COMMERCIAL FEATURE Safe testing of circuit breakers By Nils Wacklen – product manager, circuit breaker testing, Megger Sweden A fter long periods of inactivity, a circuit breaker must function perfectly when needed. The best opportunity to test for this is the first-trip test. The rated operating sequence (also known as the standard operating duty or standard duty cycle) is the specified operating sequence that the circuit breaker must be able to perform at specified ratings. Breaker manufacturers normally specify these sequences and corresponding rated times, which are defined in IEC 62271-100. Many regulations and laws require all electrical equipment to be grounded on both sides before any maintenance work is carried out. During circuit breaker maintenance, however, the most basic and important test – main contact timing – is usually performed without this basic safety prerequisite. The reason is simple; conventional technology does not offer a safe way of carrying out timing tests on circuit breakers. Now, however, DualGround/ DCM technology offers a much safer approach to testing. First introduced in 2006, this novel testing technique offers greatly increased safety for personnel, and is suitable for use with circuit breakers of all types. It is non-intrusive and does not require any special preparatory information. Despite these benefits, the basic method of working and the interpretation of results remain unchanged, except that testing is much faster and easier. With DualGround testing, it is possible to carry out timing tests on the main contacts of a circuit breaker while both sides of the breaker are grounded. This means that personnel can be kept at a safe distance from dangerous voltages – a safe area can be created around the circuit breaker under test and clearly marked with security fencing. Even though safety protocols dictate that both sides of a breaker should be grounded during field tests, conventional timing methods require the 110 ground to be lifted on one side of the breaker to allow for the test instrument to sense the change in contact status. This means that the test cables and the instrument form a path for capacitive coupled current while the test is being performed. In contrast, the DualGround method allows reliable measurements to be made while both sides of the circuit breaker are grounded, which makes the test faster and easier. The DualGround technique also makes it possible to test circuit breakers in gas insulated switchgear (GIS), generator and transformer applications where conventional timing methods require the removal of jumpers and busbar connections. Timing measurements are difficult to make with both sides of a circuit breaker grounded. However, the DualGround or dynamic capacitance measurement (DCM) method of timing gives accurate and dependable results, even when the ground resistance is low. There is no lower limit on ground loop resistance; even if the ground loop has a lower resistance than the main contact/arcing contact path, the method still works. This is particularly important when testing GIS breakers and generator breakers, and is also relevant for air insulated switchgear breakers that have fairly low resistive grounding appliances, say a couple of milliohms. The reason for the superiority of the DualGround/DCM test technology is that it uses a high frequency to achieve resonance in the test circuit. The resonant frequency varies when the circuit breaker changes state, and this variation can easily be used to determine when the contacts close and open. There are other technologies that use dynamic resistance measurement (DRM) to time circuit breakers with both sides grounded. A current is injected and the voltage drop across the circuit breaker is recorded, allowing the resistance to be calculated. The breaker state is determined by evaluating the resistance graph against an adjustable threshold. If the resistance is below the threshold, the breaker is considered closed and if it is above the threshold the breaker is considered open. Difficulties arise when it comes to setting this threshold since it has to be below the ground loop resistance (which is initially unknown) but above the resulting resistance of the arcing contact (which also is unknown) and the ground loop in parallel. This is because, according to the IEC standard, it is the closing/opening of the arcing contact that determines the breaker’s operation time, not the closing/opening of the main contact. The difference between main and arcing contact operation time can, depending on contact speed, be as much as 10 ms. Further, a 95 mm2 copper grounding cable of length 2.0 x 10 m has a resistance of about 3.6 mΩ, neglecting connection resistances. Arcing contacts also usually have resistance ranging from a couple of milliohms up to about 10 mΩ depending on the type of breaker and the condition of the arcing contact. Taken together, these factors make setting the thresholds for DRM testing an almost arbitrary task. When both sides of a circuit breaker are grounded, a loop is formed with a large area exposed to the magnetic fields from surrounding live conductors. These fields induce a current, which can be as large as tens of amps, in the circuit breaker/grounding loop. This is a very significant proportion of a test current of, say, 100 A. If the evaluation threshold is close to the limit, such an induced current would definitely impair the accuracy of the timing results. In contrast, the DualGround/DCM method for contact timing is completely unaffected by 50/60 Hz induced currents and interference. ESI This article is an extract from The Circuit Breaker Testing Guide available at www.megger.com ESI AFRICA ISSUE 1 2014