POWER QUALITY

Mystery Problems Are Often Related to Grounding

How many times have you heard "I thought you'd already fixed that" when electrical maintenance personnel have to deal with a recurring problem? The disabled unit is repaired and all local cabling and grounding checks out, everything looks fine and works well. However, since the real cause of the failure is not uncovered, predictably, the problem reoccurs. Then the problem is tossed into the category of "bad power quality".

Mystery Problems Are Often Related to Grounding
In statistical records, we find that 80 per cent of power quality problems involve grounding. In fact, not surprisingly, it turns out that proper grounding is more important than surge protection in ensuring reliable electronic function.

The consequences of poor grounding are significant because ground is a reference for logic in any electronic device. This means that any irregularity in the ground can directly affect performance of the logic at the chip level.

In fact, a comparatively small change in ground can produce a large effect in equipment operation.

For instance, computers operate on the simple binary or "on/off" system. If logic 1 is triggered by a 5 Volt potential, then low levels of noise can confuse these signals. Even if impulses are only as high as 50V, they can do severe damage to the chip which is referenced to that ground.

Typical Grounding Problem Prevention Strategies
The reasons for grounding are several, the most important of which is providing personnel safety. The second is protecting assets, and a third is assuring equipment performance. Traditionally, three approaches to grounding have been used:

• Bond everything
• Rely on standards and specifications
• Applying isolated ground

Bonding everything involves a systematic and careful bonding of each structural point in a building, as well as each piece of equipment, cable and metallic surface. This approach was developed by Bell Laboratories (USA) in the 1950s and is still being used by the TelCo/cell industry today.

If a single point is overlooked, it can allow a ground potential to develop -- circulating ground current -- and problems result. Outside the TelCo industry this approach is often badly executed because it is time-consuming and therefore costly. It needs to be constantly maintained through any structural or equipment changes your facility experiences and you need to ensure that connections remain solid. Even within the Telco industry the application is often disputed especially in relation to lightning protection.

An alternative approach is to rely on standard specification codes designed for personnel safety rather than system performance. In North America this code views the main frequency to be so low that it is considered equal to DC and consequently grounding is designed on resistance only and does not allow for the requirements of high frequency impulses and noise.

The problem with this arrangement is that while it prevents injury to humans, equipment can be damaged by the smaller voltages which can build up.

However, as indicated in recent code changes, the isolated ground system remains isolated only to the first point of reference.

Unfortunately, the isolated ground approach is often misunderstood. The term Ôisolated' only applies to connections between equipment and the first point of reference. The system can be enhanced by using a ground connection suitably installed between the reference point in the distribution panel and the ground reference in a separately derived source. However, this is not a requirement of an isolated ground system.

Only one of these approaches addresses all requirements effectively and that is the isolated ground system.

How to Find the Culprit
If fixing the affected component alone does not solve the case, sleuthing is necessary. A power quality specialist will be able to gather the clues. Here are two examples of how effective detective work can lead to an inexpensive solution.

The Mystery Behind Crazy Computers Every Other Friday
A testing laboratory for a major medical service suffered repeated attacks of malfunctioning computers at regular intervals during the first few months of computer room operation. The units were fixed, parts were exchanged, and still the problem persisted. The computer service technician could not find the overriding fault until the regularity of the problem was noticed and a power quality specialist was called in. The specialist looked at the equipment, the computer room, the site facility and studied the logs. The computers were compromised every second Friday.

What was special about the schedule? The investigation revealed that on each Friday a delivery van brought supplies to the Cold Room. The Cold Room door was propped open to allow access for deliveries, the temperature rose and caused the back-up compressor to kick-in. Right next to this compressor was the "special ground wire" for the new computer room. The noise induced on this wire by the compressor was directly delivered to the isolated ground bus for the computer. The inexpensive solution was to remove the connection for this wire from the IG-ground bus and rely on the connection to the transformer bonding point.

The Case of the Flying Flag and Blown Interface Boards
A commercial equipment supply operation in a typical industrial complex was facing blown interface boards whenever a thunderstorm passed over. Surge protection was installed and did help to some extent, but the problem continued to occur. Both the head office and warehouses were located in the facility and each was rechecked by the power quality specialist. Everything appeared fine. The mystery deepened until the relationship between the storms and the flagpole in front of the building was noticed. The flagpole attracted lightning which dissipates through the earth like a wave, changing ground potential as it passes. It turned out that different parts of the computer system were connected to different parts of the building. This meant that when the wave of the lightning discharge passed under the building, severe voltage differences were created on different parts of the system. Extending the IG Receptacle System from the Computer Room to the associated equipment protected the system from lightning.

Application of appropriate technology
As we have shown, when an electrical problem occurs repeatedly for no apparent reason, it takes extra detective work to get the complete picture and to actually find the problem. It also takes informed evaluation of the strategic options to solve the problem.

If equipment exhibits recurring faults, you need to question its suitability for the application. Even the best equipment is no good if it is compromised by incorrect grounding or other system incompatibilities. Equally, the best electrician can be undermined if the analysis of conditions does not extend beyond the affected equipment. The application of inappropriate technology worsens the problem and increases costs.

For example, a power factor correction capacitor bank was installed at a facility to reduce demand costs and improve power quality. When the system was switched to adjust for changing load conditions,z the unit caused a sag and switching transience that damaged the equipment. This problem could have been avoided by choosing a better power factor filter and by installing the unit differently.

It is essential to think "outside the box" and understand the structural and procedural conditions in and outside the plant. A process of elimination should take place following a clear and complete methodology. Finally, appropriate tests should be carried out to check for impedance and other factors.

Solving the Case
Many of these mystery problems are put down to "bad power". It would be more accurate to say that our plants and facilities are more susceptible to poor power quality as they increase in technological sophistication. These kinds of power quality problems will likely increase after deregulation of the electricity market. Spikes and other interference will magnify the consequences of poor grounding.

A good power quality specialist can analyze your facility, perform proper testing and recommend the most effective solution. This need not be expensive or complex and will likely solve more than the initial problem. The results will increase reliability and reduce unscheduled downtime.