Changing currents

The high harmonic currents produced by electronic loads result in a neutral current through cables and busbars. To control the temperature of the cables and busbars, double-size neutral conductors and current derating may be required. However, the high-harmonic load-current will pollute the supply so extra equipment may be required at the load.

Component standards assume that the phase currents in three-phase cables and busbar trunking are balanced, so the neutral conductor carries no current. Hence the use of a conductor for neutral, which is only 50% of the phase conductor size.

In addition the IEE Wiring Regulations (BS 7671) state in Section 524-02-02: "In a polyphase circuit the neutral conductor shall have a current carrying capacity for the maximum current likely to flow in it under normal operating conditions.

"When assessing the maximum likely neutral current account shall be taken of:

• inequality of phase loading;
  
• inequality of power factor in each phase;
  
• harmonic currents in the neutral conductor."

  The standard further states in Section 524-02-03: "In a discharge lighting circuit and polyphase circuits where the harmonic content of the phase currents is greater than 10% of the fundamental current, the neutral conductor shall have a cross-sectional area not less than that of the phase conductors."

Electronic non-linear loads produce a high harmonic content of the load current. The third harmonic content for a switching-mode power supply (as used in computers) is approximately 70% of the fundamental current.

If the loading of the three phases is balanced by carefully connecting the single-phase loads, in the worse case scenario the phase current can be approximated by:

(I(L)12 + I(L)32)1/2
= (1·02 + 0·72)1/2
= 1·22

The neutral current can be approximated by:

(I(L1)3 + I(L2)3 + I(L3)3)
= (0·7 + 0·7 + 0·7)
= 2·1
so Ineutral/Iphase
= 2·1 / 1·22 = 1·72

Therefore, the neutral conductor should be almost twice the cross-sectional area of the phase conductors when the load is formed completely by the use of electronic equipment.

Reducing the heat
The extra heat produced by a neutral current causes a current derating to maintain the total heat produced at the same level, hence the temperature rise within the cable or busbar limits.

The effects on the rating factor caused by the change in neutral current due to harmonics is outlined in table one, below.

Who's responsible?
The Electricity Association's Engineering Recommendation G5/4 gives system designers the responsibility to ensure the electrical network user does not pollute the supply with harmonics in the neutral.

However, the system designer does not manufacture the cables, busbar or other components and is normally not the building owner or occupier. Often it is the consulting engineer or occasionally the electrical contractor that is responsible for the design of the electrical system as installed in a building.

This may require additional equipment in the form of harmonic filters at the load. With the reduction in neutral current, this may allow the use of standard 100%-rated cables and busbar, while complying with BS 7671.

Further information is available on www.beama.org.uk.