ohms, TN-S (cable sheath) 0.8 ohms, TT system 21 ohms

( Table A L Table B or C )

Sizing Cables Conduit and Trunking REV4.1 32 Example

A circuit supplying a DB where a multi core armoured cable is clipped direct using 50 metres of multi core 25.0mm² 70°C armoured thermoplastic insulated cable. The bunched CPC conductor size is 16.0mm². The Ze is 0.5Ω. Calculate the earth loop impedance in Ohms at the maximum operating temperature.

i) Write down the formulas and obtain the values for each part.

Ze = 0.5Ω; R1+R2 must be calculated; L = 50m ii) Obtain the value for R1 and R2 in Ohms

Using Table 9A we can see that the resistance, in milli-ohms per metre, of 25.0mm² and 16.0mm² is 1.877mΩ/m.

iii) Obtain the multiplier value from table 9C

The line and earth conductors are part of a thermoplastic multicore cable so are classed as incorporated in a cable or bunched giving us a value of 1.20. Input all values into the R1+R2 formula

iv) Input the values into the main Zs formula and calculate Zs at the maximum conductor operating temperature

Turn to Table 9A of your Tables from BS7671 and Onsite Guide and complete the questions on the next page to gain some understanding of earth loop impedance calculations.

Multipliers are used by the designer and are required to allow for one of the following:

Table 9B – Used so the designer can give values of resistance at the ambient temperature expected during the tests (20⁰C is classed as 1)

Table 9C – Used so the designer can give values of resistance at the conductor’s maximum operating temperatures

Note: If the R1 and R2 value is measured this can be added to Ze to give the total Zs

1. A circuit supplying a DB using 50 metres of multi core 70°C armoured thermoplastic insulated cable. The line and earth conductors are both 25.0mm². What is the expected Zs at the maximum operating temperature?

2. A motor circuit where single core conductors are installed in p.v.c conduit using 40 metres of 90°C thermosetting single core.

The line and earth conductors are both 4.0mm². What is the expected Zs at the maximum operating temperature?

3. A cooker circuit where 10m of cable is installed in building fabric using 70° thermoplastic (p.v.c) insulated and sheathed flat cable with protective conductor. The line and earth conductors are 6.0mm² and 4.0mm² respectively. What will be the expected Zs at 10^oC?

4. A power circuit where a 2.5 mm² cable is installed on a tray using multi core 90°C armoured thermosetting insulated cable.

The circuit length is 80 metres.

What will be the expected Zs at 5^oC?

Complete the following exercise to determine the earth loop impedance (Zs) of the circuits.

Assume in all cases that the Ze = 0.3 Ω. You will need your “Tables from BS7671 and the on-site guide” appendices. You must show all working out and state the R1 and R2 values and show what Zs is for each circuit.

Sizing Cables Conduit and Trunking REV4.1 34 Earth fault current

Once the Zs has been calculated we then calculate the earth fault current (If) using the calculation below and then ensure that the device will disconnect within the given time using the time / current trip curves in the tables from BS7671. If the time is less than the maximum allowable for the circuit (i.e. At 230v, 0.4 or 5 seconds) we can be sure that it will disconnect in time so that it provides protection from indirect contact.

We can also ensure that the protective device’s short circuit fault current capacity has not been exceeded.

Example

A 230 V circuit is protected by a 15 A semi-enclosed (BS3036) fuse and has an earth-fault loop impedance of 1.6 Ohms. What will be the maximum earth fault current?

This level of earth-fault current will cause the fuse to operate quickly. From the time / current trip curves in BS7671, Fig 3.2A (more on these next) the time taken for the fuse to operate will be about 0.15 s. Any load current in the circuit will be additional to the fault current and will cause the fuse to operate slightly more quickly.

However, such load current must not be taken into account when deciding

disconnection time, because it is possible that the load may not be connected when the fault occurs. Therefore if the earth loop impedance is higher this will restrict the flow of fault current meaning the protective device will take longer to operate.

To gain some appreciation of fault current calculations complete the questions on the next page.

I f - term used to describe a circuit’s earth fault current, in amps.

U o - term used to describe the nominal voltage to earth, in volts.

Z s – term used to describe the earth fault loop impedance, in ohms.

Zs If = Uo

Zs amps

If Uo 143 . 75 6

. 1 230 =

=

=

1. A circuit with a BS3036 overcurrent protective device rating of 30A has an R1+R2 value measured at 0.79 Ω.

2. A power circuit with a BS88

overcurrent protective device rating of 32A has an R1+R2 value

measured at 0.56Ω.

3. A BS88 63A fuse protects a feed to a three-line socket. The measured R1+R2 value is 0.2 Ω.

4. A lighting circuit is protected by a BSEN60898 – 10amp Type B MCB. The measured R1+R2 value is 1.5Ω.

5. A radial power circuit is protected by a BSEN60898 – 32amp Type C MCB. The measured R1+R2 value is 0.8Ω.

Complete the following exercise to determine the maximum earth fault current (If) of the circuits.

Assume in all cases that the Ze = 0.3 Ω. You must show all working out.

Sizing Cables Conduit and Trunking REV4.1 36 Time / current characteristics and disconnection times

In appendix 3 of BS7671 there are graph-like tables that represent the time / current characteristics of the main types and rating of circuit protection. They are used to determine the time it takes a device to operate under a certain amount of fault current.

If you look at the time/current curve you will find that the scales on both the time (seconds) scale and the prospective current (amperes) scale are logarithmic and the value of each subdivision depends on the major division boundaries into which it falls.

For example, on the current scale, all the subdivisions between 10 and 100 are in quantities of 10, while the subdivisions between 100 and 1000 are in quantities of 100 and so on. This also occurs with the time scale, subdivisions between 0.01 and 0.1 being in hundredths and the subdivisions between 0.1 and 1 being in tenths, etc.

If you look at Fig.3.2A in your “Tables from BS7671 and the on-site guide”

appendices you can see that current, in amps, is represented along the bottom (Χ-axis) and along the side (Y (Χ-axis) is the time in seconds.

Also available on each graph is a quick reference table that displays the main disconnection times and the required amount of current to achieve those times.

Each line between two points represents a value in the lower of the two units

Three lines above the number 10 represents the value 40……

Remember:

• Any circuit rated at 32A or less must disconnect within 0.4 seconds

• A distribution circuit or circuit exceeding 32A must disconnect within 5s

Example

Find out the expected disconnection time of a circuit that is protected by a 30A BS3036 fuse when 70 amps of fault current flows.

i) Obtain the correct time/current graph for the selected protective device from Appendix 3 of BS7671.

Look at Fig.3.2A. Find the 30amp BS3036 fuse trip characteristic line.

ii) Identify the fault current value on the Χ axis and follow it to the point where it crosses the selected fuse rating curve.

Identify 70 amps on the Χ-axis. Follow 70 amps upwards until it crosses the 30amp curve.

iii) At the point where the fault current crosses the fuse rating curve follow the line across to the Y axis. Identify the value of time in seconds.

Follow the point where 70amps crosses the fuse line and track it (left) to the Y axis and obtain the time. The disconnection time is expected to be 20 seconds.

Over-current device and

You should now have a good grasp of the way we determine the disconnection times of a few devices based upon fault current levels.

What if the disconnection time is higher than the maximum allowed?

What can the designer do to ensure that shock protection is afforded?

Complete the exercise below using the “Tables from BS7671 and the on-site guide” appendices to determine the operation times of different devices.

Sizing Cables Conduit and Trunking REV4.1 38