The Impact of a Short Circuit on Conductor Performance

Over-current and shorting are an ever-present risk to power wires and cables. The hope is that circuit protection can quickly respond to over-current conditions fast enough to prevent any damage to the circuit components. Wire conductors have a temperature rating, limited by the plating, because that is the established temperature for long-term reliability. But what happens when a single wire is exposed to repeat over-current conditions? Does the latest generation of circuit protection sufficiently protect the wiring?

In a recent set of tests, Lectromec examined the impact of over-current conditions on wire conductivity. These tests sought to determine if repeat over-current conditions would result in a measurable resistance increase. The background of these tests, the test setup, and the results are discussed here.

Similar Tests

For any testing to be conducted, it is important to find an existing test that closely matches the conditions to be evaluated. The obvious benefit is to compare the results against an existing set of tests carried out as part of product qualification. The SAE AS4373 contains standards for tests that are used for product qualification and which are similar to those conducted in this evaluation. For example, Time to Smoke and Smoke Resistance are crucially different in the properties being evaluated as well as the practical conditions each test aims to simulate.

In the Time to Smoke test (AS4373 Method 507), amperage increases are applied to the current in 5A steps until smoke is observed. The circuit resistance is changed in order to achieve the desired increase in current.

In the Smoke resistance (AS4373 Method 513) test, a resistance value for the test wire at an elevated temperature is estimated based on the wire’s physical properties. The current is adjusted to achieve this resistance and temperature and the wire is held at these conditions for 15 minutes. Though the wire’s resistance is considered in this test, it is representative of a much longer-term latent fault within a circuit.

Both existing tests look for visible damage (smoke) due to a slow, long-term increase in current/ temperature while in use. No SAE standard currently exists for measuring the effects on a wire conductor due to a rapid change in current/ temperature such as a short circuit/fault condition.

Test Set-up

In an effort to evaluate the possible effects a short circuit fault condition may have on a typical wire, Lectromec carried out a series of experiments. This experiment was performed on a 20ft sample of 20AWG AS22759/34 wire.

The sample wire was connected on one end through one of Lectromec’s arc track resistance test units to a generator, with the other end of the sample connected to the neutral return. The Lectromec arc cabinet provided a means for a controlled change of current between trials by selecting the desired circuit resistance. The short circuit duration was limited by a 7.5A thermal circuit breaker.

A data logger was connected to the circuit and recorded the current output during each trial.

Results

Tests were conducted at five different currents increasing the current applied in each trial. Prior to testing and after each trial, the wire’s resistance was measured by following the AS4373 Method 403 for conductor resistance.

The recorded data allowed for the determination of each trial’s duration and maximum current. One may note that test duration decreases with an increase of current, as the thermal circuit breaker reacts more quickly to the higher temperature created by high current events.

In-Series Resistance (Ω) (not including connecting wires)	Current Peak (A)	Test Duration(ms)	Resultant Wire Resistance (Ω)
Pre Test – Initial Resistance Measurement	–	–	0.094
2.0	73	583	0.094
1.5	93.7	293	0.096
1.0	121	201	0.095
0.5	190	89	0.095
0.0	345	30	0.094
0.0	356	663	0.093
0.0	377	706	0.093

Two tests at the maximum current (the second and third trials with in-series resistance of 0.0 Ohms) were run without circuit protection, leaving them to remain at the fault condition for a longer duration. The extended duration provided insight into the effects such a fault condition may have on an unprotected circuit wire. Those extended tests represent a total of more than 40 times longer exposure to over-current conditions.

Summary

Lectromec determined that there was no appreciable long-term change to the conductor resistance due to repeated short circuit faults. A thermal circuit breaker will successfully protect the wiring in a circuit during a fault event. Even when exposed to the fault condition for a longer duration at high current, the wire showed no significant change in resistance.

It is useful to note that AS22759/34 wire used is tin plated, meaning that it has a lower temperature rating than silver or nickel plated wire. It is safe to say that these materials, rated for higher temperatures, would similarly remain unaffected when subject to a short circuit fault condition.

While it is possible that additional high current cycling might degrade the conductor performance, seven high current events yielded no appreciable change during this evaluation. For those seeking an ISO accredited laboratory to support your aircraft evaluation needs, contact Lectromec.