Importance of Insulation Resistance in Marine Electrical Systems

Insulation resistance is one of the critical readings of marine electrical equipment systems and serves as the best guide to indicate the health of the electrical machinery.

As the name suggests, the insulation resistance is the ability of the insulation material to resist the current flow. With time, the insulation begins to age, and it causes deterioration in the performance of the insulation. Harsh operating environments where the electrical insulation is exposed to extreme operating temperatures, moisture, and chemical contamination, as experienced on a ship, will accelerate the deterioration process. It’s extremely critical to always know this electrical condition (IR) of the insulation in ship’s electrical equipment at all times to avoid any accident such as electrical shock, fire, short circuit, etc.

Insulation resistance is measured between the insulated conductors and earth and between conductors.

The insulation resistance is measured by the equipment known as megger, which is a high resistance meter with a test voltage of about 500 volts dc. Megger can be of the mechanical type with the hand driven operation or digital type with battery operation and electronic voltage charger.

The 500 V test charge is suitable for testing equipment which is rated for 440 volts AC.

Megger is typically used for “Spot type” test to measure the insulation’s dielectric condition at a given moment in time. The test is performed by applying a current limited DC test voltage between the conductors (e.g. Windings) and the chassis of the equipment (Ground).  Any current leakage is to be measured across the insulation’s dielectric materials. The current may be measured in Milli-amps or Micro-amps and then calculated into Meg-ohms of resistance. The lower the current value, the greater the insulation resistance.

The equipment to be tested for insulation resistance must be disconnected from the live power supply and the supply to be locked down to prevent any accidents.

On ships, the insulation resistance of all the motors, alternator and other electrical machinery is checked from time to time and values are logged as a part of planned maintenance system. The insulation resistance of the machinery reduces with increase in temperature. The reasons for the increase in temperature may be due to dust deposits on the windings or improper ventilation. The resistance is checked between the windings U-V, V-W, W-U and between U & earth, V& earth, W& earth.

On ships, machines, transformers, appliances and other apparatus are to have, at both ambient and operating temperatures, an insulation resistance of at least:

3 x∙ rated voltage in V = MΩ

Output in kVA + 1000

The minimum values of test voltage and insulation resistance are:

Rated Voltage Min. Test Voltage (V) Min. Insulation Resistance (MW)
Vr < 250 2 x Vr 1
250< Vr < 1000 500 1
1000< Vr < 7200 1000     Vr    +1


7200< Vr < 15000 5000     Vr       +1



Each power and light circuit is to have an insulation resistance between conductors and between each conductor and earth of not less than

Minimum Insulation Resistance

Up to 5 amps load 2 MΩ
10 amp load 1 MΩ
25 amp load 400,000 Ω
50 amp load 250,000 Ω
100 amp load 100,000 Ω
200 amp load 50,000 Ω
Over 200 amp load 25,000 Ω


Interior communication circuits shall have an insulation resistance between conductors, and between each conductor and ground of not less than the values provided below

Voltage Range 115V and above –  1 MW

Voltage Range Below 115V  – 1/3 MW

Important points related to Insulation Resistance:

Circuits which depend on the maintenance of high insulation resistance for their correct functioning shall not be used unless special precautions are taken to maintain the high insulation resistance; such circuits shall be provided with means to check the insulation resistance.

The main propulsion circuit shall be provided with ground leakage indicating devices that will operate when the insulation resistance is 100,000 ohms or less.

The insulation resistance of all generators shall be measured both in the cold condition and in the warm condition immediately before and after running at normal full load.

When a cable is spliced to replace a damaged section of the cable, before replacing the damaged section, the insulation resistance of the remainder of the wire is measured, and it is determined that the condition of the insulation is unimpaired.

The readings are logged down, and the graph is plotted, and the trend of insulation resistance is checked. If the reading is reduced to a megger value, then the windings have to be checked and cleaned, and the readings are to be retaken.

A separate Electric Power Insulation Resistance Test log book is provided to the ship. The insulation resistance checks can be documented on the same and kept on board as a permanent reference for future insulation resistance tests.

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About Author

An ardent sailor and a techie, Anish Wankhede has voyaged on a number of ships as a marine engineer officer. He loves multitasking, networking, and troubleshooting. He is the one behind the unique creativity and aesthetics at Marine Insight.


  1. First we should find good earth point by placing two probe at any two point on the motor body. And test the continuity. After that we should commence the insulation reading.

  2. When testing insulation resistance of generators – high or low voltage – it is essential to detach all direct (meaning, upstream of output circuit breaker) connections to voltage regulators and certain protective devices. High voltage DC may cause serious damage if this is not done. Fortunately, disconnection is often as simple as removing the correct fuses.
    Thank you for all the hard work you do maintaining this website. You do a magnificent job.

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