Insulation Resistance Testing

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Insulation Resistance Testing

When it comes to electric testing, several tests can be conducted by certified electricians to confirm the efficiency and safety of an electrical installation. However, one basic test that stands out the most is the insulation resistance test.

This test not only checks the wiring of an installation but also makes sure that the wires are fit to carry heavy loads without causing excessive heat.

Why is this important? Well, movement creates heat via friction, and in the same way, the movement of electrons through a wire with resistance generates heat. Normally, this heat output is balanced, and the wires can easily dissipate heat without damage. However, when the resistance fails, the wires can quickly lead to electric faults, short circuits, or worse, fires.

In this guide, we will dive deep into the concept of insulation resistance testing, what it is, what’s involved in it, the testing process, and more!


Insulation resistance testing or IR testing is usually the first type of electrical test that certified electricians run on installations.

The result of this test is so important that it determines whether an installation is fit to run other tests! Insulation resistance testing is performed using a megohmmeter or other similar testing instruments that can show the resistance and load of an installation.

Let’s go over some of the most important processes in insulation resistance testing:


To run an insulation resistance test, a certified electrician will attach a megohmmeter or a “megger” to a circuit.

The megohmmeter is specifically designed to accurately measure the load and resistance of a circuit and display it according to the specified standards set by the manufacturer of the tester.

An electrician may also carry a British standard handbook to accurately determine the testing load and time required for the insulation resistance test.


Before the test is performed, the electrician may run a continuity test. In essence, a continuity test is a quick way to check if a circuit is open or closed.

If the circuit is closed, this confirms that the circuit has no load and that the resistance test can be performed. If the continuity test fails, then this would mean that there is an internal issue in the installation and that the power box needs to be thoroughly checked by a certified electrician for errors.

Once the preliminary tests are completed, the electrician will de-energise the installation and make sure that the system is isolated from the main power source.


Once everything is in order, the electrician will continue with the insulation resistance test.

The electrician will first connect the test leads of the resistance tester to the nodes within the installation. In most cases, one lead is connected to the equipment being tested while the other is connected to the ground or, in some cases, the chassis of the equipment being tested.

The actual test revolves around applying a high-voltage DC current to the installation. Once the installation is flooded with electricity, the current flow is recorded and displayed by the megger. Normally, the duration of the test can range from a couple of minutes to even hours. The duration depends on the type of electrical instrument being tested.

For most home installations, the duration of a typical insulation resistance test is usually around a minute.

As the name suggests, the megohmmeter displays the insulation resistance in megaohms. This value can then further be processed to give an accurate reading of the insulation resistance according to various standards.

For example, AC motor windings, heavy-duty machines, and power boxes can have different values that need to be corrected using the appropriate standards. A certified electrician will know exactly how to interpret and calculate the results to get an accurate reading.

Test Analysis

If you watch a certified electrician perform this test, you will notice that they write down the readings on a form. This form is extremely useful in not just keeping track of the readings, but also in helping the electrician calculate and interpret the final test results.

The test results are compared to the standards set by the British Standards. A low value may indicate that the installation is wearing down and that there could be signs of degradation. On the other hand, a higher value indicated that the insulation is effective and that the installation is in good health.


The primary role of any electrical tests is to determine the health of the electrical installation. In the case of insulation resistance testing, this test mainly involves testing the resistance within the wires of the installation.

Resistance tests are incredibly important in determining the overall health of the installation. So, if the insulation resistance test fails, there would be no point in conducting the other tests since the installation will not be deemed fit!

An insulation resistance test can help highlight potential problems that may lead to serious issues.

Insulation Resistance tests can help identify the following faults:

  • Faulty wiring,
  • Overloading, overheating, or short-circuiting,
  • Damaged cables,
  • Damage due to water/moisture ingress.

Here is why you need to conduct regular insulation resistance tests:

Integrity of Installation

The role of insulation testing goes beyond testing for mere resistance in a circuit. This test can help determine whether an installation is valid or not!

The integrity of an installation is important for not just making sure that the system works as intended, but also that it doesn’t pose any risks related to electrical failures, shocks, or fires.

An insulation resistance test can help electricians confirm that an installation is free from defects like cracks, breaks, cuts, or moisture-induced damage. These faults may not cause immediate problems, but if left unchecked, they can wreak havoc and have the potential to cause quite a lot of damage.

Current Leak Prevention

Insulation testing can test more than the safety of an installation! It also tests for efficiency.

For example, current leakage is the leading cause of electric shocks, damage, and fires. These leaks may also lead to inefficiencies and potentially even higher electricity costs.

If the insulation resistance test reveals lower values, then this would indicate that the electrical installation has leakage points which could cause short circuits, equipment damage, and excessive heat generation (which leads to fires).

Detects Depreciation

Electric equipment is bound to depreciate. No matter how perfect the installation is, if it generates heat and is exposed to the environment, then it is bound to deteriorate with time.

Insulation resistance testing can help highlight this type of depreciation in installations. The test results directly relate to the efficiency and health of the overall installation. If you get lower values, then the system is bound to have several inefficiencies due to deterioration.

Compliance with Standards

Finally, insulation resistance testing also helps homeowners comply with regulations. Legally speaking, everyone with access to electricity needs to perform this basic test as part of a larger series of tests to comply with the appropriate regulations.

These tests can not only help keep things running smoothly, but they can also prevent serious damage and ensure the safety of the installation as well as the people occupying a space powered by the installation.


As mentioned above, electrical components are prone to general wear and tear, especially when subjected to regular use, like in commercial properties. Even in the case of residential properties, some components might be more susceptible to depreciation than others.

For example, HVAC systems are always connected and usually always on. These systems are power intensive and can also be sensitive to surges. This is why these systems, in particular, may lose resistance over time!

Insulation Resistance tests and general electrical tests aim to:

In general, there are three safety checks involved in electrical testing and inspections:

User checks

User checks are a great informal way of testing an installation without getting into a lot of technical details. These tests can be performed by laypeople – so, if you notice anything off like flickering, sparks, or overheated socketsf, this would count toward a typical user check.

The goal of user checks is to detect obvious signs of damage such as damage to the cables, plugs, and wall outlet, burn marks on the equipment, plugs, sockets, frayed cables, and water ingress damage.

Visual inspections

A visual inspection is an informal but useful way to ensure the safety of an installation by taking an overview of the health of the electrical system. Visual inspections can help highlight warning signs. Here are some of the most important warning signs:

  • Flickering, persistent electrical sounds, or fluctuating lights: if the lights have a buzzing sound when turned on or flicker when in use with other electrical devices, then this would indicate an electric fault in the wires or the connection.


  • Discolouration, excess heat, and smoke: All three of these signs are a strict no-go! Discolouration that involves blackish hues or smoke near the outlets may indicate a fault in the electrical system. A build-up of heat occurs due to a fault in resistance or general damage to wires or electrical equipment.
  • Warm wall outlets: outlets should never be warm! If the outlet is hot or even warm without use, then this would indicate that the socket and the general installation need to be thoroughly checked by a certified electrician.
  • Circuit breaker trips: if circuit breakers trip without the presence of a surge, then this would indicate that the system needs to be checked for voltage or general electrical faults. In this case, the breakers might trip often and may even fail. Call a certified electrician to check if there is a voltage fluctuation issue due to incorrect installations or resistance failure.
  • Burning and odd odours: Smell smoke or burning odours? Call in the professionals! Outlets and circuit boards never carry any smell. So, if you notice anything outside of the usual, you can assume that something is burning inside. Always make sure that you call emergency services at the first sign of smoke or fire.

Detailed inspections – Electrical Installation Condition Report (EICR)

For more serious and comprehensive electrical testing, we recommend that you go for an Electrical Installation Condition Report (EICR).

EICRs are far more detailed than a visual assessment. These reports thoroughly check electrical installations for faults that visual inspections may overlook.

These tests are extremely thorough and cover basics such as insulation resistance testing along with other complex tests.

Due to their detail and thoroughness, trained NICEIC-approved electrical contractors, like Calder Electrical, use electrical inspection checklists to guide them through the EICR. These checklists help the electrician(s) comply with the wiring regulations and can help them record, analyse, and share the test results according to the appropriate SOPs.

If the inspections identify any faults with the system, the fault is categorised into one of three codes, code C1, C2, and C3, depending on the level of danger there is.

  • Code C1: indicates that the property is high-risk and immediate action is required.
  • Code C2: indicates that there is potential danger and remedial action is required at the earliest.
  • Code C3: indicates that the system could use some improvements and that investigation is required at the earliest.

If the property passes the electrical inspection, you will be provided with an EICR Certificate of Safety which states that all your electrical installations are safe and up to date.

The certificate will also contain information about various electrical tests. For example, you can easily navigate through the certificate to find the values for the insulation resistance test. These records are important because they can be shared with other certified electricians for future testing or for when you want major changes in the installation down the road.


We have been providing electrical services to residential, commercial, and industrial properties for over 40 years and are specialists in all aspects of electrical installations, maintenance, and testing.

All our work is fully insured, and we work to meet the standards set by the NICEIC to make sure all your electrical installations meet the current regulations, and all the notifiable work is signed off by the local authorities.

If you have unplanned issues within your home or business, we can provide a call-out service to rectify and solve any problems such as circuit tripping, loss of power, broken items that need repairing, or to check if you feel something is unsafe or a potential hazard.

Our team can generate a Visual Inspection Report (VIR) to identify anything visual that does not comply with the standards or looks unsafe, as well as a full Electrical Installation Condition Report (EICR) where we will identify and test each circuit to make sure they are not deteriorating and are safe for use within the current standards and regulations set by the BS 7671.

We can also provide PAT testing of appliances to make sure that any items such as kettles, computers, and microwaves are safe for use in the home and workplace.

Contact us here or call us at 0800 612 3001 to get in touch with our professional electricians for more information on insulation resistance tests and the importance of regular electrical maintenance and testing for your property.

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