Surge protective device Q.

[olly]

I have a guy that has a million $+ home with a lot of expensive appliances and he lives up in the mountains where there is pretty heavy lightning storms from time to time. Also, he has a back up generator installed on the home

My knowledge is below average on SPD's. Two questions:

1) Do SPD's have a problem with "dirty power" a generator produces.

2) What specific model would you recommend for this application?

Thanks!

 

[GoldDigger]

1. The answer depends equally on two variables: the design of the specific SPD and the exact waveform of the generator.

a. If the generator output waveform regularly contains voltage spikes that exceed the threshold of the SPD, the SPD will not last very long.​

b. If the generator has a "squarish" but otherwise clean output waveform, it will contain relatively high order frequency harmonics. This may not interact well with an SPD that includes capacitive filtering. Those filter elements may end up carrying a continuous current higher than they are dessigned for.​

c. Numerous considerations others will put forward that I have missed.​

2. The engineering department of the SPD manufacturer may well have specifications or tech notes on the subject. I don't.

 

[paulengr]

I have a guy that has a million $+ home with a lot of expensive appliances and he lives up in the mountains where there is pretty heavy lightning storms from time to time. Also, he has a back up generator installed on the home

My knowledge is below average on SPD's. Two questions:

1) Do SPD's have a problem with "dirty power" a generator produces.

2) What specific model would you recommend for this application?

Thanks!

1. No. Actually generators produce quite clean waveforms. A surge arrester uses up some of its life absorbing surges. It’s a consumable. But where a $20 surge strip may last one surge, bigger ones last longer.

So 2, not sure what you could possibly be meaning.

Generac sells by far the most popular generators but Cummins is a step up in reliability. CAT shows up in some commercial projects. Their engines are decent but the Italian generators they use are just OK. On bigger ones they use Katolite.

 

[junkhound]

2) What specific model would you recommend for this application?

I by MOVs from mouser or digikey and mount with minimal lead lengths.
Sizes and numbers of movs depend on how well you want to protect. Inductance and mounting are key.
A commercial box with foot long leads is near worthless.

Probably not per code*, but I mount my MOVs directly across the breaker outputs to the box, < 1 inch leads.
* except for QO and similar breakers violates the single iwre code provision for one thing, but is the BEST way to mount.

For a typical 20 kA lightning strike, at least 2 parallel B2 size block movs if you can fit them near the breakers with < 1" leads, otherwise inline cox with conduit from panel to MOVs box. .

 

[junkhound]

PS: Also, some of the semiconductor devices in the modern appliances will blow below the MOVs voltage knee.
Had lightning strike son's house via a nearby tree, tree arced to house to AC feeder. Had small movs at the AC which did not get damaged, yet a couple of FETs in the AC blew.
Only way to protect against that is semiconductor limiters and LC filters or isolation transformer, large and $$$

 

[Fred B]

Not sure what exactly you are looking for but have a customer that was blowing out refrigerator circuit boards at least once or twice a year from surges over many years. I've been using a SPD from PSP Product, it is Gen compatible, US made and offers a very high warrenty against surge damage on the residential models and has service rated products. Installed one on that customer's service now 3 yrs ago and he hasn't had a problem needing to replace the circuit board for his fridge since.

 

[paulengr]

PS: Also, some of the semiconductor devices in the modern appliances will blow below the MOVs voltage knee.
Had lightning strike son's house via a nearby tree, tree arced to house to AC feeder. Had small movs at the AC which did not get damaged, yet a couple of FETs in the AC blew.
Only way to protect against that is semiconductor limiters and LC filters or isolation transformer, large and $$$

This demonstrates one of the issues with your design.

You are using components which are not Listed. That’s a huge Code problem. Worse you are using only the MOV. Are you aware of what happens when the MOV fails? It turns into a dead short. That is why Listed assemblies must be fused. You are just being cheap in all the wrong ways.

The lead length is important to be sure but the MCOV on those commercial devices is at the end of the pigtails. That’s how UL tests them. So your concern is valid but lacking interpretation.

Finally you apparently grasp the concept of lead length but remember a MOV is in parallel with the equipment protected along with the leads. So the range you are talking about has some seriously long leads from the MOV and all that branch circuit wiring counts. Regardless of size, spacing, insulation, etc., the surge impedance of residential wiring works out to above 20 V per inch. So if your range is 10 feet from the breaker, the voltage it sees increases by 20 x 10 x 12 = 2400 V over the knee.

THAT is the lead length you need to be focused on and why your attempts to shorten leads failed to protect a load. So-called “whole house” surge arresters protect the panel they are connected to. That’s why they always tell you it’s no substitute for surge protection at the loads. There is some “sharing” effect between surge arresters but if there isn’t one at the load, you have no protection. You cannot ignore the laws of physics.

Finally knee voltage is not a steep drop off to 0 resistance in a MOV or really any surge arrester. There is still impedance there and the overvoltage, though severely reduced, is not zero.

 

[junkhound]

Thanks for the lecture professor, although you got a few items wrong and need to review maxwell's equations.

 

[Jraef]

SPDs (Surge Protective Devices) come in three “Categories” based on what /where you want to protect.

Cat C would go at the Service Entrance, aka “whole house” protection. It would be rated for the highest energy surges and spikes, like what would come from a nearby lightning strike (nothing will protect from a direct hit). You should always get one that includes a “hit counter” and an alarm or indicator that tells you it is no longer functioning.

Cat B would go on any distribution or sub panels, intended to protect against low level spikes and surges that were too low for the Cat C to react to. Get one that at the very least has indication that it is still active and good.

Cat A would be the little Surge Protectors or strips that you plug into the wall, then plug your end devices into. These are the ones that are generally “one hit wonders” that essentially sacrifice themselves the first time they are hit. If you have Cat B and C, these things are basically useless. If you are a renter and can’t do B or C, then they are slightly better than nothing.

 

[olly]

Not sure what exactly you are looking for but have a customer that was blowing out refrigerator circuit boards at least once or twice a year from surges over many years. I've been using a SPD from PSP Product, it is Gen compatible, US made and offers a very high warrenty against surge damage on the residential models and has service rated products. Installed one on that customer's service now 3 yrs ago and he hasn't had a problem needing to replace the circuit board for his fridge since.

Could you tell me exactly which SPD it was that you used? Thanks for the reply!

 

[ggunn]

In a related story...

Lightning once struck a transformer on my block while my electric dryer was running. The voltage spike blew out the heating element in the dryer.