CEP Troubleshooting & System Testing 5 — Questions and Answers
Question 1: During commissioning of a fire alarm system, all smoke detectors on a loop show a trouble condition immediately after installation. The most likely cause is:
- The detectors are not UL listed for the application
- An end-of-line resistor is missing or has incorrect value (Correct answer)
- The fire alarm control panel battery is low
- Humidity in the building is above 60%
Correct answer: An end-of-line resistor is missing or has incorrect value
Addressable and conventional fire alarm loops require a proper end-of-line resistor to establish correct loop supervision; a missing or wrong-value resistor causes a trouble fault on the entire loop.
Question 2: A ground-fault protection relay on a 480V solidly-grounded system operates at 5A sensitivity. A line-to-ground fault of 300A occurs. The relay should:
- Not operate because 300A exceeds the relay's range
- Operate because fault current exceeds pickup of 5A (Correct answer)
- Operate only if zero-sequence current is confirmed by a separate sensor
- Remain sealed because the fault self-cleared below 600A
Correct answer: Operate because fault current exceeds pickup of 5A
Ground-fault relays operate when zero-sequence current exceeds their pickup setting; a 300A fault far exceeds the 5A pickup, so the relay will operate.
Question 3: An infrared scan of a 600A fusible disconnect reveals the phase B fuse clips are 40°C hotter than phases A and C under balanced load. The recommended action is:
- Record for trend analysis and recheck in 90 days
- Immediately de-energize, inspect and tighten or replace the fuse and clips (Correct answer)
- Increase ventilation around the disconnect
- Replace the fuses with the next higher ampere rating
Correct answer: Immediately de-energize, inspect and tighten or replace the fuse and clips
A 40°C temperature differential is a serious finding indicating high contact resistance that can lead to fuse failure, fire, or arc flash; immediate corrective action is required.
Question 4: A technician is verifying the operation of a transfer switch that moves a critical load from utility to generator. After a utility outage, the load transfers but a 6-second delay occurs before the generator feeds the load. This delay is:
- A fault indicating the transfer switch relay has failed
- Normal — the standard time delay prevents retransfer during momentary outages
- Caused by the generator reaching rated voltage and frequency before transferring (Correct answer)
- A result of the transfer switch being wired in open transition
Correct answer: Caused by the generator reaching rated voltage and frequency before transferring
Automatic transfer switches include a generator time delay to allow the genset to stabilize at rated voltage and frequency before closing the transfer switch onto the load.
Question 5: A variable frequency drive faults on 'DC bus overvoltage' during motor deceleration. The most effective corrective solution is:
- Increase deceleration ramp time to allow energy to dissipate (Correct answer)
- Increase motor insulation class
- Reduce the VFD input voltage by adjusting a transformer tap
- Install a phase rotation relay upstream of the VFD
Correct answer: Increase deceleration ramp time to allow energy to dissipate
During deceleration, the motor acts as a generator, pumping energy back into the DC bus; extending the decel ramp time reduces the rate of energy return and prevents overvoltage trips.
Question 6: A step-voltage test (hi-pot) is performed on a new cable installation at 2.5 times rated voltage per NETA/IEEE standards. The leakage current increases sharply after the third voltage step. This indicates:
- Normal charging current behavior expected on long cables
- Insulation weakness or void that may lead to breakdown under sustained overvoltage (Correct answer)
- The cable exceeds its ampacity rating
- The cable shield is properly grounded
Correct answer: Insulation weakness or void that may lead to breakdown under sustained overvoltage
A sharp non-linear increase in leakage current during a step-voltage test indicates ionization within insulation voids or a weak spot that may break down under continued overstress.
Question 7: After replacing a failed protective relay on a substation feeder, the relay is put back in service without performing functional testing. During the next actual fault, the relay fails to operate. The root cause of this outcome is:
- The fault was below the relay pickup setting
- Failure to perform commissioning tests after replacement to verify correct settings and trip output wiring (Correct answer)
- The replacement relay model was more sensitive than the original
- The utility did not notify the site of the fault event
Correct answer: Failure to perform commissioning tests after replacement to verify correct settings and trip output wiring
NETA and IEEE C37 standards require functional commissioning tests including injection testing and trip circuit verification after any relay replacement to confirm settings and correct wiring.
CEP Troubleshooting & System Testing 5
During commissioning of a fire alarm system, all smoke detectors on a loop show a trouble condition immediately after installation.
The most likely cause is: