Electrical Safety: Understanding Grounding as a Fire Safety Necessity

Improper grounding of electrical equipment may result in conditions that could become a source of ignition, and ultimately lead to fire loss. There are many misconceptions about grounding of electrical equipment. Although many people think of it only in terms of reducing the hazard of electrical shock, that is not the only function an equipment grounding system performs. A properly installed system will allow the overcurrent devices, whether they are fuses or circuit breakers, to open in time to prevent excessive heat buildup and other damage to the equipment. If the grounding system is not properly installed, significant damage may occur before the overcurrent devices operate, if they operate at all. The equipment grounding system must provide a path for the ground-fault currents to return to the source. This ground-fault return path is then part of the circuit and provides a means to open the overcurrent device.

Short Circuits and Ground Faults Often Confused

Although electricians, fire investigators, and inspectors often incorrectly identify all insulation failures as short circuits, many of these so-called "shorts" are actually ground faults. Short circuits are best defined as insulation failures between two circuit conductors, whereas a ground fault is a failure between one conductor of the circuit and a grounded surface.

Short circuits are the less common of the two types of failure, since two insulation failures must take place. The overcurrent device protecting the circuit will open the circuit quickly when a short circuit occurs. Normally, short circuits produce very little damage beyond that found at the point where the actual short circuit occurred.

Ground faults, which require only a single insulation failure, are far more common. They can be much more destructive than short circuits, especially when equipment grounding is not properly installed. A poor ground return path can result in an arcing ground fault that may persist for a considerable length of time. Arcing faults cause excessive heating, and particles of hot metal may be expelled from the arc. Both the heat and the hot metal particles may ignite combustibles. Heating during ground-fault conditions may also cause insulation on other conductors to soften and fail. A true short circuit may then occur, and the overcurrent device will open the circuit.

A fire may be well advanced before the secondary short circuit occurs. The fire investigator may point to the shorted conductors as the source of ignition when the cause of the fire was actually a ground fault. If the short circuit had developed earlier, the fuse of the circuit breaker might have opened in time to prevent the fire.

Point of Fire Origin Improperly Identified

Another scenario that may result in fire occurs where arcing and sparking or heating take place at some point on the system other than the site of the actual ground fault — for example, at loose joints or connections in metal raceways, or where metal cable armor is not properly terminated. This damage could occur some distance from the actual ground fault. Misleading evidence of arcing at a loose connection may cause the fire investigator to incorrectly identify the site as the true location of the first fault.

Even in connections that appear to be tight, there may be excessive heating due to high resistance across the connections. This high-resistance connection could be the result of corrosion or could occur at a point where paint has been left on conducting surfaces. These hot spots may be a source of ignition, or they may cause additional ground faults or short circuits to develop. The fire investigator may identify them as the point of fire origin, and the fire cause as an electrical failure in that area. He is correct, but the true cause of the fire was a ground fault that the investigator did not discover. The same type of arcing and heating may also be found at points of casual contact between a poorly grounded raceway and other grounded metal surfaces.

What causes all this arcing and heating in a raceway that seems to be well-grounded? The answer is poor grounding -- a ground return path that does not satisfy Section 250-51 of the National Electrical Code ^® (NEC). This section of the NEC requires the ground path to meet three conditions: It must be permanent and continuous; it must have the capacity to safely conduct any fault current likely to be imposed on it; and it must have sufficiently low impedance to limit the voltage to ground, and to facilitate the operation of the protective devices.

Many of these same words could be used to define a circuit. A circuit should be permanent and continuous, have adequate capacity, and be of low impedance. The ground return path provides one side of a circuit during ground-fault conditions. Under fault conditions, the ground-fault return path must carry the current in the circuit long enough to clear the overcurrent device protecting the circuit. The time involved may be a fraction of a cycle or as long as several seconds, depending on the characteristics of the overcurrent device.

Ground-Fault Return Path Requires Proper Attention

All the good installation practices applied to a normal circuit must also be applied to the ground-fault return path, since all the rules of basic electricity that affect a normal circuit also affect the ground-fault return path for the short period that it is required to act as part of the circuit. Even installers who take extreme care with each connecion of the circuit conductors may fail to recognize the need for the same attention when they work with the ground return path. When separate grounding conductors are used, each connection must be given the same attention that is given to the circuit conductors. If raceways and enclosures are used for the ground return, couplings and connectors must be made tight. Bonding jumpers may be required around knock-outs and some types of fittings.

In some cases, we may also fail to observe the basic rules of electricity and their effect on the ground return path when it performs its function as a circuit. A typical case occurs where electrical equipment is mounted on, and in good contact with, building steel. Since the electrical service is grounded and bonded to the building steel, isn't the equipment mounted on the building frame grounded, too? And isn't there a ground return path through the building steel that could replace normal grounding?

The answer to the first question is Yes, the equipment is grounded, but the ground return path does not satisfy NEC Section 250-51 because of the high-impedance return path. The second question is covered by NEC Section 250-58(a), which prohibits the use of the building steel as the ground return path.

Again, we must refer to basic electricity rules to find the reason. Section 300-3(b) requires all conductors of a circuit to be run together. Remember that the equipment grounding is part of the circuit under ground-fault conditions. Because of the magnetic fields around AC conductors, it is necessary that all the conductors be kept together. This allows for mutual cancelling of the fields. When current flows in a single isolated conductor, the magnetic field is not cancelled and the impedance of the conductor is increased. The greater the distance between conductors in the circuit, the greater the impedance. This phenomenon must be considered if we attempt to rely on the building steel as a ground return.

Under ground-fault conditions, the ground return path through the building frame is not in close proximity to the conductor delivering energy into the fault. The greater the distance, the greater the impedance, and as a result, Section 250-51 is not complied with. In this case, we do not have a low-impedance ground return path, and the fuse or circuit breaker may not open. While the building steel may provide a ground, it does not provide a low-impedance ground return path.

The installer should understand how the equipment grounding system functions as part of a circuit under fault conditions. The ground path must comply with Section 250-51. To achieve proper grounding, the installer must be aware of the effects of the basic laws of electricity on the circuit.

The fire investigator should understand the basic requirements for equipment grounding and learn to recognize conditions that may lead to a high-impedance return path. The investigator should also understand that ground faults can produce damage that, if misread, can lead to the conclusion that a short circuit caused the fire. By understanding high-impedance return paths, he may be able to pinpoint the exact cause and provide more accurate fire investigation reports.

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Dann Strube, Fire Journal 8, no. 1 (January/February 1989).

Photo 1 courtesy of IAEI; Photo 2 courtesy of Mark R. Hilbert.