I thought I would touch on this, because I have had to deal with this problem countless times, and not just with fire alarm system installations, but with other security, electrical, or data center infrastructure hardware/equipment. Even more interesting, troubleshooting and fixing them can be as simple as pie or as difficult as finding a needle in a haystack, depending upon the severity of the problem and the size of the system or cable-plant.
In layman's terms, here are some electrical "basics":
1) Every "direct current" (DC) source (ex: batteries) has a "positive" (+) and a "negative" (-) connection.
2) Most "alternating current" (AC) sources (ex: a standard single-phase 120VAC wall outlet) have a "hot" (~) and a "neutral" (~) connection.
3) Electricity wants to move from it's source (positive or hot), through some type of circuit, and eventually to what we call "ground" (negative or neutral).
4) Usually, every modern-day or properly designed circuit (DC or AC) also has an alternate "earth ground" connection (the 2nd or "back-up" path to ground).
So, what is a "Ground-Fault"?
A ground-fault occurs when there is an issue or problem with the circuit, equipment, and/or wiring that causes the 2nd or back-up "earth-ground" path/connection to become fully or partially "energized", draining some or all of the moving electricity to ground. Grounding problems may also occur due to 1) unwanted multiple paths to ground or "ground loops", 2) resistance to ground, or even 3) complete loss of the path to ground.
There are two types of ground-faults: a "positive" ground-fault and the more common "negative" ground fault.
1) a positive ground-fault occurs when a positive or hot connection, or any part of the energized circuit, makes contact with earth ground prematurely. This kind of ground-fault will usually disrupt the electrical flow of the circuit causing a short, and usually will trip a circuit breaker or fuse, ...but not always!
2) a negative ground-fault occurs when a negative or neutral connection after the energized circuit makes contact with earth ground prematurely. This kind of ground fault usually does not disrupt the circuit's electrical flow and can go "undetected" in most cases. It can also be more difficult to trace, find, and fix, even with a TDR, especially with hundreds of feet of wiring going through cable tray or conduit.
Better (and newer) equipment is usually designed to alert you to any problems within its infrastructure. Other equipment (legacy) may not have any provision in its circuitry to let you know otherwise. When I was at Interland (now Peer1) back in 2003, one of our Liebert HVAC units in the South wing of our Atlanta Data Center shut down and stopped running, and I couldn't get it to start back up. I called our usual service technician to come out and fix the unit. To my surprise, I later found our tech still working on the unit after several hours, unable to find the problem. Eventually, the problem was traced to a thin 24-gauge wire running down the left-side of the unit through small holes in the framework. What did we find? The insulation on the wire going thru one of the holes had worn-away over time due to the unit's vibrations, and the "exposed" copper of the wire was touching one side of the hole on the unit's metal chassis. A "Positive Ground-Fault", undetected by the logic circuit and controller (LCD screen) of the Liebert unit.
Even though the data center industry as a whole is a big proponent and user of Liebert products, one has to ask this question: "Why in the world did a world-reknowned manufacturer of a $100,000 HVAC/CRAC system NOT install simple 5-cent rubber grommets in the holes in the steel framework to prevent this from happening?"
With "high-voltage" circuitry, the issue becomes more critical, as the power for countless servers and equipment can be affected. Usually a good electrical monitoring system (stand-alone or part of a power distribution or UPS system) will alert you to any problems, including ground faults.
However, for older buildings & electrical cable installations incorporating legacy infrastructure, life all of a sudden becomes "much more fun" (I am being sarcastic). The typical picture (especially in the 90's) is one of multiple electrical contractors & vendors being called upon to work together and combine their electrical designs & equipment to form the final working infrastructure in a data center that was "supposed" to have (at that time) CO class grounding specifications in place or better. The real story is, I have seen enough unbelievable infractions and flaws in a data center electrical plant that would make any "aware" individual scared to work in one.
Even with grounding in place, the presence of even partially-energized negative ground-faults in a data center electrical plant, especially uncorrected for prolonged periods of time, can be extremely dangerous & hazardous, creating the potential for shock, electrocution, and even flash-burns for humans, as well as under/over voltage & current issues, harmonic distortion, and phase problems that all have the potential for destroying or great reducing the life of servers and equipment. EX: At NetRail's Atlanta facility, the initial presence of multiple paths to earth ground (building steel) utilizing insufficient gauge wiring and bonding created a partial potential in certain areas of the datacenter, which was discovered by our master electrician and myself when each of us got shocked by gripping on to the overhead runway and inadvertantly brushing-up against the chassis of a Chloride UPS. Keep in mind, the consequences COULD have been worse. The path(s) to "earth" ground for high-voltage installations must be reviewed and maintained properly and in accordance with all manufacturer's specifications and electrical & building codes.
TESTING FOR GROUND FAULTS:
Use a standard digital multimeter to measure any sign of an unwanted "potential" (voltage) between two points. These are usually source to chassis/box/conduit or negative/neutral wiring, and chassis/box/conduit or negative/neutral wiring to earth ground. Also remember that a potential other than the norm also translates into a variance in current and/or resistance (remember ohm's law), so you may want to check for resistance readings that are abnormal as well. You may also need to disconnect wiring from circuit boards or devices to do your measurements, if possible. WARNING: High-current/high-voltage measurements should only be conducted by a licensed electrician or other trained/certified individuals who have the proper knowledge, equipment, and protective gear to do so. Do NOT test circuits higher than 120/240VAC 40VA with a standard digital multimeter.
All comments are welcome. Feel free to expand or say anything that you would like to share in this discussion....
--- Chuck Cohen ---