Power Quality

Wiring

If you suspect that a wiring problem is causing your power quality issues, the electrical service entrance to the facility should be checked for faulty or loose connections first. A poor connection will result in heating and arcing of the connection and the wire's insulation may be burnt or discolored. With the use of an infrared thermograph scanner, poor connections can be detected before permanent damage occurs.

Another problem associated with wiring is excessive voltage drop at the equipment end due to incorrect wire sizing. The voltage drop may cause the input voltage level to be too low for the equipment to operate properly. To avoid this problem, always check that the existing wire is properly sized when replacing or adding new equipment. The voltage drop should always be considered when designing a new building and future electrical growth should also be factored into wire sizing. For critical electronic loads, use separate electrical feeders and branch circuits so that these sensitive loads will not see voltage excursions created by the operation of larger non-electronic loads such as air conditioning equipment, fan motors, etc.

The diagram below illustrates wiring methods, which provide a range offer varying degrees of effectiveness of isolation.

Common versus Separate Feeders. (Source: Federal Information Processing Standards Publication #94) (Photo courtesy of Kansas City Power and Light)

Nonlinear loads such as computers produce third harmonic currents that require special consideration. Third harmonic currents from three single-phase loads will add-in the neutral conductor and may cause excessive heating of the conductor. To overcome this problem, the following configurations are commonly used:

  • Run a separate neutral conductor for each phase in a three-phase circuit that serves single-phase nonlinear loads.
  • When a shared neutral must be used in a three-phase circuit with single-phase nonlinear loads, use a neutral conductor with approximately double the phase conductor capacity.
  • Use delta-wye transformers designed for nonlinear loads to limit the penetration of high neutral currents. These transformers should be placed as close as possible to the nonlinear loads.
  • Place filters at individual loads to control third harmonic current. This offers an alternative to expensive wiring changes.
  • If a major portion of the load is nonlinear and a conventional transformer is used, the dry type transformer should be derated. In addition, some manufacturers supply "K-factor" transformers that are specifically designed to serve particular nonlinear loads.