Explanation

In circuits protected by an RCD and MCB one or the other or both devices may operate depending upon the level and type of fault.

This simulator shows how a 230V circuit protected by a 16A type B breaker and a 30mA RCD reacts when various values are changed via the sliders.

There are 3 sliders. The 2 red sliders combine to alter the L - E insulation resistance. The sliders set the value using Standard Form notation but display in long hand near the top right of the tool (values are changeable from 0.1 to 9900000Ω) whilst the black slider alters the resistance of a purely resistive load such as a heater (1 to 200Ω).

By adjusting these sliders, values such as power (wattage of the heater) and the currents flowing in the phase, neutraI and earth conductors change. The effects of these changes can be hidden or displayed by ticking/unticking various checkboxes.

Sliders can be dragged to approx positions and then fine tuned by using left and right arrows.

Tasks

The following steps should be carried out and your observations recorded in the answer sheet which is downloadable via the Download link below. The completed answers sheet should then be emailed as an attachment to Robert.Eyre@Hull-College.ac.uk

• With the Fault Resistance set to maximum investigate how changing the Load Resistance value alters both the power consumed by the load and how the currents in the 3 conductors change.
• With the MCB checkbox checked alter the Load Resistance to its extremes investigate at what points the MCB decides to do something different. Write down these values
• Investigate how these thresholds would change with different breaker ratings and different types (B, C and D)? Write down your answers
• It is often handy when testing Insulation Resistance to have an idea what value certain loads would show up as so that you can easily differentiate between faults and appliances left connected. Use the Load Resistance slider to find out the resistance you would expect get in a P to N Insulation Resistance test if a 3kW Immersion were left connected during the test.
• Write down the Load Resistance value that gets nearest to 3kW and the current this would draw.
• Investigate 4 others you might encounter and for each write down their wattage, the current they would draw and the resistance value they would give.
• With the RCD checkbox checked and the load set to this near 3kW level start altering the Fault Resistance values to find the 3 thresholds were the RCD state changes.
• Record the fault resistance levels where these changes occur.
• For each of these thresholds record the three values of the currents (P,N & E).
• Write down what you notice with these sets of values.
• Investigate why these 3 levels were chosen and write an explanation for each.
• With both the MCB and RCD checkboxes checked reduce the Fault Resistance to a very low level. At what level of resistance do both the RCD and MCB operate?
• Write down why both would want to operate and what is likely to happen if this happened.