Danger of Open Service Neutral
According to the NEC, metal parts of service equipment shall be grounded to the earth. The purpose of grounding the metal parts to the earth is to protect persons and property from fires by limiting voltage on the metal parts from lightning [250.4(A)(2)].
In addition, the grounded (neutral) service conductor shall be grounded to the earth at service equipment. For the purpose of limiting the voltage imposed by lightning, line surges, or unintentional contact with higher voltage lines. Grounding is intended to shunt potentially dangerous energy into the earth from the system [250.4(A)(1)].
Note: The grounding of the grounded (neutral) conductor to the earth also helps the electric utility clear high-voltage ground faults when they occur, but reducing the utility system ground resistance.
Purpose of Neutral
Because electric utilities are not required to install an equipment grounding conductor to service equipment, a grounded (neutral) conductor shall be run from the electric utility transformer to each service disconnecting means and this conductor shall be bonded to the service disconnect enclosure [250.24]. The grounded (neutral) service conductor serves as the required effective ground-fault current path necessary to ensure that dangerous voltage from a ground fault will be quickly removed by opening of the circuit protection device [250.4(A)(3) and 250.4(A)(5)].
Hazard of Open Service Neutral
If the grounded (neutral) service conductor is opened or not provided at all, objectionable neutral current will flow on metal parts of the electrical system and dangerous voltage will be present on the metal parts providing the potential for electric shock. This dangerous electrical shock condition is of particular concern in buildings that contain swimming pools, spas and hot tubs.
In addition, when the grounded (neutral) conductor from the secondary of a transformer is open or not present, the operating voltage for the loads on one line will rise, while the operating voltage for the other line will drop. Another hazard exists from over or under line voltage, and that is a fire from excessive heat.
Equipment Voltage Rating
Manufactures typically rate equipment at 115V +-10-15%, which means that 115V rated equipment should have a continuous operating voltage between 100V and 135V. Because electrical equipment of the inductive type (motors, computers, electronic ballast, etc) can be damaged or destroyed from over voltage, and resistive loads will only be damaged from over voltage, we must be sure that the voltage remains stable within the equipment voltage rating.
The voltage distribution on the circuits and the touch voltage on metal parts from an open service grounded (neutral) conductor are dependent on the impedance of the loads on Line 1, Line 2, and ground resistance as measured by a ground resistance meter.
For example, the voltage distribution of a 3-wire, 120/240V service where Line 1 is 100A, Line 2 is 50A, and the ground resistance is varied will be:
As we can see from the above table:
the operating voltage for the loads on Line 1 is below the equipment rating,
the operating voltage for the load on Line 2 are above the equipment rating, and
the touch voltage is excessively high and dangerous.
Download a free spreadsheet http://mikeholt.com/free/neutral3wire.xls to determine the above values.
One last point, if the grounded (neutral) service conductor (which serves as the effective ground-fault current path) is opened or not provided at all (this happens when people think that a neutral is not required if there are no line-to-neutral loads), a ground fault cannot be cleared. The result is that metal parts of electrical equipment, as well as metal piping and structure steel will become and remain energized at line voltage. In this case 120V.
Let me summarize by saying that the ground resistance must be as low as possible, but a low resistance ground does not reduce dangerous touch voltage to a safe level from a ground fault! Yes, a lower resistive ground will reduce touch voltage if the grounded (neutral) is opened, but not likely to a safe value.
P.S. If the grounded (neutral) service conductor is open, neutral current will flow onto the metal parts of the electrical system. When this occurs in a wood frame construction building, neutral current seeking a return path to the power supply will travel into the moist wood members. After many years of this current flow, the wood will be converted into charcoal (wood with no moisture) and ultimately it can result in a fire. This condition is called pyroforic-carbonization.
By the way an AFCI will protect against the arc caused by an open grounded (neutral) service conductor. What will happen is that the electronic sensing element of the AFCI will be destroyed from over voltage!
Mike Holt Comment: Please send me your feedback and comments. Mike@MikeHolt.com
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