Protection against final circuits arc faults in conductors and connections (AFDD)

According to IEC 60364‑4‑42:2024 (see clause 426.3), it is required to install Arc Fault Detection and Protection Devices (AFDDs) to mitigate the risk of fire caused by final circuits arc-faults.

This requirement applies only to the following circuits supplying socket outlets:

Circuits supplying locations intended for people to sleep (facility for elderly persons, persons with disabilities, hotels, elementary schools, kindergarten, dwellings)
Circuits supplying locations classified BE2 for risk of fires, due to the nature of processed or stored materials (barns, wood-working shops, paper factories ...);
Circuits supplying locations with irreplaceable goods (e.g. museums, historical buildings, archives)
Circuits supplying workplaces in highrise buildings
Circuits supplying agricultural premises with livestock

The standard recommends the other final circuits to be also protected against the effect of arc-faults by AFDDs.

Origin of arc faults in conductors and connections

When a cable is locally damaged or an electrical connection comes loose, there are two main types of arc faults which initiate a fire:

Series Arc fault
Parallel arc fault

Series Arc fault

(see Fig. F81)

This phenomenon results from an arc between two parts of the same conductor (see Fig. F79).

Fig. F79 – Series Arc

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 (see Fig. F80).

Fig. F80 – Arc fault generation

Fig. F81 – Example of a carbonized connection

Parallel arc fault (Resistive short circuit)

(see Fig. F83)

This phenomenon happens between two different conductors (see Fig. F82).

Fig. F82 – Parallel arc fault

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.

Fig. F83 – Illustration of a resistive short circuit

Situations where series or parallel arcs may occur

These phenomena can occur in the following situations (see Fig. F84):

Fig. F84 – Situation increasing risks of fire

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 Faults Detection and Protection Devices

The common feature of series and parallel arc faults is the ignition of the fire by arcs.

For this reason, early detection of the presence of arcs is a means to prevent them from turning into a disaster.

How does AFDD work?

The arc fault detection device 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.

The IEC 62606 international standard defines Arc Fault Detection and Protection Devices (AFDDs) which detect the presence of dangerous electric arcs and disconnect the circuit to prevent initiating the first flame.

Speed is of the essence as an electrical arc can degrade in a flash (literally), igniting any nearby inflammable material and causing a fire. According to IEC 62606, arc fault detection and protection devices shall react very fast in case of arc faults and isolate the circuit within limited time (see Fig. F85).

These dangerous electric arcs are not detected by residual current devices, circuit breakers, or fuses.

Fig. F85 – MCB vs AFDD tripping curve

The arc fault detection device continuously monitors numerous electrical parameters of the circuit it protects (see Fig. F86), to identify typical signs of dangerous electric arcs (see Fig. F87).

Fig. F86 – General principle of Schneider Electric arc fault detection devices

Fig. F87 – Waveforms of the electric currents that could indicate the presence of potentially dangerous arc faults

Examples of electrical parameters monitored by AFDDs:

The current of the arc,
The duration of the appearance of the arc (very short durations, for example, are characteristic of the normal operation of a switch),
The irregularity of the arc (the arcs of brushed motors, for example, are fairly regular and as such should not be considered dangerous),
The 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. F88),
The presence of high frequency disturbances at various levels, which typically indicates the passage of current through heterogeneous materials, such as cable insulation

Fig. F88 – Typical waveform of electric arc. Arc voltage (black) and current (green)

Arc fault detection and protection device types

AFDDs contain a disconnection system that interrupts the current in case of an arc fault, thus preventing fire from starting.

According to IEC 62606 standard, three methods of construction of the arc fault detection and protection devices are listed (See Fig. F89):

Fig. F89 – Methods of construction of Arc fault detection devices

Arc fault detection unit with opening means

These devices comprise an AFD unit and opening means, but do not provide overcurrent or residual current protection. They are intended to be installed in series with a circuit breaker or an RCBO (See Fig. F90).

Fig. F90 – AFD unit with opening means installed in series with an RCBO

AFDD as one single device

There devices comprise an AFD unit integrated in a protective device such as a MCB, RCCB or RCBO (See Fig. F91)

Fig. F91 – AFD unit embedded in an MCB (AFDD as one single device)

[a] Single-line diagram
[b] Example of product (Schneider Electric)

Arc fault detection unit

These devices are AFD Units intended to be mechanically coupled with a protective device, such as a MCB, RCCB or RCBO. (See Fig. F92).

Fig. F92 – Example of arc fault detection unit (Schneider Electric)

Installation of Arc fault detection devices

Arc Fault Detection Devices (see Fig. F91b) are intended to mitigate the risk of fire caused by the presence of arc fault currents in the final circuits of a fixed installation and installed in electrical switchboards.

According to IEC 60364‑4‑42:2024 (see clause 426.3) it is required to install Arc Fault Detection and Protection Devices (AFDDs) to mitigate the risk of fire caused by final circuits arc-faults. This requirement applies only to the following circuits supplying socket outlets:

Circuits supplying locations intended for people to sleep (facility for elderly persons, persons with disabilities, hotels, elementary schools, kindergarten, dwellings)
Circuits supplying locations classified BE2 for risk of fires, due to the nature of processed or stored materials (barns, wood-working shops, paper factories ...);
Circuits supplying locations with irreplaceable goods (e.g. museums, historical buildings, archives)
Circuits supplying workplaces in highrise buildings
Circuits supplying agricultural premises with livestock

The standard recommends the other circuits to be also protected against the effect of arc-faults by AFDDs.

It is recommended that AFDDs be installed at the place of origin of the low voltage final circuit to be protected (i.e., switchboard of an electrical installation).

More specifically, the installation of the AFDD is highly recommended to protect circuits with highest risk of fire, such as:

Protruding cables (risk of knocks)
Outside cables (greater risk of deterioration)
Unprotected cables in secluded areas (like storage rooms)
Aging, deteriorating wiring or wiring for which the connection boxes are inaccessible.
Electrical final circuits in cases of renovation.

To know more about arc fault detection devices, you can download the white paper "How Arc Fault Detection Devices Minimize Electrical Fire Threats"

AFDD application examples

Hotel

Example of standard hotel room application:

In this example :

The fire prevention in case of insulation failure is provided by the 30 mA RCD (30 mA <= 300 mA)
The 30 mA RCD also provides fault protection and additional protection for all final circuits
The AFDD protects all required circuits against arc fault according to IEC 60364-4-42:2024

Fig. F93 – Hotel room single line diagram - AFDD usage example

External links

For more information on electrical fire origins, and the latest solutions to mitigate the risks, download the Electrical Fire Prevention Guide (PDF)