Low Impedance Path (01-25-2K)
By Mike Holt, Published in Power Quality Magazine
The National Safety Council estimates that approximately 300 people in the United States die each year as a result of an electric shock from low voltage systems (120 or 277 volt circuits). People become injured and death occurs when voltage pushes electrons through the human body, particularly through the heart. Death can occur in less than 1 second if the touch potential is as little as 50 volts and the current flow through the body is over 50 milliamperes.
To protect against electric shock, dangerous voltage on metal parts of the electrical system and the building from a line-to-ground fault must be quickly removed by opening the circuit's overcurrent protection device (trip the breaker or blow the fuse). Since death can occur in less than 1 second from ventricular fibrillation, it is critical that the overcurrent protection device open quickly.
The time it takes for an overcurrent protection device to open (clear the phase-to-ground fault and remove dangerous voltage) is inversely proportional to the magnitude of the fault current. This means that the higher the ground-fault current, the less time it will take for the overcurrent device to open and clear the fault.
Example: A 120 volt, 20 ampere circuit breaker will clear a 120 ampere line-to-ground fault in approximately 1/100th of a second (impedance of the fault path is 1 ohm, I = E/Z, I = 120 amperes), but it will take 30 seconds to clear a 40 ampere line-to-ground fault (impedance is 3 ohms, I = E/Z, I = 40 amperes). If the impedance of the fault path was 6 ohms, then only 20 amperes of fault current would flow (I = E/Z, I = 120 volts/6 ohms) and the circuit breaker would never open to remove dangerous voltage on the metal parts, Figure 1. Graphic not posted on internet.
REMINDER: Dangerous voltage from line-to-case faults cannot be removed by grounding the energized metal parts to the earth! This is because the earth is a poor conductor whose resistivity is around one billion times that of copper [IEEE Std. 142 Section 2.2.8] and the earth does not permit sufficient fault current to flow back to the power supply to open the circuit overcurrent protection device, Figure 2.
Low Impedance Fault Path
As we can see, the impedance of the fault current path plays a critical and vital role in removing dangerous voltage from metal parts by opening the circuit overcurrent protection device in less than 1 second. To open the circuit overcurrent protection device in less than 1 second, the fault current path must have sufficiently low impedance to allow the line-to-ground fault current to rise to a value of at least 5 times (some books recommend 2x and others 10x) the rating of the overcurrent protection device.
The impedance of the fault current path must be permanent and electrically continuous, capable of safely carrying the maximum fault likely to be imposed on it, and it must have sufficiently low impedance to facilitate the operation of overcurrent devices under fault conditions [110-10, 250-2(d)]. This is accomplished by bonding the metal parts of the electrical system together [250-90] and to the power supply system grounded (neutral) conductor in accordance with Section 250-24 for service equipment and Section 250-30(b) for separately derived systems, Figure 3.
Example: A 100 ampere disconnect is located 200 feet from the electrical system, has a feeder with No. 3 THHN for the ungrounded conductors and a No. 8 for the low impedance fault current path [250-122]. If a line-to-ground fault occurs at the disconnecting means, the fault current would rise to a value of approximately 583 amperes (I = E/Z, I = 120 volts/0.206 ohms), Figure 4.
Result: The 100 ampere circuit protection device should open within 1 second, thereby removing dangerous voltage from the metal parts of the electrical system.
The requirements for creating and maintaining the low impedance fault current path are contained in the following NEC rules.
Section 250-96(a) General Requirements. All metal parts that serve as the low impedance fault current path (such as raceways, equipment and enclosures) must be effectively bonded together to assure electrical continuity [300-10] and nonconductive coatings such as paint, enamel, tarnish, etc., on contact points and surfaces must be removed [250-12]. The standard practice of driving a locknut tight is considered sufficient in removing paint and other nonconductive finishes to assure proper electrical continuity.
CAUTION: For the overcurrent protection device to operate properly (clear the fault), the low impedance ground fault path must have sufficient capacity to conduct safely any fault current likely to be imposed on it [110-10 and 250-122]. Reducing washers (donuts) are not listed for this purpose; therefore, bonding jumpers must be installed around this high impedance path, Figure 5.
Section 250-118 Low Impedance Fault Path. The low impedance fault current path can consist of any of the following: Figure 6,
(2) Rigid Metal Conduit
(3) Intermediate Metal Conduit
(4) Electrical Metallic Tubing
(6) Flexible Metal Conduit [350-14]**
(8) Liquidtight Flexible Metal Conduit ***
(9) Armored Cable
(10) Mineral Insulated Cable
(11) Metal Clad Cable
(12) Cable Trays
(14) Other metal raceways
* When the low impedance fault current path consists of a conductor, it can be solid, stranded, bare, covered or insulated and it is sized in accordance with Section 250-122. In addition, the conductor must be installed within the same raceway, cable or trench with the other circuit conductors to ensure a low impedance fault current path [300-3(b), 300-5(i), 300-20(a)].
** In non-hazardous locations, if the ground return path of the flex does not exceed 6 feet, and the circuit conductors are protected by overcurrent protection devices rated 20 ampere or less.
*** In non-hazardous locations, if the ground return path of the liquidtight does not exceed 6 feet for 3/8 inch and 1/2 inch liquidtight contains conductors protected by a protection device rated 20 ampere or less, or 3/4 inch through 1 1/4 inch liquidtight contains conductors protected by a 60 ampere or less protection device.
Section 250-102 Bonding Jumper. Where bonding jumpers are necessary to maintain the low impedance ground fault path, they shall:
(a) Material. Be of copper or aluminum, or other corrosive-resistant material.
(b) Attachment. Terminate by exothermic welding, listed pressure connectors, listed clamps, or other listed means and be accessible [250-8].
(d) Size. Sized to Table 250-122 to the largest overcurrent protection device for any circuit contained in the raceways, Figure 7.
(e) Flexible Raceways. Bonding jumper can be outside the flexible raceway if the jumper is routed with the raceway and it does not exceed 6 feet in length.
Author's Comment: The NEC has specific bonding requirements to maintain the low impedance path for:
- Service equipment and raceways [250-92].
- Enclosures and raceways containing circuits over 250 volts [250-97].
- Raceways in hazardous (classified) locations [250-100].