Electrical Testing – Continuity, Insulation – Aircraft

Continuity testing

What is the purpose of continuity testing?

The purpose of this task is to test for a break in the circuit.

There are two methods of performing this task. Either by a continuity tester ( a 3 volt battery and test bulb or L.E.D.) or by performing the milli-volt drop test using a volt meter. The latter is the preferred method but you must know both and always use the method as defined in the maintenance manual.

Before testing any circuit, The following should be performed:

  • All power removed
  • All circuit breakers, fuses and switches are in there necessary positions with respect to the maintenance manual. Note, Circuit breakers must be closed to test that circuit but some circuit breakers may need to be tripped to isolate that circuit to prevent damage to other sensitive circuits that may be connected Directly or in-directly,


If performing the milli-volt drop test, you may now apply power as required to put that circuit into its usual working condition but

  • Always work progressively and systematically
  • Always try to start nearest the supply end of the circuit if the circuit must be tested in sections.

Continuity tester method

To perform a continuity test using this method you must simply connect the positive side of the battery to one end of the circuit, connect the bulb or L.E.D. to the negative side of the battery and then connect the end of the circuit, that is not connected to anything, to the bulb. If there is an unbroken circuit, the bulb will light up and vise versa. This must always be performed on a circuit isolated from other power sources.

Millivolt drop test

This test is more preferred because not only will it tell you if you have a circuit, but it will also signify the condition of the circuit. This test is accomplished by referring to the maintenance manuals for the allowable voltage drop and test connection points. Once you have consulted with the maintenance manual and put the aircraft circuit breakers, fuses and switches in the desired configuration, Then connect your voltmeter to the test connection points and take your volt reading. Now compare your results with the allowable limits in the aircraft manual.

Please remember the following:-

  • The milli-volt test requires the circuit to be powered up to its normal operational load so there is a hazard of Electric Shock so you must be aware of electrical safety and how to protect and insulate yourself.
  • If the circuit fails the test, check the circuit for corrosion, condition and that the terminals are clean and clamped correctly.

Insulation testing

Insulation testing is performed to check that the insulation of the wires within a circuit has not degraded or failed. This task is performed by following the directions in the maintenance manual of which normally follow the steps laid out below:-

  • Disconnect all power sources from the aircraft which includes the aircraft’s own batteries as well as external power.
  • Main power switches should be in the normal stand alone position. i.e. External power off, battery and generators switched on.
  • Circuit breakers should be in the position as required by the maintenance manual. Remember that although you must have the circuit to be tested with no circuit breakers pulled.
  • Some circuits which are voltage sensitive might not appreciate 300 volts being put up them and must be isolated as per the maintenance manual. This may be by means of physically unplugging them or just by pulling their circuit breakers.
  • All ancillary equipment that might be affected by the circuit(s), must be isolated or disconnected prior to the test.

The tests now differ for the two types of systems in use, The Double pole and the single pole, The latter being the system commonly in use today, but because there are still old aircraft flying and you must know everything, as well as have a maintenance manual, both methods are shown below:

Single pole system

Place one lead connector on to the earth ( normaly the casing or aircraft structure and then the other lead connector to the wires or leads of the circuit to be tested. If the resistance is less than allowed in reference to the aircraft manual, then the exact wire which has failed must be traced and the fault located and rectified.

Please note that although this method allows a bunch of wires to be tested together to save time, It is preferred that each wire be tested individually.

Double pole system

To test the insulation on a double pole system, you should follow the following process and order. First measure the resistance between the positive and negative battery leads ( leads must be disconnected from the battery),then the resistance between earth and the positive battery lead, and then do the same for the negative battery lead.

Post Testing

After the insulation tests have been performed, then it is necessary that the wiring is connected back up to its ancillary equipment and that the circuit breakers are reset to the required positions. Once the system has been restored, then a function test must be performed in accordance with the maintenance manual before release to service.