Transcript short circuit

ADT490 TroubleShooting
Locating Shorts A
Finding a Short
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Previously, Class B initiating circuit serving the
second floor of a small apartment building
Now, we will assume there is a short on the circuit.
Again, we will imagine that the circuit is a
conventional initiating circuit
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Determination of A Short
‣
‣
A short on initiating circuits puts FACP into ALARM ->
Not Trouble
When on the site, we will be told that an alarm
sounded
‣ the Fire Service arrived to find no emergency
‣ FACP has red LED on Zone 2 - the second floor
‣ No pull station is pulled
‣ no smoke detector LED’s are on, and
‣ no heat detectors in the 4 suites have been
damaged
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Where Do We Start
1) Visual Inspection
‣ Visiting the second floor
‣ NO smoke detector has flashing LEDs.
‣
Smoke detectors have no operating voltage
‣ a short between positive and negative is present
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Where Do We Start
2) DMM Readings at the FACP
‣ Step 1
‣ DMM on 200 Vdc & read across Zone 2 terminals.
‣
We get a reading of 0.0 Volts.
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Expected with short on the circuit –>
voltage cannot exist on a shorted circuit
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Graphical Resistor Substitution
Short developed & indicated by double-arrow,
and FACP has Zone 2 in Alarm.
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6
Where Do We Start
‣Disconnect red wire from Zone 2 “+ ve”
terminal
‣Set DMM to read current and read current.
‣High, probably between 30 - 40 mA
‣-> high-current limit set by FACP circuitry.
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Step 2
•Disconnect circuit.
•Reset panel from Alarm
•Goes into Trouble -> circuit open
-> short in wiring.
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Step 3 & 4 - Resistor Substitution
•Ensure that Panel is good
•Problem in Field
•Install Resistor with same value as EOL
•Check Resistance of wire -> 0.3 ohms
- >Fault in Wiring
4.7KΩ
Substitute Resistor
Looking for a Short
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Short - no clues to location.
Smoke detectors No LEDs flash -> Why?
Operating manual station useless -> Why?
Boolean Search good approach
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Looking for a Short
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Remove temporary EOL
Connect Zone 2 to FACP
FACP goes back into Alarm.
Silence the signals.
Go to the second floor and review how the circuit is laid out.
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15
Boolean Search – Step 1
•
•
•
•
Go back to panel
Try a System Reset
If Reset -> short between Test Point (TP) & EOL
Reset fails -> short between TP & FACP
Suite 202
Suite 204
FA
CP
Suite 201
Test Point
Suite 203
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Boolean Search – Step 1
•Short between Test Point and FACP.
‣Reading is 4.8KΩ.
‣See the EOL – this part of circuit good.
FAC
P
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Boolean Search – Step 2
•Re-install smoke detector
•Find a mid-point to FACP.
•First smoke detector good location - next ‘shoot.
FAC
P
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Boolean Search – Step 3
• Short is still between you and the FACP.
• Short in raceway between you and first station, or
• in raceway from station down to the FACP
• Re-install smoke detector
• Remove manual station - do third ‘shoot.
FAC
P
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Boolean Search – Step 3
‣See the 24 Volts DC from panel - circuit is okay
‣Towards the EOL, circuit is also clear.
‣So where is the short???
•The short was cleared by removing the pull station.
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Boolean Search – Step 3
‣Look closely at station
‣The button has NO spring return when pushed.
‣Contacts are permanently closed –> shorts circuit.
‣Damage result of forcing station closed after pulling it.
‣Discovered by operating station and testing button, when
first arrive on second floor -> hindsight
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Class Discussion
1)
What other ways could a short have been caused?
a)_____________________________________
b)_____________________________________
c)_____________________________________
2)
Could you modify or change “search” to find a short?
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_______________________________________
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Class Discussion
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3)
What reading is expected if we looked
toward the FACP with our DMM set to resistance in
Step 3?
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_______________________________, but circuit
would have to be disconnected from FACP before
we start.
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4)
Could we have done this without looking for
panel voltage?
_____________________________________________
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Class Discussion
5) On the sketch, use symbol to indicate a conductor
ground point, such that a short circuit condition is
created.
Suite 202
Suite 204
FAC
P
Suite 201
Suite 203
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Finding a Short on a Signal Circuit
‣Look for short on signal circuit:
‣What clue does FACP give - where to search?
‣_____________________________________________
‣What panel voltage expected toward panel on clean
circuit?
‣_____________________________________________
‣Assume FACP has 4 signal circuits - three used.
‣How to determine circuit with fault?
‣_____________________________________________
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Discussion - Shorted Signal Circuit
‣Review architecture
‣Signal devices - types?
‣Has diode in series with activating coil in device.
‣“Blocking diode” block supervisory current (reverse bias)
‣-> supervisory current flows through EOL at fixed level
‣sensed by the FACP as the “normal” condition.
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Discussion - Shorted Signal Circuit
‣FACP reverses voltage polarity & direction of current
‣Diodes alters approach to troubleshooting.
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Discussion - Shorted Signal Circuit
‣Initiating devices - simple resistance looking for EOL.
‣Blocking diodes can conduct current in one direction
‣With many devices on circuit parallel resistance low
‣Could be between 32 ohms for 65 devices (16 Ω for 120)
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Discussion - Shorted Signal Circuit
‣Supervisory voltage levels will vary by manufacturer
‣Could be 24 Vdc on a signal circuit.
‣Could be as low as 1 to 2 V dc or as high as 20 Vdc.
‣Supervisory level verified prior to the Boolean Search.
•Signal circuits are arranged same as Initiating circuits
(i.e. in zones).
•One signal circuit can serve all floors in a building.
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Discussion - Shorted Signal Circuit
‣Good design more than one signal circuit serve a floor
‣Failure of one circuit will not totally remove signalling
for the floor.
‣In our case:
‣Two signal circuits serve occupied floors, and a third
circuit serves a 1-level underground garage.
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Example
‣There are 4 bells per floor; 5 in garage. A riser follows:
NAC1
NAC2
NAC3
EOL
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Example
‣If a short develops on one of three signal circuits:
1)FACP will in Trouble - not be in Alarm
2)No Zone Alarm LEDs will be lit – no zones are affected
by the short on a signal circuit.
3)The System Trouble LED and a yellow LED lit marked
“NAC” or “Signal” or “Circuit”
4)No double ground short -> Ground Fault LED OFF.
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37
Example
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FACP display:
Problem on one circuit. But which one?
Verified by “resistor substation”
One signal circuit is not used - EOL is
connected across the “spare” circuit.
Why?
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38
Example
‣FACP signal circuit connections (Class B).
3.9K
NAC1
NAC2
NAC3
NAC4
•Value of EOL on spare (NAC4).
•To find shorted ->Remove circuit wires & replace a 3.9 kΩ.
•If Trouble LED goes out, then this is the Troubled circuit.
•Replace the NAC2 circuit with 3.9KΩ resistor
->Trouble LED goes out.
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Example
‣FACP shows Normal System Status.
‣So the Trouble is on NAC2.
‣Remove resistor from NAC1 & re-connect circuit wires.
‣FACP remains Normal.
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Example
‣NOTE:
‣If it went back into trouble, then the problem is on 2 circuits,
NAC1 and NAC2.
‣Could happen if two circuits are cross-connected through a
short as a result of a chafed cable in a conduit or junction box.
‣Area of the building is served by NAC2?
‣With older installations original documentation is scarce.
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Example
‣Disconnected NAC2 from FACP.
‣Put the FACP into alarm & check floors and garage.
‣Only half bells on floors & all of the bells in garage ring.
‣NAC2 operates half of bells (i.e. 2 bells) per floor.
‣Note the bells not operating.
‣What kind of trouble is on NAC2?
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Example
‣Open close to FACP, or a short anywhere on the circuit.
‣Set DMM to resistance and connect to wires from NAC2.
‣Result 0.6 Ω - very low reading, not even 1 Ohm.
‣Lower than blocking diode.
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Example
‣Manufacturers often label terminals with signals
operating –normal ->voltage reads reverse.
Wires for NAC2
+
NAC1
Indicates short 0 to 5 ohms,
across conductors.
+
NAC2
+
NAC3
+
NAC4
Find normal
supervisory voltage
circuit known good spare circuit. Need
for Boolean Search
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Example
‣Note:
‣DMM shows a negative in supervisory
‣Positive in the alarm state -> reverses voltage polarity
‣Forward bias the blocking diode in each bell.
‣Voltage increases to 24Vdc to ring bells.
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Example
XX
X
‣Short exists on NAC2.
‣Reconnect NAC2
‣Start Boolean Search R
‣Removing device middle of circuit.
‣“X” suitable Test Point (TP) to start.
‣Remove bell
‣Measure toward FACP with DMM Volts DC.
‣If -6VDC
‣Short between TP and EOL.
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Example
XX
X
‣Put NAC2 into alarm
‣ “bell X” removed
‣3 bells between TO & FACP ring
‣Short circuit removed from the FACP.
‣Could take 3.9KΩ resistor
‣Connect across circuit going back to FACP.
‣Trouble LED extinguishes -> satisfied this
part.
‣Conclusion:
‣The short beyond “bell X”
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Example
Re-connect “bell X”
Move half way toward EOL.
Bell marked “XX” - next TP.
Remove completely
XX
Do all the bells ring to this point?
None of NAC2 bells are ringing.
Measure toward FACP for voltage
Should be6 VDC
X
Measure 0.01 VDC
Conclusion:
Short re-appeared
-> between “bell XX” &“bell X’.
Leave “bell XX” disconnected.
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Example
Remove one bell on 3rd floor “Bell XXX”
Now put NAC2 into alarm again.
XX
All the bells up to this point operate.
Is the short in wiring between bells XX and
XXX?
XXX
A
Measure resistance at point “A” -> infinity.
No short in wiring between the two bells.
“Bell XX’ was completely removed
X
When re-installed short re-appeared.
Not in “bell XX’
Not in wiring up to EOL.
Short in “bell XXX’
look closely at “bell XXX”.
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Example
‣Check if bell good
‣Read across both directions - Ohmmeter on low scale.
‣No resistance either way.
‣Short is across bell
‣Replace bell with identical device.
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‣NAC2 into alarm -> works.
‣FACP still
in trouble
‣Remove circuit
‣Take
‣Two
wires
circuit resistance reading.
shorts on circuit?
‣Meter
reads infinite resistance.
‣There
is now an open.
‣Don’t panic, just re-connect the bell on the fourth floor!
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What Else Could Have Been Done?
•Confusion from reverse bias readings of blocking diodes.
•Could have isolated NAC2
•Removing other working circuits, NAC1 and NAC3,
•Substituting 3.9KΩ resistor.
•Allows NAC2 bells to ring during testing for short location
•Reduces confusion for what bells are ringing.
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of
presentation