Some Types Of Power Source Problems

Some types of problems are associated with the power or signal circuits. These can be arranged into broad categories as shown in the listing below.

A. Power Failure: extended loss of power due to:

1. Severe weather
2. Network failure (human mistake; accidents to line apparatus)

1. Line faults cleared by automatic reclosures (tree limbs or animals, etc.)
2. Open-transition switching by the utility or customer

1. Harmonic waveform distortion = distortion of supply voltage waveform from a true sine wave, generally repetitive from cycle to cycle. (May be due to transformer connection or some non-linear load in the vicinity.)
2. Out-of-Range voltage disturbance ("voltage regulation"). This is a persistent voltage level at the service entrance which is too high or too low, as defined by the Service Standards in effect. This can be due to:
   • Incorrect tap setting on the transformer or a malfunctioning tap-changer-under-load (TCUL).
   • Capacitor switching (or failure to switch when scheduled).
   • Excessive load current on primary, secondary, feeder, or branch circuit.
   • Brownouts: a lowering of system voltage required by energy conservation policy.
   • Improper voltage regulation on stand-by or emergency generators in operation at the time.
3. Frequency problems (either over or under).
   • Incorrect setting or inadequate regulation of the governor of the standby or emergency generator supplying the power at that time.
   • A region-wide problem on the interconnected utility system (such as the ill-famed Mid-East Power Blackout during the initial stages of the problem).

1. Voltage spikes and drops, due to one or more of these:
   • Short circuit on the system
   • Lightning stroke and power follow-through
   • Switching of capacitors on the system
   • Primary feeder switching
   • Intermittent shorts and arcing faults
   • Poor ground, resulting in erratic grounding
   • Locally-induced transients due to motor starting, etc.
   • Locally-generated static electricity
2. Electrical Noise Disturbances, due to:
   • Radio Frequency Interference (RFI): High frequency electromagnetic radiation picked up on the power lines acting as antennas and bringing the RFI signal to the computer.

     The source can be a C-B radio, another radio transmitter, a microwave oven, an induction heater, diathermy device, etc.

   • Electromagnetic Interference (EMI): This is EM interference of an erratic sort produced by electrical equipment in the vicinity and transmitted to the computer over the power line. This noise can be caused by the operation of a switch, a relay, a thermostat, motors, SCR's, triacs, etc.

     EMI can be "conductance coupled" to the computer when the device causing the EMI is on the same circuit as the computer. This includes the common neutral of a 4-wire or a 3wire system.

     "Inductively coupled" EMI is passed from one circuit to the computer circuit by capacitance (or, less likely, inductively).

E. Transmission problems on the data line. These include:

1. Cross-talk: signals coupled inductively or capacitively from one line to another signal line. This can be caused by using a common grounded signal line conductor for both circuits.
2. Interference between data lines. Here, the interference can be of any type, not just cross-talk of signals.

1. Follow the practice specified by the National Electric Code (NEC) rules. Consider these as minimum requirements and go on from there.
2. Ground conductors should be direct, separate, have low impedance, and generally go to the same ground reference bus as the building electrical system.
3. Do not use multiple grounding wires to different grounds. Either use an isolated ground OR the system ground, but not both. For safety, it is almost always preferable to use the system ground.
4. Use conductive flooring or carpeting in the vicinity of the computer (and any other solid-state, small-signal device). The static charge one can accumulate while walking across a non-conducting floor covering can destroy FET's and other delicate solid-state devices.
5. Use a separately-derived ground for the sensitive apparatus. This involves the use of an isolating transformer and a grounded secondary winding to serve this apparatus. Each of these devices should be grounded to this separately-derived ground by its individual grounding conductor.