The Right and Wrong Way (06-25-01)
By Mike Holt, www.mikeholt.com
What's a neutral?The IEEE defines a Neutral as the conductor that has an equal potential difference between it and all other conductors (hot wires). An example of a neutral wire would be the white/gray wire of a single-phase 120/240 volt system or a three-phase 4-wire Wye system, Figure 1-1. However, the white/gray wire of a 4-wire, Delta system is not a neutral conductor because the potential difference between each of the ungrounded conductors to the white wire are not all the same, Figure 1-2.
Note: Graphics not contained on the Internet. For more info, order my grounding book/video http://www.mikeholt.com/catalog/necpro.htm or 1-888 NEC Code.The proper term for the white/gray wire is the "Grounded Conductor." It gets its name from the fact that at the electric service, transformer or other separately derived location, the white or gray wire is connected to the earth (grounded), Figure 1-3.
The rules for the use and identification of the grounded (neutral) conductor are contained in Article 200 of the National Electrical Code. Section 200-6 specifies that the grounded conductor must be identified with the color white or gray, Figure 1-4.
Author's Comment: When you call the white/gray wire a neutral, and it's not, then you are using the term "neutral" incorrectly. Most people do this and it's a generally accepted practice, but it's technically not the correct term to use under all conditions.
What's the purpose of the grounded conductor (neutral wire)?The purpose of the grounded (neutral) conductor is to permit line-to-neutral loads, such as 120 and 277 volt circuits and it serves as a current-carrying conductor to carry return (neutral) current, Figure 1-5.
In addition, a safety ground wire is provided for electric services (meter locations), therefore the grounded (neutral) conductor is intended to provide a low-impedance ground-fault return path to clear ground-faults, Figure 1-6.
Why does the NEC require neutral-to-case connections?
Neutral-to-case connections are required by the NEC to provide a low impedance path to open the circuit overcurrent protection device and ensure that dangerous voltage on metal parts is quickly removed. To open the circuit overcurrent protection device, the ground-fault path must have sufficiently low impedance to permit ground-fault currents to reach a level at least five times (preferably 10 times) the overcurrent protection device's rating. This is accomplished by bonding metal parts to each other and then bonding the neutral to the metal parts (neutral-to-case) at the source of power, Figure 1-1.
Author's Comment: The time it takes for a circuit breaker and fuses to clear the fault (open) is inversely related to the magnitude of the current. This means that the greater the current magnitude, the quicker the overcurrent device will open. The following values are examples of the time it takes for a 20A circuit breaker to open: Figure 1-2.
Fault Current Level Time to Open
25A 500 seconds
40A 30 seconds
125A 0.002 seconds.
Where are neutral-to-case connections required by the NEC?
Neutral-to-case connections are required by the National Electrical Code at the Service Disconnecting Means and at Separately Derived Systems.
Service Disconnecting Means. Because electric utilities are not required to install a safety ground wire, a neutral-to-case connection must be made at the service disconnecting means [250-23(b)] so that the neutral can serve as the safety ground wire for ground-fault current. This is accomplished by installing the main bonding jumper (a screw or strap provided by the service equipment manufacturer) from the grounded (neutral) terminal to the metal case of the service equipment (main disconnecting means), Figure 1-3.
DANGER: The accepted practice of omitting the safety ground for service conductors and using the neutral-to-case connection at the electric service is very dangerous, because if the service grounded (neutral) conductor is open we lose our safety ground. This can cause all metal parts of the electrical system and the building to become energized providing the potential for electric shock as well as fires, Figure 1-4.
Transformers or Other Separately Derived Systems. A neutral-to-case connection is required at separately derived systems, such as transformers or generators located in buildings [250-26(a)]. This is accomplished by bonding the neutral terminal of the derived system to the safety ground. This is done at transformers by installing a bonding jumper from the "X0 terminal" (neutral) to the transformer case lug, or at the panel on the load side (secondary) of the transformer, Figure 5.In no case can the neutral-to-case connection be made at both locations because this would permit neutral currents to flow onto the safety ground path, Figure 1-6.
Author's Comment: If no transformer or other separately derived system exists in a building, then the only required neutral-to-case connection is at the electric service (meter location), as indicated in Section 250-23.
Does the NEC prohibit neutral-to-case connections?The National Electrical Code contains the following rules that prohibit neutral-to-case connections:
Objectionable Current - The NEC prohibits neutral current to flow on equipment grounding conductors [250-21(a), 250-6(a)], Figure 1-1.
Load Side of Service - A grounding connection shall not be made to any grounded (neutral) conductor on the load side of the service disconnecting means [250-23(a), 250-24(a)(5].
Separate Buildings - A grounded (neutral) conductor shall not be connected to equipment grounding conductors at separate building disconnecting means [250-32(b)(1)].
Author's Comment: The NEC in Section 250-61 [250-142] specifies that neutral-to-case connections are permitted within the service disconnect [250-23(b), 250-24(b)], the separate building disconnect [250-24(a) Exception No. 2, 250-32(b)(1)] and at separately derived systems [250-26(a), 250-30(a)(1)].
What are the dangers associated with neutral-to-case connections?A neutral-to-case connection at other than a separately derived system (e.g., transformer or generator) creates a condition where neutral current and ground-fault current will flow through conductive metal parts of a building or electrical system. This current flow can cause death from electric shock, property damage from fires, as well as power quality issues.
Electric Shock. Death from electric shock occurs in a matter of minutes when electric current passes through the heart causing it to stop pumping blood to the brain (ventricular fibrillation). Ventricular fibrillation occurs when current (30 milliampere, +-10 mA) flows through the heart for approximately 1/6th of a heartbeat. When the service grounded (neutral) conductor is opened (wind storm, ice, etc.) all metal parts of the electrical system will become energized* and the system loses the ability to clear ground-faults. This dangerous voltage condition is of particular concern in building with pools, spas and hot tubs (personal experience), Figure 1-2.
*To determine the actual voltage on the metal parts you need do some fancy math calculations and a spreadsheet should be used to accommodate the variable conditions. An Excel spreadsheet is available upon request (Mike@mikeholt.com).
Electric Shock- Electric shock can occur if the service grounded (neutral) conductor is open because the required neutral-to-case connection at the service equipment [250-23(b), 250-24(b)] permits neutral current to flow onto the metal parts of the electrical system.
Example: If the service neutral is open, and a person is in series to a lamp (touches any grounded metal part of the electrical system), the voltage across the lamp (144 ohms) and the human body (2,400 ohms) will equal the voltage drop of the circuit (voltage source), Figure 1-3.The following steps explain how to calculate the voltage distribution (resistors in series).
Step 1. Calculate the circuit resistance total: Rt = R1 + R2
R1 = Lamp 144 ohms (R = E2/P), R2 = Person 2400 ohms
Rt = 144 ohms (lamp) + 2400 ohms (person)
Rt = 2544 ohms
Step 2. Calculate the circuit current: I = E/Rt
E = Voltage source 120 volts, Rt = 2544 ohms
I = 120 volts/2544 ohms
I = 0.0472 amperes, or 47.2 milliamperes
Step 3. Calculate the voltage across each of the loads: E = I x R
I = 0.0472 ampere, R = 144 ohms and 240 ohms
Lamp Voltage = .0472 amperes x 144 ohms, = 7 volts
Person Voltage = .0472 amperes x 2400 ohms, = 113 volts
Electric Shock from No Safety Ground - If the service grounded (neutral) conductor is open, the low-impedance path used to clear ground-fault current is lost. Under this condition (open service neutral), a ground-fault will not be cleared and all metal parts of the electrical system will be energized to line-voltage, Figure 1-4.
Fire. A fire is created when heat is sufficient to cause ignition. In electrical systems, heat is generated whenever current flows. The temperature rise is dependent on the square of the current flow (I) and the resistance of the material (R), as well as the duration of the current flow (I2R). A neutral-to-case connection (even if it meets the NEC requirements) can cause a fire, and sometimes an explosion, due to an electric arc if the grounded service (neutral) conductor is open.
When the service (neutral) conductor is open, neutral current flows onto the metal parts of the electrical system because a neutral-to-case connection (main bonding jumper) is required within the service disconnect enclosure [250-23(b), 250-24(b)]. When the service grounded (neutral) conductor is opened in wood frame construction, neutral current seeking a return path to the power supply travels into the moist wood members. After many years the wood is converted into charcoal (wood with no moisture) because of the heat generated from the current flow. The ignition temperature of the wood is decreased and the temperature of the wood is increased because of neutral current. A fire caused by this condition is called pyroforic-carbonization, Figure 1-5.
Author's Comment: I can't create an acceptable graphic to demonstrate how pyroforic-carbonization causes a fire by an open service neutral. However, if you would like to order a video (it's fascinating) showing actual fires caused by pyroforic-carbonization, call my office 1-888 NEC CODE.
RecommendationsTo protect against electric shock and fires from an open service neutral, an equipment grounding conductor (sized to Table 250-94, 250-66) and a separate grounded (neutral) conductor should be installed to the building service disconnecting means. At the building, the grounded (neutral) conductor and equipment grounding conductors must be separated (no neutral-to-case connection), except at the secondary side of separately derived systems.
Author's Comment: Installing a separate equipment grounding conductor and not bonding the neutral to the service equipment enclosure is a violation of NEC Section 250-23(b) [240-24(b)]. Maybe the 2002 NEC will be revised to permit this safer installation.
If a safety ground wire is installed to electric services and there is no neutral-to-case connection, an electric shock cannot occur, Figure 1-6 and 1-7.
Author's Comment: This same practice (separate neutral and ground) should be followed for feeders and branch circuits supplying power to remote buildings or structures.
Why does the NEC require a neutral-to-case connection at service equipment and permit the grounded (neutral) conductors to be used for equipment grounding when it's so dangerous?I don't know, but I think this is the way electrical services have been installed for over 100 years and the electric utilities don't want to increase their cost by having to install an additional safety grounding conductor. However, maybe someone reading this article will have the influence to change the 2002 NEC.
In the past few issues of PQ I explained the National Electrical Code requirements as well as the dangers of improper neutral-to-case connections. In this issue, we will review the power quality problems caused by improper neutral-to-case connections, particularly electromagnetic interference to sensitive electronic equipment and elevated equipment case ground voltage.
VOLTAGE BETWEEN METAL PARTS TO EARTH
Proper Neutral-to-case ConnectionWhen a neutral-to-case connection is properly made in accordance with the NEC, the voltage between the any metal part of the electrical system to the earth will be almost zero volts, Figure 1-1.
Improper Neutral-to-case ConnectionThe National Electrical Code requires a neutral-to-case connection to be made at service equipment only and that there shall not be any neutral-to-case connection on the load side of service equipment [250-23(a), 250-24(a)(5)], except as permitted in Section 250-61 [250-142].
Author's Comment: Because of confusion on proper grounding and bonding, many electricians install the main bonding jumper that is supplied with the panelboard on the load side of service equipment making an improper neutral-to-case connection (I personally did it many times).
When a neutral-to-case connection is made at the load side of service equipment in violation of the NEC, the feeder neutral current will divide and return on both the feeder's metal raceway as well as the feeder's neutral conductor (parallel path). This improper neutral-to-case connection permits neutral current to return on the metal parts of electrical equipment (i.e. metal raceways), Figure 1-2.
When neutral current (or any ac current) travels on the metal parts of electrical equipment, the electromagnetic field generated from the flow of alternating current is not able to be canceled. This uncanceled electromagnetic field can negatively impact sensitive electronic devices, Figure 1-2.
IMPORTANT: Improper neutral-to-case connections create parallel paths for neutral currents on the metal parts of the electrical system, including any metal shielding of low-voltage and limited-energy cables!
Author's Comment: There is the unproven health issue on to humans from electromagnetic fields.
Elevated Ground Voltage
When a neutral-to-case connection is made at the load side of service equipment in violation of the NEC, the voltage difference between the equipment ground and the earth will rise to equal the voltage drop of the neutral conductor at that location in the electrical system. The elevated ground voltage can be calculated by the following formula:
E(Voltage Drop) = I(Current) x R(Resistance)
Author's Comment: Today's office buildings contain large quantities of single-phase nonlinear loads such as personal computers and laser printers. These loads, when on a Wye 4-wire 3-phase system, produce odd triplen harmonic currents that add (instead of cancel) on the neutral conductor causing the neutral conductor to carry elevated neutral current. In addition, when high amperage loads such as laser printers or copiers cycle on, they cause rapid increases of neutral current which elevates the voltage on the grounding conductor.
Feeder. If a neutral-to-case connection were made at the panelboard in violation of the NEC, the feeder neutral current would divide and return on both the feeder metal raceway and the feeder neutral conductor. Under these conditions, the voltage difference between any part of the electrical system to the earth will have risen to 1.25 volts (the voltage drop of the feeder neutral), Figure 1-3.
Author's Comment: The voltage drop of the feeder neutral conductor was 3 volts in Figure 1-1, but the parallel paths for the neutral current reduces the feeder voltage drop to 1.25 volts (for the purpose of example) in Figure 1-3.
Branch Circuit.If a neutral-to-case connection were made at the receptacle in violation of the NEC, the branch circuit neutral current would divide and return on both the branch circuit metal raceway and the branch circuit neutral conductor. Under this condition, the voltage difference between any part of the electrical system to the earth would have risen to some value more than 3 volts but less than 4.5 volts.
VOLTAGE BETWEEN NEUTRAL AND GROUND
Proper Installation.In a proper neutral-to-case installation, the voltage between the neutral conductor and any metal part of the electrical system will be equal to the voltage drop of the neutral conductor in accordance with the following:
1. At service equipment, the voltage difference between the neutral conductor and the service equipment case will be 0 volts, Figure 1-4.
2. At panelboards, the voltage difference between the neutral conductor and the equipment grounding conductor (panelboard case) will be equal to the voltage drop of the feeder neutral conductor, which is 3 volts in Figure 1-4.
3. At branch circuits, the voltage difference between the neutral conductor and the equipment ground (ground contacts of the receptacle) will be equal to the voltage drop of the feeder and branch circuit neutral conductors, which is 4.5 volts (3 volts feeder and 1.5 volts branch circuit) in Figure 1-4.
Author's Comment: Computer and copier manufacturers insist (for warrantee purposes), that the voltage between the neutral and the ground contacts of the receptacle should not exceed 1 to 3 volts, and some manufacturers actually specify that the voltage must not exceed .5 volt! This is practically impossible to achieve in the real world without violating the NEC and I wonder if these manufacturers have any clue as to why they contain this requirement in their specifications. Maybe this subject should be explored in depth in the future.
Feeder. If a neutral-to-case connection were made at the panelboard in violation of the NEC, the voltage difference between the panelboard case ground to the neutral conductor would be 0 volts. Under this condition, the voltage difference at the receptacle contacts would only be equal to the voltage drop of the branch circuit conductors, Figure 1-5.
Branch Circuit. If a neutral-to-case connection were made at the receptacle in violation of the NEC, the voltage difference between the grounding contacts of the receptacle to the neutral conductor would be 0 volts.
Author's Comment: At service equipment, the voltage between the neutral-to-case will always 0 volts.