Testing for safe and efficient branch circuits












Branch Circuit Testing

Branch circuit wiring and testing practices are primarily code driven with little thought as
to why such stringent requirements are necessary. But these practices are necessary to
ensure safe and efficient branch circuits. Hidden problems within a branch circuit can
result in fire, electrocutions and equipment failure.

Fires

Based on the National Fire Protection Association (NFPA) and the US Consumer
Product Safety Commission data, there was an estimated 406,000 residential structural
fires in 1997, resulting in an estimated 3,390 civilian deaths and 17,775 injuries.
Approximately 9% of the structural fires and 7% of the deaths were determined to be the
result of the electrical distribution system. Residential fires were by far the biggest
problem, accounting for 97% of all structural fires and 87% of deaths.
1


The most common cause of residential electrical fires is problems within the branch
circuit wiring. These problems resulted in 14,600 fires, 420 injuries and 110 deaths in
1997.
2
















1
Fact Sheet on Fire in the US and Canada, National Fire Protection Agency (NFPA) 1997
2
1997 Residential Fire Loss, Consumer Product Safety Commission, 1997
250
1360
40,300
Total
10
350
160
320
420
Injuries
10
4,600
Other
30
9,900
Lamp, light Fix.
10
4,900
Switch, Outlet
90
6,300
Cord, Plug
110
14,600
Installed Wiring
Deaths
Fires
Electrical
Distribution
250
1360
40,300
Total
10
350
160
320
420
Injuries
10
4,600
Other
30
9,900
Lamp, light Fix.
10
4,900
Switch, Outlet
90
6,300
Cord, Plug
110
14,600
Installed Wiring
Deaths
Fires
Electrical
Distribution

Testing for safe and efficient branch circuits

Written by Chad Reynolds, Product Manager,
IDEAL Test and Measurement Division
Arc Fault Circuit Interrupters

There are two main causes of fires to installed wiring within the electrical distribution
system. The first is arcing within the circuit. An arc fault is characterized by an erratic
flow of electricity. Because normal breakers were designed to protect against short
circuits, arc faults occurring in damaged cable can continue undetected. These leads to
hazardous situations such as high temperatures that could ignite nearby combustible
materials
3

To offer protection against these conditions, the 2002 edition of he National Electrical
Code (NEC), requires the installation of Arc Fault Circuit Interrupters (AFCI) in bedroom
circuits in new residential circuits.
4
Currently, the only devices that meet the new NEEC
guidelines are Arc Fault breakers. These breakers, which are manufactured by a
number of companies, have a special circuit within the breaker to detect arc fault
conditions on the branch circuit.

These devices should be tested upon installation to ensure that the breaker will
adequately protect the circuit. An independent arc fault tester simulates an arc fault
condition on the line to determine if the breaker will protect the circuit.

High resistance connections

The second major cause of residential fires is a high resistance point in the circuit, such
as a loose connection, poor splice or defective electrical device. Current flowing through
these high resistance points causes heat to build up behind the wall. This can create a
smoldering fire if there are combustible materials nearby, and no way to dissipate the
heat.








Identifying potential problems

Most fixed wiring and receptacle hazards are hidden from inspection. A visual
inspection in the rough-in stage of residential construction may identify obvious
problems, such as a staple cutting through the conductors, but they may not identify a
loose wiring connection or a bad splice.

Normal instrument testing of a static circuit reveals little about the quality of wiring or the
integrity of the circuit. However, testing under load and calculating the voltage drop can
identify 90% of these hidden defects.


3
IEC Fact Sheet, Arc-Fault Circuit Protection, Illinois Electric Council, Fact Sheet #28
4
NEC code Articles {210.12 (A)}
Loose wire connections create a high
resistance point within the electrical
system, which can lead to a breakdown
in insulation or even a fire. Voltage Drop

Voltage drop is a measure of how much a circuits voltage fluctuates (or drops) once a
load is applied. Voltage drop can be calculated by comparing a voltage measurement
with no load on the circuit to a voltage measurement under full load.

The voltage drop calculation will be most accurate when no-load conditions are
compared to full load conditions. When using a digital multimeter to calculate voltage
drop, remove all loads from the circuit to take the no-load measurement. For the full
load measurement, use a space heater or some other appliance that will draw close to
15 amps.




Voltage Drop can also be measured with a SureTest® Circuit Analyzer. It uses a
patented technology to place a full load onto the circuit without tripping a breaker or
causing any interruption to equipment on the line. The SureTest compares the voltage
measurement at full load, with a measurement at no load and calculates the voltage
drop.









How Much Voltage Drop is Acceptable?

The National Electrical Code (NEC) recommends that the combined voltage drop of the
electrical system (branch circuit and feeders) not exceed 5% for optimum efficiency.
5
It
is important to note that this is a recommendation and that local inspectors, or other
governing bodies, may use their own judgment on an acceptable level of voltage drop for
the electrical system.

For example, the Philadelphia Housing Development Corporation (PHDC) requires
contractors to calculate the voltage drop prior to installing blown insulation in existing
homes
6
. If the voltage drop is 10 % or higher contractor must replace/repair the circuit
prior to proceeding with the insulation.

Prior to instituting this requirement, half a dozen smoldering fires resulted from the blown
insulation installations. In the 2,500 homes insulated during a two-year period after this
electrical integrity test was instituted, there were no fires reported. At least 15 other
municipalities have followed the PHDC's lead in requiring the load test as part of their
winterization programs.


5
NEC code Articles {210-19(a) FPN No. 4} {215-2(d) FPN No. 2
6
Kinney, Larry "Assessing the Integrity of Electrical Wiring" Home Energy Sept/Oct 1995: 5,6
% Voltage Drop = (V
(no-load)
V
(load)
)/ V
(no-load)
Voltage drop at a full
load can be easily
taken by simply
plugging the SureTest
into a receptacle.
Troubleshooting a Circuit

Troubleshooting to identify the cause of the high impedance within the electrical system
is actually quite simple. First measure the voltage drop at the furthest receptacle from
the panel on the branch circuit under test. If the voltage drop is high, than further
investigation is necessary.

Testing the remaining receptacles in sequence, from next furthest from the panel to the
closest to the panel, will identify the problem.

If the voltage drop reading changes significantly from one receptacle to the next, then
the problem is a high impedance point at or between the two receptacles. It is usually
located at a termination point, such as a bad splice or loose wire connection, but it might
also be a bad receptacle.

If the reading steadily decreases as you get closer to the panel, with no significant
decreases between receptacles, then the wire may be undersized for the length of run,
or for the load on the line. Check at the panel to see if the wire is sized per code, and
measure the current on the branch circuit.

The reading may not decrease at all from the last receptacle to the first. This would
indicate that the problem could be at the first splice, or at the panel itself. Most poor
panel connections show up as hot spots on the panel. These can be