DIGITAL MEGOHMMETER ( M98126501-03 / 04A ) TECHNICAL SPECIFICATIONS OPERATING INSTRUCTIONS
DESCRIPTION The CIRCUTOR model MD-5060 digital electronic ohmmeter is a versatile, robust and easy-to-use equipment. It uses an efficient well experienced technology, which provides reliable, safe and accurate measurements up to 4,000,000 MΩ @ 5kV, with 10 test voltages: 500, 1000, 1500, 2000, 2500, 3000, 3500, 4000, 4500 and 5000 V. Its microprocessor-based development makes the equipment operation easier and enables the introduction of functions such as auto-range, memory, test voltage measurement, polarization and absorption Index indication, etc. Other feature that is important in this megohmmeter is negative voltages that refer to the zero potential terminal (R), for detecting moisture by means of the Evershed effect. Due to its reduced dimensions and weight, power supply autonomy and mechanical resistance, this equipment is suitable for fieldwork, and under extreme weather conditions. It is easy to carry, very simple to be operated and resistant to violent treatment, which unavoidably includes frequent bumps, extremely high or low temperatures, intense vibrations during transportation over rough roads, long exposure to direct sunlight, dust, sand and other wind-carried particles, etc. Everything without affecting its accuracy, comparable to the best laboratory instruments. CASE Moulded in high dielectric resistant plastic material. Apart from the output terminals there are no operator-accessible conducting parts. Terminals are placed on the farthest and most protected position of the box. HIGH VOLTAGE LIGHT INDICATOR A light indicator displays the occurrence of high voltage on the output terminal during the measurement and keeps lit until the discharging process has been completed. RANGE The current meter sensibility of such megohmmeters allows reliable readings of very high electric resistance. Therefore, the maximum measurement value of 4,000,000 MΩ @ 5kV can be achieved. GUARD TERMINAL Allows the measurement of very high electric resistance values, eliminating the effect of parasite resistance whose influence is to be minimized. SHORT-CIRCUIT CURRENT The equipment provides a 1.2 ma ± 0.2 ma short-circuit current. 2
DIGITAL DISPLAY Alphanumeric LCD where the measurement results, the measuring unit, the analogue indication by means of a bar-graph and messages to the operator are displayed. MEGOHMMETER ACCURACY The measurement typical inaccuracy is smaller than ± 5% of the read value, between 1 MΩ and 100,000 MΩ @ 5 kv. TEST VOLTAGE STABILITY The high voltage generator features electronic stabilization, thus keeping the test voltage constant for open circuits, with typical tolerance smaller than ± 2%, within the entire power supply voltage admissible variation range and for any weather condition within the specified limits. Due to the measurement system used, the output voltage error does not spread to the electric resistance value, since its reading is done by dividing the real voltage applied to the sample by the current value effectively circulating through it. POWER SUPPLY The MD-5060 megohmmeter is powered by built-in a rechargeable Ni-Cd battery. BUILT-IN CHRONOMETER It features time indication (in minutes and seconds) on the display left hand side. A beep is activated at every 30 and 60 seconds and every 10 minutes. ABSORPTION INDEX After 1 minute, pressing the A I key, the display shows the ABSORPTION INDEX value, calculated using the following formula A I = R 60 seconds R 30 seconds POLARIZATION INDEX After 10 minutes, pressing the P I key, the display shows the POLARIZATION INDEX value, calculated according to the following formula P I = R 10 minutes R 1 minute 3
AUTO POWER-OFF After 15 minutes' operation, the instrument turns itself off automatically, thanks to its unique auto-power-off system. RS 232 OUTPUT PORT Baud rate: 4800 bps 8 bits - non-parity 1 stop bit (8,n,1) Each line sent consists of the following information: Time Test Voltage Resistance value A typical line can feature the following format: 02:30 2.50 KV 5.89G Time is measured in minutes and seconds counted from the measurement start. Each line ends with CR LF characters If any condition occurs preventing the measurement from being done, the resistance value should be replaced by a message indicating the fault condition. A serial printer, a data collecting device, a communication software equipped PC, etc. can be connected to the megohmmeter RS232 output port. KEYPAD AT THE PANEL RIGHT HAND SIDE CASE AND CASE On/Off KEY: The first key from bottom up. When pressed, it turns the equipment on. "MEMORY" KEY. When pressed, the instrument displays the last resistance reading done with a sound signal. P I KEY. When pressed, the instrument displays the POLARIZATION INDEX value. A I. KEY. When pressed, the instrument displays the ABSORPTION INDEX value. "VOLTAGE (Vdc)" KEY. When pressed, it indicates that the instrument is at the VOLTMETER function, so it is displaying the real voltage applied to the resistor being tested. "BATTERY" KEY. It displays, when pressed, that the equipment is checking the rechargeable battery status. The case is made of high dielectric- and impact-resistant plastic material. A resistant synthetic leather carrying case provides safe and suitable transportation for the equipment and its accessories. The megohmmeter can be operated without being removed from its case. 4
DIMENSIONS AND WEIGHT 260 x 155 x 99 mm (cabinet outside dimensions without case). 1.8 kg (3.5 kg including accessories) SUPPLIED ACCESSORIES Each ohmmeter is supplied with: Negative voltage output cable. Return terminal cable. GUARD terminal cable with jaw connector. High voltage cable with acrylic probe. Fine synthetic leather case. Operation manual in English. Power supply for the battery charger ( 90 to 240 Vac). Computer connection cable. (RS232 port) Rechargeable Ni-Cd battery. OPERATION ENVIRONMENTAL CONDITIONS Room temperature can range from 0 C to +50 C, and moisture reaching its saturation limit, without affecting the equipment operation. 5
OPERATING INSTRUCTIONS 1) Check if there is no voltage across the points where the megohmmeter will be connected to, nor between them and the ground. 2) Connect the red banana jack terminal (red cable) to the megohmmeter (-V) terminal. 3) Connect the black cable to the megohmmeter (+R) terminal, with the yellow terminal to the G terminal. 4) The green terminal (GUARD) will not be always used. CIRCUTOR Application Note #32 explains the usage of Guard terminal for minimising the parasite resistance effect, whose influence one intends to minimise. If the measurement is performed between two parts, which are not grounded (for example: a transformer high and low voltage windings), the megohmmeter Guard terminal should be grounded for defining the instrument potential. Whenever a measurement is performed, the +R or the guard terminal should be grounded, but not both simultaneously. If none of these terminals are grounded, the megohmmeter can be subjected to a higher potential, hazardous to the operator; and the reading can result unstable and unreliable. If both terminals are simultaneously grounded, they will be short-circuited, and the megohmmeter will not perform the measurement. 5) Connect the free ends of the cables in use (red and black jaw connectors) to the element to be measured. 6) Choose the test voltage using the VOLTAGE SELECTOR rotary selecting switch.. 7) Check if the "MEMORY" key is free. 8) Press the "On/Off" key. This key should remain depressed and the "HIGH VOLTAGE" light indicator will be on, indicating that the megohmmeter internal generator is applying a high voltage to the element being tested. 9) For a few seconds, the self-scale smart system will search for the most convenient scale for the value being measured. Meanwhile, the display will show the message " WAIT..." Then, the display will show the measurement result using both digital (measurement value and unit) and analog displays. At 30 and 60 seconds' time, the equipment will sound a beep for reminding the operator to register the measurement at that time. If the measured value exceeds 4000 GΩ @ 5 kv, the OVERRANGE message will be read. 10) During the measurement, check if the battery is charged. Press the "BATTERY" key and check the display information. If the charge is enough, it will read "BATTERY OK" or "LOW BATTERY", should it be recharged. The analogue bar-graph will give an approximate idea of the remaining charge percentage (at least 60% is required for a normal operation). 11) VOLTMETER : By pressing the VOLTAGE key, the instrument should measure the voltage effectively applied to the element being tested. 12) MESSAGES TO THE OPERATOR : The display will show the following messages to the operator: MEASURING UNIT AND RESISTANCE VALUE. TIME. Indicates the test time on the display right hand side. OVERRANGE. Whenever the measured resistance is greater than 4000 GΩ @ 5kV. 6
"WAIT..." The operator should wait, because the megohmmeter is automatically searching for the most convenient scale. LOW BATTERY. Battery should be recharged. VOLTAGE. Voltage value and unit effectively applied to the load. M. The instrument is at the memory function and, therefore, holding the value of the last performed measurement on the display. P I. POLARIZATION INDEX. A I ABSORPTION INDEX. SOUNDS MADE BY THE MEGOHMMETER: Change of scale BEEP. Each time the equipment changes scale automatically, a beep is heard indicating the change. Low battery BEEP, together with the corresponding intermittent warning on the display. Memory BEEP. If the memory key is activated, an intermittent BEEP will be heard. 30 seconds' elapsed time BEEP. 60 seconds' elapsed time and ABSORPTION INDEX BEEP. 10 minutes' elapsed time and POLARIZATION INDEX BEEP. Auto-power-off BEEP. The equipment has turned off automatically. ANALOGUE BAR-GRAPH. Located on the display lower part, it analogically indicates the resistance value being measured. 0 10 20 30 40 50 60 70 80 90 100 In the example above, the display should read 62.5 MΩ (if automatically set on the range up to 100 MΩ). BATTERY CHARGER. This equipment is provided with an external power supply (90 to 240Vac) and a built-in auxiliary smart circuit for charging batteries. On the panel there is a two-colour LED, indicating when the battery is being charged (red light), and when is already charged (green light). The instrument is delivered with a charged battery, but we recommend to provide an initial charge following these instructions: Connect the power supply cable plug to the connector placed on the equipment right hand side. The battery charger will operate correctly whenever the power supply is between 90 and 240 V AC. On the front panel, a red LED will remain lit until the battery is fully charged, then this LED will change to green colour. The battery normal charging time is 4 (four) hours, from discharged condition to full charge. If the battery is fully charged, it will take approximately 10 minutes to change from red to green light. 7
ABSORPTION INDEX A I : On this test, the megohmmeter should remain on, applying high voltage to the sample for 1 minute (60 seconds). After this time, the operator must press the A I key for reading the ABSORPTION INDEX value on the display. If he anticipates and press the key before one minute is elapsed, the display will show A I =???. The ABSORPTION INDEX is the ratio between the insulating resistance value measured at 60 seconds' time and the value measured at 30 seconds' time and it is useful for winding preventive maintenance (on transformers, motors, generators, etc.). A I = R 60 seconds R 30 seconds POLARIZATION INDEX P I : On this test, the megohmmeter should remain on, applying high voltage to the sample for 10 minutes. After this time, the operator must press the P I key, so the megohmmeter shows the P I value on the display.. If he anticipates and press the key before 10 passed minutes, the display will show P I =???. If he presses it after 10 minutes until 15 minutes, the P I will be shown. The POLARIZATION INDEX is the ratio between the insulating resistance value measured at 10 minutes' time and the value measured at 1 minute time. P I = R 10 minutes R 1 minute Approximately 5 minutes after the 10 minutes BEEP has sounded, the equipment will turn off automatically if no new function is selected (no key is pressed or reset). The POLARIZATION INDEX is useful for preventive maintenance for detecting insulation resistance degradation due to excessive dust, grease, or action of chemical or physical agents, etc. 8
APPLICATION NOTE # 32 USE OF GUARD TERMINAL IN MEGOHMMETERS When insulation resistance measurements are performed with megohmmeters, specially with high-sensitivity instruments measuring high resistance values, the use of the GUARD terminal avoids the harmful influence of stray resistances. In order to better explain the function of this terminal, let us start reviewing the megohmmeter basic circuit diagram of Fig. 1. Vt Ri +V I -R A Rx Guard Fig. 1 Where: V : DC high-voltage generator Ri: Generator internal resistance A : Indicator meter (microammeter) 9
The unknown resistance (Rx) is connected between Vt and R terminals. Its value determines the current passing through the circuit, which in turn is indicated by the microammeter. The value of Rx can be determined as follows: V Rx = - Ri I In many cases the resistance to be measured is in parallel with other stray resistances which influence on Rx should be minimum and if possible, zero. A typical example of this situation is when the insulation resistance between primary and secondary windings of a transformer mounted inside a metal housing is to be measured. Rx: Insulation resistance between primary and secondary winding. R1: Insulation resistance between primary winding and housing. R2: Insulation resistance between secondary winding and housing. If megohmmeter (terminals Vt and R) is connected to transformer terminals A and B, and considering that the resistance of the turns on each side of the transformer may be disregarded as compared with the insulation resistance between primary and secondary windings measured by megohmmeter, Rx appears to be in parallel with (R1 + R2). The situation is changed if we connect the transformer housing to GUARD terminal. 10
Then the resulting circuit will be: R i V I R A R A R x 1 R 2 G FIG. 3 In the circuit of Fig. 3 it may be noted that R1 is in parallel with a low-value resistance (the one from the built-in microammeter) therefore its influence is reduced during reading. Through resistance R2 circulates a current which is not passing through the meter and consequently does not affect the reading. In fact, current through R2 originates a certain error, since it creates an additional voltage drop in R1 which was not regarded during megohmmeter calibration. As regards the practical use of megohmmeter, it shall be considered that if R1 and R2 are higher than 100 MΩ, any value of Rx will be measured with an error lower than 10%. For example: Let us consider Rx = 3.000 MΩ and R1 = R2 = 100 MΩ, the reading without using the GUARD terminal would be 187.5 MΩ, which is quite wrong. On the other hand, if the GUARD terminal is properly used, we would have 3.000 MΩ. 11
SAFETY CONSIDERATIONS All installation specification described at the previous chapters named INSTALLATION AND STARTUP, INSTALLATION MODES and SPECIFICATIONS. Note that with the instrument powered on, the terminals could be dangerous to touching and cover opening actions or elements removal may allow accessing dangerous parts. This instrument is factory-shipped at proper operation condition. MAINTENANCE The CVM-144 does not require any special maintenance. No adjustment, maintenance or repairing action should be done over the instrument open and powered and, should those actions are essential, high-qualified operators must perform them. Before any adjustment, replacement, maintenance or repairing operation is carried out, the instrument must be disconnected from any power supply source. When any protection failure is suspected to exist, the instrument must be immediately put our of service. The instrument s design allow a quick replacement in case of any failure. TECHNICAL SERVICE For any inquiry about the instrument performance or whether any failure happens, contact to CIRCUTOR s technical service. CIRCUTOR S.A. - After-sales service Vial Sant Jordi, s/n 08232 Viladecavalls (SPAIN) Tel - + 34 93 745 29 00 fax - + 34 93 745 29 14 E-mail : central@circutor.es 12