Electricity + Control February 2018

CABLES + ACCESSORIES

tain time against the additional investment cost for over- sizing. For cables, the chosen size shall be determined taking into account the cost of losses that will occur during the working life of the cable against the initial cost of the cable. In some appli- cations (particularly industrial), the most economical cross-section of a conductor may be several sizes larger than that required for thermal reasons.’ Reference is then made to IEC 60287-3-2 [3]‚‘Eco- nomic optimisation of power cable size’.This, howev- er, like many other current approaches in the stand- ards, assumes that the load profiles are accurately known. Yet this is rarely ever the case. So what can be done to improve this and other related situations? Individual consumer A standardised load profile for homes exists with- in the IEC range of standards, namely H0. This, however, is only to be used for the connection of the entire property, for instance from the distribu- tion network to the meter. Behind the meter, the power is distributed to the final circuits in a very uneven manner in terms of both time and space. Consider, for example, the socket in the laundry room. If that is intended solely for the washing ma- chine, the load of this circuit can be estimated fair- ly accurately. During a two-hour wash cycle, there is approximately half an hour in which the machine consumes a current that represents a substantial load for the installation cable (see graph). The length of the

If the cable is upsized from 3*1,5 mm 2 to 3*2,5 mm 2 , the annual cost of the losses can be reduced to 50 cents (see Table 2). This is a sav- ing of 20 cents per year. The present cost of 13 m of a MYM-J 3*1,5 mm 2 cable is around €12,50. A similar cable with 3*2,5 mm 2 conductor sections would cost €16,80.The payback period of the latter cable compared to the first one would be more than 20 years. The potential in this case is obvious- ly very limited because there is only a substantial load on the circuit for around 60 hours per year. Table 1: Calculation of the annual line losses with a standard cable of 1,5 mm 2 … Line length 13,0 m Conductor cross section 1,5 mm 2 Washes per year 125 Electricity price 29,7 ct/kWh Share of line losses 1,65% Electricity cost per wash 34,29 ct Annual electricity cost 43,56 €/a Line loss cost per wash 0,56 ct Annual line loss cost 0,70 €/a Table 2: … and with upgraded cross-section of 2,5 mm 2 , resulting in a saving of 20 cents per year! Line length 13,0 m Conductor cross section 2,5 mm 2 Washes per year 125 Electricity price 29,7 ct/kWh Share of line losses 1,19% Electricity cost per wash 34,13 ct Annual electricity cost 43,17 €/a Line loss cost per wash 0,40 ct Annual line loss cost 0,50 €/a The nett return however is that the stringent re- quirements to limit the voltage drop result in en- ergy savings. In the original document, the house- hold profile is further explored. Office floor An office building produces completely different values. In the example chosen (again built in 1983, see Table 3 ), a floor is protected with 3 * 100 A. Each outer conductor is split between seven final circuits of 10 A each. If the annual consumption of 91 466 kWh in 2015 is divided according to the specified methods, this gives a mean utilisation of around 20%. This exceeds the corresponding value for the above-mentioned terraced house by

cable from the dis- tribution box to the laundry room is es- timated to be 13 m. The increase in elec- trical resistance due to the rise in tem- perature is ignored, and the ambient temperature in the laundry room is as- sumed to be 20°C. The monitoring val-

ues (see graph) consist of the active, reactive and apparent power recorded in one second intervals. The instantaneous current is calculated from the apparent power and the rated mains voltage. The energy loss in the line during the respective sec- ond follows from this. The sum of all the individual values per second then results in the energy loss for one wash. At 125 washes per year, the result- ing energy loss on the laundry room circuit will be 70 EUR cents (see Table 1 ).

8 Electricity + Control

FEBRUARY 2018