There are some questions I find myself answering every time I teach a course for electrical journeymen and home inspectors. I’ve collected the questions, answers and explanations here with the hope that my fellow home inspectors will develop a better understanding of some of the issues we identify during a home inspection and why we do so. During the article review process, the ASHI Technical Committee provided comments, which also appear here.
Q. Why is it important to “float” the neutral in a sub panel?
Answer: If the neutral conductor is not “floated,” and instead is connected to the bonding system at a sub panel, there will be objectionable currents imposed on the bonding system. These currents can cause injury, death or fire. (“Floating neutral” means there is no electrical connection between the neutral bar and the bonding system; the neutral bar must be mounted on non-conductive spacers.)
Explanation: The neutral conductor carries current back to the source (utility). When the neutral is connected to the bonding system in a sub panel, the bonding system becomes a parallel path with the neutral and shares the return current. Neutral currents will flow on the metal parts of the system and/or the equipment bonding conductor. When the current flows in parallel paths, there may not be sufficient impedance (total resistance of the path) to clear a ground fault. (Not to be confused with a ground fault as relates to a GFCI, this ground fault is an accidental connection between a hot wire and the bonded metal parts of an electrical system.)
If metallic conduit is used as the equipment bonding system and the fittings are loose, heat will be generated, possibly leading to a fire hazard. Anyone working on the system runs the risk of being shocked if the bonding conductors or conduits are disconnected, or, if the person makes contact simultaneously with a grounded object and the conduit or bonding conductor. Making contact with a grounded object provides another parallel path through the person.
The neutral conductor should be connected only to the equipment bonding or grounding system at the first means of disconnect and supply-side metering equipment. This connection is made with a bond screw, strap or jumper. A neutral/ground/bond connection downstream from the service disconnect will induce objectionable currents on the equipment bonding system and create an unsafe condition. Sub panels are to be connected with a separate bonding conductor or
a solidly connected metal piping system to provide a low-impedance ground fault current path and all equipment bonding conductors are to be separate from the neutral.
Q. Why is a false ground unsafe at a three-wire (grounded) receptacle outlet?
Answer: The grounding prong of a grounded receptacle outlet should be connected to a separate grounding conductor or grounded raceway system.
Explanation: Sometimes this situation is found when a grounded receptacle is installed in an ungrounded two-wire system. When a false ground (a jumper from the neutral to the ground terminal at an outlet) is made, the grounding prong becomes a parallel path with the neutral. The grounding conductor of the appliance cord is connected to the metal frame of the appliance, which lays out the path from the grounded prong through the cord to the frame. The metal frame becomes a parallel path with the neutral. Any contact between the frame of the appliance and a grounded object can result in current flow. If the contact point is a person, then there is potential for fatal shock.
Q. Why is it dangerous to use a Nonmetallic-Sheathed cable (NM) grounding conductor as a neutral?
Answer: Using the grounding conductor as the neutral creates an ungrounded circuit. There is no grounding conductor available for clearing ground faults.
Explanation: Sometimes this is found at retrofit ceiling fan installations or where an outlet has been added at a switch box. Also, it can be found at well pumps and AC disconnects where one leg of a 240-volt circuit is tapped and the grounding conductor is used as a neutral to run a light or 120-volt receptacle. The frame of the fan, yoke of the device and metal box are not grounded (bonded). Just as with the false ground, any contact between these parts and a grounded object will result in current flow. Anyone working on the circuit would not expect neutral current to be flowing on the grounding conductor and could get shocked or electrocuted if he or she disconnected it. In the event of a ground fault on the metallic parts of the connected equipment, there would be no way to clear the fault and the metallic portions of the equipment would become and remain energized, creating a very hazardous condition.
Q. Is it legal to install grounded receptacle outlets on a two wire ungrounded system?
Answer: The National Electrical Code (NEC) allows the installation of grounded receptacles on a two-wire system if a Ground Fault Circuit Interrupter (GFCI) is installed at the beginning of the circuit, either as a breaker or a feed-through receptacle at the first receptacle in the circuit. All receptacle outlets installed downstream must have a label stating that they are GFCI-protected and that there is no equipment ground.
Explanation: A GFCI does not need an equipment ground to function and provide protection. The person using the outlet will be protected, but the equipment will not be grounded. The GFCI will trip if there is an imbalance between the hot and neutral return currents, thereby protecting the circuit.
Q. Why can’t a separate ground wire from a water pipe to the outlet box be run in an ungrounded system to allow installation of a grounded receptacle outlet?
Answer: The NEC has strict rules for ground wires where they can be connected and how. Installing a separate ground wire may be possible if all conditions are met.
Explanation: The NEC states that the equipment grounding conductor of a grounding-type receptacle outlet shall be connected to:
- Any accessible point on the grounding electrode system.
- Any accessible point on the grounding electrode conductor.
- The equipment grounding terminal bar in the enclosure where the outlet branch circuit originates.
- For grounded systems, the grounded service conductor within the service equipment enclosure.
- For ungrounded systems, the grounding terminal bar within the service equipment enclosure.
Summary: When inspecting any electrical system, we must be aware of the type of wiring method. An older metallic flexible cable (BX) or conduit system may or may not have a low-impedance path sufficient to clear ground faults; it depends on how tight the fittings are and how the system was installed. BX fittings may not be rated for grounding. A two-wire non-metallic cable system will not have an equipment-grounding conductor.
When we inspect a two-wire system that has grounded receptacle outlets, we have to determine what, if any, equipment-grounding system exists. In a two wire, non-metallic cable system, any receptacle outlet that indicates a ground when inspected is suspect. A metallic conduit system may have a low-impedance grounding system. The adequacy of the system will not be known unless the system is tested. The three-prong testers, commonly used by inspectors, will not indicate the impedance of the grounding system. They will indicate the presence of a ground, but not the adequacy of the grounding system.
Grounding and bonding of an electrical system is very important, and separation of the neutral and bonding systems is just as important. It is up to each home inspector to learn and understand these systems so he or she can determine if there are areas of concern to the inspector and the client. Through education and experience, a home inspector will be able to identify the conditions and make sound recommendations to the client.
From the ASHI Technical Committee
During the review and editing process, members of ASHI’s Technical Committee often comment or expand on what is covered in an article. The committee provided the following comments for this article.
Q. Why is it important to “float” the neutral in a sub panel?
Metal piping systems are considered a good bond if the following requirements are met:
No concentric (combination rings) ‘knock outs’ or KOs (a removable piece of a box or panel that can be “knocked out” to allow for the entry of cables) are used to terminate the conduit in the box.
All paint has been removed from the surrounding the connection.
If concentric KOs are used in the box, a grounding bushing with a tail terminating in the box in a grounding lug should be used to ensure a good bond.
Q: Why is a false ground unsafe at a three-wire (grounded) receptacle outlet?
The connected neutral/ground of the appliance will normally carry current equal to that of the “hot” when the appliance is in use; however, if the frame is connected to the neutral, there will be “zero potential” between the neutral and the frame unless a person makes a better path for the electricity to flow back to the source.
Q: Why is it dangerous to use a Nonmetallic-Sheathed cable (NM) grounding conductor as a neutral?
In the event that a grounding conductor being used as a neutral is disconnected and the appliance is in the “on” position, a person making contact with the ground will get shocked or electrocuted if he/she makes contact with a grounded surface.
Note 1: When the neutral is connected to the bonding system in the subpanel, the bonding system does become a parallel path with the neutral; however, if the resistance of the neutral is lower than the resistance of the bonding system, most return current will flow on the neutral. There will be some current flowing on the bond system.
Note 2: Typically, any current flow to ground when protected by a GFCI circuit will trip the circuit when the current flow exceeds the rating of the GFCI device, usually in excess of 5 Ma. Resistance of the bonding circuit only plays an important role when a different path of current flow is less than that of the bonding circuit, e.g., a secondary source for the path of the fault. Conclusion: When there is a ground fault, the GFCI will trip.
2A. Industry standard for low or acceptable “low impedance” (resistance) is 25Ω (ohms) or less. When there is a “hot” or live conductor that is in contact with a ground or grounded conductor, and the current flowing exceeds the limit of the fuse or circuit breaker, the fuse or circuit breaker will open (blow), stopping the flow of electricity.