Introduction to Application Examplesback to learnmore

Using electronics outdoors for measuring, controlling, monitoring or communicating will expose them to unusual conditions that can compromise equipment reliability. Precautions such as good grounding and careful use of overvoltage protectors can eliminate or greatly reduce equipment disruption, damage or destruction. This section discusses concepts related to over-voltage protection and provides examples relating them to particular applications.

Low Impedance Grounds- Safety grounds required by electrical codes can carry ten or sometimes hundreds of amperes of current to ground without developing a significant voltage drop between the equipment ground and earth ground. However, using electronics outdoors increases the probability of exposure to lightning or lightning induced currents. Such currents are ground referenced, i.e., trying to get to earth ground and can easily reach amplitudes of thousands or sometimes tens of thousands of amperes. Now granted these currents flow only for tens of microseconds, but during this time high impedance grounds carrying them can develop surprisingly high voltages. Low impedance grounds can be achieved using a nearby ground rod and grounding connections from equipment to the rod that have minimal resistance and inductance. Electrical resistance and to some degree inductance are directly proportional to wire length to wire length and inversely proportional to wire cross-sectional area. This means a ground wire’s impedance is lowered by making it shorter and fatter.

Wires are Antennas- Intentional antennas designed to pick up electromagnetic signals such as AM, CB, FM, shortwave radio or TV are essentially electrical conductors in which the signal induces a current. Since these signal sources have limited radiated power and are typically located some distance away, the antenna must be sized, arranged and oriented to pick up enough signal to be useful. Other electrical conductors such as wires to outdoor equipment used to carry power, telephone communications or control signals also pick up energy from such sources. However, the induced currents are very small because these wires are inefficient receiving antennas and the source is distant and limited in power. These very small currents seldom affect equipment operation. However, a nearby lightning strike radiates signals with much higher power levels over short distances. The inefficiency of these wires as receiving antennas is of little help in preventing significant lightning induced currents in all exposed wires. Such currents flow through system impedances developing high voltages which stress insulation causing disruptions, damage or destruction. Protectors designed to withstand and reduce transients to tolerable levels can be inserted at both ends of each wire between the wire and susceptible circuits.

Wires can Isolate-Wiring to, from and within equipment cabinets is sized based upon the maximum operational currents expected. This means the wire’s resistance and inductance are low enough to not interfere with normal operation. Surge currents albeit brief have amplitudes that can easily exceed 10 or 100 times operational levels. Such high currents flowing in wire resistance and inductance selected for operational current results in large voltage drops from one end of the wire to the other. Thus if one end of the wire is clamped to a safe voltage level by an overvoltage protector, the other end, if unprotected, can still rise to a high voltage level. This is not necessarily problematic if the component or circuit on the other end has excellent overvoltage immunity. For applications with susceptible circuits on both ends of the wire, two protectors are recommended, one on each end.