POWER QUALITY Q&A

Answers to Frequently Asked Questions About Power Quality

Question:
We are constructing a new building where sensitive electronics will be installed. If the electrical distribution system is installed according to the Electrical Safety Code, will this be sufficient from a power quality standpoint?

Answer:
The Electrical Safety Code is a minimum standard which defines the method and materials that must be used in the installation of electrical equipment and the wiring of machinery and facilities. Its prime focus is safety. When an inspector is reviewing an installation, he or she is looking for conformity with the Code and more specifically correct wire sizing, fusing, clearances, and grounding. If we look closely at the Code, there is very little that concerns itself with the quality of the power we hope to receive.

Therefore, if we use the Code as our specification for defining power quality installation practices, we will be quite disappointed with the results.

So how do we get the results we want; a safe reliable power system that works with our sensitive electronic equipment? We have to define what is needed to achieve our goal and communicate it to those performing the installation work. One way to do this is to find an electrical engineer or other experienced professional who knows power quality issues and how to mitigate the potential problems. Engage them to write a specification that details the materials and methods that cover all your power quality concerns. Include this specification in all your electrical specification packages for bidding.

In this way, the installation contractor knows exactly what is expected, can bid accurately, and will provide you with an installation that will be reliable and trouble-free. Of course, as with any contract, you must ensure that the installation follows the specifications.

In the end, you will get the system you want and will find that the extra cost involved is minimal.

Question:
What is the best type of Uninterruptible Power Supply (UPS) to use for my Computer Centre?

Answer:
Typically, for computers we are trying to protect against a data loss from momentary outages. For longer-term outages, we are trying to buy time so that open files can be closed and our operating system brought down in an orderly fashion.

One available type is a Stand-by type system. In this type, the computer runs normally from a clean AC line. This AC supply also charges a set of batteries that remains in a stand-by mode. If an outage occurs, a static (or solid state) switch connects the battery through an inverter to the computer load. When the AC line comes back on, the switch reverts back to its normal position and the computer is fed from the AC power supply. With this type of system, there is a short discontinuity of power during switching even though the static switch reacts very quickly. This switching may affect the computer hardware or the integrity of the data.

Another, more preferable solution would be to use a Floating type system. In this type a reliable AC source feeds a rectifier or battery charger which holds a charge on a set of batteries. An inverter uses this DC power to synthesize an AC waveform that is used to power the computer. Therefore, the computer is 'floating' or drawing inverted power from the batteries under normal circumstances. Hence, when a loss of AC power occurs, the computer continues to operate on the batteries without any switching or discontinuity. When the power returns, normal operation resumes.

Regardless of the type of UPS selected, it is vital that the UPS provide a clean sinusoidal output. A higher end UPS will do this. A check of its specifications will confirm this. A lower end UPS designed for general back-up service will give a rougher, if not square wave type output. Transients and harmonics associated with a non-sinusoidal output may create undesirable power quality effects.

Looking at the big picture, there may be other things to consider. Are there other systems that may affect the computer's operation such as air conditioning systems? If these cease to operate during power failures, will they cause a computer failure? Some strategy may have to be developed for them.