Mobile Phone Fires

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Mobile Phone Fires

I noted the picture here attached on some recent social media posts and the comments arising. I thought the circumstance rather prevalent given everyday use of mobile phones and the like.

Anyhow, I do agree with the general context of the message, which was to warn against the practice of charging mobile phones with the phone left beneath a pillow or sandwiched between bed sheets, duvets, cushions etc.

Fundamentally there are two (2) electrical phenomena, which bring about an electrically induced fire, i.e. Joule Heating, commonly referred to as ohmic or resistive heating and electrical arcing. More often arcing is preceded by heating.

So what are resistive heating and electrical arcing?

Resistive heating is the process by which the passage of an electric current through a conductor produces heat. This heating phenomenon occurs in all electrical circumstances, yet is ‘controlled’ by awareness and good design.

The ‘conductor’ in the case of the mobile phone charger is the electrical cable and the electrical current is the entity, which flows from the charger to the phone to charge the phones battery with ‘electrical energy’. Further electrical resistance is the entity that affords resistance to the flow of electrical current and given normal copper cables, the magnitude of electrical resistance will be low.

Now if the copper cable becomes damaged or ‘kinked’, electrical resistance increases with a consequence that electrical current flow decreases and resistive heating increases. This is similar to creating a ‘kink’ or restriction in a hosepipe when conveying water.

The effect of this is that if heat cannot be readily dissipated from the locality of increased resistance, i.e. the ‘kink’, heating will continue and temperature of the locality will increase to the point where most probably the plastic coating of the cable will melt, the surrounding pillow, cushion or duvet material begins to carbonise and scorch, and eventual flash point for fire ignition is reached.

Of course if heating still continues to attain the melting point of copper, the metallic constituent of the cable transposes from a solid to liquid and of course due to gravitational forces, parts of the molten copper cable will flow, i.e. physically part, creating an air gap in the once continuous solid copper cable.

One could assume that given an air gap that this would negate the process of heating since electrical current flow would cease. However, given sufficient electrical charge exists across the air gap, the air can ionise and start to conduct electrical energy once more. In simple terms air ionisation is the phenomena whereby air becomes a conductor of electricity, similar to the manifestation of ‘lighting’ in weather storms.

In these circumstance we refer to ‘small scale’ electrically induced lightening as ‘electrical arcing’, which is characterised as a plasma, which may produce intense blue/white visible light and exists at an extremely high temperature in the order of 20,000 oC, a sure fire igniting phenomena.


About Dr Robert Brown:

Eur Ing Dr Robert Brown (Robert) is the Executive Director of Fraser George and Associates Limited and is a Consultant and Forensic Engineer in the fields Electrical Electronic and Control Engineering.

Robert is an accomplished professional Electrical Expert Witness having prepared and presented many court compliant reports and presented oral evidence within the High Court, Crown Courts and County Courts.

Robert is a registered Chartered Electrical Engineer, specialising in the fields are control engineering, parameter data capture and accurate data ‘representation’. Robert graduated from Sheffield Hallam University with a first class honours degree in Electronic Systems and Control Engineering and followed this by completing doctorate research in Electrical Engineering, again at Sheffield Hallam University where Robert undertook research studies in the field of Disintegration and Arcing in Electrical Fuses. Robert is now considered to be a leading world expert in the field of electrical circuit protection systems, the phenomena of electrical fuse operation and the attributes of clearing ‘fault’ electrical current circumstances. Robert has published my pure and applied research studies in peer-refereed journals and at conference.

In his spare time Robert is a Formula 1 motor racing enthusiast and enjoys outdoor activities especially hill walking and scrambling, golf and fishing. Robert is a brass band enthusiast, a tuba player and band manager of Hatfield and Askern Colliery Band. When time allows Robert also enjoys travelling and reading any books associated with travel and popular science.


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