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NEMA Surge Protection Institute

These FAQs cover: * General * Grounding & surge protection * Installation

* Surge protection with filtering

General What is the difference between a Surge Arrester and an SPD?

Surge Arrestor and SPD are often used interchangeably and with similar meanings.

However, the two terms are defined differently by NEC, UL, IEEE/ANSI, etc. Surge Arrester is a surge suppressor installed before (on the power side) the first breaker connected to the distribution panel, and SPD is a surge suppressor installed before (on the power side) the first breaker connected to the distribution panel. It is installed only on the back (load side).

Using a surge suppressor listed as an SPD under UL 1449 on the power side of the first circuit breaker violates the NEC.

Meanwhile, secondary surge arresters can be installed on both the power side and the load side of the primary circuit breaker. (Surge arresters are mostly installed outdoors and do not have fail-safe functions.

What do the various items listed on the SPD mean and which ones are important?

UL has outlined minimum requirements, some of which should be considered when selecting an SPD:

* Nominal voltage - Must match the voltage of the system where the SPD is installed.

* MCOV - The highest voltage that the SPD can withstand without clamping.

* Single surge current rating - For 8/20 waveform, the maximum surge current that can be withstood once without the limit voltage changing by more than 10% at the specified surge current and without any decrease in performance. The number of parts depends on the mode of the SPD. Since the types are different, the capacity is indicated by mode.

* SVR - Suppression Voltage (Limiting Voltage). According to UL 1449, SPDs must be marked with SVR.

It is expressed as 330, 400, 500, 600 V, etc., as a voltage that appears at a given surge current and lead length. SVR shows a suppression performance similar to that of SPD.

This test was conducted at 8/20 waveform of 500A, so it cannot predict actual situation that occurs at rated current, etc.

A 110V SPD with SVR 400V is not better than a 220V SPD with SVR 600V. Also, even if it is the lowest SVR SPD, it is of no help if the lead length is too long and installed incorrectly.

* Enclosure Rating - The NEMA rating for the enclosure must match the environmental conditions in which the SPD is installed.

What are Surge, Transient, and Temporary over-voltage and what are their typical characteristics?

These are all sometimes used interchangeably as overvoltage.

The key difference between these terms is the duration of the waveform. Transients are short in duration (<10 μs) and have very little energy.

These transients appear as voltage switching spikes, which have very small energies but are sufficient to cause junctions in semiconductor substrates to break.

On the other hand, surge (> 10 us & < 1 ms) has a large energy and can destroy or burn home appliances or electronic components. TOV occurs when an accident occurs in the power grid of a power company and has a long duration (ms). (seconds to hours) can cause extensive damage.

(CAUTION) UL 1449 requires that SPDs not catch fire or compromise safety in these situations. SPDs are not designed to protect against TOV.

References http://www.nemasurge.com/help.html (NEMA Surge Protection Institute)

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