# Fault Diagnosis

Fault Diagnosis questions are likely if you are applying for a position as an electrical engineer or technician. They are used in assessing skills for technical personnel who need to be able to find and repair faults in electronic control systems.

These types of faults are often caused by the failure of a single component within the system, but because the component has failed internally it is impossible to identify it by examining it. This means that the technician needs to use a logical process of elimination in order to isolate the faulty component.

The ability to approach a maintenance or repair problem in a logical and systematic way to to locate the fault is critical when working with complicated systems like those found in aircraft, ships and communications equipment.

Fault diagnosis questions generally have two separate components. Firstly, a grid which describes the functions of a series of switches.

This grid shows four switches and their effect on four numbered inputs which may be either on or off. The first switch (diamond symbol) inverts inputs one and two. That is, if the input is on, it is turned off and if it is off it is turned on. The other switches act in a similar way as detailed in the grid. If a switch is not working then it has no effect on the input which passes through unchanged.

The second component is a flowchart showing: the input, switches, and the output. Each of the switches acts to alter the input before the next switch in the series.

In this example flowchart, all of the inputs (numbered 1, 2, 3, 4) are on. This is indicated by them being shown in white.

When these inputs pass through the first switch 1 & 3 are inverted (i.e. switched off).

These modified inputs (1=off, 2=on, 3=off, 4=on) then pass through the second switch.

The second switch inverts inputs 3 & 4, which gives (1=off, 2=on, 3=on, 4=off).

These modified inputs (1=off, 2=on, 3=on, 4=off) then pass through the third switch.

The third switch should invert inputs 2 & 4, giving (1=off, 2=off, 3=on, 4=on).

However the third switch is not working so the output from switch two is not changed.

The answer to this question is therefore ‘C’ – because switch type ‘C’ is at fault.

#### Key Points

• Fault Diagnosis questions are likely if you are applying for a position as an electrical engineer or technician.
• Faults are often caused by the failure of a single component within the system.
• This cannot be identified visually and must be identified by a process of elimination.
• Fault diagnosis questions consist of a grid and a flowchart.
• You need to look at the output and identify the component that has failed and produced the unexpected output.