CIE-A-200 Control Panel Engineering and Commissioning Manual
56
Earth Faults
General
If the control panel detects a short circuit or leakage current to earth/screen from one of its supply
rails (either 0V or +Ve) an
Earth Fault
will be displayed.
The voltage between 0V and Earth SCRN can be checked with a DVM. The normal voltage is 1.7V.
The panel will display an earth fault if this voltage is below 1.5V (caused by current leakage to 0V) or
above 2.5V (caused by current leakage to +Ve).
Remove the circuit cables connected to the control panel one at a time and reset the system. When
the circuit with the earth fault is disconnected, the earth fault will clear within approximately 30
seconds of being reset.
Note that if a computer is connected to the control panel, an earth fault may be generated at the
control panel. If the connection is permanent, an optical isolator must be used between the PC and
the control panel.
A short circuit (low resistance: expected to be less than 5 ) may be identified and tracked with a
digital test meter between either the positive core or the negative core and the screen of that circuit.
It is not possible to override the earth fault monitoring as it is important for correct system operation.
If it is shown that the earth fault is on the addressable device loop, then disconnect it from the control
panel and investigate it with a digital test meter.
Loop
–Ve to Screen
If the fault is a
short circuit from screen to loop
–Ve
then it will be easily identified and rectified
with a digital test meter.
An investigation of the resistance reading between loop
–Ve and earth/screen at the control panel
for each end of the addressable device circuit should give a good indication of the location of the
short circuit.
Eg, if the resistance reading at Loop end 1 were 9 ohm, and at Loop end 2 were 3 ohm then it may
be estimated that the short circuit lay approximately 9/(3+9) = ¾ of the way round the addressable
device circuit from Loop end 1. Investigating that area, and introducing a split to the Loop
–Ve as
required, will allow it to be tracked and rectified.
Loop +Ve to Earth
If the fault is a
short circuit from screen to loop +Ve
then it will only be identified with a digital test
meter at that individual length of cable due to the inbuilt short circuit isolators in each device.
To locate this manner of fault, split the +Ve core of the loop at approximately a half-way point on the
loop, then reinitialise with only Loop End 1 connected to the control panel.
Even though the initialisation will fail due to the enforced open circuit, the earth fault will only be
indicated if it is located within the section of the cable which was initialised. The split may then be
reinstated in another position and the process continued, until the section of the system suspected
of containing the earth fault is small enough to allow each portion of cable to be individually tested
with a digital test meter.
