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GEORGE GEORGEVITS: Tame that flame

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54Electric blankets are just one source of danger in domestic settings. In the first of two articles, George Georgevits explains the risks when power goes awry.

Quite apart from any loss of life, a house fire has the potential to cause damage that runs into millions of dollars.

There will probably be water or foam damage from fire-fighting efforts. In addition, smoke and soot can spread to many rooms, coating everything with a black film and leaving behind an acrid smell.

Cleaning up the mess and restoring the building will be an expensive and time-consuming exercise.

For a fire to start, an energy source must provide a high ignition temperature near combustible material. Electricity is one such source, and in this article we examine some of the ways in which it can start a fire.

 

Some basics

Electricity flows in a circuit, and the power dissipated is governed by the circuit elements, namely the source voltage and the circuit resistances.

These are related by Ohm’s law:

V = I x R (equation 1)

where V is the source voltage, I is the current flowing in the circuit (in amps) and R is the total resistance in the circuit (in ohms).

When a current flows in a circuit, power in the form of heat is dissipated in all of the resistive elements in that circuit. This is called Ohmic heating.

The power P (in watts) dissipated in a resistance R is given by the relationship:

P = I2 x R (equation 2)

Or, substituting from Ohm’s law above:

P = V x I (equation 3)

Resistive elements in a circuit include the intended load (light globe, heater, motor, stove element) plus cable conductors, joints and connections, power points, switches, circuit breakers, fuses, etc.

The temperature of all resistive elements rises when a current starts to flow. This is offset by heat lost to the surroundings through radiation, conduction and convection.

Under normal conditions, the temperature rise in cables and connections, etc, is minimal, because the associated resistances are very low compared with the load resistance (assuming the circuit elements have been correctly rated).

As an example, a low-cost 240V fan heater typically has three settings: cool, heat 1 for low heating and heat 2 for maximum heating. On the cool setting, the appliance uses 24W, on heat 1 it uses 1200W and on heat 2 it uses 2400W. These settings correspond to line currents of 0.1A, 5A and 10A.

When the heater is used on the high setting, the line cord will get warm after some time, indicating that the current is not far short of its maximum rating.

The energy consumed by an electrical device is given by the product of the power dissipated by it and the time for which it has been running:

Energy = power x time

An electricity bill shows how much electrical energy has been used in kilowatt/hours (kWh).

 

Common forms of electricity

Electricity is mostly provided to domestic premises in the form of single-phase 240V 50Hz alternating current.

Owners of a big house or commercial building using lots of power can apply to the local distribution network service provider for a three-phase connection to the electricity grid.

Some substantial appliances (eg: ducted air-conditioners, large electric motors) require a three-phase connection.

Other electricity sources encountered in daily life include car batteries and the batteries in portable devices such as laptops, mobile phones and tablets.

Batteries are commonly specified by two electrical parameters: the terminal voltage and the storage capacity in amp/hours. For example, a car may be fitted with a 12V battery of 45 amp/hours. A cordless phone typically has a 2.4V rechargeable battery of 800 milliamp/hours.

Under some fault conditions even small batteries can put out a large amount of energy in a very short time, thereby being a potential source of ignition for a fire. For some common examples, do a web search on laptop or mobile phone fires and examine the images.

In short, all sources of electricity should be treated with care and respect, regardless of voltage.

Causes of electrical fires

Electrical fires can start in two ways: faults caused by overheating due to ohmic losses and faults caused by arcing.

Sometimes these are a result of component failure. Some instances can involve cabling termination faults.

Overheating due to ohmic losses. If sufficient current flows through a resistance (such as a high-resistance joint), then the power dissipated in it (as given by the second equation above) can heat it to a temperature capable of igniting combustible material in the vicinity.

For a cable joint, this is usually the PVC insulation on the conductors.

Overheating due to arcing. Dry air acts as an insulator. Its breakdown strength is about 25kV per inch. If a conductor carrying a voltage approaches another conductor at a different voltage and the breakdown strength of the intervening insulation is exceeded, an arc will form.

The current flowing in the arc is limited by other elements in the circuit. Arc temperature is strongly dependent on the amount of current flowing. It can exceed 5000C for a modest current of only a few amps.

This is enough to melt any known substance. At a current of only 50A, the arc temperature can exceed 10,000C, which is hotter than the surface of the sun.

 

Circuit protection

Most electrical circuits are fitted with some form of over-current protection (eg: fuse, circuit breaker).

Unfortunately, this type of protection is of limited use in preventing electrical fires.

Consider a typical 240V general purpose outlet circuit as found in most homes and offices. It is usually fitted with a 15A fuse or circuit breaker at the switchboard.

If the load or the cabling develops a short-circuit fault, then a large fault current flows and the circuit protection is activated.

Now consider electric blankets, which are usually powered directly from the 240V mains and typically dissipate 60W when the controller is set to full power. Using equation 3 the load current is 0.25Amps. Using equation 1, the blanket resistance is 960 ohms.

The electric blanket heating element is a length of resistance wire woven into the blanket in a zig-zag pattern (Figure 1). However, the two legs of the circuit are close to each other to minimise magnetic field radiation.

Resistance wire is relatively brittle and can fracture due to repeated mechanical stress (eg: kids jumping on the bed). If the wire breaks and a short circuit occurs a quarter of the way from where the power is applied to the blanket, then only a quarter of the resistance remains in the circuit (240 ohms).

As a result, four times as much current flows (1A). According to equation 3, this remaining quarter will be dissipating 240W, or four times as much as the whole blanket did before the fault. Thus the heating is now far greater, and more concentrated.

This smaller heating part of the blanket is probably covered by other bedding, which will act as a thermal insulator. The blanket will heat up until the bedding catches fire. Figure 2 shows the result.

In such a situation, the 15A circuit breaker will not prevent a fire because there is not enough current flowing in the circuit to trip the breaker.

This is one good reason to never leave an electric blanket switched on if it is unattended for an extended period.

Electrical arcing faults tend to occur on 240V (and higher voltage) circuits. This is because the higher voltage will arc across a larger air gap. If such an arc occurs from the active conductor to earth, a large fault current flows and the circuit protection will trip.

If it occurs across the terminals of a series circuit element such as a switch, it will continue to arc until the component falls apart or the air gap becomes so large that an arc can no longer be sustained. Fires caused by such faults are rare.

The second part of this series will provide more examples of how electrical fires can occur – and what to do about preventing them.

About George Georgevits

George Georgevits

George Georgevits, BE (Hons), manages his communications engineering consultancy Power and Digital Instruments, established in 1980. PDI specialises in lab and field testing and troubleshooting of cabling systems and components, as well as general electronics, communications and power engineering. Contact PDI on +61 2 9411 4442.

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