Location of the hipot test station
The next step is determining where the test station will be located. The test area should be isolated from the factory assembly area. It should be located away from foot traffic to assure the safety of passersby and, of course, the safety of the station operator. Operator distractions should be kept to a minimum and the area should be conspicuously marked with internationally approved signage, such as “DANGER HIGH VOLTAGE.” During testing, the hipot tester itself should have indicator lights to denote when high voltage is present.
There should be ample and reliable power supplied to the test station. Verifty that the power wiring meets electrical code requirements for polarization and grounding. Always use an outlet that has a properly connected protectionground and make sure this ground has been tested to insure a low impedance path to the panel ground and earth bonded ground. Operator injury may result if the hipot tester is not connected to earth ground properly.
The work area and bench surface should consist of nonmetallic materials; which means that metal work surfaces should be avoided, and metal objects should not be placed between operator and DUT. All other metal objects should be grounded or be out of the test area all together. An ESD mat is not a recommended platform for your test
station, as it may cause erroneous readings for leakage and is unnecessary in this application. In addition, the test equipment should provide for immediate and safe removal of the output voltage using internal discharge circuity at the conclusion of the test or if the test is interrupted. Never remove power for the hipot tester. If there is a power interruption, use extreme care in any contact with the DUT.
The safest approach is to leave the DUT connected to the hipot tester until power is restored and the tester can conduct its discharge function.
Operation safety considerations
The test station should have suficient space for the tester and the DUT without the operator having to reach over the DUT to access the tester. The tester should be at least three inches away from the wall to provide proper aiflow for the unit. ldeally the DUT should be isolated from the operator and tester. For larger DUTs, which are wheeled to the test station, the cart should be non-conductive and have locking wheels. (This also applies if the tester needs to be wheeled to the DUT.) Keep the area clean and neat and arrange the equipment so that it is easy and safe for the operator to use.
There are many safety features that can be added to the test station to prevent the operator from encountering high voltage, such as guards or enclosures. When placed around a DUT they should be non-conducting and be equipped with safety interlocks that interrupt all high voltages when open. Interlocks should be arranged so that operators are never exposed to high voltages under any conditions.
In addition, it is easy to implement circuit palm switches that prevent the operator from encountering high voltage during testing. The basic operation of a palm switch requires the operator to use both hands to initiate a test with, potentially, a foot switch to activate the test. If one or both hands are removed from the switches while testing, the test is immediately stopped. The switches are placed directly in front of the operator and spaced shoulders-width apart.
Spacing the switches prevents an operator from trying to press both buttons down with one hand or object. No high voltage can be applied to the output terminals and DUT until both switches are pressed simultaneously. The operator cannot touch the DUT or test leads if both hands are on the palm switches. The palm switches are connected to the digital /O on the hipot tester. When the switches are in the down position the start is enabled. Once one switch goes up the safety interlock is enabled, terminating the output voltage of the hipot test. This method is safe, quick and effective.
Figure left ilustrates two alternative approaches to setup of a benchtop hipot test. In Figure 7a, the DUT is placed on the test bench and a combination of palm switches and a foot switch ensure that the operator cannot make contact with the DUT while the test is underway. The operator is wearing safety glasses. As a practical matter, the use of palm switches is typically restricted to short-duration tests done on a repetitive basis with a series of DUTs. If this test set up is used for longer tests, operators will find a way to defeat the palm switches.
In figure right, the DUT is placed under a protective cover with interlock to isolate the operator during the test. The use of an enclosure is a more reliable means to assuring operator safety, particularly when testing requires longer time periods.
More elaborate test stations can include a hipot testers interlock. One safety method that utilizes the interlock is a light curtain, which is an infrared light beam that will open the interlock if anyone interrupts any part of the beam. The output of the light curtain is connected to the interlock terminal on the hipot tester. If the interlock is open, high voltage is immediately terminated. The light curtain is placed in between the hipot tester or the DUT and the operator. For the operator to touch the high voltage they would have to pass through the light curtain, hence opening the interlock, which will terminate the high voltage.
If the hipot tester is placed behind the light curtain there must be a way to start the test. A foot switch is an easy solution. Keep in mind you must ensure that nobody can reach the high voltage by going around the light curtain.