8. Insulation testing
8.1 General
The insulation resistance test is the test specified by international standards for checking the quality of insulation
between electrical conductors in an electrical installation.
The MFT-X1 can test insulation at 50 V, 100 V, 250 V, 500 V and 1,000 V. It can also test at a wide range of
intermediate values using the VAR option. Most common values are:
500 V: Used on all new 110 V, 230 V and 400 V wiring installations
250 V: Used on existing installations where there is a risk of 500 V causing damage to electronic circuits
In all cases, the instrument can supply a test current of 1 mA or greater (but not more than 2 mA), which complies
with the requirements of IEC 61557-2.
8.2 Test starting timer
Fig 18: Insulation resistance (IR) start timer
All insulation tests must now have a timer to prevent them starting instantaneously. This is a mandatory requirement
of IEC61010, the international standard that defines electrical test instrument safety.
On the MFT-X1 the test voltage is turned on one second (three seconds for 1,000 V tests) after you press and hold
the test button. The test remains on for as long as you keep the button pressed.
When you release the test button, the test voltage is turned off and, provided you leave the test connections in place,
the circuit under test is automatically discharged.
During discharge, the display shows the residual voltage in the circuit under test in the actual voltage window. The
arc will also change to volts and show the discharge of the circuit.
NOTE : The discharge can be very fast any may not be noticeable in the display. For capacitive circuits this
can take seconds or even minutes.
If the measurement has been locked on after the test has started (see Hot key 2 below) the test voltage remains on
even when you release the test button. To turn the test voltage off, you must press the test button again or press Hot
key 2.
WARNING : When the test is locked on, the circuit under test is not automatically discharged and the
test-leads remain live and hazardous. The circuit can also remain hazardous once the test-leads have been
removed
8.2.1 Insulation test-inhibit
The instrument displays a warning if it detects a circuit voltage of 25 V or higher, and it inhibits testing if the circuit
voltage is 50 V or higher. You can change the inhibit voltage, but not the warning voltage, in set up Refer to 15.7.1
Live voltage limit on page 83.
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MFT-X1
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Insulation testing
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