types.

Overload
An overload occurs when a current flows that is somewhat too high (usually 50% to 100% too high) for the system. Overloads don't normally cause immediate, catastrophic damage. Instead, the likelihood of damage increases gradually as the duration of the overload increases. If the fault is not resolved, cables will overheat and melt, exposing bare conductors. The heat generated may be sufficient to cause a fire.

In a domestic setting, overloads usually result from using too many appliances at the same time, or plugging a heavy-duty appliance into a supply that isn't strong enough for it. An example of the latter is connecting an electric shower to a standard 13-amp plug, and plugging it into a socket.

Short-circuit
A short-circuit is a connection between live and neutral, or between live and earth, that bypasses an appliance. The connection will probably have a low resistance, and the current that can flow may be hundreds or thousands of times too high for the system. This current is usually called the fault current or short-circuit current. A short-circuit will by produced if, for example, the wires in a mains plug become loose and touch one another.

The ability to handle short-circuits is not just important to protect cables, it is part of the protection against electric shock. If a live conductor in, say, an electric kettle becomes loose and touches the metal case, we hope that a large fault current will flow. This current will flow from the live, through the case, and back to earth via the earth wire. The fault current will blow the fuse or trip the MCB, thus rendering the circuit dead. If this does not happen, then we have a potentially very dangerous situation: a metal casing with a live voltage on it.

In practice, in domestic installations overload protection and short-circuit protection are both provided by the same device: either a fuse of an MCB. Additional shock protection may be provided by an RCD. Whether a fuse or an MCB is used, when the current exceeds a certain limit for a certain time, the fuse will 'blow' (break) or the MCB will 'trip'. In both cases this will open the circuit and prevent the flow of further current. For simplicity, I will use the term 'trip' for both these events.

1.3 Over-current protection devices

1.3.1 Fuses

A fuse is a simple device that will limit the current flowing in an electrical circuit. In practice a fuse normally consists of a piece of wire of exactly the right length and thickness to overheat and break when the current gets to a particular level.

There are two sorts of fuse normally used in houses. Cartridge fuses have the wire enclosed in a sealed cylinder, with a contact at each end. You should be familiar with this kind of fuse: it's the kind that goes in a plug. Larger versions are available for distribution boards as well.

Semi-enclosed or re-wirable fuses are the kind that can be rewired with fuse wire. Although they are still widely used, they are discouraged by most authorities for two reasons: a common-sense reason and a technical reason. The technical reason will be discussed later. The common-sense reason is simply that it is very easy to rewire it with the wrong size of fuse wire, so that we end up with, for example, a 30-amp fuse 'protecting' a 5-amp circuit. This is exceptionally dangerous.

Fuses are an effective basic method of over-current protection, but they have a practical disadvantage: if a fuse blows and you haven't got one of the right rating, what do you do? Of course you won't use a fuse of the wrong rating, or wrap a bit of tinfoil around a blown one, but someone will.

Because of this failing, and for technical reasons that will be discussed, permanently-installed equipment (particularly mains distribution panels) often have electrical over-current protectors rather than fuses.

1.3.2 MCBs

The most popular of these electrical protection devices is the 'miniature circuit breaker' (MCB). An MCB can usually act as an ordinary switch as well as an over-current circuit breaker, and so has a lever on the front for manual operation. This is very convenient, and MCBs are universally used in new domestic distribution boards (and most industrial ones as well).

MCBs are available in various types1.1: '1', '2', '3', 'B' and 'C'. Each has different characteristics, and is appropriate for a particular application. In a domestic system, we will normally use a type '1' or a type 'B' device, as these are general-purpose units.

If you have a distribution board with re-wirable fuses, and don't want to replace it (yet), you can get adapters that will let you plug in an MCB in place of the fuse. If you are replacing wiring with a system that is rated on the basis that you will eventually be using MCBs and not fuses, this is a very sensible thing to do.

1.4 Fuse and MCB characteristics

Fuses and MCBs are rated in amps. The amp rating given on the fuse or MCB body is the amount of current it will pass continuously. This is normally called the rated current or nominal current. We normally assume that if the current in the circuit is lower than the nominal current, the device will not trip, however long the current is maintained. This isn't quite true, but it's a reasonable design assumption.

Many people think that if the current exceeds the nominal current, the device will trip, instantly. So if the rating is 15 amps, for example, a current of 15.00001 amps will trip it, right? This is not true. There isn't any reason why it should be true: the MCB or fuse is designed to protect the circuit cabling, and a current of 15.00001 amps won't damage a 15-amp cable. So when will it trip?

This is where things start to get interesting. It turns out that there's a rather complex relationship between the tripping current and the time for which an over-current is maintained. As an example, the relationship between time and level of over-current that will trip either a 32-amp type-1 MCB or a 30-amp cartridge fuse are shown in figure 1.1.

Figure 1.1: time for which a 32-amp MCB or 30-amp fuse will stand an over-current before tripping
\begin{figure}
\epsffile {fusecharacteristic.eps}\par\end{figure}

The horizontal axis of this graph shows the current flowing in the fuse/MCB and the circuit it is protecting. The vertical axis shows the duration for which the device can stand this current before it trips. There are a few things to note about this graph.

  • The fuse and the MCB, even though their nominal currents are similar, have very different properties. For example, to be sure of tripping in 0.1 seconds, the MCB requires a current of 128 amps, while the fuse requires 300 amps. The fuse clearly requires more current to blow it in that time, but notice how much bigger both these currents are than the '30 amps' marked current rating.

  • Neither device will trip at 30 amps, in any length of time shown on the graph, but the lines get closer and closer to 30 amps as the time increases. There is a small likelihood that in the course of, say, a month, a 30-amp fuse will trip when carrying 30 amps. If the fuse has had a couple of overloads before (which may not even have been noticed) this is much more likely. This explains why fuses can sometimes 'blow' for no obvious reason.

  • Both fuse and MCB will stand currents of over 40 amps for an hour or so.

If the fuse is marked '30 amps', but it will actually stand 40 amps for over an hour, how can we justify calling it a '30 amp' fuse? The answer is that the overload characteristics of fuses are designed to match the properties of modern cables. For example, a modern PVC-insulated cable will stand a 50% overload for an hour, so it seems reasonable that the fuse should as well.

In fact it would be very impractical to use a fuse or MCB that tripped at a current very close to the nominal value. This is because many electrical devices take higher currents for the first fraction of a second after they are switched on, compared to normal running. Take an ordinary lightbulb, for example. The resistance of all metals increases as they heat up. When the lightbulb is first switched on, its filament is cold, and it has a very low resistance. As it heats up, the resistance increases, so the current decreases. For the first tenth of a second or so, the current flowing in a lightbulb may be 5-10 times higher than its normal running current1.2.

So we have to allow some margin for start-up currents, or the fuse or MCB will tend to trip by accident, which is inconvenient.

Because the MCB trips very quickly once a particular threshold is reached, the concept of an 'instantaneous trip current' is appropriate for MCBs. This is the current that will trip the device in 0.1 seconds. For type 1 MCBs the instantaneous trip current is guaranteed to be between 2.7 and 4 time the nominal current; for type B it is 3 to 5 times the rating.

1.5 Fuse/MCB selection

When selecting the correct MCB or fuse to use, we have to consider its role in both over-current protection, and short-circuit protection. The basic principles are as follows.

Nominal current rule
The nominal current of the fuse/MCB must be less than the current rating of the cable it is protecting, but higher than the current that it will carry continuously [433-02-01]. For example, a 32-amp MCB is suitable for a current of 30 amps in a 35-amp cable circuit.

Tripping rule
A current of 1.45 times the nominal current must cause the device to trip in less than 1 hour1.3. In practice you haven't normally got to worry about this, it's the job of the MCB designer. All modern devices meet this requirement except re-wirable fuses. This is why re-wirable fuses are discouraged. These fuses normally require about twice the nominal current to blow them in one hour.

Disconnection time rule
In a short-circuit condition, the fuse/MCB must trip in less than a specified short time (see below).

The 'disconnection time' rule is the most awkward to ensure compliance with in a domestic installation; it will be discussed later. In practice it doesn't affect what rating of fuse or MCB to use, but it often affects whether to use a fuse or an MCB, and may impose the use of additional protective devices.

1.6 Example

In this example we will determine which MCB to use to protect a circuit.

Assume we are installing a lighting circuit, which will nominally have 8 light fittings of 100 watts each. The current is 800 / 230 or about 3.5 amps. 1 mm$^2$ cable appears to be appropriate, as it has a rating of at least 11 amps (see table A.1), even when concealed in a plastered wall1.4.

So the MCB must have a nominal current (that is, the current marked on its body) of at least 3.5 amps, and less than 11 amps. Furthermore, its tripping current must be less than 1.45 times 11 amps, in order to protect the cable. Looking in the manufacturer's catalogue, I find a 6 amp MCB, that has a trip current of 8.7 amps. This appears to be just right. These currents are shown in figure 1.2.

Figure 1.2: Relationship between the fuse nominal and tripping currents, and the current carrying capacity of the cable, for the example given in the text
\begin{figure}
\epsffile {fusecharacteristics.eps}\end{figure}


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