Bleed leak detection system test

[baobab72]

Hi folks
For the gurus outthere, in the 605 although i am sure it applies to other airplanes as well, we have a dual loop system around the 10 th stage bleed air manifold, both loops referred to as A and B must detect the overheat to set off the alarms. The ability of the system to discern between a short and an actual overheat can be tested by twisting a DUCT MONITORING know on the bleed air control panel on the ovhd to the LOOP A and then to the LOOP B position and grounding doing so the opposite loop. Failure of the test will trigger the related 10th DUCT EICAS message.
What does that mean in plain english? Can you clarify the grounding concept for me? Cos to me when you ground an item you complete the circuit and i dont see how that relates to the fact that you are actually testing the opposite system!!!
Many thanks
Baobab

[Escape Path]

While I'm not a 605 "guru" as you pointed, I'll have a go at it and someone else might correct if I'm wrong.

Since you need both loops to detect the overheat condition so they trigger the alarms, by grounding the opposite loop of what you selected with the knob (grounding it puts the system out of operation, i.e. breaking the circuit) you test two things: One, that the grounding is working correctly for one loop and two, you're testing the proper normal operation of the other loop. Why is the grounding necessary? If the other loop doesn't ground, it will trigger the corresponding alarm since both loops would "detect" the overheat condition in the testing.

You need to ground one loop so you can test either loop individually, otherwise you'll get the warning light/bell/whatever since they are both "detecting" an overheat condition.

Hope it helps somehow! Spanish being my native language and talking about an aircraft I have absolutely no clue may not help in my redaction, but hey...!

[grounded27]

I believe you are simply testing the function of the controllers by grounding both relays in the controller via the test switch that would normally find ground in the detector to make sure you get a flight deck effect. Arm chair troubleshooting, I would have to see the schematic. Kidde/Fenwall or Systron Donner? A fail message is usually indicated as a result of the loop K/F sense resistance change, the length of the loops are critical. S.D. a loss of pressure in the loop triggers a fail, high pressure = warning.