Current Harmonic Distortion - Breakers/Fuses

When breakers or fuses operate inexplicably, current distortion is usually the cause. Current distortion creates two environments for this to occur. The first is a perceived environment, and the second exists in reality.

Whenever the current in the power system is no longer sinusoidal, we say it is distorted, or that there is harmonic distortion. Harmonic distortion is problematic in that most of the power system's operation is based on, and dependent on, the assumption of sine waves.

This has led to a number of commonly held beliefs that are now jeopardized by harmonics. One belief is that to find out the amount of current or voltage in the system, simply get a cheap voltmeter or ammeter and make a measurement.

The actual technology used in most hand-held meters today assumes the signal being measured is sinusoidal. The meter reads the average value of the signal, assumes it is a sine wave, and adjusts it to display the value in rms. This is known as an average detecting, RMS calibrated meter, and is by far the most common type used today.

However, when harmonics are present, the adjustment used by these meters is wrong. The actual measurements may be as much as 50% in error. For example, an averaging meter may read 15 amps, but in reality the current is 27 amps. This error results from the improper measurement of distorted current.

So, an unexplained breaker trippage the result of having the wrong meter. We perceive the current to be under 20 amps, but in reality it is so high the breaker trips. The breaker is operating correctly, but the user's perception is off due to using the wrong meter.

In this case, make sure a true RMS meter is used.

Electronic equipment has high inrush or starting currents. It is possible that a high inrush current may trip a breaker or blow a fuse designed to act quickly.

Whenever breakers or fuses operate, it is important to take stock of what else is happening. Did a load turn on? Then maybe overload or inrush current is the culprit. Does the trippage happen intermittently, with no apparent load correlation? Then perhaps a wrong meter has been used and an overload condition exists that no one is unaware of.

Whatever the cause, true RMS meters must be used anytime there is current harmonic distortion.

If unexplainable operation continues, then replace the fuses or breakers with true RMS sensing devices and/or increase the time delay ratings on them. This should resolve any problems.