Does length matter?

The use of marshalling boxes for commercial lighting installations is now standard practice. As these get larger, with 12-way boxes now commonly used, many electrical contractors wrongly assume that they are limited by the length of flexible cord between the lighting distribution system in the box and the luminaire. In fact the British Standards do not specify any maximum length for the flexible cord.

There are, however, factors that need to be accounted for in the selection of the length of the flexible cord. Three key requirements are:

• voltage drop;
• protection against electric shock;
• the selection and erection of the wiring system.

Voltage drop

Regulation 525-01-01 is deemed to be satisfied for a supply given in accordance with the Electricity Safety, Quality and Continuity Regulations 2002, if the voltage drop between the origin of the installation (usually the supply terminals) and the terminals of the fixed current-using equipment does not exceed 4% of the nominal voltage of the supply.

The nominal voltage supply in the UK is 230 V. The maximum voltage drop permitted is therefore 230 V x 4/100 = 9·2 V. A mistake often made is to overlook the flexible cord length in the voltage drop calculations (see figure 1).

No shocks

For protection against indirect contact and electric shock the most common method is earthed equipotential bonding and automatic disconnection of supply. It is recommended that you always consider the influence of the length of flexible cord for the correct operation of the protective device.

To check compliance for automatic disconnection of supply at the luminaire, it must be verified that the earth fault loop impedance does not exceed the maximum tabulated values in BS 7671, or any values derived by applying the appropriate formula specified in BS 7671 for the appropriate overcurrent protective device. Manufacturers generally provide data derived from this formula.

For example, in figure 2 (overleaf) if the circuit is designed to comply with BS 7671 table 41B2 the maximum design earth fault loop impedance for the 16 A type C circuit-breaker for 0·4 and 5 s disconnection time would be 1·5 V.

Fault currents

A basic relationship between the length of a conductor, its resistance and associated fault current is that for a given conductor its resistance will increase with length and fault current is obtained by dividing the resistance into the voltage. Fault currents generate heat, therefore we must consider the length of conductor, its associated resistance and effects of fault current on the flexible cord.

Here we will focus on the protection of the circuit protective conductor (cpc) and the ability to carry earth fault currents, without thermal damage, until the overcurrent device operates.

If the cross-sectional area of the cpc has been worked out by applying table 54G of BS 7671 (overleaf) and the overcurrent protective device is providing protection against overload currents and fault currents, then no further checks are needed. Table 54G details the cross-sectional area of the protective conductor in relation to the cross-sectional area of the associated phase conductor.

However, if the overcurrent protection device is not providing protection against overload current and you have a cpc that does not comply with table 54G you need to apply the formula in Regulation 543-01-03:

S = =(I2t)/k

where:

S = nominal cross-sectional area of the cpc in mm2;

I = fault current in amperes;

t = operating time of disconnecting device in seconds;

k = factor taken from BS 7671.

The equation given above may not convey any immediate understanding of its objective. The equation is shown below after it has been rearranged into a format that clearly demonstrates the objective of the overcurrent device:

I2t < k2S2

I2t is proportional to the thermal energy let through the protective device under fault conditions;

k2S2 indictates the thermal capacity of the conductor.

If the conductor is not to be damaged, I2t must never exceed k2S2.

A quick and simple way of applying this calculation is to use the manufacturer’s I2t characteristics for the overcurrent protective device. Calculate k2S2 and superimpose this value as a horizontal line on the graph showing the protective devices I2t characteristics (see figure 3).

Provided that the fault levels are within the minimum and maximum values specified in figure 3 the flexible cord will be protected against thermal damage and comply with the Wiring Regulations.

The wiring system

Finally we must consider the selection and erection of the wiring system. Chapter 52 of the Wiring Regulations specifies requirements for:

• selection of the wiring system;
• selection and erection in relation to external influences.

For selecting the wiring system, BS 7540: 1994 provides a guide to the proposed safe use of electric cables, including flexible cords to BS 6141. This specifies that cables are selected so that they are suitable for any external influences such as ambient temperature, presence of water, mechanical stresses and others.

It is worth noting that many commercial lighting installations use suspended ceilings. If this is the case then flexible cords with pvc or similar coating should be supported clear of the framework to avoid damage to the sheath. There are several supporting systems available; one of the simplest is to use a self-adhesive cable clip.

In conclusion, there is nothing in the regulations that specifically prohibits the length of a flexible cord from a lighting distribution system to the luminaire. As in any electrical installation it is up to the designer to ensure that such an installation complies with all the necessary regulations.