Author’s Comment: Code references in this article
are based on both the 1996 and 1999 National Electrical Code. The first reference
is to the 1996 NEC and the italic reference
is to the 1999 Section number. The Code Section numbers in the graphics, however,
are based on the 1999 NEC.
In the past few issues I explained the National Electrical Code requirements as well as the dangers of improper neutral-to-ground connections. In this issue, we will review the power quality problems caused by improper neutral-to-ground connections, particularly electromagnetic interference to sensitive electronic equipment and elevated equipment case ground voltage.
VOLTAGE BETWEEN METAL PARTS TO EARTH
Proper Neutral-to-Ground Connection
When a neutral-to-ground connection is properly made in accordance with the NEC, the voltage between any metal part of the electrical system to the earth will be zero volts.
Improper Neutral-to-Ground Connection
The National Electrical Code requires a neutral-to-ground connection to be
made at service equipment only and there shall not be any neutral-to-ground
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-ground connection (I personally
did it many times).
Electromagnetic Interference
When a neutral-to-ground 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-ground connection permits neutral current to return on the metal parts of electrical equipment (i.e. metal raceways).
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.
IMPORTANT: Improper neutral-to-ground
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 about the effects of electromagnetic fields on
humans.
Elevated Ground Voltage
When a neutral-to-ground 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-ground
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).
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-ground 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-ground 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:
- At service equipment, the voltage difference between the neutral conductor
and the service equipment case will be 0 volts.
- 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.
- 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).
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.
Improper Installation
Feeder: If a neutral-to-ground
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.
Branch Circuit. If a neutral-to-ground
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-ground will always 0 volts.
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