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Is true Clamping Voltage a definitive measure of quality of a Surge Protector?

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c(n*199780) Senior Member
Feb 18, 2002
So, the question of how to choose a quality Surge Suppressor / Protector / Power Strip was always difficult to answer.

I thought I found a good deal when I looked at a TROND Surge Protector because they said it had a 350V Clamping Voltage [lower is better] but then someone said "It's not UL Listed, meaning they can claim whatever they want. Compare to Tripp Lite for example, which will state: Certified to meet strict UL standards (UL 1449 4th Edition; File E89439; Series AGIP7568; Updated 10-28-2015)

So then I found this deal:

I thought it was good because it has a Clamping level of 150 V RMS.

Why is Tripp Lite not ringing bells with their low Clamping Voltage instead of posting the Joule rating - isn't Clamping Voltage what it's all about for a Surge Protector?
Plugging a device into any outlet in your house that does not have a UL rating is just foolish.

There are MANY different UL standards, and purchasing a device that is UL listed does not guarantee the quality. What is does guarantee is that the design was tested and verified for safety by an independent laboratory. Depending on the standard, UL may request "control" of the design documents (drawings, parts lists, etc.) and place of manufacture. If the company changes any of these and doesn't get approval, then UL will remove their listing.

So - don't use anything in your home that is not UL listed. It doesn't require much detective work to see if a device has a UL listing...if it does, you will see one of these symbols on it:

UL Mark.jpg

UL is different than CE. UL is verified by an independent laboratory. A CE mark is a "self certification" by the manufacturer that the design complies with various international standards. A big difference.

Do you remember the electrical scooters that caught on fire a few years back? Yup - not UL listed. I could go on and on, but I know you purchase smart home thingies...and many of the WiFi devices ARE NOT UL listed...and are just a fire waiting to happen. The ZigBee and Z-Wave devices are required to be both UL listed and comply with the appropriate Z-Wave or ZigBee standard.

Now - for a surge protector...if it is UL listed then it is worth buying.

If you want to overanalyze this (as you tend to do), then you need to ask yourself...what is an " electrical surge"? A surge is a brief overvoltage spike in the voltage waveform in your house. This overvoltage spike causes the waveform to deviate substantially from the ideal sinusoidal waveform. An overvoltage spike is not caused by just your typical lightning strike, it could be caused by a large motor turning on or off (A/C, refrigerator, freezer, dryer), an adjacent circuit tripping, someone knocking over a power pole, power being restored after a loss of power event, etc. (Motors turning on and off are the most common causes). There are 3 "figures of merit" to look at when trying to protect from surges:

1. At what voltage does it clamp?
--- The lower the better to prevent stressing of protection circuitry within your electronic devices.

2. How fast can it clamp?
--- The faster the better to minimize the high frequency energy stressing the protection circuitry within your electronic devices. Any power input circuit on an electronic device looks like a low pass filter...meaning that it will pass low frequency (i.e. 60 Hz) energy and reject higher frequency energy.

3. How many joules can it absorb?
--- Every time a surge protection device does its job, it absorbs a certain amount of energy...measured in joules (or J). The protection circuits can only absorb so much before they are completely destroyed. So, the higher the joule rating, the better.

In general, you should be replacing any surge protection devices on a frequent basis (every few years). They are sacrificial (like break pads on your car) and are not meant to run forever. If you have sensitive electronics, I would recommend a UPS backup as well. You want one which will protect from under-voltage. An under-voltage situation can cause inrush current damage to electronics, and a surge protector does not protect in this situation.
You say replace them every few years, any way to test\know when it's time?
So when I looked at the numbers, there seems to be a general agreement that Clamping Voltage of more than 400V is too high, it won't kick in time, regardless of Joule rating, which should be at least 2,000 joules.

Well I have a Tripp Lite that is 4320 Joules but with a huge Clamping Voltage of 800 V.
So does that mean that it's useless at such a high clamping voltage!?

I have a Belkin that is 3000 Joules but with a Clamping Voltage of 500V.
500V is being looked at as still not low enough, does it make those 3000 Joules useless?

Not sure what to do with those two models which I bought many years ago, by blindly only looking at the Joule rating.

More importantly I wish to protect my router and Access Point.
My new Tripp Lite at 150V Clamping voltage and 1500 Joules should do just fine.
But I want to use a battery backup too.
So do I plug in the battery backup into my Tripp Lite?

Battery Backup says "zero clamping response time: instantaneous" - which sounds bogus, I don't think APC is even listing the Clamping Voltage on it and it lists the joule rating only at 490V:

So can I just plug in the battery backup INTO a surge protector!?
I have a 3,000 Joule Power Surge Protector but it has a high 500V Clamping Voltage, more than the at least 330V-400V usually recommended [Lower is better].

For that matter I also have a 4,320 Joule one with a huge 800V Clamping Voltage.

How useless are these Power Surge Protectors with clamping voltages so high?

Are they really just expensive power strips with no protection, since they will not "kick-in" in time to save your equipment with clamping voltages so high?

Separate question, are there any down sides to plugging them into a surge protectors with 150V clamping voltage with lower Joule rating?

Let's say you have a 150V CV but only 1000 Joule model.

So you plug it into the wall and then plug into it the 4,320 Joule one with a huge 800V Clamping Voltage.

So do you now you have 4,320 Joules [plus the 1000 Joules + 150V clamping voltage?
As said above, get yourself a good UPS. In my experience, as long as you are purchasing a "true sinewave output" UPS model then you are in good shape.

CyberPower and APC both make good products.

I use this series (https://www.amazon.com/gp/product/B00429N19W/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1) for my entertainment center, my son's PC, wife's PC, TV in bedroom etc.

I use this one (https://www.amazon.com/gp/product/B001RJEF7M/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1) for my network rack and my main PC.

Also - in general - you should not plug a surge suppressor into a UPS...there is really no need as the UPS does the same job.

You should not plug a UPS into a surge suppressor (the manuals for the UPS will tell you this).
Okay, well hold on.
"The internet" generally agrees that the minimum clamping voltage should be 330V, maybe as high as 400V, but no higher. So that would mean that if a UPS has a clamping voltage that is high, it will not provide the protection that a surge protector with a 150V does.

And that's without even talking about Joules. My APC UPS has only a 490 Joule label, and my surge protector is 1500 Joules to 2000 Joules.
So then my question stands about what would happen if I use both, and yes the companies "don't want you" doing that but several posts on the internet called marketing BS on that. So what does the math say?