What is an "OPEN" fire detection loop?

[SE210]

Dear colleagues

I have a question regarding my curret aircraft - the CRJ200.

The FCOM mentions various fire detection loop failures; shorted loop, loss of power to the loop, one loop indicatin fire/the other loop not, dual loop failure, and open loop.

Can any body explain, what an open loop is - is it a broken loop???

Brgds

SE210

[KBPsen]

Yes it is.

[flyboyike]

It is indeed.

[SE210]

Thank you for the answers.

Does a broken loop still have fire detection capability?

Brgds

Soren

[zerozero]

If the component isn't working as designed then it should be considered a failure.

[ImbracableCrunk]

Yes, a broken loop may still have detection capabilities. It depends on the type.

Eutectic salt types can have a an open loop and still function

The Fenwal CFD® System maybe wired to employ a "loop" circuit. In this case, should an open circuit occur, the system will still signal fire or overheat. If multiple open circuits occur, only that section between breaks becomes inoperative.

If it is a gas-type, then no.

[SE210]

On the CRJ200, the detector loops are electrical, but due to the fire warning test logic, I doubt that an open loop is still operable.

Brgds

SE210

[Machinbird]

On the CRJ200, the detector loops are electrical, but due to the fire warning test logic, I doubt that an open loop is still operable.
SE210, If you define operable as able to indicate a real fire, that is probably wrong.
It just won't pass the press to test function (or modern equivalent).

Don't confuse test logic with operability. If it is an open loop indication, then there is a break or open in at at least one point in the sensing element. The fact that it an electrical loop type system means than there is detection capability at each end of the sensing element.

You are not going to go flying with a bad test unless the MEL says its OK (which isn't likely).

Once airborne and experiencing an engine fire indication, the cessation of the fire indication following execution of emergency procedures would be when you would be very interested in the test circuit indications.

[SassyPilotsWife]

just a small FYI that was included in our A/C ff training materials when assigned to a FD that included mutual aid to MEM Int'l Airport.

In normal operation, both detection loops must detect a fire or
overheat condition before a fire warning is generated.

[atpcliff]

I believe on the DC-9 we COULD fly with one loop broken, as long as the other tested OK.

cliff
HHN

[bcgallacher]

Electrically operated fire warning loops are ac powered and as a result a single break or open connector will give a fault on test but will operate normally.If there are two breaks or open connectors then the section or sections of the loop between the breaks will be inoperative but the remaining loops will still operate.A short will give the same indication as a fire or overheat condition.

[rigpiggy]

loop will still function, but may not come on till a higher temp is sensed.

[ImbracableCrunk]

May not come on until a higher temp is sensed? How do you figure?

[SE210]

Thank you for your answers.

The CRJ200 has dual loop detection in several zones.

Dual loop system will allow for dispatch with one loop U/S.

Normally (in dual loop) both loops will have to detect fire in order to trigger a fire warning. If one loop shorts or loses pwr, the system will automatically revert to single loop operation. There will be no indication for this.

Fire Fail caution light can come on for two reasons:

- One loop sensing fire/one loo not sensing fire.
- Both loops have failed.

During Fire Warning test, if the applicable FIRE WARNING light + FIRE FAIL caution light comes on - this indicates single loop operation due to a shorted loop, or a loop with no power.

During Fire Warning test - if the applicable FIRE WARNING light and the FIRE FAIL caution light DO NOT come on - this indicates an open loop. The system is still in dual loop and will not be able to detect fire, unless both loops (including the open one) detects fire. By manually swithing to the "normal" loop, the pilot can make a succesful Fire Warning test and dispatch with an open loop.

Pretty complicated system

[NSEU]

A short will give the same indication as a fire or overheat condition.

Not necessarily. The fire detection system on the 747-400 sees a short as a fault. The system discriminates between a fire and a fault by the rate at which the resistance changes. Quick changes are normally seen as shorts.

[spannersatcx]

with an inoperative loop you set the 'loop select' switch on the fire control panel to isolate the defective loop In the good old days, not so much now!

[rigpiggy]

Unsure which manual I was reading, my stack of books makes for light reading most firewire systems measure the drop in resistance between an exterior sheath, and the interior wire, it uses salt coated ceramic beads or porous matts as an insulator. with a shorter wire length you may require a higher temp to get the same drop.

from the manufacturers website

At a temperature more than 100° F (56° C) below the alarm point, the
impedance between the center conductor and the outer sheath is very high.
When heated to the alarm point the salt melts resulting in a very low impedance.
The elements return to high impedance as they cool.