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ADT490 TroubleShooting Locating Grounds A. 2 Introduction ‣ Fire Alarm Panels cannot tolerate grounds ‣ A ground on any fire alarm circuit is detected.

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Presentation on theme: "ADT490 TroubleShooting Locating Grounds A. 2 Introduction ‣ Fire Alarm Panels cannot tolerate grounds ‣ A ground on any fire alarm circuit is detected."— Presentation transcript:

1 ADT490 TroubleShooting Locating Grounds A

2 2 Introduction ‣ Fire Alarm Panels cannot tolerate grounds ‣ A ground on any fire alarm circuit is detected by the FACP and a System Trouble (yellow LED) condition is displayed ‣ Location - initiating, supervisory, signal, ancillary or power supply. ‣ Recent panel manufacture - separate indicator -> yellow “ground fault” LED. ‣ Ground can develop with any circuit conductor making metallic connection with a piece of mounting hardware (i.e. junction or back box, or metallic raceway

3 3 Introduction ‣ Call a hard ground - a very low resistance contact ‣ Can develop if a conductor is exposed to wetness within a metal electrical box (i.e. junction box, 4 x 4 or octagon box) ‣ Water leaking laden with contaminates that are very conductive (i.e. salts, carbonates). ‣ If contaminated water allowed to collect in box, eventually makes contact with terminal on device or it’s connections, thereby providing a path to ground. 3

4 4 Importance of Ground Detection ‣ Single ground fault on a circuit may causes moderate current increase, placing an extra burden on the FACP’s power supply ‣ A single ground fault allowed to “progress” to a double ground fault ‣ ground on both positive & negative circuit conductors ‣ SHORT developed between conductors ‣ double-ground (really a short circuit) happens on an Initiating circuit: ‣ an Alarm condition will result ‣ voltage goes to 0, all electronic devices (smoke detector) on circuit not have power. 4

5 5 Importance of Ground Detection ‣ If double - ground happens on a Signal circuit: ‣ a Trouble condition will result ‣ voltage goes to 0 ‣ signals without voltage cannot operate ‣ Ground fault must be investigated immediately to: ‣ prevent nuisance alarms, or ‣ failure of fire alarm system to operate signals during emergencies 5

6 6 What Panel Tells Us - Ground 1. FACP displays Trouble condition & Trouble buzzer is on. 2. Recent panels (12-15 years old), separate indicator (LED) designated as “Ground”, “Ground Fault”, “Gnd”, “Earth” or something similar, will be on 3. Very recent panels, third Indicator may indicate when ground occurred on signal circuit or initiating circuit. 6

7 7 How to Use Ground LED ‣ What’s with this “Ground Fault” LED? ‣ How can we use it to troubleshoot a ground? ‣ FACP ‣ operates “Ground Fault” LED when ground is detected, and ‣ extinguishes LED if ground is no longer detected. ‣ Can use LED as tool -> which circuit has a ground fault by systematically removing circuits until ground fault LED turns off. ‣ Feasible in smaller systems (up to 20 zones), but any circuits prone to grounding conditions from dampness or water damage are first to be investigated. 7

8 8 Finding Grounded Circuits ‣ Removing a circuit that is “contaminated” with a ground results in “ground fault” indicator LED turning off. ‣ Because the circuit has been removed, System Trouble LED remains lit. ‣ CAUTION: There may be more than circuits with ground. ‣ For this reason, DO NOT replace circuit when removed -> may itself have a ground. 8

9 9 8 Zone System ‣ 8-zone FACP terminal strip, & all 8 zones used. ‣ LED display on front of the FACP shows a System Trouble condition and a Ground Fault condition. ‣ Can assume that Trouble condition is caused by Ground Fault -> no other trouble indicators on. ‣ Start disconnecting circuits with Zone 1 9

10 10 8 Zone System 10

11 11 8 Zone System ‣ Continue to carefully tag and removing circuits. ‣ When we disconnect Zone 7, we get this: 11

12 12 Locating Actual Ground ‣ We are confident we have found circuit that has developed a contact to ground -> Zone 7. ‣ Zone serves top floor of a small apartment building. ‣ On floor - three apartments, a party room, sauna, small whirlpool and fitness centre. ‣ Visit floor, everything is in order, no evidence of leaks or any other damage. 12

13 13 Locating Actual Ground ‣ Sketch of Zone 7 devices as installed below: ‣ Focus on front panel display -> Ground Fault and System Trouble LEDs only 13

14 14 Locating Actual Ground ‣ Step 1 – Which conductor Grounded – “+ve” or “-ve”? Ground affecting entire circuit. ‣ Easier if we were able to search for ground on just one conductor - either positive wire or negative. ‣ Removing positive wire or negative wire from the FACP will not tell us anything -> they share one contact with ground because they are actually connected together through EOL resistor. ‣ Imagine Zone 7 circuit to be in a straight line 14

15 15 Locating Actual Ground ‣ It now looks like this: 15

16 16 Locating Actual Ground ‣ Disconnect to Zone 7 EOL ‣ Now only one conductors has ground contact ‣ Go back to FACP and disconnect positive wire. ‣ Does the “Ground” LED turn off? ‣ If it does, then ground contact is on positive wire 16

17 17 Locating Actual Ground ‣ Note Zone 7 Trouble LED -> indicating an open circuit. ‣ It is open -> EOL is totally disconnected ‣ Positive wire at the FACP. ‣ Note that ground on the Zone 7 Initiating circuit did not cause the Zone 7 Trouble Led to turn on 17

18 18 Locating Actual Ground ‣ Step 2 – Finding the Grounded point ‣ Now we use Boolean Search method. ‣ With 2 technicians (technician & helper), Can begin with having one person on to 7th floor and other at FACP. ‣ Re-connect positive wire to positive terminal of Zone 7. ‣ The Ground Fault indication returns. 18

19 19 Locating Actual Ground ‣ The technician to the 7th floor, selects mid-point on circuit and opens circuit by removing a device -> Fitness Room heat detector. ‣ Now Zone 7 Trouble (open) and Ground Fault are on with the System Trouble. 19

20 20 Locating Actual Ground ‣ Next device removed is Hallway smoke detector -> easiest, most accessible device. ‣ According to panel technician, Ground Fault Led is now off. ‣ Ground condition is between Hallway smoke detector and Fitness Room heat detector (i.e. wiring to Party Room detector). 20

21 21 Locating Actual Ground ‣ Step 3 – Verifying contact to Ground ‣ Section of circuit looks like “straight line”. ‣ Circuit leaves disconnected Hallway Smoke Detector, goes through a length of ½” EMT to Party Room Heat Detector, then through another length of conduit to detector in Fitness Room. ‣ There is a ground contact somewhere along this section of circuit wiring. 21

22 22 Locating Actual Ground ‣ Set DMM to read resistance ‣ Clip one lead of our meter to one end of positive wire that was connected to the Hallway west Smoke Detector – not the positive wire coming from the Panel, but the positive wire heading down the circuit. ‣ Then touch metal raceway containing the wiring (or back-box), with our other lead. ‣ Read a low resistance contact between the wire and the metal pipe – a reading of 3.8 Ohms ‣ Measured resistance “ with respect to ground” 22

23 23 Locating Actual Ground ‣ Grounding point could be between Test Point and Party Room detector, or from Party Room detector onward. ‣ In Party Room, disconnect circuit leaving detector. 23

24 24 Locating Actual Ground ‣ Now our DMM displays this: ‣ There is no longer a circuit between wire and pipe. 24

25 25 Locating Actual Ground ‣ To confirm location of grounding point, connect DMM on wire in Party Room and take another reference with respect to ground. 25

26 26 Locating Actual Ground ‣ There is confirmation of ground contact on the positive conductor in raceway between the Party Room detector and Fitness Room detector. ‣ Appropriate action: Replace cable, using existing cable as a “pull cable”, if possible 26

27 27 Determine Cause ‣ Raceway may not be a straight run between devices as but there may be elbows or pull elbows along the way. ‣ The cable could been stretched over a sharp edge inside the conduit hardware that cut into cable and wire insulation ‣ Bringing wire into electrical contact with the conduit ground path. ‣ Care must be taken replacing cable to ensure that it is not stressed or stretched when it is pulled into the conduit run. 27

28 28 Why Not Take Voltage To Panel? When troubleshooting an “open” or a “short”, there were substantial changes in circuit voltage supplied by the FACP when either of these faults occurred. Expected voltage dropped to almost zero if there was an open or a short between Test Point and FACP Not so with a ground 28

29 29 Why Not Take Voltage To Panel? ‣ On a conventional circuit, either initiating or signaling, a single ground does not have a major effect on the circuit’s supervisory voltage or current. ‣ RECALL: FACP resistively “floats” above earth ground. ‣ The negative side of a circuit, although it is sometimes labeled as “0”, is not an earth ground connection 29

30 30 Why Not Take Voltage To Panel? ‣ The FACP enclosure is earth grounded through the main 120 VAC connection. ‣ All raceways and metal sheath-protected cables connected to FACP are earth grounded through the bonding conductor ‣ This resistive separation of FACP control board and circuits from earth ground reduces the effect of a ground on supervisory voltages and currents 30

31 31 Why Not Take Voltage To Panel? ‣ In the middle of an initiating circuit and looking toward the FACP with a voltmeter and measuring circuit current at FACP under normal circumstances, meter readings would typically look like this: 31

32 32 Why Not Take Voltage To Panel? ‣ Cannot take current measurements because it would require an open in the circuit while performing Boolean Search routine. ‣ Recall: Even though ground has appeared, there is still circuit continuity through the EOL and therefore Zone 7 32

33 33 end of presentation

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