Protection against arc faults in cables and connections (AFDD)
Fires of electrical origin
The European Fire Academy (http://www.europeanfireacademy.com/) estimates that the 2,250,000 fires (total number of fires) that occur in Europe each year represent more than 4,000 deaths and 100,000 injuries.
The buildings involved are residential buildings in more than 80 % of cases.
Electricity is a very regularly identified cause of domestic fires.
Depending on the country and the investigation methods, but also depending on the means of identification, the proportions of electrical fires are:
- 13 % in the United States (www.nfpa.org);
- 25 % in France (www.developpement-durable.gouv.fr);
- 34 % in Germany (www.ifs-kiel.de/);
- 40 % in Norway (www.sintef.no).
Causes of fires of electrical origin
Electrical fires are caused by overloads, short circuits and earth leakage currents, but also by electric arcs in cables and connections.
When a cable is locally damaged or an electrical connection comes loose, there are two phenomena which can initiate a fire due to an arc:
Carbonization
(see Fig. F76):
Whenever a conductor is damaged or a connection is not properly tightened, a localized hot spot occurs which carbonizes the insulating materials in the vicinity of that conductor.
Carbon being a conductive material, it enables flow of the current which becomes excessive at various points.
Since the carbon is deposited in a non-homogeneous manner, the currents which pass through it generate electric arcs to facilitate their paths. Then each arc amplifies carbonization of the insulating materials, a reaction thus occurs which is maintained until the quantity of carbon is high enough for an arc to inflame it spontaneously.
Resistive short circuit
(see Fig. F77):
Whenever the insulating materials between two live conductors are damaged, a significant current can be established between the two conductors, but it is too weak to be considered as a short circuit by a circuit breaker, and is undetectable by residual current protective devices as this current does not go to earth.
When passing through these insulating materials, these leakage currents optimize their paths by generating arcs which gradually transform the insulating materials into carbon.
The insulating materials thus carbonized then amplify the current leak between the two conductors. Thus, a new chain reaction occurs, amplifying the quantity of arc current and carbon until the first flame appears from the carbon lit by one of the arcs.
The common feature of these phenomena is ignition of the fire by arcs which inflame the carbon: that is why detection of the presence of arcs is one way to prevent them from turning into a disaster.
These dangerous electric arcs are not detected by residual current devices nor by circuit breakers or fuses.
These phenomena can occur in the following situations (see Fig. F78):
Power supply cord subjected to excessive forces (by furniture or a position)
Power supply cord defective following inappropriate or excessively numerous operations
Cable weakened at connection
Accidental damage to a cable
Power sockets in poor condition
Ageing of cable protective devices
Loose connections
Cables damaged by their environment: UV, vibrations, moisture, rodents.
Arc fault detectors
The arc fault detector (see Fig. F79 ) technology makes it possible to detect dangerous arcs and thus protect installations.
Such devices have been deployed successfully in the United States since the early 2000s, and their installation is required by the National Electric Code.
Since 2013, the IEC 62606 international standard defines Arc Fault Detection Devices (AFDDs) which detect the presence of dangerous electric arcs and cut off the circuit's power supply to prevent initiating the first flame.
The arc fault detector monitors in real time numerous electrical parameters of the circuit that it protects in order to detect information characteristic of the presence of dangerous electric arcs.
For example, distortion of the current signal (sine) at the time of its zero crossing is characteristic of the presence of an electric arc: the current flows only after the appearance of an arc which needs a minimum voltage to be created (see Fig. F80).
Installation of Arc Fault Detectors
Arc Fault Detection Devices (AFDD) are designed to limit fire risks caused by the presence of arc fault currents in the final circuits of a fixed installation.
They are installed in electrical switchboards, mainly on the circuits supplying the power sockets of bedrooms and living rooms of residential buildings, and are especially recommended in cases of renovation.
It is also recommended to install them in the following buildings:
- Buildings with a risk of propagation of fire (e.g. buildings with forced ventilation);
- Buildings with a high density of occupation (e.g. cinema theatres);
- Buildings with evacuation difficulties;
- Buildings which store flammable materials or potentially explosive materials
(e.g. buildings storing wood, the paper industry).
Since 2014, International Standard IEC 60364 - Electrical installations of buildings - Part 4-42 recommends the use of AFDDs:
Excerpt from the IEC 60364-4-42 standard
"It is recommended that special measures be taken to protect against the effects of arc faults in final circuits:
- in premises with sleeping accommodations;
- in locations with risks of fire due to the nature of processed or stored materials, i.e. BE2 locations (e.g. barns, wood-working shops, paper factories);
- in locations with combustible constructional materials, i.e. CA2 locations (e.g. wooden buildings);
- in fire propagating structures, i.e. CB2 locations;
- in locations where irreplaceable goods are endangered.
In a.c. circuits, the use of arc fault detection devices (AFDDs) in compliance with IEC 62606 will satisfy the above-mentioned recommendation."
