HOW SURGE EVENTS HAPPEN
In a normal equipment environment there are reasonable threats from Electrostatic Discharge (ESD) and lightning transients which can cause loss of data integrity and permanent damage to equipment if not controlled.
ESD threats are generated by personnel movement, which causes triboelectric (rubbing causing a separation of charge) charges to accumulate on equipment or clothing and then be discharged through hand or tool "touch". This discharge can represent several thousand volts at 10 to 30 amps of current. There are two primary threats from an ESD event. These are the peak current of the discharge and the resulting electromagnetic field. Should the input to the device not be protected from this ESD threat, as little as 100 milli-Joules can cause permanent failure to an input device at the silicon die level.
Lightning also can create a large disturbance and delivery destructive energy to the equipment. These lightning events cause differential voltages to develop as a result of inductance in the protective earth ground path to the equipment. During a direct lightning strike it is possible to measure peak currents into the Kilo Amp range. These events can occur both at the facility as well as outside the facility along utility power lines. When they occur on the power line outside of the facility peak currents can be extended in time due to the additional inductance of the power line. A normal event may be 10 usec zero to peak, with a decay of 200 usec. These currents are called longitudinal. The Cisco surge protection cable will adequately prevent this transient damage from occurring to within the industry standards for lightning protection devices. A good equipment chassis protective ground is strongly recommended to assure adequate protection.
EFT disturbances occur as a result of arcing contacts in electro-mechanical switches and relays commonly found in an industrial environment. The electro-mechanical switches are used to connect and disconnect inductive loads. Like ESD, EFT can be especially fatal on data and I/O lines. The fast rise time of the EFT pulses demand similar protection as ESD pulses.
CONFIGURATIONS THAT ARE SUBJECT TO SURGE EVENTS
There are several hardware configurations and applications that are more subject to damage from ESD events. These configurations include the following:
- Locations Subject to Electrical Storms-Any location where electrical storms are common is a good candidate site for surge protection on the serial ports;
- Unshielded Cables-If unshielded cables are used to connect to the serial port, it is much more likely that electrical discharges will be picked up by the cabling and damage the serial port;
- Long Cable Runs-The longer the cable, the more susceptible it is to surge events, even if the cable is shielded;
- Outside Wiring-If the cable runs outdoors, it is more susceptible to surge events;
- Different Earth Grounds-If the router and the device at the other end of the serial cable are connected to different earth grounds, then the cable can become a conduit for current that equalizes different ground potentials;
- Installations with Multiple Serial Port Failures-If an installation has experienced multiple serial port failures in the past, it is a good candidate for surge protection;