Re: House Power Failures and Mac

J.J. O'Shea wrote:
On Fri, 15 Feb 2008 02:26:20 -0500, bud-- wrote
(in article <b852d$47b53d30$4213eb82$4625@xxxxxxxxxxxxx>):
Depending on the quality of electrical service in your area, a
surge protector's MOVs might last 3 years. Or they might last
18 months.
.
Plug-in suppressors are readily available with very high ratings at relatively low cost. "Grossly undersized" is a red herring. If plug-in suppressors only last 3 years you are buying junk or are in an extremely high lightning area.

Bingo. There's a reason why Florida Power & Light is also known as Florida Flicker & Flash. I saw more surges in my first six months in Florida than I'd seen in my last two years in Jamaica, and it's not as though JPS (the Jamaican power utility) was famous for delivering clean power.

..
Parts of Florida do sound ‘exciting’.

Francois Martzloff, was the NIST guru on surges and wrote the NIST guide also wrote many technical papers. One looks at a MOV (simulating a plug-in suppressor) at the end of a 10-50 meter branch circuit. The surge is 2,000-10,000A. There is an arc-gap at the source end with a breakdown voltage of 6,000V. US systems will have arc-over at panels (and receptacles) at about 6,000V. Arc-over dumps a large percentage of the surge to earth. This is where much of the surge energy is dissipated if there is no service panel suppressor. [w_ asks where the energy is dissipated and ignores this explanation.] Branch circuit impedance greatly limits the surge current to the MOV.

In all cases the energy dissipated at the MOV was less than 1J except for a 10M branch circuit and, ironically, the lower current surges below 5,000A. Contrary to intuition, at all branch circuit lengths the energy dissipation at the MOV was lower as the surge current went up. That was because the MOV also clamped the voltage at the source spark gap. Higher current surges forced the voltage at the gap up faster causing it to break down faster and dump more of the energy to earth. With the short branch circuit and lowest surge currents, the MOV prevented the gap from arcing over at all. The max energy dissipation at the MOV was 22J.

(Protection is by clamping the voltage at the MOV, not absorbing surge energy. Energy is absorbed in the clamping action.)

I would expect you have a service panel suppressor which should take the main hit. Even without one, a plug-in suppressor with high ratings should have a long life. The 1500J rating of your suppressor is for a single event. If the energy hits are a small fraction of 1500J the cumulative rating is far greater than 1500J.



Like you I would rather have a high rating plug-in suppressor ahead of a UPS. Plug-in suppressors with very high ratings are readily available. I would rather not have the UPS try to provide surge protection. Which unit takes the surge hit is actually determined by the MOV clamp voltages (which Martzloff says are set unnecessarily low).

–-
bud--


.

Relevant Pages

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