Protection against arc faults in cables and connections (AFDD)

Origin of arc faults in cables 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

(see Fig. F85)

This phenomenon results from an arc between two parts of the same conductor (see Fig. F81).

Fig. F81 – Serial 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 (seeFig. F83).

Parallel arc fault (Resistive short circuit)

(see Fig. F84)

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.

Fig. F83 – Arc fault generation

When passing through these insulating materials, these leakage currents optimize their paths by generating arcs which gradually transform the insulating materials into carbon.

Fig. F84 – Illustration of a resistive short circuit

The common feature of these phenomena is ignition of the fire by arcs that is why detection of the presence of arcs is one way to prevent them from turning into a disaster.

Fig. F85 – Example of a carbonized connection

These phenomena can occur in the following situations (see Fig. F86):

Fig. F86 – 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 Devices

How does it 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.

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.

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 devices shall react very fast in case of arc faults and isolates the circuit within limited time (see Fig. F87). These dangerous electric arcs are not detected by residual current devices nor by circuit breakers or fuses.

Fig. F87 – MCB vs AFDD tripping curve

The arc fault detection device monitors in real time numerous electrical parameters of the circuit that it protects (see Fig. F88), in order to detect information characteristic of the presence of dangerous electric arcs (see Fig. F89).

Fig. F88 – General principle of Schneider Electric arc fault detection devices

Fig. F89 – Anomalies in electric currents that could indicate the presence of potentially dangerous arc faults

For example:

• The current of the arc (a series arc is dangerous as soon as its value equals or exceeds 2.5 Amps).
• 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. F90).
• The presence of disturbances at varying levels of different high frequencies is characteristic of the passage of a current through heterogeneous materials (such as cable insulation).

Fig. F90 – Typical waveform of electric arc. Arc voltage (black) and current (green)

Arc fault detection device types

AFDDs are assembled with a disconnection system that interrupts the current in case of arc fault, thus, they prevent fire to start.

According to IEC 62606 standard, three methods of construction of the arc fault detection devices are listed (See Fig. F91):

Fig. F91 – Methods of construction of Arc fault detection devices

• AFDD as a single device, including an arc fault detection unit and opening means and intended to be installed in series with a protective device, that can be a MCB or RCBO (See Fig. F92)

Fig. F92 – AFD unit with opening means installed in series with an RCBO

• AFDD as one single device, including an arc fault detection unit and a protective device such as a MCB or RCBO (See Fig. F93)

Fig. F93 – AFD unit with MCB

• Arc fault detection unit that shall be assembled on site, with a protective device, such as MCB or RCBO. (See Fig. F94).

Fig. F94 – Schneider Electric arc fault detection unit

Installation of Arc fault detection devices

Arc Fault Detection Devices (see Fig. F95) 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, to protect circuits supplying power sockets and lighting and are especially recommended in cases of renovation.

Since 2014, International Standard IEC 60364 - Electrical installations of buildings Part 4-42 makes the following recommendations surrounding the installation and application environments of AFDDs in residential and commercial buildings:

• In locations with sleeping accommodations (e.g., hotels, nursing homes, bed-rooms in homes)
• In locations with risks of fire due to high quantities of flammable materials (e.g., barns, wood-working shops, stores of combustible materials)
• In locations with combustible constructional materials (e.g., wooden buildings)
• In fire propagating structures (e.g. high-rise buildings)
• In locations where irreplaceable goods are housed (e.g., museums)

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.

To know more about arc fault detection devices, you can download a white paper covering this topic: http://www.schneider-electric.com/en/download/document/EDCED117020EN

Fig. F95 – Schneider Electric AFDD

Conclusion

In addition to protection against electric shock, protection against fire is another main target of a proper design of electrical installation. The minimum requirement is the coordination between overcurrent protective device and conductor permanent and short-time withstand. (See Chapter Sizing and protection of conductors ). But other protective measures are recommended to reduce the risk of fire due to electrical installation.

• For main circuits and distribution circuits, more sensitive earth fault detection than the one necessary for protection against electric shock is recommended.
• For terminal circuits where mechanical withstand of conductor is lower, where the number of connection is higher, where portable equipment can be supplied, in addition to RCD, Arc fault detection device is recommended.

The table below tries to position the different types of current-based protections required or recommended for low voltage installations.

	Protection against electric shock			Protection against thermal effect / Fire			Protection against overcurrent (IEC 60364-4-43)
				BE2 location	Other location
Earthing system	Basic protection	Automatic disconnection	Additional protection		Terminal Circuit	Distribution Circuit
TNC	Insulation of live part / Barriers enclosure	OCPD	NA	NA	NA	RS GFP	OCPD (overload and short-circuit)
TNS		OCPD/RCD	RCD 30mA	300mA RCD AFDD	AFDD	RCD
TT		RCD	RCD 30mA		AFDD	RCD
IT		OCPD/RCD	RCD 30mA		AFDD	RCD

Fig. F96 – Standard voltages between 100 V and 1000 V (IEC 60038 Edition 7.0 2009-06)