The DET2/2...the Only Ground Tester for Large Ground Systems

Securing valid measurements when testing large ground systems such as those in substations and power stations requires that proper techniques and instrumentation be used. A substation must have a very low ground resistance to reduce excessive voltages developing during a fault which could endanger the safety of nearby people or damage equipment. The nature of substation grounding systems and related conditions make testing far more complex than on a simple ground rod.

Following are the three key challenges in testing substation ground systems:

• The physically large area of a substation ground system results in large "resistance areas" and, consequently, long distances to the test probes; ideally, the current test probe should be placed 10 times the maximum distance on the ground system (eg: 3,000 feet for a 300 foot square ground grid).
• The large "resistance areas" typically give ground resistance values of less than 0.5W ; test instrument resolution is critical if small variances in readings are to be observed.
• Large electrical networks contain noise consisting of the frequency of the power utility and its harmonics, plus high frequency noise from switching, etc. and induced signals from other sources; most ground testers only reject noise of a single frequency (usually 128 Hz) which is adequate in most situations because it avoids harmonics of standard line frequencies; unfortunately, it is often not adequate in substations; this type of interference can cause significant measurement errors.

Problem #1 can be addressed by using the Slope Method because it enables the measurement of large ground systems without finding the "flat" portion of the characteristic curve. The DET2/2 was designed to address problems #2 and 3.

• The DET2/2 is the only ground tester with 1 mW measurement resolution; when measuring small differences between low readings, this extra digit of resolution makes the measurement of these variations significantly more accurate and suitable for use with published tables.
• The DET2/2 is the only ground tester to offer variable test frequency (105 to 160 Hz) to help remove any stray noise that could affect the reading; in addition, it has 40 V peak-to-peak interference suppression and uses a sophisticated filtering system that can reject more noise than any other available ground tester.
• The DET2/2 offers selectable high test current (to 50 mA) to improve the signal-to-noise ratio when necessary.