Breakdown can be defined as a condition where voltage discharges across or through the insulation and causes excessive current flow. Traditionally, a Hipot tester is designed to monitor and measure the current flow generated by this type of catastrophic insulation failure. A Hipot with its current trip point exceeded will indicate failure and high voltage will immediately be shut down.
Preceding a dielectric breakdown, corona or high impedance arcing may form around a conductor. In some environments corona can be defined as a luminous discharge caused by the ionization of the air. Corona is a partial breakdown caused by a concentration of electrical stresses at the edge of an electrode in an electrical field. High impedance arcs and corona generate high frequency pulses which ride on the low frequency wave. These pulses may have a frequency ranging from less than 30 kHz to more than 1 MHz, and may be very short in duration. Many times these pulses are much less than 10 microseconds (See Figure 1). These short duration pulses or spikes may not immediately result in a disruptive discharge causing the current to increase or the output voltage to drop. IEC 60601-1 for medical electronic equipment states the following: “During the test, no flashover or breakdown shall occur. Slight corona discharges are ignored, provided that they cease when the test voltage is temporarily dropped to a lower value, which must be higher however, than the reference voltage and provided that the discharges do not provoke a drop in the test voltage.” Please keep in mind that although an agency might allow a DUT with corona to pass the Hipot test, this corona may be an indication of a potential problem in the insulation system.
Arc Detection
The geometry of an arc is not a constant. For example, breakdown voltages may vary greatly between two rounded surfaces or two sharp points which have the same gap spacing. The impedance and distributed capacitance of the circuits between the point where the arc is generated and the detector may also effect the di/dt (rate of change of current versus time) of the current waveform being monitored by the arc detector. The amount of voltage, rate of rise, polarity, and the waveform all effect the speed in which corona and arcing conditions occur. Temperature, humidity, and atmospheric pressure all influence the voltage at which corona begins as well as breakdown voltage levels.
An Arc Detection system incorporates a high pass filter circuit that only responds to high frequencies that are greater than 10 kHz. These high frequency signals are fed into a comparator and checked against the sensitivity level adjustment that the user selected during setup. If this level is exceeded an interrupt signal is fed into the CPU, which shuts down the Hipot in 400 microseconds (See Figure 2). While the leakage and overload detection circuits are always active, some instruments allow the user to shut off the arc detection circuit. We have found that many manufacturers may use arc detection for diagnostic or research and development purposes but on the production line it may actually be best to not use arc detection.
Many appliances such as power tools and vacuum cleaners have low level arcing conditions present as part of their normal operation. In many cases safety agencies acknowledge that low level arcing does exist and allows it in manufacturing tests. Therefore arc detection circuits used in this type of production environment could show a failure condition when indeed the product is good. On other types of products such as medical electronics, especially patient connected devices, low level arcing conditions need to be detected for safety reasons. In these applications arc detection can have real benefits.
Because of test condition variables and the lack of safety agency standards in defining maximum limits for arcing and leakage currents, IDI has taken a flexible approach in its instrument designs. On instruments that contain the functions to set both the high trip limits for leakage current and trip limits for arcing conditions, we feel the customer must have the ability to set variable limits that can be adjusted to meet specific test requirements. We also provide the customer with the option to enable or disable the arc detector circuit. This may be done independently of the high current trip circuit that Hipot testers must have for compliance testing. Arc detection when used properly and under the correct conditions can provide valuable information on product design and safety. However, the manufacturer must first determine that arc detection is applicable to their products to avoid failing products that are actually electrically safe.