1.4 Voltage Drop, Circuit Protection, and Aircraft Batteries

1.4 Voltage Drop, Circuit Protection, and Aircraft Batteries

Quick Takeaway: FAA AC 43.13-1B regulates maximum allowable voltage drops to 2% for continuous circuits ($0.56\text{ V}$ on 28 VDC buses) and 14% for intermittent circuits. Aircraft circuit breakers must be trip-free to prevent forcing faulty circuits closed. Aircraft storage batteries use either Lead-Acid ($2.1\text{ V/cell}$) or Nickel-Cadmium ($1.2\text{ V/cell}$) chemistries; NiCd batteries mandate thermal monitoring to prevent catastrophic thermal runaway caused by a negative temperature coefficient of resistance.

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FAA AC 43.13-1B Voltage Drop Standards & Wire Selection

In aircraft electrical installations, conductor wire size must be selected to satisfy two independent criteria: current-carrying capacity (ampacity) to prevent wire insulation overheating, and maximum allowable voltage drop to ensure equipment receives adequate operating voltage.

Federal Aviation Administration (FAA) Advisory Circular AC 43.13-1B, Chapter 11, establishes strict limits for maximum voltage drop between the main bus bar and the connected load:

Note: Continuous duty is defined as any circuit operating for more than two minutes continuously (e.g., navigation lights, avionics radios, pitot heat). Intermittent duty applies to circuits operating for less than two minutes (e.g., landing gear actuators, starter motors, flap motors).

Calculating Conductor Voltage Drop

Wire resistance is determined by length and American Wire Gauge (AWG) size:

$V_{\text{drop}} = I \cdot \left( \frac{R_{\text{per 1000 ft}}}{1000} \right) \cdot 2L$

where $I$ is current in Amperes, $R_{\text{per 1000 ft}}$ is wire resistance from Mil-Spec tables, and $2L$ accounts for both supply and return path lengths (or single length $L$ if using airframe ground return).

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Aircraft Circuit Protection Devices

Circuit protection devices—fuses and circuit breakers—are installed primarily to protect aircraft wiring from overheating and burning, rather than solely protecting the connected load equipment.

    [Main Bus Bar] -----[ Circuit Breaker ]==== Wire Harness =====[ Load LRU ]-----(Ground)
                                               (Protected Segment)

1. Fuses

Fuses contain a calibrated low-melting-point alloy strip (such as tin-lead or zinc) that melts when subject to excessive current. While simple and lightweight, fuses are non-resettable and must be replaced after opening. FAA regulations require spare fuses of adequate capacity (at least 50% of total installed count, minimum 3 of each rating) to be accessible to the flight crew in flight.

2. Circuit Breakers (CBs)

Circuit breakers are mechanical switching devices that open automatically under overcurrent conditions. Aircraft installations utilize two primary trip mechanisms:

• Thermal Circuit Breakers: Utilize a bimetallic strip that bends as $I^2 R$ heat builds up, releasing a mechanical latch. Thermal breakers incorporate an inherent time-delay inverse characteristic (higher overload currents trip the breaker faster).
• Magnetic Circuit Breakers: Utilize a solenoid coil that creates an electromagnetic field proportional to current. When severe short-circuit current flows, the magnetic force instantly trips the latch, providing immediate protection.

3. The Mandatory "Trip-Free" Requirement

FAA FAR Part 23/25 airworthiness standards mandate that all aircraft circuit breakers must be trip-free:

Definition: A trip-free circuit breaker will trip open under an overcurrent fault condition even if the pilot or technician manually depresses and holds the reset button in the "ON" or "CLOSED" position.

Non-trip-free breakers allow a user to hold the contacts closed during a fault, which forces heavy current through shorted aircraft wiring, leading to electrical fires inside wire bundles.

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Aircraft Storage Batteries: Lead-Acid Chemistries

Aircraft storage batteries serve as secondary power sources for engine starting and emergency electrical power following main generator failure.

Chemical Construction & Voltage

A lead-acid battery consists of secondary cells immersed in a dilute sulfuric acid ($H_2 SO_4$) and water electrolyte (specific gravity $1.275 - 1.300$ fully charged at $80^\circ\text{F}$):

• Positive Plate: Lead Dioxide ($PbO_2$).
• Negative Plate: Sponge Lead ($Pb$).

$\text{Discharge Reaction:} \quad PbO_2 + Pb + 2H_2 SO_4 \xrightarrow{\text{Discharge}} 2PbSO_4 + 2H_2 O$

Each fully charged lead-acid cell produces a nominal open-circuit voltage of 2.1 Volts per cell (fully charged $2.20\text{ V} - 2.25\text{ V}$).

• A 12 V Nominal Battery contains 6 cells in series ($6 \times 2.1\text{ V} = 12.6\text{ V}$).
• A 24 V Nominal Battery contains 12 cells in series ($12 \times 2.1\text{ V} = 25.2\text{ V}$).

State of charge in flooded lead-acid batteries is checked using a hydrometer to measure electrolyte specific gravity. Modern aircraft frequently use Valve-Regulated Lead-Acid (VRLA) or Absorbed Glass Mat (AGM) sealed batteries, which immobilize electrolyte in fiberglass mats to eliminate spill hazards and reduce maintenance.

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Aircraft Storage Batteries: Nickel-Cadmium (NiCd) & Thermal Runaway

Nickel-Cadmium (NiCd) batteries are widely used in turbine aircraft due to high energy density, flat voltage discharge curves, exceptional sub-zero performance, and low internal resistance capable of delivering 1,000+ Amperes for turbine engine starting.

Chemical Construction & Voltage

• Positive Plate: Nickel Hydroxide ($Ni(OH)_2$).
• Negative Plate: Metallic Cadmium ($Cd$).
• Electrolyte: Alkaline solution of Potassium Hydroxide ($KOH$, 30% by weight in water, specific gravity $1.240 - 1.280$).

Each NiCd cell produces a nominal voltage of 1.2 Volts per cell.

• A 24 V Aircraft NiCd Battery requires 19 or 20 cells connected in series ($20 \times 1.2\text{ V} = 24.0\text{ V}$).

Unlike lead-acid batteries, the specific gravity of NiCd electrolyte does not change significantly during charge or discharge; specific gravity cannot be used to determine NiCd state of charge.

Thermal Runaway Mechanism

The primary hazard associated with NiCd batteries operating under constant-voltage aircraft bus charging is Thermal Runaway:

 [ Constant-Voltage Bus ] ---> Charging Current Increases ---> Battery Temperature Rises
            ^                                                           |
            |                                                           v
            +--- Cell Internal Resistance Decreases (Negative Temp Coeff) <--+

1. Negative Temperature Coefficient: As a NiCd cell warms during charging, its internal resistance decreases.
2. Current Escalation: Under constant-voltage bus charging, reduced internal resistance causes the battery to draw higher charging current ($I = (E_{\text{bus}} - E_{\text{cell}}) / R_{\text{internal}}$).
3. Heat Generation: Increased current generates severe internal Joule heating ($I^2 R$), further elevating cell temperature.
4. Vaporization & Fire: The cycle accelerates rapidly into a self-sustaining feedback loop, causing electrolyte boiling, separator breakdown, cell shorting, hydrogen gas emission, thermal destruction, and airframe fire.

Prevention & Certification Requirements

FAA regulations require NiCd installations to feature battery temperature monitoring systems that alert pilots when battery temperature reaches $140^\circ\text{F}$ ($60^\circ\text{C}$) and automatically disconnect the battery from the charging bus via an inline battery sensor relay.

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Avionics Trap: Battery Shop Acid/Alkali Cross-Contamination

A critical safety rule in aircraft maintenance facilities is strict physical segregation between Lead-Acid and NiCd battery servicing areas.

Sulfuric acid fumes from lead-acid batteries permanently neutralize potassium hydroxide electrolyte in NiCd batteries, destroying their capacity. Conversely, alkaline contamination damages lead-acid plates. Dedicated tools, hydrometers, clothing, and separate ventilated rooms must be maintained for each battery chemistry.