Management

Aircraft circuit protection: A review of 5 technologies

April 29, 2014 · Michael Traskos

Two weeks ago we posted a blog on the selection process for circuit protection/wire pairs. Today, we produce a grid to help you understand common aircraft circuit protection types that can be found on aircraft.

The five device types talked about here are the thermal circuit protection, fuse, magnetic circuit protection, Arc Fault Circuit Breaker (AFCB), and distributed power. There may be other unique protection devices that are actively used.

Before assessing the pros and cons of each, it is important to understand what fault conditions circuit protection devices can interrupt. Here we identify five conditions under which circuit protection devices may activate. Note: not all devices will activate under each of these conditions.

1. Short Circuit

A short circuit occurs when there is a hard short between the high voltage side and the ground or different voltage potential. Potential hazards resulting from this include overheating of wires and subsequent faults as well as damage to equipment if not properly grounded (equipment bonding). All protective devices are designed to respond to a shorting event.

2. Overload

This is where the devices in the circuit are pulling more current than the system is designed to handle (e.g. drawing 20A on a 15A circuit). Circuit protection devices, particularly analog devices, may not catch overload conditions less than 130% of the rated current. Sustained periods of overloading a circuit can lead to wire system heating and degradation of associated components.

3. Parallel Arcing

This is the most dramatic failure event and can damage the harness, nearby systems, and affect equipment functionality. With thermal or analog circuit protection, these events can last for a long time and may not event be detected.

4. Series Arcing

Series arcing is a harder event to detect because the signature from series arcing is far more subtle than parallel arcing. It can cause thermal heating and damage as well as inconsistent equipment operation.

5. Faulty Operation

This may only be detected by software-assisted circuit protection in coordination with a health monitoring system. In this fault condition, an expected set of operational characteristics are stored in the protective device system. If the powered equipment begins to operate outside of these conditions, the circuit protection may trip to protect the device from causing damage to itself or other parts of the system.

The pros and cons outlined here are based on the performance of circuit protection devices when used properly (e.g. not placing two circuit protection devices in parallel to achieve a higher rating).

Device Type	Pros	Cons
Thermal Circuit Protection	Long service history Known performance Thermal memory (more rapid activation of circuit protection if reset soon after opening) Easily replaced Functionality checks can be easily performed Easy to identify when circuit protection has been activated	Requires a cool-down period before resetting (may be a ‘pro’ depending on your system) Temperature sensitive; will take longer to trip in colder environments Will not provide adequate protection from electrical arcing
Fuse	Fast Acting Smaller profile Cheapest option	One-time use More dangerous for replacement Easily damaged Impossible to verify performance (once you verify that it trips, it needs to be replaced)
Magnetic Circuit Breaker	Can be reset without cool-down period Faster Response time than thermal circuit protection Response time not affected by temperature changes Easy to identify when circuit protection has been activated	Heavier than all but distributed power type
Arc Fault Circuit Protection (AFCB), Software Controlled protection	Rapid Response to fault conditions Still uses thermal circuit protection as backup Some give indication of when arc fault has been detected Easy to identify when circuit protection has been activated	Cannot be placed on any circuit (some circuits have electrical characteristics that can be interpreted as electrical arcing)
Distributed Power	Software can be set to act like thermal circuit protection or any trip curve Benefits of AFCBs Can be used to reduce the amount of wire used in other configurations Can use solids state relays for bounceless operation and combine the circuit protection and switching devices into a single component Allows for remote operation of multiple systems May be combined with health monitoring system Preferred for unmanned vehicles	Can run into problems of nuisance trips Heaviest option (weight benefits improve as more circuits added) Requires more planning and design time for implementation than thermal circuit protection

It is anticipated that the newest aircraft designs will continue to capitalize on the benefits of distributed power technology. This does place a new set of challenges such as determining the Electrical Wire Interconnection Systems' (EWIS) safety and collocation of redundant/critical systems.