Try a full lesson: Finding and fixing faults

Why this task matters at Level I

NICET task 1.2.2 is troubleshooting: the panel says something is wrong, and you find it and fix it. Maintenance is close to half of the Level I exam, and troubleshooting is the part of it that most separates a technician who can be left alone on a service call from one who can't. Good troubleshooting is method, not luck. You read what the panel is telling you, you work out what kind of fault it is, and you narrow it down with your meter until you are standing in front of the bad splice.

Opens, shorts and ground faults: how each shows at the panel

There are three basic wiring faults. Each one looks different at the panel, and knowing the difference saves hours.

Open

A conductor is broken or a termination has come loose. Supervision current stops flowing, so the panel reports a trouble on that circuit.

• On a Class B circuit, every device past the open stops working.
• On a Class A circuit, the panel reaches the devices from the other end, so all of them keep working, but it still reports the trouble.
• On an addressable SLC, the panel loses communication with the devices past the open and reports each one as missing, or reports a Class A loop break.

Short

The two conductors touch each other, or a device has failed in a way that connects them.

• On a conventional IDC, a detector in alarm puts a low resistance across the circuit. A short looks much the same, so on most conventional panels a short on an IDC shows as an alarm on that zone. Some panels can tell the difference and show a trouble.
• On a NAC, the panel watches the circuit in standby with reversed polarity. A short shows as a trouble. In alarm, the panel shuts that circuit down to protect itself.
• On an SLC, a short can take down the whole loop. Isolator modules limit the damage to the section between two isolators.

Ground fault

A conductor touches earth ground: a metal box, conduit, building steel or a wet ceiling. Most panels have one ground fault detector for all field wiring, so the trouble often says "ground fault" without naming a circuit. A single ground fault usually does not stop the circuit working. Two of them on different conductors can act like a short or cause signals to appear on the wrong circuit, which is why ground faults have to be found and fixed, not ignored.

Meter technique for troubleshooting a circuit

You need a good digital multimeter, and you need to use it in the right order.

1. Read the panel first. Write down every active trouble and the circuit or address. Check the history log for when it started. A trouble that started the day the ceiling contractor arrived tells you where to look.
2. Measure voltage with the circuit connected. Is the panel putting out what the manual says on that circuit? This step checks the panel before you blame the field wiring.
3. Disconnect the circuit at the panel before measuring resistance. Remove the field conductors from the terminals and note which goes where. Resistance readings on a live circuit are meaningless and can damage the meter.
4. Measure loop resistance across the two conductors.
   • On a Class B IDC or NAC with an EOL, you should read close to the EOL value plus the wire resistance. On a NAC the appliances are polarized, so swap the meter leads and read both ways; one direction can show the appliances' diodes conducting. To read the wire's own loop resistance, remove the EOL and jumper the far end. A reading near zero means a short. A reading of OL (over limit, meaning beyond the meter's range) means an open.
5. Measure each conductor to ground. Put one lead on the conductor and the other on a good ground (the panel's earth ground terminal). A healthy conductor reads very high resistance, usually beyond the meter's range. A low reading on one conductor is your ground fault.
6. Be careful with an insulation tester. A megohmmeter puts out hundreds of volts. Disconnect every electronic device on the circuit first, or you will destroy detectors and modules. Many manufacturers say not to use one on installed device circuits at all.

Write your readings down as you go. They are your evidence, and they make the next step faster.

Worked calculation: isolating a ground fault by halving the circuit

When you know a conductor is grounded but not where, you split the circuit in half and test each half. Each test throws away half the remaining possibilities. The readings below are example values.

The circuit. A Class B IDC with 24 devices, numbered 1 (nearest the panel) to 24 (the EOL).

Step 1: confirm the fault. With the circuit lifted at the panel:

• Positive conductor to ground: 1.2 kOhm. This is the fault.
• Negative conductor to ground: OL (open, very high resistance). This is healthy.

Step 2: first split, between devices 12 and 13. Open the circuit at device 12 and test each side.

• Panel side (devices 1 to 12): positive to ground reads OL. Clean.
• Field side (devices 13 to 24): positive to ground reads 1.2 kOhm. The fault is in 13 to 24.

Step 3: second split, between 18 and 19. Reconnect device 12 and open at device 18.

• 13 to 18: positive to ground reads 1.2 kOhm. The fault is here.
• 19 to 24: OL. Clean.

Step 4: third split, between 15 and 16.

• 13 to 15: OL.
• 16 to 18: 1.2 kOhm. The fault is in 16, 17, 18 or the wire between them.

Step 5: fourth split, at 17. Test 16 to 17 and 17 to 18 separately. You find the fault on the run between 17 and 18. When you open the box at device 18, the conductor insulation has been cut against the edge of the box.

How many splits does it take? Each split halves the number of devices left. After one split you have 12, after two 6, after three 3, after four you are down to one or two. In general, the most splits you need is the smallest whole number n for which 2ⁿ is at least the device count. Since 2⁴ = 16 is less than 24 and 2⁵ = 32 is more, a 24-device circuit needs at most 5 splits. Checking every device one by one could take 24.

What the exam tests here. The method (halve and test), why you lift the circuit at the panel first, and how many steps a given device count needs.

Replacing a device and returning the system to service

Once you find the faulty device or wire:

1. Use the right replacement. The same model is safest. Any substitute must be listed for the purpose and, for two-wire smoke detectors, listed as compatible with the panel. The panel manual's compatibility list decides this, not the parts counter.
2. Match the settings. On an addressable system, set the same address and device type. On a strobe, set the same candela. On a heat detector, use the same temperature rating.
3. Repair the wiring properly. Use listed connectors in a box. Don't leave a splice buried in a ceiling.
4. Test what you touched. Test the replaced device and the circuit it is on. NFPA 72 Chapter 14 requires testing after any change, and the test must cover the device and the functions it controls.
5. Restore everything. Clear the trouble, return any bypassed outputs to normal, confirm the panel is normal, and confirm with the supervising station that they see the system in normal condition.
6. Document it. Write what you found, what you replaced, the test result and the time the system was returned to service. Tell the owner.

Check yourself

The free practice set below has ten original questions across all three Level I domains, including two on troubleshooting, each with a full explanation of why the right answer is right and why every other option is wrong.

Where to look in your own books

• NFPA 72-2022 Chapter 12: pathway classes and how each must behave under an open, short or ground fault
• NFPA 72-2022 Chapter 14: testing after repairs and replacements, and records
• The panel installation manual: troubleshooting tables, expected circuit voltages and the compatibility list