Transcript with respect to ground

ADT490 TroubleShooting
Locating Grounds A
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
2
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
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
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
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
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
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
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
8 Zone System
10
8 Zone System
‣
‣
Continue to carefully tag and removing circuits.
When we disconnect Zone 7, we get this:
11
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.
‣
Visit floor, everything is in order, no evidence of leaks
or any other damage.
On floor - three apartments, a party room, sauna, small
whirlpool and fitness centre.
12
Locating Actual Ground
‣
‣
Sketch of Zone 7 devices as installed below:
Focus on front panel display -> Ground Fault and
System Trouble LEDs only
13
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
Locating Actual Ground
‣
It now looks like this:
15
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
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
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
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
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
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.
21
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
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
Locating Actual Ground
‣
Now our DMM displays this:
‣
There is no longer a circuit between wire and pipe.
24
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
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
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
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
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
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
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
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
end
of
presentation
33