POWER QUALITY Q&A

Our Expert Offers Answers to Frequently Asked Questions About Power Quality

Question:
Why do Programmable Logic Controller (PLC) manufacturers suggest using surge suppressors on inductive loads to protect their equipment?

Answer:
When an inductive load or coil is energized, the magnetic lines of force in the increasing field generate a back EMF (electro-motive force) in the coil's windings. This can create a transient that can rise to as much as twice the normal operating voltage level. Obviously, this is not healthy for sensitive electronic equipment that can be damaged permanently if exposed to sufficiently high voltage levels.

One way to protect against these transients is to install a surge suppressor across the coil winding. There are various types of surge suppressors, each having its own characteristics and use. For AC coils an MOV (metal oxide varistor) can be used. A varistor is a device whose resistance decreases with increasing frequency.

Hence, during a fast transient the developed voltage will travel through the varistor to ground instead of developing a high potential on the high side of the coil. An RC (resistor capacitor) circuit can also be used.

The capacitor is also a device that has low impedance at high frequencies. The resistor creates a time constant characteristic that slows the transient development keeping the voltage within safe bounds.

A DC coil requires a completely different strategy. A diode is connected in reverse bias across the DC coil. Any EMF induced reverse transients will go to ground through the diode thus keeping the voltage within reasonable limits.

Question:
We have been using ground rods for portable equipment installations that we have been doing around the country.
We are looking at a more compact solution using a disk electrode. Will this be as effective?

Answer:
Artificial ground electrodes are used to ensure that we are protected against ground faults, touch and step potentials, and of course, to keep our power quality within acceptable levels.

There are a number of different ways that we can achieve this, each with its own advantages and disadvantages. The ground rod (usually about 10 feet long and 3/4 inch in diameter) is the most popular choice because of its effectiveness given its economy of material usage and ease of installation.

However, in certain circumstances where local conditions preclude its use or where there are other reasons such as yours, other methods may prove more favourable.

The resistance characteristics of various methods can be compared here simply without the rigorous mathematics. Those who wish the full treatment can refer to IEEE Standard 80-1986.

(Please refer to Figure 2). I would use a conventional ground rod of 3 metres (L) and 2 centimetre diameter (A) as a base case. A loop type electrode will require a radius of approximately .5 metres (B) to achieve the same resistance. Similarly, a full circular plate would require a radius of approximately .35 metres (D). A square plate electrode would need sides of .58 metres (E). Thickness of the plate is not a significant factor.

Because of the smaller mass of earth above the electrode, the resistance of any electrode typically doubles from the theoretical when it is near the surface.

Deeper burial is always better from a resistance and a touch potential standpoint. A ground electrode installed below the frost level and into the water table will be most effective. In difficult areas, chemical salting, concrete encasement or multiple electrodes may improve the effective ground.

But in considering the many variables we have to play with, the variable with the greatest effect is the resistance of the ground itself. The key to good grounding is understanding the type of soil where the electrode is to be installed and taking the most appropriate action to achieve it.