Is There a One Size Fits All SPD?

Overview:

Frequently, our customers will ask for a "one size fits all" Surge Protective Device (SPD), eliminating the need to stock several different part numbers to meet their customers needs. Some manufactures claim to have a one size fits all Surge Protection Device (SPD), however there is absolutely no benefit of this to the end user. Why? The one size fits all approach could in most cases actually cause damage to the equipment it should be protecting.

For example, you would not want to use a 2000A fuse as a one size fits all fuse. If you only need a 30A fuse to protect the load, clearly a 2000A fuse would be significantly over sized. In the end, the fuse would never open, making it a poor investment for the facility. Now what if a 60A fuse was used instead? In this case, the fuse might open but when it does it will not protect the load as well as a properly sized 30A fuse.

This concept is the same for an SPD. While it is possible to have a "one size fits all," it is not recommended because in most cases it will not provide adequate protection. To help make this point let's look at some part numbers and compare the ratings. To ensure our "one size fits all" SPD can be applied to all common voltage configurations we must use a 480V delta SPD, part number STT24803PDG.

This configuration could be installed on a 208Y/120V system but how much protection will it provide? Answering this question involves reviewing the MCOV and VPR ratings. MCOV is the turn on voltage of the SPD and is where it will start to clamp the excess voltage. From the table below, STT24803PDG has an MCOV of 550V line to ground which is about 450% of our circuit line to ground voltage. Typically the MCOV should be between 125% to 150% of the voltage.