Get Into the Loop

By David Herres

The Fluke 1651 is a well-engineered ergonomic loop impedance tester with an excellent range of functions, perfect for troubleshooting and verifying 50Hz systems, but the company will not bring out an American model until demand increases.

This Megger LT320 performs a broad range of measurements and presents a profile of branch circuit parameters including any harmful loop impedances. Additionally, the LT330 incorporates a USB port and with included software can download results to a computer to create certification.

Ideal's SureTest has been an American staple for years and is an excellent instrument for general electrical work. You can quickly check all installed branch circuits and devices and be assured a hot spot won't emerge down the road.

Energetic weather events such as flooding means that indoor branch circuits that were never intended to get wet may experience varying degrees of moisture. After the waters recede and ambient heat dries the connection, corrosion can set in. A slight amount of impedance at the connection interface coupled with electrical current flow always results in I squared R heat rise, which sets the stage for more corrosion.

Whether the full effects surface hours, days or years later the end result will be somewhere between an annoying callback for the electrician and catastrophic loss of property or life.

Arc fault circuit interrupters address this hazard by recognizing the unique sputtery signature of a series connection with too much impedance or a parallel fault with too little impedance, but only after these defects have caused an outage. Other impedance issues can exist unseen by AFCI and GFCI equipment Ñ an example is a poor connection within an equipment grounding conductor.

Moreover, corrosion is only one of several factors in this equation. Other problems include terminations not fully torqued or overtorqued to failure, wirenuts that disengage when wiring is squeezed back into an undersized enclosure, raceway serving as equipment grounding conductor that separates due to structural problems of the building or concrete cracking and separating. In other words, the list is immense.

Unwanted impedance in circuitry can result in either of two harmful effects, depending upon where it occurs. If a splice or termination within a phase (hot) conductor or a neutral develops a slight resistance and if the circuit is energized and a load is connected, an intense hot spot will occur. The same thing will happen if one of these conductors becomes partially severed, for example by an errant nail driven through the wall. Sometimes a hot spot will burn itself out and open up the circuit. But if there is flammable material nearby, fire often results.

Another very serious problem can result from a high impedance anywhere along an equipment grounding conductor. Since no current flows through this conductor during normal operation, this type of fault can remain dormant for years until a ground fault develops in a location such that the impedance lies between the ground fault and the service equipment where the equipment grounding conductor is connected to the neutral through the main bonding jumper.

If the impedance is great enough, the fuse or circuit breaker will fail to open and the supposedly grounded metal case of a power tool or appliance will remain energized until a person touches and completes the path to ground. Moreover, the grounding conductor's hot spot can ignite adjacent flammable material, so that there is the risk of both shock and fire.

Some of these risks are mitigated by AFCI and GFCI protection if they are in place and functioning correctly, but sometimes these conditions are not met.

Can construction and maintenance electricians address these problems? The answer is yes.

Sufficient investigation involving volt, ohm and amp readings will eventually uncover many of these problems assuming the electrician knows the layout of a wiring system including what has been installed by others, often concealed behind finish surfaces.

A further problem is that these impedances can remain hidden unless testing is performed under energized and loaded conditions so that current flow approximates real world conditions. Damaged insulation may continue to function when checked by an ordinary ohmmeter, but when subjected to higher voltage, the compromised material becomes ionized and current suddenly surges through just as lightning strikes without warning in a sultry atmosphere.

There exists a diagnostic instrument capable of quickly and accurately reporting on the status of installed wiring without even removing device faceplates. It's called a loop impedance tester.

This test is required in England and European countries that subscribe to International Electrotechnical Commission (IEC) guidelines. Standard IEC 60364 mandates loop impedance testing for all installed wiring prior to utility hookup. But the NEC, as adopted by U.S. states and many municipalities, has no such requirement. As a result, sophisticated loop impedance testers long available across the Atlantic have been slow to appear in the U.S.

The problem is that different frequency and voltage parameters require redesigned models. Since loop impedance testing has not been required in the United States, demand has been sluggish. But news of these remarkable instruments is spreading and they will be seen more in the United States, especially if a future NEC mandates their use or at least makes mention of them in an advisory note.

Loop impedance testers perform a variety of measurements and display the results. Exact features vary widely among different makes but what they have in common is that they are line powered and perform the tests under varying loads so that realistic results are obtained. The tests are performed very quickly (typically within two half cycles) so that the electrical supply is not disrupted.

The procedure is to begin a loop impedance test at the last outlet in each daisy chain, then work back toward the panel until the impedance lies within ordinary voltage drop limits. That reveals the location of a bad wirenut, loose termination or other fault. Then connect right at the panel to ascertain available fault current. These are two of numerous available procedures which are quick and easy to perform but highly informative.

Fluke's Model 1650 is a highly capable instrument but not suited to U.S. line characteristics. It is very well regarded in England and other IEC compliant countries, but the company has no plans to bring out a North American model unless loop impedance testing becomes NEC mandated and/or demand increases.

The Ideal SureTest 61-165, which Grainger sells for $330, though not called a loop impedance tester by the manufacturer, offers an advanced array of features including:

• Arc fault testing.
• Tests for shared neutrals.
• Measures voltage drop with 12A, 15A and 20A loads.
• Measures true RMS.
• Measures line voltage.
• Measures peak voltage.
• Measures frequency.
• Measures ground to neutral voltage.
• Measures ground impedance.
• Measures hot and neutral conductor impedances.
• Verifies proper wiring in three-wire receptacles.
• Identifies false (bootleg) grounds.
• Tests GFCIs for proper operation.
• Conducts testing without disturbing sensitive loads.
• Verifies isolated grounds (with 61-176 adapter).

The instrument is less than 6.5 inches tall, 3 inches wide and less than 1.5 inches deep. It also comes with a two-year warranty.

Megger has solved the compatibility problem by bringing out the LT 300 series high current loop impedance testers. Since these models operate at 16 Hz to 400 Hz at various voltages, they work on U.S. and Canadian wiring as well as that found in Europe. At $625 to $850 depending on the exact model, this instrument performs IEC-mandated tests and is useful for U.S. electricians and inspectors. Features include:

• Overcurrent devices are not tripped by testing.
• Three-wire low current non-tripping loop impedance test provides measurements from 0.01 ohms to 2 Kohms with a resolution of 0.01 ohms up to 10 ohms.
• Two-wire high current test is provided where GFCI connection is not an issue.
• All tests are auto-ranging with no range changing needed.
• Three-phase safe; the instrument is not damaged if connected across phases.
• The LT310 operates over a voltage range of 100V to 280V for single-phase testing.
• The LT320/330 operates over a voltage range from 50V to 480V for single- and three-phase testing.
• Frequency and phase rotation can be measured with the LT320/330. Supply voltage is measured.
• Prospective fault current is displayed.
• LEDs indicate correct supply and test lead connections.
• Meters are rubber armoured for durability.
• A user guide in the lid provides basic information.
• The LT330 can save over 1,000 test results to internal memory. These results can be downloaded into a computer to CSV (comma separated variable) spreadsheets and can be imported into Microsoft Excel. For this purpose there is a USB connection. If certification is required, results can be downloaded into Megger Powersuite Professional with a range of features for creating professionally finished documentation.

By performing these tests, electricians can offer a higher quality installation and know they are providing enhanced functionality and safety for years to come. And as a troubleshooting instrument, the loop impedance tester is awesome so as tools go, one you should have now!

Definitions

Loop: An electrical path from phase to neutral and ground, often dividing into unintended segments. Unlike a circuit, which conforms to the original design, a loop may include harmful parallel ground paths. Any ground loop impedance opposes overcurrent device function.

Impedance: Total opposition to current flow including resistance and capacitive or inductive reactance. These latter elements vary with frequency so that square waves and arc faults, with very fast rise and fall times, may set the stage for various unwanted harmonics, voltage spikes and surprising impedance fluctuations.
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