Download High-Impedance Neutrals

High-Impedance Neutrals
Author: Brian D’Andrade, Ph.D., P.E., CFEI
In 2013, John Hall wrote a report on electrical fires i that was published by the National Fire
Protection Association. ii The report stated:
In 2011, an estimated 47,700 home structure fires reported
to U.S. fire departments involved some type of electrical
failure or malfunction as a factor contributing to ignition.
These fires resulted in 418 civilian deaths, 1,570 civilian
injuries, and $1.4 billion in direct property damage. In
2007–2011, home electrical fires represented 13% of total
home structure fires, 18% of associated civilian deaths,
11% of associated civilian injuries, and 20% of associated
direct property damage.
Electrical distribution or lighting equipment accounted for
6% of 2007–2011 home structure fires, ranking fourth
among major causes behind cooking equipment, heating
equipment, and intentional.
Given those statistics, it is not surprising that Exponent consultants are regularly involved in a
variety of electrical fire investigations. In particular, consultants in Exponent’s Electrical
Engineering and Computer Science practice are the key to fire investigations when there is
involvement of an electrical device because they are both professional electrical engineers and
certified or trained as fire and explosion investigators. The combination of the two skills is highly
useful when complex failures may appear to be unresolved at first glance. Fire investigations
involving electrical equipment and wiring can be complex for many reasons, including scenarios
where the cause is not near the origin of the fire patterns, and extensive fire damage to the
suspect electrical device or system hampers electrical characterizations.
Some electrical failures are obvious, but the insidious
ones are most appropriately investigated using a
systematic and scientific approach, as detailed in the
National Fire Protection Association’s Guide for Fire
and Explosion Investigations (“NFPA 921”). A highimpedance neutral is one potentially insidious fault,
because the cause of the fire may not be located at its
origin, and an effort to thoroughly investigate the
building wiring, for example, can be economically
daunting.
An electrical fault that is hard to determine when there
is extensive fire damage, is an increased resistance
(or high impedance) at a connection or at some point
along a current path. Areas of increased resistance
are prone to overheating due to basic resistive losses,
which is similar to the heating mechanism in a toaster
oven. If there is minor, localized damage at a plug for
example, this type of fault can be identified by a
process of elimination or with basic visual inspection;
however, electrical characteristics of the suspected fault would likely not provide meaningful
insight to the original electrical state of the system before the fire.
If the fire damage is substantial, it will be difficult to locate the point of the overheating or
determine exactly what overheated, because of the inability to perform basic electrical
characterizations (e.g., resistance measurement) caused by residual char or changes to the
damaged electrical system. Detecting a high-resistance fault is complicated further because it
affects the voltages across devices. In such a scenario, the fire origin may be at a different
location from the high-resistance fault.
A highly simplified multi-wire branch circuit is shown in Figure 1 to help provide an intuitive
understanding of how increasing resistance along the neutral conductor, which is typically the
grounded conductor iii in a residence, affects the voltage across equipment such as a lamp bulb.
Power from a grounded power source is fed into the circuit via two lines and a neutral. Ideally,
the resistance, X, of the neutral between the main bonding jumper iv and the node at VA is very
low, such that VA = 0 V. If X increases, VA will become increasingly negative in the circuit
shown in Figure 1. For example, VA has a nominal –10 V when X = 18 ohms, so the voltage
across the bulb will be 130 V = 120 V – (–10 V). Figure 2 shows more values of VA for various
values of X.
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The increased resistance may be caused by a variety of factors, including corrosion, loose
connections, and damaged conductors, for example.
Figure 1. A simplified circuit diagram of a multi-wire branch circuit where the resistance along the neutral
is X ohms between the main bonding jumper connection and the node where the electrical devices are
connected.
Figure 2. The voltage at VA, shown in Figure 1, is a function of the resistance, X, along the neutral
between the main bonding jumper and electrical devices. The graph illustrates the value of VA for various
values of resistance X.
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In the above scenario, it is uncertain that the high-resistance neutral fault will be found if the
bulb fails when 130 V, instead of 120 V, is applied across the bulb and if a fire occurs at the
location of the bulb. There may be some observable overheating of the neutral wire, but the
location of the overheating may be hidden in the building structure.
The simplified circuit in Figure 1 is only for pedagogical purposes, but it provides quantitative
insight into the magnitude of resistance that may be considered high resistance for this type of
electrical fault. Broken neutrals, which occur when X has a magnitude of thousands or millions
of ohms, are well known electrical faults, but this example raises the awareness of electrical
faults that occur when neutrals are not broken but still have high resistance.
An even more precarious situation can occur if the high-resistance area is located along the
utility neutral or the service neutral between the main bonding jumper and the grounded source
(which is typically a utility transformer for residences). In this second scenario, the magnitude of
the voltage at the main bonding jumper may increase above 0 V. v Cable TV, telephone, and
satellites are all required to be bonded to the building ground terminal, so there is the possibility
that increased voltage at the main bonding jumper will affect telecommunication electrical
systems that are bonded to ground.
Occasions do arise when the wiring at a fire scene can be energized and electrical
measurements can be made to provide useful data for fire investigators. In those cases, one
may be able to measure branch circuit electrical characteristics with an AC load tester to
determine whether the circuits have adequate performance characteristics, such as low
resistance of circuit conductors. ANSI/ASHRAE/IES 90.1-07 - 2010 is an energy standard for
buildings, except low-rise residences; it provides requirements for voltage drop in feeders and
branch circuits. vi
Also, the National Electrical Code (NEC), 2014 Edition, FPN 4 to rule 210.19 and FPN 2 to rule
215.2, discusses voltage drops of 2% (feeders) + 3% (branch circuits). Note that informational
notes in the NEC are not rules. In residences, it is common to measure voltage drops greater
than 10% with a 15-amp load. Such a large voltage drop is typically due to long lengths of wire
used to form the branch circuit.
Additionally, one can use a ground clamp meter to get an estimate of the utility and service
neutral resistances. A ground clamp meter placed around the grounding electrode conductor
measures the total resistance in a circuit path, and thereby simultaneously measures the
ground contact resistance plus the utility neutral resistance plus the service neutral resistance.
In Section 250.53 of the NEC, ground contact resistance must be less than 25 ohms;
otherwise, there shall be a supplemental ground electrode. Therefore, one typically expects to
measure less than 25 ohms when the ground clamp measurement is performed at a site
investigation.
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This article has focused on one of the more complex and not readily identified causes of
electrical fires that require engineering expertise to identify. Causes of other electrical fires
could be readily visible or easier to identify. These fires, in most cases, are preventable by
following the guidelines offered by the NFPA on how to prevent more common electrical fires,
including:
•
•
•
•
•
•
Replace or repair damaged or loose
electrical cords.
Avoid running extension cords
across doorways or under carpets.
In homes with small children, make
sure your home has tamper-resistant
(TR) receptacles.
Consider having additional circuits or
outlets added by a qualified
electrician, so you do not have to
use extension cords.
Follow the manufacturer's
instructions for plugging an
appliance into a receptacle outlet.
Avoid overloading outlets. Plug only
one high-wattage appliance into
each receptacle outlet at a time.
•
•
•
•
If outlets or switches feel warm, if
fuses blow or breakers trip
frequently, or if flickering or dimming
lights are observed, call a qualified
electrician.
Place lamps on level surfaces, away
from things that can burn, and use
bulbs that match the lamp's
recommended wattage.
Make sure your home has ground
fault circuit interrupters (GFCIs) in
the kitchen, bathroom(s), laundry,
basement, and outdoor areas.
Arc-fault circuit interrupters (AFCIs)
should be installed in your home to
protect electrical outlets.
Summary
Electrical fires account for a significant number of home fires and can be complex. A most likely
cause based on reasonable engineering investigative procedures may be appropriate when
losses are small, but a more extensive investigation should be considered when the losses are
high or when there is significant injury or death. Exponent electrical engineers provide technical
support in electrical fire-related matters, including matters involving improper wiring and highresistance neutrals.
Contribution Authors
Brian D'Andrade, Ph.D., P.E., Senior Managing Engineer
202.772.4907 • [email protected] • Bio
Dr. D’Andrade has a diversified set of expertise in electrical, electronic,
computer engineering, software engineering, optical engineering, and
microelectronic systems.
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i
Hall, J.R. Jr. 2013. Home electrical fires. National Fire Protection Association: Fire Analysis &
Research.
ii
An electrical fire is defined in Hall (2013, ibid) as a structure fire that involved some type of electrical
failure or malfunction as a factor contributing to ignition.
iii
A grounded conductor is defined as “a system or circuit conductor which is intentionally grounded”
(NFPA 70, Article 100, 2014).
iv
A main bonding jumper is defined as the “connection between the grounded circuit conductor and the
equipment grounding conductor at the service” (NFPA 70, Article 100, 2014).
A grounding electrode conductor is defined as a “conductor used to connect the system grounded
conductor or the equipment to a grounding electrode or to a point on the grounding electrode system”
(NFPA 70, Article 100, 2014).
A grounding electrode is defined as a “conducting object through which a direct connection to earth is
established” (NFPA 70, Article 100, 2014).
v
This is due in part to fact that the contact resistance between earth and the ground electrode is not zero.
vi
8.4.1.1 Feeders. Feeder conductors shall be sized for a maximum voltage drop of 2% at design load.
8.4.1.2 Branch Circuits. Branch circuit conductors shall be sized for a maximum voltage drop of 3% at
design load.
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