Explanation

This simulator has sliders which allows you to alter the external impedance (Ze), the supply voltage and circuit length in metres

Ze can be altered from 0.01 to 30Ω in 0.01Ω increments.

Supply Voltage can be altered from 216V to 253V in 1V increments.

Circuit Length can be altered from 0.25m to 100m in 0.25m increments

On changing these sliders the other elements of the circuit alter automatically as does the fault current to earth.

Individual characteristics and or steps can be hidden or displayed by ticking/unticking various checkboxes.

Instruction

Earth Fault Loop Impedance or Zs comprises 3 components

1. Ze which is outside of the control of the designer/installer but has general upper limits depending upon Earthing System feeding the installation.
2. R1 (resistance of phase conductor from consumer unit to furthest point on circuit), which is within control of the designer/installer as the CSA and /or length of circuit can be altered.
3. R2 (resistance of CPC from consumer unit to furthest point on circuit), which is within control of the designer/installer as the CSA and /or length of circuit can be altered.

Ze can be altered to simulate likely values present at the incoming supply.

R1 and R2 are altered indirectly by changing the Circuit Length slider (as the length increases the values of both R1 and R2 increase in proportion to length chosen- note they increase at slightly different rates with R2 increasing more quickly to emulate it being the slightly smaller CSA such as found in twin & CPC cables)

As the Zs value increases the Prospective Earth Fault Current decreases and vice versa.

There is a sweet spot or range for each breaker. The Zs value needs to be below an upper limit of Zs in order to create a sufficiently high fault current to operate the fuse or breaker quickly BUT ALSO be above a Zs value that would create too high a fault current that the breaker or fuse is not capable of safely clearing.

In order to disconnect within a given time of 0.4s or 5s a circuit needs to have a sufficiently low Zs value. These max Zs values are dependent upon the type of device and its rating. These values are given in a downloadable table available via the link at the bottom of this page.

Note all calculations display to 2dp which means some values displaid mid calculation may be a bit misleading.

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 supply voltage set to 230V and all checkboxes checked alter the sliders to simulate a variety of scenarios from a very short length circuit fed by a consumer unit in a building fed from TNCS system having a substation close by to a long circuit fed by a TT system. For each one.

1. See how the Zs value and Prospective Earth Fault Current (PEFC) change.
2. See which of the following sizes of BS60898 type B 6k breakers would be suitable to use, whether they would disconnect within 0.4 secs at that Zs and where not suitable state why and what could be done to protect those circuits. Remember too high a fault current is also an issue.
• 6A
• 16A
• 32A
• 40A

Use the simulator to calculate Zs, PEFC and suitability of breakers as described above for the following scenario (all at Supply Voltage of 230V).

1. Highest Ze value on TNS system with 1m long circuit.
2. Highest Ze value on TNS system with 10m long circuit.
3. Highest Ze value on TNS system with 100m long circuit.
4. Highest Ze value on TNC-S system with 10m long circuit.
5. Highest Ze value on TNC-S system with 10m long circuit.
6. Highest Ze value on TNC-S system with 100m long circuit.
7. 30Ω Ze value on TT system with 10m long circuit.
8. 30Ω Ze value on TT system with 30m long circuit.
9. Ze value of 0.01Ω with 100m long circuit.
10. Ze value of 0.01Ω with 50m long circuit.
11. Ze value of 0.01Ω with 2.5m long circuit.
12. Ze value of 0.01Ω with 0.25m long circuit.