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A high 3rd harmonic current can cause a high neutral current

Goals and Symptoms

An incorrect wiring connection or 3-phase load unbalance issue may cause resultant current flowing on neutral conductor. Apart from these possibilities, high 3rd harmonic current present on each phase may also cause high neutral current.

Causes and Fixes

Harmonics overload the neutral wiring
3-phase building wiring consists of 3 hot (or phase) conductors, a ground conductor, and a neutral conductor. Single phase loads are connected between the different hot conductors and the neutral conductor. Therefore, the neutral conductor serves as the "common" return for all of the single phase load currents. It is a property of three phase power systems that if each of the three hot conductors has a nearly equivalent load, that the neutral current will be nearly zero due to the fact that each phase current is "out of phase" with the other. In other words, the load currents "cancel out" in the neutral wire. In North America, sometimes the building wiring design takes advantage of the cancellation and the neutral wire is sized smaller than the hot wires. Unfortunately, the harmonic currents created by computers cause the operation of this system to change. Computers generate a substantial amount of 3rd harmonic current. Due to the mathematical phase properties, third harmonic currents add instead of cancel out on the neutral wire (see Fig.1). Therefore, in a building with a large number of Personal Computers installed, the neutral wire can carry much higher currents than it was designed for. In fact, the harmonic current alone in the neutral wire can be larger than the full rated current of the power wiring. This is the most critical problem relating to harmonics and PCs. Note that the data above shows that while it is unlikely for the neutral current to exceed the phase current, the neutral current can reach the phase current value in a PC environment. For this reason it is essential that neutral undersizing never be used in an office environment.

Fig.1 Phase relation between fundamental voltage (upper) and 3rd harmonic current (lower)


There are a number of approaches to avoiding harmonic problems. These include:

Specifying EQUIPMENT that does not create harmonics

  • In the case of networking EQUIPMENT, the problem is solved because of the IEC regulations. In the case of PCs, it is more difficult since a large amount of the harmonic contribution comes from the monitor. One approach is to use PCs and monitors with lower harmonic pollution, such as the use of LCD monitors or laptop PCs. This avoids both building wiring and transformer problems.

Oversizing neutral wiring
  • In modern facilities the neutral wring should always be specified to be the same capacity as the power wiring (or larger). This is in contrast to the electrical codes which may permit undersizing the neutral wire. An appropriate design in the case of a large Personal Computer load like a call center is to specify the neutral wiring to exceed the phase wire capacity by about 50%. Particular attention should be paid to wiring in office cubicles. This protects the building wiring, but does not help p