Protecting Protectors - AWS310 - AWS310-SITE - AWS430 - FOC201 - RME111 - RWCC - RWS200

Grounding and Lightning Protection in Vaisala Outdoor Installations Technical Reference

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Transient current can destroy metal oxide varistors, gas discharge tubes, and transient voltage suppression diodes, or they deteriorate over time. They can fail-safe by shorting the connection and protecting the electronics. Include overcurrent protection in the series with the surge protection component.

There are two philosophies in overcurrent protection in case of a shorted protective component:
  1. Install a series overcurrent protector (usually a fuse) and after it an overvoltage protector component. When the protector shorts and blows the fuse, a visit to the site is needed to repair the protector. The equipment is protected from further transients before the maintenance visit, but this method reduces system availability.
  2. Overvoltage protectors (with fuses) are connected in parallel with the power/data line. When the fuse blows the overvoltage protector is disconnected from the line and system seems to operate normally. The situation can remain unnoticed until the next transient arrives and possibly destroys the protection of the system. Several commercial surge protection devices (SPD) are connected this way to maximize the short-term system availability.

The threshold voltage of gas discharge tubes (GDT) tends to creep lower after numerous trigger cycles. In certain applications, you need to test them periodically if the threshold voltage is critical for the system.

It is best to design overvoltage protecting devices that you can easily and safely replace several times during the technical lifetime of the system.