WHITE PAPER:
LOOSE NEUTRALS AND GROUNDING
Contributed by Chris Mullins
A loose neutral is a common but dangerous single-phase problem. The situation often presents as a power quality problem
but in reality it’s a safety issue that demands immediate attention. The basics of loose neutrals have been described in
a previous white paper; here the more complicated situation
involving ground path conductance is shown.
This is shown in Figure 1. Here we have 1600 W of load on
leg 1, and 1800 W on leg 2. That loading is represented by 9
ohms and 8 ohms, respectively. The leg currents are 13.3 A
and 15 A, with a neutral current of (L1-L2) = 1.7 A. With an
open neutral, that 1.7 A cannot f low to the transformer. The
voltage on L1 and L2 can be computed as 240 x (R1/(R1+R2))
= 240 x 9/(9+8) = 127 V, and 113 V. The artificial neutral is
7 Volts above the real neutral. One leg has increased by 7 V,
and the other decreased by 7 V. A more severe difference in
leg currents would cause a larger voltage shift.
LOOSE NEUTRALS
GROUNDING
In a classic loose or open neutral situation, there is an open
in the neutral conductor between the distribution transformer secondary and the customer meter base. This open may be
permanent or intermittent. The difference in load currents
between the two 120V legs f lows through the neutral. The neutral provides a low-impedance connection to the center tap of
the transformer, and is the mid-point in the 240V winding that
divides the voltage equally between the two 120V legs.
The situation changes when we take into account grounding.
Almost all residential services will include a ground rod at the
meter base. The neutral and ground are bonded at the service
entrance (meter base). Additionally, the transformer center
tap is likely grounded at the pole or transformer pad. These
grounds provide a parallel path for current to f low from the
service entrance to the transformer center tap – through the
ground itself, as shown in Figure 2.
ABSTRACT
With an open neutral, the difference current cannot f low to the
transformer winding. Instead, an artificial neutral is formed.
The voltage at this artificial neutral depends on the relative
size of the 120V loads. Essentially, a voltage divider circuit
is formed, and the artificial neutral potential depends on that
divider. As the loading becomes different on the two legs, the
artificial neutral voltage drifts closer to one end of the transformer winding. This gives rise to the classic symptom of one
leg rising in RMS voltage, and the other falling, in equal and
opposite amounts.
Figure 2.
The electrical resistance through the ground varies widely with
soil characteristics, ground rod depth, and distance from the
transformer ground to the meter base. Typical ranges are single-digit ohms to hundreds of ohms. This resistance may also
change with weather, especially rain or snow. As the ground
resistance increases, the circuit approaches the limiting case of
a complete open on the neutral.
Figure 1.
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WP#367
Assuming a relatively low ground resistance of 10 ohms, the
artificial neutral voltage changes from 7 Volts to approximately 5 Volts. This reduces the L1 and L2 shifts enough to stay
just inside the ANSI C84 voltage regulation limits (114 and 126
V), and consequently less noticeable in general. Any higher
ground resistance would push the voltage excursions higher, and in the limiting case an infinite resistance (a complete
break) it is back to the original 7 Volts.
© 2023 Power Monitors, Inc. • Call Us: (800) 296-4120 • www.powermonitors.com
WHITE PAPER:
LOOSE NEUTRALS AND GROUNDING
Keep in mind that the ground resistance effect depends completely on the size of the loads. In this example we have a 10
ohm ground resistance, and 9 and 8 ohms of load. The ground
resistance is comparable in magnitude to the loads themselves.
With a lightly loaded house, for example at night, the load resistances could be much higher. If the ground resistance is much
lower than the loads, the ground functions more like a real
neutral. If the ground resistance is much higher than the loads,
the circuit approaches regular open neutral behavior.
In general, the ground path either has little effect, or tends to
lower the open neutral voltage shifts, thus masking the root
issue. The lower the ground resistance compared to the loads,
the more masking occurs.
The ground current causes an additional, more serious problem
than neutral masking. Because there is current f lowing through
the ground resistance, there is a voltage drop across it. This
can cause a voltage to appear on grounded objects in the home.
In the above example, 5 VAC on the ground might be enough
to give a tingle or small shock to someone. In that example, a
ground resistance of 10 ohms was assumed – this very much on
the low side of the range. A more typical ground may be in the
hundreds of ohms. With loads in the kilowatt range, that level
of ground resistance is approximately an open neutral, placing
nearly full artificial neutral voltage on local grounds. This can
be enough voltage to produce a shock hazard to anyone in the
home. In some cases, this symptom from customers may be the
only symptom.
The ground situation may be even more complex if multiple
homes are fed from the same transformer, and have other
grounds in common, such as cable television or gas lines. For
cable TV fed by coax, the coax ground braid is connected to
earth ground somewhere close to the service entrance. Multiple
houses on the same secondary will have a common ground path
through the coax braid. If one home has an open neutral, current can f low through that home’s coax braid back to another
house, where the coax is bonded to the neutral. That neutral
then provides the path back to the transformer secondary, as
shown in Figure 3, from https://www.electrical-forensics.com/
Open-Neutral/Open-Neutral.html.
In this case, the full neutral current f lowed through the coax
braid. A typical RG-59 braid is about 1.1 ohms per 1000 feet, in
between #10 and #12 AWG. This is certainly not rated for the
neutral currents that may appear in a house; in this case the coax
insulation melted due to the excessive heat (Figure 4). The coax
may provide enough of a neutral to mask the open neutral condition until the coax itself fails. Measuring the current through
coax with a clamp-on ammeter may reveal this situation.
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WP#367
Figure 3.
Figure 4.
CONCLUSION
The classic loose neutral symptom is the rise in one voltage leg
paired with the same size fall in the other voltage leg. Customer symptoms include f lickering lights, step increase in light
brightness, and frequent UPS operations. Grounding complications may mask these symptoms, while introducing new ones.
In particular, reports of getting a tingle or shock from grounded objects requires immediate attention and may be a symptom
of the ground carrying neutral current while the neutral is
intermittent or open. Always check the loose neutral graphs in
any single-phase PQ investigation, but also keep in mind that
ground current may mask the voltage shifts. A check of coax
current, and ground voltage may help determine the presence
of an open neutral.
© 2023 Power Monitors, Inc. • Call Us: (800) 296-4120 • www.powermonitors.com