In harsh industrial environments maximization of operations is imperative. The downtime, damage, & destruction caused to mission critical systems costs businesses millions of ruppees each year.
When selecting an SPD for a given application, there are several considerations that must be made:
- Application - Ensure that the SPD is designed for the zone of protection for which it will be used. For example, an SPD at the service entrance should be designed to handle the larger surges that result from lightning or utility switching.
- System voltage and configuration - SPDs are designed for specific voltage levels and circuit configurations. For example, your service entrance equipment may be supplied three phase power at 480/277 V in a four-wire wye connection, but a local computer is installed to a single-phase, 120 V supply.
- Let-through voltage - This is the voltage that the SPD will allow the protected equipment to be exposed to. However, the potential damage to equipment is dependent on how long the equipment is exposed to this let-through voltage in relation to the equipment design. In other words, equipment is generally designed to withstand a high voltage for a very short period of time, and lower voltage surges for a longer period of time. The Federal Information Processing Standards (FIPS) publication "Guideline on Electrical Power for Automatic Data Processing Installations" (FIPS Pub. DU294) provides details on the relationship between clamping voltage, system voltage, and surge duration.
As an example, a transient on a 480 V line that lasts for 20 microseconds can rise to almost 3400V without damaging equipment designed to this guideline. But a surge around 2300 V could be sustained for 100 microseconds without causing damage. Generally speaking, the lower the clamp voltage, the better the protection.
Surge current - SPDs are rated to safely divert a given amount of surge current without failing. This rating ranges from a few thousand amps up to 400 kiloamperes (kA) or more. However, the average current of a lightning strike is only approximately 20 kA., with the highest measured currents being just over 200 kA. Lightning that strikes a power line will travel in both directions, so only half the current travels toward your facility. Along the way, some of the current may dissipate to ground through utility equipment.
Therefore, the potential current at the service entrance from an average lightning strike is somewhere around 10 kA. In addition, certain areas of the country are more prone to lightning strikes than others. All of these factors must be considered when deciding what size SPD is appropriate for your application.
However, it is important to consider that an SPD rated at 20 kA may be sufficient to protect against the average lightning strike and most internally generated surges once, but an SPD that is rated 100 kA will be able to handle additional surges without having to replace the arrester or fuses.
Standards - All SPDs should be tested in accordance with ANSI/IEEE C62.41 and be listed to UL 1449 (2nd Edition) for safety.