The following is an extract from my Grounding versus Bonding book.

A neutral-to-ground 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 at www.MikeHolt.com, go to the Free Stuff link, and download the touch voltage calculator.

Electric Shock - Electric shock can occur if the service grounded (neutral) conductor is open because the required neutral-to-ground connection at the service equipment 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-ground 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-ground connection (main bonding jumper) is required within the service disconnect enclosure. 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.