Alternating current (ac) loop impedance testing, a testing procedure long-established in Europe, is only recently beginning to find its way into U.S. practice. Best known as "loop testing," it addresses the rapidly expanding development and deployment of increasingly sophisticated electronic equipment, which places greater demands on instrumentation and testing capabilities.
Loop testing is a quick, convenient, and highly specific method of evaluating an electrical circuit for its ability to engage protective devices (circuit breakers, fuses, GFCIs). It has been mandated in Europe but overlooked in the United States because it isn't required.
A "loop" is not the same as a circuit. A circuit conforms to a design, whereas a loop may define itself by including unsuspected elements where current has found parallel paths to ground. Because a ground loop determines the effectiveness of protective devices, it is crucial to be able to measure it, in order to detect and correct problems.
Picture a ground-fault loop as follows: A fault occurs, current travels through the grounding conductor back to the service, then down the ground connector to the ground rod or grid, and into the soil. Parallel paths then exist through the soil and the grounded neutral conductor back to the supply transformer; the transformer winding and phase conductor back to the point of the fault complete the loop (Fig. 1). In order for protective devices to function properly, this loop must be of sufficiently low impedance to allow enough current to flow to activate the devices. High impedance can render protective devices useless by reducing current flow to less than is required to activate the fuse or breaker. The faulty circuit will remain energized, resulting in damage, fire, and even fatal shocks.
Various codes have set requirements without specifically mandating a test procedure. The National Electric Code, Section 250-51, requires the ground path to be sufficient to facilitate the operation of protective devices. This is most commonly implemented by use of calculations, such as point-to-point or unit methods.
Calculations are time-consuming, rare-ly performed by non-engineers, and can be done incorrectly. It becomes simpler to take shortcuts by merely assigning a circuit design that has worked in the past. Even when faithfully employed, calculations are subject to inherent errors. It is difficult to consider the entire circuit length, including branch and feeder, to the transformer. And two return paths exist in parallel between the facility and the transformer: utility neutral and earth return from the ground electrode at the service entrance. Accurate calculation of these parallel paths is difficult, and typically not done.
Yet these paths may contribute significantly to the total current in an actual fault condition. Even conductors sized according to NEC Table 250-95, while affording basic protection, can develop excessive impedance in a long circuit-and not accommodate protective devices.
The ready solution for avoiding these potential errors is to perform an impedance test. New technology permits the reading of total impedance in a given circuit exactly. U.S. Electrical workers are already familiar with the correlative concept of voltage drop. NEC Section 210-19 recommends no more than a 5% voltage drop due to impedance on a given circuit. Unsuspected parallel grounds, in addition to increasing fault currents, can reduce voltage drop on the circuit and mask high impedance. Various methods, both specific and jury-rigged, are used to determine voltage drop. But loop testers are the most accurate means of measuring impedance because they include actual circuit conditions, taking into account temperature and lost currents traveling in parallel paths.
A loop impedance tester is a marked advancement over more time-consuming and error-prone methods. In a few seconds, it gives an accurate measurement that assesses all factors that contribute to actual performance of protective devices in a fault condition. It does this by simulating a fault from "hot" to ground (ground fault) or from hot to neutral (short circuit). The tester first measures the unloaded voltage, then connects a known resistance between the conductors, thereby simulating a fault. The voltage drop is measured across the known resistor, in series with the loop, and the proportion of the supply voltage that appears across the resistor will be dependent on the impedance of the loop (Fig. 2). Speed is the key to a successful measurement. The tester completes the measurement in two half-cycles of the main supply (16 milliseconds at 60 Hz), in which protective devices do not have time to react.
An indispensable corollary measurement also performed by a loop tester is that of Prospective Short Circuit Current (PSCC). This is the maximum short circuit current that could flow in event of a fault. It is necessary information for the correct sizing of protective devices, according to NEC Sections 110-9 and 110-10, so that they are not dangerously sacrificed when called on line. The maximum point of test for PSCC is at the service entrance, while impedance is best tested at the farthest point from the incoming supply. A quick table reference indicates if impedance is sufficiently low (see Table 1), and sizes the breaker (see Table 2).
Loop impedance testing reduces a complex process, fraught with uncertainty, to a reliable task completed in minutes.