Question 1

Explain in your own words "tolerance".

The tolerance is the allowable variation from a nominal value. A tolerance can be expressed as a unit value, or as a ratio like a percentage.

NOTE: Tolerances only apply under normal operating conditions. A component that is used beyond its ratings can perform outside its tolerance range.


Question 2

Explain the difference between "power rating" and "power dissipation".

Power rating is the maximum power dissipation that a component is designed for. The power dissipation is the actual amount of power the component consumes in circuit.

NOTE: A properly selected component in a circuit operating under normal conditions will always have a power dissipation less than its power rating.


Question 3

Why are colour codes used on resistors?

Colour codes are used as a means of identifying the attributes of the resistor, like resistance, tolerance, stability etc.

Colour codes were historically used as many resistors were small and round, and so quite awkward to mark with printed or written digit values. Many resistors can be marked with digits (including small surface-mount resistors). Colour coding is mainly used today for round bodied resistors.


Question 4

What does Brown, red, orange mean as coloured bands on a resistor?

There are three bands. This code may be interpreted as a 3-band value code, or a 4-band colour code, with the lack of a 4th band meaning a tolerance ±20%. Either way, the value bands mean the same. Brown means "1", red means "2", and orange means "multiply by 1000".

The marked resistance is therefore 12000 Ω, or 12 kΩ.


Question 5

What is the advantage of a 5 band resistor colour code system?

The advantage is that a 5-band system gives you an extra significant digit in the value bands. This allows you to express values like 499 Ω, which is not possible with two value bands.


Question 6

What is the minimum voltage applied for an insulation resistance test?

This question is ambiguous, because the minimum value varies with the circumstances. In general, you won't go wrong saying 500 V, but this is a "general" minimum, and not the "universal" minimum. The test voltage may be cut to 250 V if a protective device like a VDR or varistor is preventing the voltage from reaching 500 V.


Question 7

How does length affect insulation resistance?

This question can be answered using the equation \( R = \frac{\rho L}{A} \). \( R \) is the resistance, \( \rho \) is the resistivity, \( L \) is the conductor length, and \( A \) is the conductor area.

For insulation resistance, the length \( L \) is based on the thickness of the insulation - since that is the path the leakage current takes. The area of the insulation \( A \) is proportional to the length of the cable.

To avoid confusion, the resistance equation for insulation resistance is re-written with new variables:

\( R = \frac{\rho_{\mathrm{i}} T}{2 \pi r L} \)

where \( R \) is the insulation resistance, \( \rho_{\mathrm{i}} \) is the insulation resistivity, \( T \) is the insulation thickness, \( r \) is the conductor circumference (for a round conductor, it is the circumference of the conductor), and \( L \) is the cable length.

This equation can be simplified further. \( \rho_{\mathrm{i}} \), \( T \), and \( r \) are all constant, so all of those variables can be condensed into a new resistivity \( \rho \) as shown below.

\( R = \frac{\rho}{L} \)

The value of \rho can be determined if a given length of cable and insulation resistance are known.

\( \rho = R L \)

The resistance \( R_2 \) of any other length of cable \( L_2 \) can be calculated as below.

\( R_2 = \frac{\rho}{L_2} \)

The length of cable \( L_2 \) with a given resistance \( R_2 \) can be calculated as below.

\( L_2 = \frac{\rho}{R_2} \)


Question 8

Give 3 examples of what can cause a cable to fail an insulation resistance test.

This answers will only be about the cable itself. Any termination or connection faults or errors that could cause a failed insulation resistance test are not considered.

This question is quite broad, some answers may overlap in scope.

Some factors are:

  • insulation damage;
  • insulation ageing;
  • mechanical damage;
  • moisture contamination;
  • defective insulation;
  • over heating;
  • over current;
  • over voltage.


Question 9

With the aid of a graph explain how a VDR protects electrical equipment.

A VDR designed for 230 V devices protects equipment by "clamping" the voltage across it to ~400 V. The VDR does this by "flipping" from a high resistance state below ~400 V to a low resistance state above 400 V. This value is above the 325 V peak of the mains waveform, so the VDR does not continuously conduct mains current.

The graph shows this conduction characteristic. The steeper slope below 400 V reflects a higher resistance. The shallower slope up to 400 V represents a decrease in resistance, or a larger increase in current without a significant increase in voltage.


The VDR absorbs the surge energy by turning it into heat.

A VDR can fail from attempting to absorb too much surge energy.


Question 10

What is the effect on a cable of exceeding the voltage rating?

This question is quite broad. Exceeding the voltage rating will not necessarily result in instant destruction and fire. All cables have a "safety margin" built in. The safety margin is only really there to protect against performance reductions at the rated voltage. Exceeding a cable voltage rating could cause any one or more of the following:

  • shortened cable life;
  • reduced insulation performance;
  • insulation failure e.g. "punch through" from over voltage.

I don't think any cable manufacturer will accept liability for the consequences of using cable beyond its voltage rating.


Question 11

What is the effect on a cable of exceeding the current rating?

This question is quite broad. Exceeding the current rating will not necessarily result in instant destruction and fire. All cables have a "safety margin" built in, and wiring rules further enforce safety margins. The safety margin is only really there to protect against performance reductions at the rated current.

Overcurrent protection is designed to protect a cable from thermal overheating. If the overcurrent protection is selected correctly, there should never be a problem with cables overheating.

In general, current flow causes heating through \( I^2 R \) losses. The insulation is exposed to this heat directly. The insulation has a maximum temperature it can withstand. If this temperature is exceeded, irreversible damage can result. In the worst case, the wires can melt through the insulation (PVC melts at ~150°C, copper melts at ~1085°C).

Solid insulation cannot recover from melting or charring damage. A cable with melted or charred insulation is only good for scrap.

Exceeding a cable current rating could cause any one or more of the following:

  • shortened cable life;
  • cable overheating;
  • reduced insulation performance;
  • insulation failure.

I don't think any cable manufacturer will accept liability for the consequences of using cable beyond its current rating.


Last modified: Wednesday, 20 May 2020, 4:02 PM