Megger PDA-1 Power Disturbance Analyser, Single Phase

The PDA1 is used for recording and analysing the mains power supply voltage waveform, in order to identify the disturbances which may occur. Physical Description: It is a portable, robust instrument, built into a tough "Noryl" plastic case. The front panel contains the controls for the instrument and the alpha-numeric liquid crystal display. This display shows the instructions and prompts necessary for initially setting up the conditions ready for a test run. It also indicates the limits which are set and shows relevant information about the supply as data is being logged for analysis. At the end of the test run all the accumulated information concerning what has taken place regarding the supply voltage may be displayed. A small keyboard, containing three keys marked with arrow heads, is located immediately below the display. These keys are used to input information and make selections during the setting-up and retrieval procedures. The various functions which these keys assume appear on the display. To the left of the display is the on/of key switch. The key can be removed with the switch in either the on or off positions, thus providing security for the parameter limits during a test and for the information stored after a test is finished. To the right of the display window is the display's contrast adjuster. The mains power supply lead, which will need terminating with a suitable plug, is connected toward the rear of the front panel at the left hand side. The printer unit is plugged into the DIN socket opposite to this on the right hand side. The print itself is fitted into the detachable lid of the instrument and has a paper feed push-button incorporated into the unit. High and Low Average Description: If the r.m.s. value of the a.c. voltage were to increase (or decrease) beyond pre-set limits for a time which exceeds 0,5 s (25 cycles at a frequency of 50 Hz), then this condition is considered to be a high (or low) average. Sags and Surges Description: Sags and Surges are really special cases of the low and high average conditions respectively. They occur when the a.c. voltage departs from the nominal value and is below (or exceeds) the pre-set limit for less than 0,5 s (25 cycles at frequency of 50 Hz). This includes excursions beyond the limits which are shorter than once cycle. Spikes Description: Transient high voltage pulses, known as spikes, usually occur at frequencies very much greater than that of the nominal a.c. supply. They can take the form of a single pulse of a few microseconds duration or of an oscillatory condition lasting for say five to ten pulses. The minimum spike duration which can be recorded as such is 120 ns. 'Notches', (negative going spikes in a positive part of the waveform and vice versa), are regarded in the same manner as spikes. Drop-Out Description: This condition prevails if the instantaneous a.c. voltage falls below 75 V and remains so for a minimum of 1,5 ms. A drop-out becomes a power cut if its duration is long enough. Power Cut Description: A power cut is the absence of the a.c. voltage. For this instrument is is regarded as being the condition where the voltage has fallen to less than 75 V a.c.r.m.s. which includes the no-voltage situation. A drop-out of about 300 ms (15 cycles) in a 240 V a.c. supply, or about 30 ms (1,5 cycles) in a 120 V a.c. supply, is the point where this parameter becomes a power cut. Maximum duration of a power cut is 10 hours. High and Low Frequency Description: A frequency change is deemed to have taken place if five consecutive sample measurements of this parameter taken at 0,4s intervals show it to be outside the set limits. Frequency may increase or decrease. HF Noise Description: Very high frequencies (20 kHz to 20 MHz) may become superimposed on the basic a.c. waveform. This is HF noise and is monitored every 0,5s. If it is above pre-set limits for longer than two seconds then this fact is recorded.