How is insulation resistance tested under the Spanish REBT?

Preparation and connection

Isolate, prove dead and separate conductors from earth and the usual source. For the test to earth, active conductors including neutral are linked and tested together to earth. Conductor-to-conductor tests are then made pair by pair with loads disconnected.

Where circuits contain electronics, phases and neutral are linked during the specified test so that test voltage is not applied through the equipment.

Test voltages and the scope of the minimum values

ITC-BT-19 table 3 associates these minima with installations whose combined wiring-route length does not exceed 100 m. Longer installations should be divided into approximately 100 m parts where possible. Where division is impossible, the instruction provides an inverse adjustment based on total route length in hectometres. Apply this within the full verification procedure, not as a general relaxation of the requirement.

Reading the result

Passing the minimum alone does not describe the full condition. Circuit comparison, length, moisture, connected loads and trends help locate deteriorating insulation.

For the next part of the decision, see the guides to Earth resistance and Protective continuity.

How to interpret a low insulation resistance reading

Low insulation resistance can come from a cable, moisture in a junction box or connected equipment. Testing the complete installation at once does not identify the faulty part. Professional diagnosis separates circuits and repeats the test under comparable conditions, recording the test voltage and which devices remained connected.

The 0.5 MΩ minimum quoted for installations up to 500 V comes from the ITC-BT-19 table; it is not a general quality target for new wiring. The applicable verification procedure, referenced standards and equipment may impose other criteria. A reading only just above that figure needs interpretation against circuit length, moisture and condition, rather than automatically establishing that the whole installation passes.

An ordinary multimeter is insufficient: an insulation tester applies a specified DC test voltage. All sources, including inverters and backup supplies, must be controlled before testing, and the procedure for disconnecting sensitive electronics followed. Afterwards the tested circuits are discharged and protective connections checked for correct restoration.

Step-by-step test procedure

1. 01

   Isolate, lock off and prove dead.

2. 02

   Remove or protect sensitive equipment and discharge capacitance.

3. 03

   Split the installation into identifiable sections where possible.

4. 04

   Apply the specified test voltage and record the stable reading.

5. 05

   Discharge conductors, restore connections and record the test.