Insulation Resistance Testing: Why We Do It, How to Do It, and Common Problems

Insulation resistance testing is one of the essential dead tests required by BS 7671 during initial verification and periodic inspection. Its purpose is straightforward: to confirm that the insulation between live conductors and between live conductors and earth is intact — preventing leakage currents, short circuits, and electric shock.

 

Yet it’s also the test that causes the most confusion. Low readings, false positives, the neutral conductor problem, and the risk of damaging electronic equipment all make IR testing more nuanced than it first appears. This guide covers everything you need to know.

 

Why Do We Test Insulation Resistance?

Every cable in an installation relies on its insulation to keep current flowing where it should — through the conductors, not through the insulation material and into other conductors, metalwork, or the earth.

 

Over time, insulation can degrade due to:

• Age and wear — PVC becomes brittle; rubber perishes
• Heat damage — from overloaded cables or thermal insulation contact
• Moisture — water ingress in outdoor or damp locations
• Mechanical damage — nails, screws, rodent damage
• UV exposure — cables exposed to sunlight

 

If insulation breaks down, the result is a leakage current — current flowing through a path it shouldn’t. This can cause RCDs to trip nuisance-wise, increase fire risk, and create electric shock hazards.

 

The IR test applies a DC voltage across the insulation and measures the resistance in megohms (MΩ). High resistance = good insulation. Low resistance = potential problem.

 

Test Voltages and Minimum Values

Regulation 612.3.2 of BS 7671 specifies the test voltage and minimum acceptable insulation resistance for different circuit voltages:

 

 

For the exam, the key figures to remember are:

• Standard 230V circuits: test at 500V DC, minimum 1.0 MΩ
• SELV/PELV circuits: test at 250V DC, minimum 0.5 MΩ

 

In practice, a healthy domestic circuit should read well above 1.0 MΩ — typically 200 MΩ or higher. If you’re getting readings between 1 and 2 MΩ, the circuit technically passes but should be investigated, as this suggests deteriorating insulation.

 

The Three Test Connections

The IR test must be carried out between all combinations of live conductors and earth. The circuit must be dead and isolated before testing.

 

 

The three tests are:

1. Line to Neutral (L–N) — checks insulation between live conductors
2. Line to Earth (L–E) — checks insulation between line and protective conductor
3. Neutral to Earth (N–E) — checks insulation between neutral and protective conductor

 

Before testing, you must:

• Isolate the circuit from the supply
• Disconnect or switch off all current-using equipment (lamps, appliances, electronic devices)
• Set all switches to the ON position (so you test the complete circuit, not just up to a switch)
• Disconnect any electronic devices that could be damaged by the 500V test voltage (dimmer switches, PIR sensors, RCDs with electronic trip, smart home devices)

 

The Neutral Conductor Problem

This is the single most common cause of confusion in IR testing. When you test between Line and Neutral, any load that’s still connected across L and N creates a parallel path through its element or winding. This dramatically lowers the IR reading — even though the cable insulation is perfectly fine.

 

 

How to Avoid False Low Readings

• Unplug all appliances from sockets before testing
• Remove all lamps from lighting circuits (or disconnect them at the lampholder)
• Switch off immersion heaters, towel rails, and any hardwired loads
• Disconnect electronic equipment that can’t handle the test voltage
• If the L–N reading is still low but L–E and N–E are high, suspect a remaining connected load

 

The Practical Shortcut

On large installations, disconnecting every load can be impractical. A common approach is to link Line and Neutral together at the distribution board and test them as one conductor to Earth. This gives you an L+N to E reading.

 

This method confirms the insulation between live conductors and earth, which is the most safety-critical measurement. However, it does not test the insulation between L and N — so it shouldn’t be the only test on initial verification.

 

False Positives and False Negatives

False Positives (Low reading, no actual fault)

Cause	Why it happens	Fix
Appliances still connected	Parallel path through load	Disconnect all loads
Neon indicators in switches	Low resistance through neon	Disconnect or accept as normal
SPDs (surge protection)	SPDs have low L-E impedance	Disconnect SPDs before testing
Moisture in accessories	Damp junction boxes or sockets	Dry out and retest
Long cable runs	Very long circuits accumulate leakage capacitance	Accept if above minimum; test individual sections

 

False Negatives (Good reading, fault present)

Cause	Why it happens	Risk
Fault only appears under load	IR test is at low current; fault needs heat/vibration to manifest	Fault may cause RCD tripping in service
Intermittent damage	Mechanical damage that makes contact only sometimes	Circuit may fail unpredictably
Testing only at the DB	Fault is beyond a switch in the OFF position	Set all switches ON to test complete circuit

 

Interpreting Your Results

IR Reading	Interpretation	Action
200+ MΩ	Excellent — new or well-maintained insulation	None — record and move on
2–200 MΩ	Acceptable — insulation is satisfactory	Record; note for comparison on next inspection
1.0–2.0 MΩ	Marginal — technically passes but investigate	Check for moisture, damaged cables, or connections
Below 1.0 MΩ	FAIL — insulation is inadequate	Investigate and repair before energising
0 MΩ	Dead short — insulation has completely failed	Major fault — locate and repair

 

Sequence in the Testing Process

Insulation resistance testing is the third test in the BS 7671 testing sequence:

1. Continuity of protective conductors (R1+R2)
2. Continuity of ring final circuit conductors
3. Insulation resistance ← you are here
4. Polarity
5. Earth fault loop impedance (Zs)
6. RCD operation

 

It must be carried out as a dead test — before the circuit is energised for live tests (Zs and RCD).

 

Key Regulations

• Reg. 612.3 — Insulation resistance testing requirements
• Reg. 612.3.2 — Test voltages and minimum values (Table 61)
• Table 61 — Minimum insulation resistance values
• Reg. 612.3.3 — Where SELV or PELV is used
• Chapter 61 — Initial verification sequence

 

Practice and Further Study

Insulation resistance testing is a core Part 6 topic. Test your knowledge:

• Part 6 — Inspection and Testing quiz

 

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