Aircraft wire failure and incident investigation, Part I

The hazard of aircraft wire failure and electrical arcing will exist for as long as there is a need to deliver electrical power through wires. While electrical arcing is a common occurrence, the factors that impact an arcing event are often not well understood. This article reviews each step involved in electrical arcing, from what initiates it to its effects on the system.

Precondition to Arcing

For arcing to occur, it is necessary that some portion of the circuit wiring be exposed. This article will focus on wiring and not failure at connection points.

There are a number of mechanisms that can create the preconditions for an arcing event to occur:

1. Age related degradation of the wire insulation

Degraded/aged wire insulation may result in radial cracks to the conductor.

2. Overheated Circuit

Overheated circuits (or wire bundles) can melt the insulation and expose the conductor.

3. Fluid Exposure

Some wire insulations degrade very quickly when exposed to certain fluids.

4. Error in manufacturing

The finished wire product has breaches in the insulation.

5. Damage during Installation or maintenance

Working with wiring harnesses can result in damage to the insulation

6. Abrasion

A bundle that is not properly secured can sway and rub against structures or components. There are other mechanisms that lead to wire insulation damage, but they are typically subsets of the mechanisms listed above.

The Arc Begins

Once the power wire has been damaged to a point where the conductor is exposed and there is a low resistance path to a different potential, arcing can begin. This different potential can be a grounded object (such as a component, structure, or a ground wire) or another power wire on a different phase.

Electrical Arcing

Once the arcing starts, the destruction of insulation and conductor immediately begins. These materials are often released into the immediate atmosphere and become part of the arc plume. The arc plume is a hot ionized gas that provides a low resistance path for the electrical arcing. Furthermore, this plume expands out from the arcing event and can damage nearby objects.

Electrical Arc Parameters

There are only a few parameters that directly affect the electrical arc and its progression. These include:

1. Distance from the power source

This affects the fault current and energy available for arcing.

2. Wire type

Some insulations have a propensity to limit the arc damage, but none can fully eliminate the damage from an arcing event

3. Circuit protection

Depending on the type and rating, this will determine the arc duration

4. Number of power/ground wires in bundle

The initial arcing event can result in the damage to other wires in the bundle and bring those wires into the arcing event.

Arc Termination Conditions

After the arc starts, there are only a few mechanisms that can halt the arcing event. The first one is the activation (tripping) of the circuit breaker. Depending on the type of the circuit breaker and the fault current, this might trip quickly or take several seconds before it opens. The other mechanism for ending an active arcing event is increasing the separation between the two conductors (or conductor and ground). The separation distance necessary is dependent on the voltage and the duration of arcing event.

Parameters Affecting Damage

Aside from the arcing event, it is necessary to consider the parameters that affect the damage level. These include:

1. Target materials

The structure or component material can impact the duration of the arc as well as the extent of the damage.

2. Tube pressurization

Different pressure levels can impact heat transfer and subsequent damage to pressurized tubes.

3. Contamination

Dirt, debris, and fluids can impact how much damage is done. These can either ignite and cause additional damage or limit the impact.

4. Separation distance and segregation materials

The physical separation distance impacts how much energy or direct arcing can impact the object. Furthermore, segregation materials (such as wire harnesses sleeves) also impact the damage level.

Possible Effects of Arcing

While this is the last part of the electrical arcing event, this is probably the most important part to consider. After all, if the wire arcs and has no negative impact on a system, it does not matter. But more often than not, an arcing wire will cause physical and/or functional damage. A functional damage can lead to un-commanded activation of devices or systems by transferring energy to other circuits or can result in loss of system functionality. A physical damage can mean damage to structure, to other wires in a bundle, or even damage and breach of hydraulic/fuel lines. The level of severity varies depending on the type of damage and can only be assessed through a full risk assessment of the wiring system. This risk assessment must combine the consequence of failure with the probability of failure. This is possible with tools such as Lectromec’s EWIS RAT (Electrical Wire Interconnection Systems Risk Assessment Tool).