Major tech MT550 Instruction Manual - Page 8

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6. INSULATION RESISTANCE MEASUREMENTS

6.1. Measurements at 200MΩ/250V

This is the voltage used for the majority of insulation resistance tests on

normal installation requirement. To measure insulation resistance, press

the test button to power on the tester. The LCD will displayed the

insulation resistance. Section VII indicated that subdivision of large

installations might be necessary because of the large number of parallel

insulation resistance. In such a case, an installation may be divided into

sections, each being separately tested. Each section must have not less

than fifty outlets, an outlet being a switch, socket, lighting point etc. A

switched socket counts as one outlet. The minimum acceptable

insulation resistance is 1MΩ. For a large installation, the capacitance of

the insulation will be high, and it will take longer for it to become

charged by the direct testing voltage. Care must be taken not to take a

reading until there is a steady reading, indicating that the charging

process is complete.

Note: The charge stored in the insulation will be discharged

automatically when the test button is released. Be careful not to turn

the range switch knob whilst the test button is pressed, or the

instrument will be damaged.

6.2. Measurements at 2000MΩ/1000V

Some specifications require testing at 1000V. This voltage must also be

selected where the supply voltage of the installation is between 500V

and 1000V. First, set the range switch to 1000V and then proceed as

indicated in a above for 500V testing. The above note also applies to

testing at 1000V. In addition the following applies.

Note: Make sure that the circuit under does not include components

Which will be damaged by the 1000V applied. Many normal components

of an installation are likely to be damaged if tested at 1000V. Examples

are power factor correction capacitors, low voltage mineral insulated

cables, electronic light dimmers, electronic ballasts and starters for

fluorescent lamps etc...

For hands free operation a lock power on feature is incorporated on the

press to test button. Set LOCK button to lock test voltage, Pressing it

again will switch it off.