1/23/23, 5:12 PM
How to Locate Ground Faults Fire Alarm System | Fire Alarms Online
Saturday, September 5, 2015
How to Locate Ground Faults Fire Alarm System
What is
the worst service call you can possibly receive? In my mind it is the infamous ground fault on a fire alarm system. These can either be
a quick fix (pending it's locked in and you are familiar with the circuit pathways) or a very time consuming correction. If the ground fault
is minuscule due to compromised wire insulation, water or vibration causing the ground fault to swing in and out, you could potentially
be looking at a long day.
Note there are two types of ground faults.
An absolute short to ground is caused by a portion of the conductor touching a solid earth ground. This is the case with
pinched our cut insulation, wires stripped back to far, strands of your conductor poking through tape, etc. In all these cases,
the actual copper of the conductor is touching physical ground.
The other form of ground fault is commonly known as a "soft ground fault". These faults are typically caused by moisture or
compromised insulation where the copper is not necessarily touching ground however the insulation/voltage threshold is
lowered and can be the cause of a ground fault. Water can also be the cause by penetrating the insulation through hairline
fractures and bridging the gap between the copper and ground.
How ground faults are determined by control panels and multi-meters
A fire alarm control panel uses an internal 12-24 volts DC to seek out ground faults on all connected circuits including power, SLC, IDC
and NAC. Most ohmmeters used for troubleshooting fire alarms systems have an internal battery with an output voltage of 1.5 - 3 volts
DC (In the picture below, you will notice the analog ohmmeter I am using for this article puts out 1.628 volts DC). In cases where the
insulation/voltage threshold is below 12 volts but above 3 volts, the ohmmeter will not detect the ground fault. If the ground fault does
register on the ohmmeter, it will show some conductance to ground where a clean circuit will show infinite ohms or an open circuit.
With a soft ground fault present on the conductor, the ohmmeter will show a reading in the range of high K-ohms (K) or Meg-ohms
(M). Note the fact that these readings can be unstable and hard to decipher if the technician is not completely seasoned and
experienced with their multi-meter.
www.firealarmsonline.com/2015/09/quickest-way-to-locate-ground-faults-on.html
1/11
1/23/23, 5:12 PM
How to Locate Ground Faults Fire Alarm System | Fire Alarms Online
The solution to quickly locating a soft ground fault on a fire alarm system.
In order to locate a soft ground fault as easily as the fire alarm control panel, we will need a ohmmeter that has a slightly higher output
voltage than the factory range of 1.5 - 3 volts DC. This higher voltage output will allow you to locate the source of the ground fault
more quickly and accurately saving you a great deal of time in the field. Once you know what conductor the ground fault is on, you can
use the industry standard of breaking circuits in half until the fault is isolate to its source.
How to build your own high voltage output ground fault detection meter.
Below I have outlined step-by-step instructions on how to build your own ground fault meter using materials found at any hardware
store for around or below $30.00.
www.firealarmsonline.com/2015/09/quickest-way-to-locate-ground-faults-on.html
2/11
1/23/23, 5:12 PM
How to Locate Ground Faults Fire Alarm System | Fire Alarms Online
Above is a picture of all the necessary equipment to build a very simple to operate fire alarm ground fault tester. The equipment in its
entirety is as follows:
1 - Analog ohmmeter (roughly $15.00)
4 - 9 volt Batteries (roughly $7.00)
4 - 9 volt Battery Connectors with Flying Leads ($.49)
1 - Set of Ohmmeter Test Leads ($6.00)
1 - Heat Shrink Tubing (< $1.00)
1 - Limiting Resistor (< $1.00)
1 - Set of Velcro Straps ($5.00)
Equipment you should already have in your arsenal
Digital Multi-Meter
Soldering Iron
Electrical Tape
Wire Strippers
Lets get started with the build!
Step #1:
www.firealarmsonline.com/2015/09/quickest-way-to-locate-ground-faults-on.html
3/11
1/23/23, 5:12 PM
How to Locate Ground Faults Fire Alarm System | Fire Alarms Online
Use your digital multi-meter (set to read current) to take a current draw of your analog ohmmeter. To be more detailed, you need to
find out the current used by the analog ohmmeter to read exactly zero Ohms or a dead short. Move your positive test lead on the
digital multi-meter to AMPS and turn the selection dial to DC Amps. Now place the two test leads of the digital multi-meter into the test
lead ports of the analog ohmmeter. When done correctly as pictured below, your analog ohmmeter should show zero Ohms and
your digital multi-meter will indicate the amps needed to show true zero. In the picture below the digital multi-meter shows a reading of
0.016 Amps. Remember this number for determining the limiting resistor value in step #3.
Step #2:
In order to achieve the higher output on our new analog ohmmeter, we will be using the four 9 volt batteries in series. Remember when
batteries are connected in series the voltage doubles and the Ah remains the same. So in the case of four 9 volt batteries in series, we
will achieve 36 volts DC. Start out by stripping and connecting the four 9 volt battery connectors with flying leads in series. Once
connected, I highly recommend a solid bond via solder finished off with a clean heat shrink tubing. See the below pictures for
examples.
www.firealarmsonline.com/2015/09/quickest-way-to-locate-ground-faults-on.html
4/11
1/23/23, 5:12 PM
How to Locate Ground Faults Fire Alarm System | Fire Alarms Online
www.firealarmsonline.com/2015/09/quickest-way-to-locate-ground-faults-on.html
5/11
1/23/23, 5:12 PM
How to Locate Ground Faults Fire Alarm System | Fire Alarms Online
Step #3:
Now that we know what amount of voltage we have with our four 9 volt batteries in series (36 volts) as well as the current used to read
absolute zero on the analog ohmmeter (found in step #1), we can use Ohm's Law to determine our limiting resistor value. Ohm's Law
states that E (volts) / I (current) = R (Resistance). In this case 36 volts / 0.016 amps = 2,250 or 2.25K. For the purpose of this article, I
will be using a 2.2K resistor. When you attempt this project, you may need to slightly increase/decrease the value of your limiting
resistor in order to read absolute zero. Remember, lower value resistors will move your analog ohmmeter needle closer to the right (0
ohms).
Step #4:
We will now need to insert this limiting resistor in series to the negative test lead of our analog ohmmeter. I recommend cutting the
negative test lead 5 - 7 inches away from its meter connection side. Once the lead is cut, splice the resistor in series, solder the
connections and finish off with heat shrink tubing. IMPORTANT, make sure the limiting resistor is placed between the analog
ohmmeter and the 36 volt battery connectors. See below picture.
www.firealarmsonline.com/2015/09/quickest-way-to-locate-ground-faults-on.html
6/11
1/23/23, 5:12 PM
How to Locate Ground Faults Fire Alarm System | Fire Alarms Online
Step #5:
This next step is preference as you can clean up the wire connections anyway you see fit. For this article, I just taped them up. Once
you get your ground fault meter built, you will want to play with some different ideas that will insure longevity and ease of use in the
field. With the negative test lead disconnected from the analog ohmmeter, connect all four of your 9 volt batteries to the connectors.
Once connected, use your digital multi-meter to verify 36 volts insuring your connections are correct.
www.firealarmsonline.com/2015/09/quickest-way-to-locate-ground-faults-on.html
7/11
1/23/23, 5:12 PM
How to Locate Ground Faults Fire Alarm System | Fire Alarms Online
www.firealarmsonline.com/2015/09/quickest-way-to-locate-ground-faults-on.html
8/11
1/23/23, 5:12 PM
How to Locate Ground Faults Fire Alarm System | Fire Alarms Online
Step #6:
Now that you have verified the correct output voltage, connect the negative test lead to the analog ohmmeter. With the ohmmeter set
to Ohms, short the test leads together. You should get absolute zero. In the picture below, I have the 36 volt battery pack Velcro
strapped to the back of the analog ohmmeter. You will see both test leads are shorted and the ohmmeter is reading absolute zero.
The picture below shows the new output voltage once your fire alarm ground fault tester is complete. If you remember from earlier in
the article the original analog ohmmeter put out 1.628 volts DC whereas the new setup is showing 39.53 volts DC.
www.firealarmsonline.com/2015/09/quickest-way-to-locate-ground-faults-on.html
9/11
1/23/23, 5:12 PM
How to Locate Ground Faults Fire Alarm System | Fire Alarms Online
This 39.53 volts is a combination of the 36 volt battery pack as well as the internal battery within the analog ohmmeter. This higher
voltage will make locating a ground fault a lot easier as well as increase your accuracy tenfold. Also note the output voltage is still low
enough to not damage devices and appliances on fire alarm circuits. Make sure when using this tester, you disconnect all circuits from
control boards. To locate a ground fault, look for continuity to ground on each circuit. This new analog ohmmeter will show infinite
ohms when the conductor is not exposed to an earth ground. If the insulation is compromised and/or the copper is directly connected
to ground the ohmmeter will indicate 0 ohms. If a soft ground fault is present, the meter will fluctuate from infinite (left) towards zero
(right). This fine movement should allow you to track down circuits with faults in half the time.
Good luck with your build and be sure to let us know if this new fire alarm circuit tester helps you in your daily troubleshooting.
Make sure to join our Facebook page and let us know your thoughts.
RELATED POSTS:
How To Use A MultiMeter For Fire A...
Reading Electrical
Current With A M...
Ground Fault Found
On PAM Relay Leg
www.firealarmsonline.com/2015/09/quickest-way-to-locate-ground-faults-on.html
Cable Toner Kit
Doubles As Fire
Spe...
Fire Alarm
Calculations
10/11
1/23/23, 5:12 PM
How to Locate Ground Faults Fire Alarm System | Fire Alarms Online
Voltage Drop For
Fire Alarm Systems
Free Fire Alarm Forum
Kyle Schuler, SET at 9/05/2015 03:56:00 PM
Labels: fire alarm troubleshooting, ground fault, insulation tester, megger, multi-meter, Ohm's Law, ohmmeter
1 comment:
Unknown January 31, 2020 at 5:47 AM
This is a GREAT IDEA! I've been the go-to guy for tracking down ground faults at my work because I've had good success at
it over the years. No doubt, a lot of my patience gets eaten up while trying to hone in on the actual source of a ground fault
condition, and this is often exacerbated by the fact that most of the systems I work on have no as-built drawings to work off
of. I've only come across a few ground faults that couldn't be picked up on my meter in the kΩ or MΩ range, but your idea of
ramping up the voltage when reading continuity to ground on an isolated circuit would be excellent for those occasions.
Here's a good one for you: A few times while troubleshooting a ground fault condition, I've found two or three circuits having
very minor earth faults. Cumulatively, they breach the FACP's threshold and initiate a ground fault response, but if any one of
them is removed from the bunch, the ground fault disappears. Crazy, right? And the labor to resolve all sources of ground
fault was a multiple of the number of circuits that were affected. I'm glad I didn't have to pay those bills!
Reply
www.firealarmsonline.com/2015/09/quickest-way-to-locate-ground-faults-on.html
11/11