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What is the difference between Surge Arrester and SPD?

What is the difference between Surge Arrester and SPD ?

Surge Arrester and SPD are often used interchangeably with similar meanings. However, the two terms are defined differently in NEC, UL, IEEE/ANSI, etc.

Surge Arrester (Secondary arrester) is an SPD installed in front (power side) of the primary disconnect connected to the service entrance panel, and the SPD is installed only behind the first breaker (load side). It is said that Using an SPD registered as a TVSS by UL 1449 on the power side of the first breaker would violate the NEC. Meanwhile, the secondary surge arrester can be installed on both the power side and the load side of the main breaker.

(Surge Arrester is mostly installed outdoors and does not have a fail-safe function .)


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

UL has summarized the minimum requirements, and some things to consider when selecting an SPD are as follows.

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

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

* Single surge rating current - For an 8/20 waveform, the maximum surge current that can be withstood once without the clamping voltage changing by more than 10% at a given surge current and without reducing performance. Since the quantity and type of parts vary depending on the SPD mode, it is expressed as capacity by mode.

* SVR - Suppression voltage (limiting voltage). According to UL 1449, TVSS must be marked with SVR. It is the voltage that appears at a given surge current and lead length and is expressed as 330, 400, 500, 600V, etc. SVR shows similar suppression performance to SPD. Since this test was conducted at an 8/20 waveform of 500A, it cannot predict the actual situation that occurs at the rated current. The 110V SPD for SVR 400V is not better than the 220V SPD for SVR 600V. Also, even if the SPD has the lowest SVR, it is of no help at all if the lead length is installed incorrectly and too long.

* Enclosure rating - The NEMA rating for the enclosure must match the surrounding environment where the SPD is installed.


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

These are all sometimes used interchangeably as overvoltage. The important difference between these terms is the duration of the waveform. Transients have a very short duration (< 10 usec) and have very little energy. These transients appear as voltage switching spikes, and although their energy is very small, they are enough to cause a junction on a semiconductor substrate to break.

On the other hand, surge (> 10 use & < 1 msec) has high energy and causes destruction and charring of home appliances and electronic components.

Temporary over-voltage (TOV) occurs when an accident occurs in a power company's power system, and has a long duration (msec to seconds to hours), causing very significant damage.

(Caution) UL 1449 stipulates that the SPD will not catch fire or impair safety in these situations. SPD is not designed to block TOV.


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