In 2015, Dixon Robin (then with ATF), David Powell (Sytek Consultants), and Jason Karasinski (FRT) published an article entitled, “The Preliminary Results of the Investigation into Self Actuation of Light Switches during Fure Exposure” in The Fire Scene, the peer-reviewed semi-annual magazine published by the New York State Chapter of the International Association of Arson Investigators. In the article, the authors describe how they created a series of tests to see if circuits controlled by a light switch, which were de-energized prior to a fire, could become re-energized if the light switch was sufficiently damaged by fire heat. The implications of the test results, if successful, could mean an added layer of contextualized analysis would be necessary when conducting arc surveys. The test results showed that 71% of the switches tested in live burn scenarios exhibited self-actuation as a result of heat exposure and re-energized circuits that were originally open and deenergized.
The common single-pole residential light switch is a mechanical device used to open or close a circuit to allow the flow of electricity to lights or appliances. The construction and components of a single pole switch are very similar across most manufacturers. They typically utilize a polymer handle that can be flipped from “ON” to “OFF”, used to operate one set of contacts that open and close the circuit. There are two typically brass components within the switch. The smaller piece holds a fixed electrical contact and a screw terminal. The larger piece also has a screw terminal and supports an arm (contact beam) which has a movable electrical contact at the end. When properly mounted in the switch body, the contact beam has sufficient spring force to close the electrical contacts. So, if there is no outside force, the beam arm will close the contacts and complete the circuit (therefore be in the ”ON” position). A polymer handle, the switch that we see protruding from the wall, is used to open the circuit (or to turn things “OFF”) by pushing a small polymer finger into a coil spring. This then creates the “over center” toggling action used in the switch.
Initially a bench scale test was used to test the hypothesis that exposing a single pole light switch to heat would damage its polymer components and allow a switch originally in the “OFF” position to fail and actually allow the contacts to connect and create a closed circuit. In this test, the switch was monitored in real time by an X-ray video as well as a digital multimeter. The switch was exposed to constant heat from a heat gun and once the switch’s frame reached approximately 342 F, the switch contacts went from several million Ohms of resistance to under .2 Ohms. The video showed that the mechanical toggle did not function during the heating, but that the spring pressure of the contact beam pushed the toggle handle (which was softened by the heat) forward to allow the contacts to close.
A series of live burn tests, utilizing single compartment burn cells (12’x12’x8’) allowed to burn into post-flashover conditions, were then conducted. In 28 switch exposures, 20 switches (mounted in boxes and with faceplates in a GWB-sheathed wall) failed due to heat exposure and had self-actuations allowing circuits to be re-energized. Most switches that self-actuated did so very late in the fire timeline. On average, self-actuation did not occur until the last 12% of the fire’s timeline. The self-actuation was observed real time as lights outside each burn cell and connected to individual switches, turned on late in the fire event. Subsequent X-ray analysis of some of the switches also confirmed the failure of internal components.
Live burn tests (using single compartment burn cells) at ATF’s Fire Research Laboratory in 2015 and again at the National Fire Academy in 2016, replicated the original test results.
The conclusions drawn from these tests included a) compartment fires may cause light switches to self-actuate and re-energize otherwise deenergized circuits; b) the self-actuation during the fire is not the result of mechanical movement but the result of the softening and distortion of the thermoplastic parts of the switch which are under pressure by the forces of the contact beam; and c) there are many variables to this scenario as the proximity of the switch to the area of origin, the duration of the fire, the intensity of the fire, and whether the flashover transition occurred all play a role in whether self-actuation will occur.
It is important to recognize that this study is not complete, and the results are not comprehensive. The burn cells are not representative of typical compartments in most residential structures. Importantly, it is not recommended that investigators conduct destructive examination of switch components outside of a joint examination. Nondestructive techniques, such as X-rays, should be considered.
