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TRUE POWER #218287 03/11/17 11:22 AM
Joined: Mar 2005
Posts: 1,213
S
Member
What is this homeowner trying to protect against? I gather what he's really looking for is a 59 relay (overvoltage protection). You can set it to match the CBEMA curve and give way better protection than triggering a shunt-trip from an SPD that's designed only to protect against a particular type of transient condition. They're not cheap, though; I'm not sure how much they cost because I've never specced one one individually; I usually buy combo relays that cost $1000 or more.

Your other option is to bus-tap the SPD directly to the panel busbars. Since an activated SPD is a dead short, this will cause the main breaker to trip on any significant transient, no shunt trip feature necessary. I would imagine any Type 1 SPD should be safe in this application, but check the rating to make sure it doesn't require a 15A breaker or something.

I spec industrial-grade SPD on every project I do, and even put a consumer-grade whole house surge protector on my own home for $25 a few years ago, the basic SquareD 240V 3W model. [I put mine on a 30A 2P breaker.] Cheap peace of mind and yet another "best practice" that should be in every home but is rare because of nickle-diming...

Last edited by SteveFehr; 03/11/17 11:35 AM.
TRUE POWER #218288 03/11/17 01:16 PM
Joined: Jul 2004
Posts: 9,919
Likes: 30
G
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If you have decent surge protection and a significant level of redundancy, transient damage will be rare if not non existent. It goes far deeper than simply hanging an SPD on the mains tho.


Greg Fretwell
TRUE POWER #218289 03/11/17 02:21 PM
Joined: Apr 2002
Posts: 7,380
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I'll take a 'back seat' on the spec part, and on what I feel is Greg's expertise.

I can't visualize using a SPD across the buss that would create a dead short to trip a 100 or 200 amp main. My gut leans toward the dry contacts and the shunt trip main, IF that's what the client wants.



John
HotLine1 #218290 03/11/17 03:58 PM
Joined: Mar 2005
Posts: 1,213
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Originally Posted by HotLine1
I'll take a 'back seat' on the spec part, and on what I feel is Greg's expertise.

I can't visualize using a SPD across the buss that would create a dead short to trip a 100 or 200 amp main. My gut leans toward the dry contacts and the shunt trip main, IF that's what the client wants.
SPD/TVSS don't "stop" transients, they work through voltage drop. The more current that's flowing through the wire, the greater the voltage drop, and the smaller the voltages experienced by the circuit(s) the SPD is protecting. Consequently, fault currents can be pretty high when they activate, 20kA or more. Plenty to trip a 200A breaker if the surge is that high. You want redundancy, you want SPD on the main panel, on the sub panels, and on power strips at the equipment. Damage from transients is pretty rare when the protections are designed right.

And yeah, 20kA+ is a LOT of current for a small device. The MOVs inside them can be damaged by high energy transients. Some of the really high end SPD include monitoring and will work through many small events or a few large ones and tell you when they're degraded and need to be replaced.

Last edited by SteveFehr; 03/11/17 04:05 PM.
TRUE POWER #218343 03/21/17 09:13 PM
Joined: Jul 2004
Posts: 101
J
Member
To the original point, you can get a number of high-end SPDs that have alarm contacts in them, and those alarm contacts could certainly be wired to a Shunt Trip on the Main CB so that if the SPD ever did fail, the contacts would trip the Main CB. But the big problem I see with this is that on all the ones I have seen, the alarm contact ALSO changes state when the power fails. Now technically, if the power fails, you also lose the power necessary for the Shunt Trip coil to function. But because of the way that works, I'm not sure you could ever energize the breaker in the first place. That's because the Main CB feeds the panel, which then feeds the SPD. So the alarm contact on the SPD will remain in the tripped state until it says "Ready", which means power is applied to the SPD. But that means the Main CB must close first, and any delay in the SPD alarm changing state means the Shut Trip will shut the Main CB off again immediately. You could get around this with a push button that opens the ST coil circuit I suppose, but then EVERY time the user wants to re-energize after a power failure, someone has to stand there and hold the button down to bypass the SPD shunt trip circuit, then energize the Main CB, then let go of the button AFTER the green "OK" light on the SPD comes on. That won't happen right on a dark and stormy night at 3AM...

I suppose that could also be done via a timer that delays every time power is energized, but the whole thing is getting a little ridiculous.


JRaef
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