ELECTRICIANS USE megohmmeters to perform insulation-resistance tests on equipment such as motor circuits, motor windings and coils, and high-power cables. These tests are often called "megger tests." The main value of an insulation-resistance test lies in the charting of data from periodic tests, corrected for temperature, over the life of the equipment. This helps detect any deteriorative trends. The test objective is to measure the total resistance between any two points separated by insulation.
Leakage currents from cables that carry high voltages can prove very dangerous. But typically, fire alarm systems use power-limited fire alarm circuits that do not present such a hazard. And, megohm insulation testing will only detect faults to ground on such circuits.
According to the Electric Power Research Institute (EPRI), electrical faults cause 47 percent of motor failures. No similar statistics exist for fire alarm circuits. The "assumed problem" for fire alarm circuits is a myth.
In spite of this, some designers and certain authorities having jurisdiction require contractors to perform megger testing on fire alarm circuits based on hearsay or opinion rather than facts or Code requirements.
Only section 9.7, Public Fire Alarm Reporting Systems, contains requirements for megohm testing in NFPA 72
Inquiries to the Electrical Apparatus Service Association, Inc. (EASA) reveal that no reported issues exist with regard to fire alarm circuitry that would warrant megohm testing of fire alarm wiring.
The obvious concern of those in our industry requiring megohm testing of fire alarm circuits is that fire alarm cables can suffer damage after being pulled into raceway or unprotected across ceilings. Those specifying megohm testing apparently feel that the resulting grounds or short circuit faults could cause safety issues or unreliable operation of the fire alarm system. As stated previously, there are no reported injuries of technicians working on fire alarm system circuits. Because most if not all of the systems produced today are power limited, the safety concern seems unwarranted.
In addition, given the self-diagnostics provided with most of the fire alarm system control units manufactured today, the fire alarm control unit (FACU) will undoubtedly provide the necessary cable fault informationground faults, short circuit faults, or open circuit conditions. Then, if a problem becomes apparent, the technician can use megohm testing as a tool to test the suspected circuits.
Depending on the cable rating, a 500v megger can only be used if the manufacturer suggests it and the cable is rated at 600v to 1000v. The manufacturer should provide the acceptable test value to use. For 300v cable or lower, a technician could perform a dc. test. Without careful application and without a genuine reason for conducting megohm tests, this kind of testing seems inappropriate and may indeed damage equipment and cable if performed incorrectly.
But, again, no safety or reliability need appears to justify the routine megohm testing of fire alarm circuits.
NFPA 70B-2006 section 21.5.2 states in part that "It should be recognized that, as the name implies, over-potential or high-potential testing is intended to stress the insulation structure above that of normal system voltage. The purpose of the test is to establish the integrity of the insulation to withstand voltage transients associated with switching and lightning surges and hence reduce the probability of in-service equipment failures. Direct voltage over-potential testing is generally considered a controlled, nondestructive test in that an experienced operator, utilizing a suitable test set, can often detect marginal insulation from the behavior of measured current. It is therefore possible, in many cases, to detect questionable insulation and plan for replacement without actually breaking it down under test. Unfortunately, some insulations might break down with no warning. Plans for coping with this possibility should be included in the test schedule."
As a cautionary note for those required to perform megohm testing on fire alarm circuits, do not test circuits with any devices such as smoke detectors or control equipment (monitor modules, control modules, etc.) connected, as the high voltages will destroy all connected electronic equipment.
It would seem that requiring megohm testing of fire alarm circuits is a little like requiring a sledgehammer to drive a thumb tack.
Acceptable methods exist to test the fire alarm wiring other than performing a megohm test. These alternatives will provide continuity information to the installer to ensure that nothing has damaged the wiring during installation. These alternate methods include using a high-quality volt/ohmmeter to test continuity and measurements to ground.
No one has reported fire alarm circuit safety issues. In many cases, the FACU can find most, if not all, wiring faults. If necessary, a simple volt/ohm meter testing can find other troublesome faults. On this basis, it would appear that no one can really justify the expense or need of megohm testing.
The reality of megohm testing is that unless the engineer or the authority having jurisdiction feels extenuating circumstances exist, there is no need to perform megohm testing of installed fire alarm wiring.
Source: NFPA Journal