SR Products ETL121375 Nominal Temperature Rating - Manual (Page 2)

ELECTRICAL CHARACTERISTICS
The ETL® contains a bridge wire initiated pyrotechnic heating element. The 14μ bridge wire has a
nominal resistance of 10 to 30 Ohms. The bridge wire is multi-purpose, serving as the ignition source for the
chemical heater core and providing a method for non-destructive testing. The design of the bridge wire has
been optimized, requiring minimal energy to function while remaining safe to handle, transport and install. Care
should be exercised in handling and installing the ETL® with particular attention to the following electrical
characteristics: Avoid proximity to electrostatic discharge or heavy induction fields. Keep the yellow lead wires
twisted or otherwise shunted unless performing resistance checks or until making final connection. Keep the
ETL® inside the packaging box when not in use. ETL®‘s should be wired in parallel to normally open circuit
contacts, which keeps both lead wires disconnected from the intended actuation energy source. While it is
possible to continuously monitor the bridge wire, it is not recommended. Even minimal continuous input current
through the bridge wire will cause long term “baking” of the internal chemical core, and allow ingress for
undesired current at levels sufficient to cause unwanted actuation of the device. Monitoring even at extremely
low levels defeats Faraday shield protection and allows the possibility of unwanted input currents in the event
of ground loop surges, electrical spikes, or proximate lightning discharges. If it is absolutely necessary to
monitor the ETL®, input current must be limited to <1 milliampere continuous current with all possibility of
current surge (including ground loops) isolated or eliminated. Some possibilities might include using <1
milliampere test input current on a timed interval check basis or use of optical isolation circuitry. ETL® ‘s
subjected to continuous monitoring or extremely harsh environment may have a decreased shelf life and
should be replaced at some predetermined maintenance Interval.
The ETL® may be safely resistance checked using any commercial Ohmmeter, VOM, or bridge tester
using an input current of ≤10 milliamperes total input current into the sample within the resistance range of 10
to 30 Ohms. Resistance should be checked between the individual ETL® yellow lead wires (or at the normally
open contacts after assuring that the ETL® is disconnected from all circuits). There should be an open circuit
between either lead wire and ground. Insulation resistance is >2 Megaohms @ 600 VAC.
It is possible for the chemical heat core to leave a conductive path after functioning. Some residual
resistance could cause the core to continue to draw a minimal current from a supply transformer or battery. If
this is an installation consideration or problem it will be necessary to fuse the system upstream to
accommodate it. Assure that any such fusing will not prevent any ETL® in the system from having less than
the minimum trip current (0.2 ampere minimum) available.
ETL375F
Rev. D
2/24/12
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