Caution - Unintended Electrical Paths - PSC 2000781J MkII MLB Operating Procedure

17. Caution - Unintended Electrical Paths

Several customers have created unintentional electrical paths with the power and measurement circuit while
attempting to operate the Lightband. The Lightband's internal limit switches were unintentionally bypassed and
the Lightband motors powered longer than anticipated.
The most common causes for these electrical paths include, but are not limited to:
1. The Lightband motors share a common pin 9 that can connect them under certain instances.
2. Most oscilloscopes have a common ground. Therefore, all 'negative' probes are connected to each other.
Consider this when connecting the voltage probes.
3. The user changed the voltage probe connection pins during Set-for-Flight from those recommended by PSC
and inadvertently bypassed the limit switches, deploying the Lightband.
4. The user connected the oscilloscope ground to earth ground and inadvertently bypassed the limit switches.
The Lightband thus deployed instead of the expected set-for-flight.
5. The user chose to measure current not by current probes but by measuring voltage drop across a resistor and
inadvertently bypassed a limit switch.
PSC recommends using an oscilloscope with isolated channels whenever possible. It greatly simplifies monitoring
and reduces the risk of unintended electrical paths.
The figure below shows a Set-for-Flight operation performed using an oscilloscope with common probe grounds.
Current can bypass the first open switch and run through the voltage probes to the second stow switch. The
resulting voltage and current profiles will make it appear as if both switches are synced even though they may not
actually be. Although this is not detrimental to the Lightband, it is an example of unintended electrical paths.
In the circuit below the stow A limit switch changes to NC, but power can still flow from pin 4 back to pin 8 and
through the Stow B limit switch to pin 9 until the Stow B switch changes to NC.
Figure 17-1: Set-for-flight oscilloscope common probe warning
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