How to design an electrical circuit according to British Standard BS 7671

What is the BS 7671 ?

British Standard BS 7671 ?"Requirements for Electrical Installations. IET Wiring Regulations", is the national standard in the United Kingdom for electrical installation and the safety of electrical wiring in domestic, commercial, industrial, and other buildings, also in special installations and locations, such as marinas or caravan parks and medical locations

How do we begin to design?

?1. Determine the design current Ib.

?2. Select the rating of the protection In.

?3. Select the relevant rating factors (CFs).

?4. Divide In by the relevant CFs to give tabulated cable current-carrying capacity It.

?5. Choose a cable size to suit It.

?6. Check the voltage drop.

?7. Check for shock risk constraints.

?8. Check for thermal constraints

1-Determine the design current Ib .

?2. Select the rating of the protection In.

Having determined Ib, we must now select the nominal setting of the protection In such that In >Ib

?3. Select the relevant rating factors (CFs).

Ambient temperature Ca

The cable ratings quoted in the IET Regulations are based on an ambient

temperature of 30°C, and so it is only above this temperature that an

adverse correction is needed

Grouping Cg

When cables are grouped together, they impart heat to each other. Therefore, the more cables there are the more heat they will generate, thus increasing the temperature of each cable

Semi-enclosed fuse factor Cc

This factor, equal to 0.725, should be used where the protective device is a semi enclosed (rewireable) fuse

Thermal insulation Ci

if a cable is totally surrounded by thermal insulation for more

than 0.5 m, a factor of 0.5 must be applied to the tabulated clipped direct

ratings. For less than 0.5 m, derating factors shown in Table 6.3 should

be applied.

?4. Divide In by the relevant CFs to give tabulated cable current-carrying capacity It.

?5. Choose a cable size to suit It.

?6. Check the voltage drop.

For single-phase 230 V, the voltage drop for lighting should not

? exceed 3% of 230 V = 6.9 V and for power should not exceed 5% of

? 230 V = 11.5 V

-For three-phase 400 V, the voltage drops should not exceed 12 and

?20 V, respectively.

?7. Check for shock risk constraints.

The actual loop impedance Zs should not exceed those values given

in the IET Regulations.

This ensures that circuits feeding final circuits up to 32 A will be disconnected, in the event of an earth fault, in less than 0.4 s

?8. Check for thermal constraints

The IET Regulations require that we either select or check the size of a

circuit protective conductor (cpc) against tabulated

values, or calculate its size using an equation.

Or Selection of cpc using tabulated values

1-for line conductors up to and including 16.0 mm2, the cpc should be at least the same size

2-for line conductors over 16.0 and up to 35.0 mm2, the cpc should be at least 16 mm2

3-for line conductors over 35.0 mm2, the cpc should be at least half this size.