Line Surge and Hash Protection for Hi-Fi Equipment (May 1979)

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by F.J. Stifter [President Electronic Specialists, Inc. Natick, Mass.]

Lightning, heavy machinery, and power outages can create damaging power line surges which may damage expensive audio components. Hash is created by hundreds of sources including tools, appliances, microprocessors, business machines, and defective wiring. This article discusses basic causes of surges and hash, effects these phenomena have on component performance, and cures which can be incorporated to reduce undesirable effects.

Power Line Surges

Surges on the a.c. power line are of ten caused by lightning. A direct hit on the power line is not required to create a voltage surge many times the normal line voltage.

Lightning is the discharge of two differently charged electrical objects, generally "cloud to earth" or "cloud to cloud." It is estimated the electrical potential between the discharge points can exceed several million volts and the current in the arc or "lightning bolt" can reach one million amperes.

This is an instantaneous power of over one trillion watts or over 100 million horsepower! Little wonder that power lines can have large, damaging surges and transients induced by lightning.

Heavy machinery and switch gear can create significant surges when power is interrupted. While nowhere near as impressive as lightning, these surges and transients are often large enough to cause damage. Surges may also accompany loss or restoration of power during a power outage situation. Start and stop of refrigerators, air conditioners, or shop tools may also cause minor line surges.

Effects of Surges

Power line surges cause damage in several ways. Lightning can induce extremely high voltages between the conductors carrying power to any device, and such a surge behaves essentially like a very high power-line voltage. Audio gear power transformers can have extremely high secondary voltages developed. Rectifiers may be damaged, power transformers may short, and voltage regulators may be destroyed. In short, a "domino effect" could occur, wiping out large sections of electronics. Even turntable motors may be permanently damaged.

Merely because a device is switched "Off" is no guarantee against damage.

The voltage surge may be sufficiently high to arc across switch contacts.

This, in effect, applies high voltage to the equipment with the same disastrous results.

A different situation caused by lightning can result in a common-mode voltage surge, where both power lines are brought up to a high potential relative to earth. When applied to equipment, this high voltage may cause arcing between conductors and ground, destroying cables, switches, or other equipment components. Power transformer insulation may be punctured, rendering the component use less.

Surges caused by heavy machinery or power outages usually are much less damaging than lightning-induced surges. Nevertheless, costly damage may result from these surges and may closely parallel those described for lightning surges.

Cures for Line Surge

Unfortunately, there is no complete cure for power line surges or hash.

ON Power line surges induced by lightning, heavy machinery, or power outages can be controlled by the addition of zeners, thyristors, varistors, or other forms of lightning arrestors.

These devices should be placed across the a.c. line and between the a.c. lines and ground. Limited protection can be obtained by placing a high-power zener network across power supply secondaries or even between rectified outputs and ground. Audio equipment a.c. line cords should be removed from wall sockets during electrical storms for the utmost protection.

Commercial surge protectors are also available, and many audio enthusiasts add surge protection as a convenient insurance measure. For a low, one-time cost, valuable audio equipment can be protected against the ravages of power-line surges. We make one model that is mounted in a line-cord package and can be used directly at the equipment to be protected (see Fig. 1).

Fig. 1 -- Line cord surge suppressor and hash filter.


A lot of power line hash is created by arcing in various types of electrical equipment. Tools, motors, appliances, and other small electrical devices are notorious offenders. Even low-quality audio equipment may put out line hash.

Various signals can also be picked up by power lines functioning as an antenna and can create unwelcome interference. Commercial broadcast stations, police, fire, marine, business radios, and CB sets can be the source of this type interference.

Unfortunately, quite a lot of equipment can create hash which gets into the power line during normal operation. A normally operating police or CB radio, that meets all present FCC specifications, may induce fairly strong signals into power lines. If this gets into FM or other equipment and interferes with proper operation, the net result is chaos, frustration, and frayed tempers. All such interference is commonly referred to as hash. In many cases the source is not known; indeed, the source is usually unaware of trouble being caused.

Another common source of hash can be found closer to home. Over a period of time many light sockets, wall sockets, line-cord plugs, wire connections, and the like can become loose, defective, or corroded. Although common and simple hash sources, these offenders are often overlooked. Some times this hash is well camouflaged.

When a suspected hash source is shut off, the hash disappears. Presto! The shut-off equipment must be at fault.

Hours later, when the hash has not been cured with various schemes, the hash may be reluctantly accepted as unavoidable. Actually, shutting off the machine stopped electrical current from flowing through the noisy connection or socket, and all traces of hash stopped! When investigating hash or interference problems, don't overlook: Sporadic problems caused by lightning or electrical storms, fluorescent lamps, SCR or triac controlled lights, motors and power supplies, welders, diathermy, nearby internal combustion engines, OR-(reader fills in the blank).

Effects of Hash

Hash can cause garbled FM reception or undesired signals in the audio equipment. Direct damage will seldom result from line hash or interference.

Reduced enjoyment of favorite programming and undesired noises on taped programs are the major damage.

Severe cases, of course, may render all reception impossible.

Hash on intercoms, PA systems, radio, or TV, can result in poor communications, personnel annoyance, or morale deterioration. Cost, while difficult to assess, is none the less real.

Cures for Power Line Hash

Hash filters can be installed at the device suffering hash interference, and, often, this completely eliminates interference problems. A more desirable approach is elimination of hash at the source.

Defective equipment wiring, lamp sockets, and electrical outlets should be replaced or repaired. Filters should be installed on noisy tools, appliances, or equipment. A line cord mounted filter (see Fig. 1) can be utilized at either the victim audio equipment or the hash producing equipment, and models are available which incorporate both the line hash filter and surge suppressor.

Under normal operating conditions, all covers and shields supplied by the manufacturer should be securely fastened in place. This is especially important for FM equipment.

Often, tying all audio equipment to ground will reduce interference from outside sources. When installing a ground system be careful to avoid ground loops, as these may induce sys tem hum. Generally, it is advisable to start with one piece of equipment tied to a good ground; then proceed with a second piece tied to the same ground; and so on. Check system operation after each step.

(Source: Audio magazine, May 1979)

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