NEC Questions and Answers based on 2017 NEC - December 2018

Figure 01

Here's the follow-up to yesterday's newsletter.
This includes the answers to the questions sent, so you can see how you did.

Note: The answers to these questions are based on the 2017 NEC. Underlined text indicates a change in the rule for the 2017 NEC.

Q1. According to the NEC, what is a supply side bonding jumper (SSBJ)?
A1. The SSBJ is the conductor on the supply side of the service or separately derived system overcurrent protection device that ensures electrical conductivity between metal parts and the grounded conductor [250.2].

Q2. What is the reason stated in the NEC for connecting electrical systems to the earth (grounding)?
A2. (A) Solidly Grounded Systems.
Electrical power systems are grounded (connected to the earth) to limit the voltage induced by lightning, line surges, or unintentional contact by higher voltage lines and stabilize the system voltage to earth under normal operation [250.4(A)(1)].

Author’s Comment:
• System grounding helps reduce fires in buildings as well as voltage stress on electrical insulation, thereby ensuring longer insulation life for motors, transformers, and other system components.

Note 1: To limit imposed voltage, the grounding electrode conductors shouldn’t be any longer than necessary and unnecessary bends and loops should be avoided.

Note 2: See NFPA 780, Standard for the Installation of Lightning Protection Systems for grounding and bonding of lightning protection systems.

Q3. What is the Code rule regarding the bonding of electrically conductive materials and other equipment?
A3. Bonding of electrical equipment is covered by 250.4(A)(2), (3), and (4). Metal parts of electrical equipment are grounded to reduce arcing within the buildings/structures from induced voltage from indirect lightning strikes.

DANGER: Failure to ground metal parts to earth can result in induced voltage on metal parts from an indirect lightning strike seeking a path to the earth within the building—possibly resulting in a fire and/or electric shock from a side flash.

Author’s Comment:
• Grounding metal parts helps drain off static electricity charges before flashover potential is reached. Static grounding is often used in areas where the discharge (arcing) of the voltage buildup (static) can cause dangerous or undesirable conditions [500.4 Note 3].

Metal parts of electrical raceways, cables, enclosures, and equipment must be connected to the supply source via an effective ground fault current path [250.4(A)(3)]. See 250.4(A)(5).

Author’s Comment:
• To quickly remove dangerous voltage on metal parts from a ground fault, the effective ground fault current path must have sufficiently low impedance to the source so fault current will quickly rise to a level that will open the branch circuit overcurrent protection device.
• The time it takes for an overcurrent protection device to open is dependent on the magnitude of the fault current. A higher fault current value will result in a shorter clearing time for the overcurrent protection device. For example, a 20A overcurrent protection device with an overload of 40A (two times the 20A rating) takes 25 to 150 seconds to open. The same device at 100A (five times the 20A rating) trips in 5 to 20 seconds.

Electrically conductive materials likely to become energized, such as metal water piping systems, metal sprinkler piping, metal gas piping, and other metal piping systems, as well as exposed structural steel members, must be connected to the supply source via an effective ground fault current path.

Author’s Comment:
• The phrase “likely to become energized” is subject to interpretation by the authority having jurisdiction.

Q4. What does the Code mean by an “Effective Ground-Fault Current Path”?
A4. The effective ground-fault current path is made by bonding together the metal parts of electrical raceways, cables, enclosures, and equipment together and to the supply source in a manner that creates a low impedance path for ground fault current that facilitates the operation of the circuit overcurrent protection device [250.4(A)(5)]. 

Author’s Comment:
• To ensure a low impedance ground fault current path, all circuit conductors must be grouped together in the same raceway, cable, or trench [300.3(B), 300.5(I), and 300.20(A)].

Because the earth isn’t a low impedance path for fault current, it isn’t suitable to serve as the required effective ground fault current path, therefore an equipment grounding conductor of a type recognized in 250.118 is required to be installed with all circuits.

Example: What’s the maximum fault current that can flow through the earth to the power supply from a 120V ground fault to metal parts of a light pole without an equipment grounding conductor that’s grounded (connected to the earth) via a rod having a contact resistance to the earth of 25 ohms?
Solution:
I = E/R
I = 120V/25 ohms
I = 4.80A
Answer: 4.80A

DANGER: Because the contact resistance of an electrode to the earth is so high, very little fault current returns to the power supply if the earth is the only fault current return path.Figure 01
Result—the circuit overcurrent protection device won’t open and all metal parts associated with the electrical installation, metal piping, and structural building steel will become and remain energized.

Q5. What are the Code permitted methods for the termination of grounding and bonding conductors?
A5. (A) Permitted Methods [250.8(A) & (B)].
Equipment grounding conductors, grounding electrode conductors, and bonding jumpers must terminate in one or more of the following methods:
(1) Listed pressure connectors
(2) Terminal bars
(3) Pressure connectors listed for grounding and bonding
(4) Exothermic welding
(5) Machine screws that engage at least two threads or are secured with a nut (6) Self-tapping machine screws that engage at least two threads
(7) Connections that are part of a listed assembly
(8) Other listed means

(B) Methods Not Permitted. Connection devices or fittings that depend solely on solder aren’t allowed.