What is insulation resistance?
Insulation resistance is how strongly the insulation resists leakage current. Measured in megohms with a DC test voltage, it is the clearest early warning that moisture, damage or ageing is wearing down a transformer long before it causes a fault.
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Insulation resistance (IR) is the resistance that the insulation of a series isolation transformer offers to leakage current. An insulation tester applies a DC test voltage and reads the result in megohms or higher. Three readings are normally taken: primary to earth, secondary to earth and primary to secondary, with each winding’s leads tied together. See Transformer Testing and Insulation Testing.
A high, steady value means the insulation system and potting are sound. A low value, or one that keeps falling from test to test, points to moisture, contamination, damage or ageing, and is often seen well before an earth fault appears on the circuit. Readings change with temperature and damp, so record the conditions and trend each result against the baseline. The test voltage, time and minimum reading come from the nameplate, the manufacturer and the specification. Test only a de-energised circuit that is isolated, locked out and proved dead, and discharge before touching anything.
Frequently Asked Questions
Q1What is insulation resistance in AGL?
The resistance the insulation of a transformer, cable or connector offers to leakage current, measured with an insulation tester that applies a DC test voltage. It is read in megohms or higher. A high, steady value means the insulation is sound; a low or falling value warns of moisture, damage or ageing.
Q2Which insulation resistance readings are taken on an isolation transformer?
Typically three: primary to earth, secondary to earth and primary to secondary. With each winding's leads tied together (and, for the two earth readings, the other winding normally tied and earthed as the manufacturer's procedure directs), they check the primary insulation against the core and housing, the secondary insulation against the same, and the barrier between the two windings that keeps the secondary isolated from the loop.
Q3What does a low or falling reading mean?
That more current is leaking through the insulation than before, usually because of moisture, contamination, physical damage or ageing. A falling trend is an early warning, and is often caught long before it becomes an earth fault on the series circuit. Investigate, retest and replace or repair under site procedures.
Q4What reading counts as acceptable?
The test voltage, test time and minimum reading come from the nameplate, the manufacturer's data and the applicable specification, not from a general rule. Compare each result with the baseline and the previous tests as well as the minimum, and record temperature and conditions, because damp and heat lower readings.
Q5How do I take an insulation resistance test safely?
Only on a circuit that has been isolated from the CCR, locked out, tagged out and proved dead, with the transformer disconnected as the procedure requires. Apply the specified test voltage for the specified time, log the reading, then discharge the equipment before touching it. Never test on an energised circuit.
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