what is the joule rating on surge protectors?

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The best defense is a good offense, huh? ;)

One of the parameters of a MOV is the Maximum Continuous Operating Voltage which is the voltage at which a MOV starts to conduct, specifically the voltage (on average - lot of individual variation) at which the current is 1ma. The MOV Joule rating is the single shot energy absorption that shifts the MCOV by 10% (lower) which can be considered a performance limit. At lower (but still relatively high) energy hits, the MOV can progressively degrade.

With lower (but still relatively high) energy hits the MOV can have a much longer life. See w_'s post:

Fig 7-11 which gives the number of pulses that can be absorbed (1 to 1 million) at different surge current vs duration combinations - from which surge energy can be calculated..

bud--

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Excellent data sheets, thanks.

Figures 7-11 imply permanent change in the voltage-current curve with as little as a single zot, but culmulative effects over megazots at lower energies. They say that the device "continues to function" but I can't imagine many applications that wouldn't eventually notice a series of 10% changes. Looks to me that MOVs do accumulate damage. I'd have to analyze the curves to understand whether it's total lifetime joules that damage an MOV, or some other relationship. Maybe later.

It absorbs what it absorbs. The math is simple.

A lightning strike has typical numbers like 200 megavolts cloud-to-ground and 1e9 joules of energy, sometimes a lot more. Most of that energy is dissipated in the bolt itself, and very little in the earth. The dissipation in the earth doesn't really matter, except to note that it represents a minute fraction of the energy involved.

I don't like MOVs because of their leakage, their very soft conduction curve, and their tendency to accumulate damage.

John

MOVs have other problems such as too much capacitance (see datasheets). But MOVs have superior life expectancy compared to gas discharge tubes (GDT). A rather unique 'MOV like' solution that replaced GDTs was once standard in telephone protection. That solution has since been obsoleted by a semiconductor solution. Some examples of semiconductor solutions include Sidactors and products from Sematech:

Above solutions are when protector's capacitance could impact signal integrity. AC electric does not have same restrictions. MOV technology is a best AC electric protection when considering other parameters such as life expectancy and costs.

MOVs are typically selected so that only serious and destructive transients cause an MOV response. Lesser transients should be made irrelevant by protection inside appliances. A typical transient frequency would be maybe one transient every eight years. A number that changes significantly even within a same town due to variations such as geology.

If frequency of transients is significantly higher (ie central FL or some parts of WV), then joules are increased so as to improve MOV life expectancy exponentially. Joules are a ballpark number to estimate MOV life expectancy.

In the 1970's, I worked at an outfit that used a lot of high-voltage stuff, and they used MOVs, which caused more problems than they solved. One day, one of the engineers saw a press release for Transzorbs, and we tried one, and within days had issued ECO's to replace every MOV in the product line with a Transzorb.

Hope This Helps! Rich

Both of these are technically inaccurate. The current follows _ALL_ possible paths, which share the current in proportion to their conductance. (i.e., the reciprocal of resistance.)

Cheers! Rich