AC loop impedance testing

Key Concepts

Faulty wiring causes unsafe conditions.

Testing a live circuit with utility power yields a genuine ac impedance reading under real-world conditions.

Thoroughly testing the entire loop reveals faulty wiring and other sources of improper impedance.

Sections: Testing the loop Low impedance necessary Agency recommendations

Electrical safety testing isn’t just for the sake of bureaucratic compliance. It has practical implications that literally protect lives. Improvements in electronic instrumentation have gone a long way toward eliminating the perceived “nuisance factor” in such testing.

A tragic instance of the failure to insure safety was reported in the IAEI News (International Association of Electrical Inspectors) recently. A worker was electrocuted by a vending machine. Investigation revealed that a receptacle was incorrectly wired. It was neither grounded nor connected with correct polarity. A legal responsibility existed to maintain the machine in safe working order. Both the ungrounded condition and reversed polarity could have been discovered in mere seconds by the use of an ac loop impedance tester.

Testing the loop

Long mandated in the United Kingdom and Europe but not well known in the U.S., ac loop impedance testing is accomplished by the use of a specialized handheld instrument that briefly connects a known resistance between “hot” and ground, and indicates the impedance of the entire ground loop (Fig. 2).

Fig. 2. AC loop impedance testing indicates the impedance of the entire ground loop.

The unloaded voltage is measured first. Then the voltage drop caused by the impedance is measured. These two values are compared, and the difference is used to calculate total loop impedance. The test is performed so rapidly, within two half-cycles of the utility sine wave (16 millisec), that protective devices are not tripped, and no disruption of the electrical service occurs.

By testing a live circuit with utility power, rather than employing a battery source in the tester to merely approximate a fault, the loop tester gives a genuine ac impedance reading under real-world conditions. The instrument (Fig. 3) calculates the loop impedance according to the following equations.

V test = V supply – ( IR loop )

therefore:

R loop = ( V supply – V test ) / I

or:

R loop = Difference in Voltage / Test current

Fig. 3. Loop impedance is calculated by dividing the difference between supply and test voltages by test current.

By simulating a fault, the loop that is measured includes all possible paths to ground. In typical wiring, this includes the grounding conductor, ground bus at the service entrance, local ground electrode, soil in parallel with the neutral back to the utility transformer, transformer ground and secondary winding, and the hot back to the point of test (Fig. 4).

By thoroughly testing the entire loop, the procedure reveals faulty wiring and other sources of improper impedance that can be missed easily by doing point-to-point continuity tests. The latter are dependent upon, and therefore limited by, the operator’s knowledge of the electrical system. In the tragic case cited previously, a loop tester would have over-ranged to expose the absence of ground. It would also have indicated the reversed polarity, thereby averting the possibility that the ground would be connected but the receptacle left in an unsafe condition.

Low impedance necessary

The test takes only seconds to perform. A three-prong test lead plugs into the receptacle (alternate alligator-clip leads can be used to test at a panel), and the test button engaged. A digital display shows the measurement in seconds. Other warning symbols typically include the presence of voltage between neutral and ground, which is likely to be caused by impedance from common problems such as loose or corroded connections, damaged conductors, poor splices, excessively long circuits, or undersized wires.

Low impedance to ground is necessary to allow sufficient fault currents to flow to allow protective devices (breakers, fuses) to sense the imbalance and consequently trip. But that is only half the responsibility. The protective devices must also be sized properly, so they can handle the maximum fault current long enough to operate without disintegrating. Inadequately sized breakers can explode under the impact of fault current. But with a mere change of the selector switch, the loop tester performs this operation as well. It uses the data it has already gathered to calculate a prospective short circuit current (PSCC), which is the maximum amount of current that could flow with a dead short at that point. Protective devices should then be sized according to the industry-accepted “500% Rule.” The PSCC should be no more than 5 times the rating of the protective device in order to clear a fault within 1 sec. The test is most thorough when performed at the panel , as this is where current flow is greatest. Refer to Table 1 to determine if the impedance is sufficiently low. Refer to Table 2 to determine if the breaker is the appropriate size.

Traditionally, this vital information is gained only by a series of separate tests with standard instruments. In practice, implementation is often minimized or avoided altogether. Familiar wiring procedures are followed and expected to suffice. Loop testing overcomes this challenge by facilitating a complete test for ground-fault clearance in a single operation — even alerting the operator to problem conditions that hadn’t been considered at all. If impedance in the ground loop is too high, fault current may be reduced below the threshold of sensitivity for protective devices. Current continues to flow to ground, overheating connections, starting fires, and creating dangerous voltage gradients over supposedly “dead” surfaces, in addition to sacrificing electrical efficiency.

Agency recommendations

A 5-yr study of nonresidential fires, conducted by the National Fire Protection Association (NFPA), tabulated an average 162,400 fires per year, resulting in 205 deaths and $2.9 billion damage. A significant portion (10.2%) were caused by the electrical system.

The National Electric Code (NEC) recommends a maximum 3% voltage drop for feeder circuits (section 215-10, note #4) and maximum 5% for feeder plus branch circuits (section 215-2, note #2). In other terms, IEEE states that any conductor or branch circuit should not exceed 0.25 ohms. Accordingly, the loop test is best performed at the farthest point in a circuit. This is where voltage drop is greatest. The percentage voltage drop in the circuit is calculated using the following equation.

% voltage drop = V (no load) – V (under load) / V (no load) The NEC discusses the matching of protective devices to maximum short circuit currents in sections 110-9 and 110-10, which include the advisory: “It is important that the test conditions match the actual installation needs.”

Loop impedance can be calculated without the use of a special tester, but it is a tedious exercise more likely to be honored in the breach. It is easier to submit a similar design that worked in the past. The entire circuit must be considered — including the branches, feeder, and known circuit distances, along with other data. The utility neutral is in parallel with the earth return through the soil, making an accurate calculation of these parallel paths difficult. Disregarding the earth return results in higher calculated impedance. Also, actual fault currents may be greater than anticipated. The net result can be typified by an insurance company investigation that revealed that of 30 industrial facilities, three exceeded the 5%, and two had no grounds.

Voltage drop equals heat, which equals lost energy and fire hazard. For example, consider an 8.3% voltage drop, which would exceed the NEC specification by 3.3 percentage points, possibly from a loose connection. This would equal 4 V from a 120-V supply. Assuming a 15-amp circuit, the addition of a mere 0.27 ohms produces a point source of 60 W of heat. In close and/or dusty confines, the heated connection could generate a fire.

Similarly, switch-mode power supplies can generate harmonics that cause current on the neutral. This effectively increases the load on branch circuits, and can result in fire. Induction motors have a low starting impedance that can pull six to eight times the full-load current rating. Once up to speed, these motors generate a back emf that raises impedance and reduces current to the rated value. If the branch circuit has high impedance due to wiring problems, the resultant voltage drop can limit current below the amount necessary to develop the required back emf. The motor continues to run at low starting impedance, heats up, and burns out or starts a fire. A 12-amp full-load rating can pull 72-96 amp on startup. A routine loose connection can generate 1.4-2.5 kW of instantaneous heat.

A dedicated loop tester represents a valuable advance in instrument technology, because it affords a comprehensive evaluation of an electrical system’s fault clearance capabilities with just two pushbutton tests.

— Edited by Jack Smith, Senior Editor, 630-288-8783, jsmith@reedbusiness.com

More Info

The author is available to answer questions about this article. He can be reached by phone at 610-676-8539, or by e-mail at jeff.jowett@avointl.com .

Maximum circuit impedance for specific circuit protective device ratings

CPD rating, amp
Circuit max. Impedance, ohms

15
1.6000

20
1.2000

30
0.8000

35
0.6800

40
0.6000

45
0.5300

50
0.4800

60
0.4000

70
0.3400

80
0.3050

90
0.2660

100
0.2400

110
0.2180

125
0.1920

150
0.1600

200
0.1200

225
0.1060

250
0.0960

300
0.0800

350
0.0680

400
0.0600

450
0.0540

500
0.0480

600
0.0400

700
0.0340

800
0.0300

1,000
0.0240

1,200
0.0200

1,600
0.0150

2,000
0.0120

2,500
0.0096

3,000
0.0080

4,000
0.0060

Courtesy of Travis C. Lindsey, Planscheck Specialist for the Clark County Nevada Building Department.

Maximum PSCC for specific circuit protective device ratings

CPD rating, amp
500% of CPD rated current

15
75

20
100

30
150

35
175

40
200

45
225

50
250

60
300

70
350

80
400

90
450

100
500

110
550

125
625

150
750

200
1,000

225
1,125

250
1,250

300
1,500

350
1,750

400
2,000

450
2,250

500
2,500

600
3,000

700
3,500

800
4,000

1,000
5,000

1,200
6,000

1,600
8,000

2,000
10,000

2,500
12,500

3,000
15,000

4,000
20,000

Courtesy of Travis C. Lindsey, Planscheck Specialist for the Clark County, Nevada Building Department.

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