If the flexible metal bellows (shown by the yellow oval in Photo 8 above) develops leakage, air is allowed into the bottle, and the vacuum is relieved. The bellows tend to crack due to the flexing of the metal when the circuit breaker opens and closes repeatedly. The vacuum bottle should contain the failure, but occasionally the hardened casing will rupture. When vacuum bottles fail, they often fail on closing, not opening.
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Vacuum circuit breakers can be tested using a DC hipot in the same manner as air-magnetic circuit breakers. There&#;s good news and bad news here. The good news is a DC hipot can be used to test vacuum bottles. What&#;s the bad news? Use the wrong DC test set, and the bottle&#;s toast!
When vacuum breakers first came out, many companies were using the DC hipot method to test their bottles, but it soon became apparent (after the first bottle or two) that something was amiss. Vacuum bottles were failing, even though the recommended test voltage wasn&#;t being exceeded. That &#;something&#; was a voltage spike produced by the half-wave rectifier circuitry used in most DC hipot testers. This spike can be up to 800% of the voltage indicated on the meter.
Due to this issue with DC tests, most manufacturers now recommend using an AC hipot test arrangement for the vacuum bottle integrity test, even though it is not normally used for testing the pole unit (primary) insulation. When performing the vacuum bottle integrity test, the specified voltage (AC or DC) is applied across the vacuum bottle for a specified period of time, and no current is recorded. If it doesn&#;t fail, it&#;s good. If it does, it must have been bad. This is known as a &#;Go, No-Go&#; test. If the bottle fails during the test, the circuit breaker in the test set will trip.
Hipot test sets that use a full-bridge rectifier do not introduce a spike into the output of the set. This type of circuitry generates a smooth DC output that won&#;t overstress bottles. Perform this test with the circuit breaker open, and apply the test voltage across the bottles, as shown in Fig. 1, performing the test for each bottle. Be sure to review the instruction manual for the specific vacuum circuit breaker you&#;re about to test so that you apply the correct voltages. One manufacturer recommends 36kVAC and up to 50kVDC &#; both for 10 seconds.
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Safety warning: Vacuum bottles may emit X radiation if they fail when high voltages are applied. Negligible emissions should result if the manufacturer&#;s recommended values are used. Higher-than-recommended voltages should never be used. When conducting these tests, stand as far from the circuit breaker as possible, placing the circuit breaker between you and the bottle.
2) Insulation resistance test &#; An insulation resistance test on low- and medium-voltage circuit breaker&#;s primary insulation will show gross defects, but to discover tracking, deterioration, and moisture, a DC overpotential test or an AC insulation power factor test is recommended. This is not to say an insulation resistance test should not be performed at all. It can be used for performing a safety check before the breaker is returned to service, and it can also be used to test control wiring,
current transformers, and potential transformers. ANSI/NETA MTS-, Table 100.1, provides guidelines for insulation resistance testing of non-inductive electrical equipment, which would include control wiring.
Caution: When performing insulation resistance tests on control wiring on circuit breakers, the high voltage must not be applied to the spring charging motor or solid-state overcurrent devices that may be connected to it. Make sure the device or circuit under test is properly isolated.
All control wiring should have its insulation resistance checked. Wiring that is even slightly out of position can be damaged by wear or cutting on sharp edges. 250V class wiring is tested at 500VDC and should have a minimum insulation resistance of 25 MΩ. 600V class insulation is tested at 1,000V and should have a minimum insulation resistance of 100 MΩ. Measure insulation resistance from the wiring to ground. On a circuit breaker, this would be the frame or chassis.
3) Micro-ohmmeter (contact resistance) testing &#; Manufacturers design contacts and their supporting structures to carry rated load current continuously and, when needed, interrupt the circuit breaker&#;s rated fault current. For this to happen reliably, the contacts, stabs, and the entire current-carrying structure must have a low resistance. As the resistance of a conductor increases, the amount of I2R losses &#; losses due to the heat created by current passing through a resistance &#; increase. Poor contact condition (pitting or misalignment) or aged lubrication in the contact&#;s pivot (circle) can increase the resistance of the current path. Note that there can be several current-carrying surfaces within the contact structure that require lubrication, depending on the specific design of the contact.
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