Prevent Electronics Damage With Proper Surge Protector Joule Rating

Updated on January 21, 2020
Glenn Stok profile image

Glenn Stok likes to research and evaluate products, and can clearly explain their features due to his background as a technical writer.


It's essential to understand how power surge protectors work so you can make the right choice. Selecting the right one will properly protect your expensive electronic equipment.

Many people think they are saving money by buying a low-cost surge protector that gives little or no protection, but excellent protection doesn't have to be expensive either. Selecting the right one is just a matter of knowing how it works.

The purpose of a surge protector is to divert voltage spikes safely to ground rather than letting it enter the sensitive circuits of your equipment.

Voltage spikes can come through the power line and or through the telephone line. So phone lines need to be protected too, if connected to your equipment.

Under normal conditions, electric lines have a steady voltage (120 volts in the United States). However, in some cases, erratic fluctuations of the voltage can occur. These are spikes in the electric power that can damage equipment.

Any voltage spikes need to be re-channeled to ground. That's what a surge protector does.

Where Do Power Surges and Voltage Spikes Come From?

Lightning is well known for creating power surges. But voltage spikes can also come from high powered equipment that is on the same circuit. These can create powerful spikes, in the electric line when they are turned on and off.

For example, if you have a refrigerator or air conditioner on the same line as your computer, you may very well be getting interference that can damage the computer. Sometimes it might just be an unexpected reboot, but it can also cause a hard disk crash that cannot be recovered.

The average surge protector will not protect your equipment from lightning strikes. If this is a major problem in the area where you live or work, then you need to consider a full-building protection with an external lightning arrestor that diverts lightning strikes to ground.

If this is your concern, check with your power company and your telephone company. Ask what precautions they have taken to protect you and what recourse you have in case their lightning protection fails.

Explanation of Clamping Voltage and Joules

What do you look for to select a good surge protector?

You need to pay attention to two things:

  • Clamping voltage - needs to be low to cut off high voltages.
  • Joules - needs to be high to increase the surge protector's life expectancy.

You want a low clamping voltage. This is the amount of voltage that is let through. You don't want to let through too high a voltage, as it will blow your equipment.

The standard clamping for a 120-volt line is 330 volts. Don't buy anything that allows more than that through.

You also want a protector that can absorb as much energy as possible before blowing itself out. This is measured in joules. The higher this number, the longer will be the life expectancy of your surge protector.

If this absorption rate is too low, you may only be protected for a single event. Then you need to get a new surge protector. That's another mistake many people make. They don't realize that the surge protector is not going to protect them indefinitely.

How Many Joules Do You Need for Good Surge Protection?

Some of the cheaper surge protectors only provide protection once. It's like a fuse blowing. You do want to protect your equipment, but you also don't want to keep buying a new surge protector each time it protects you.

If you use one that has a sufficiently high joule rating, then it should last through many power surges and spikes.

I suggest a rating over 3000 joules. With anything less, your surge protector may not survive a spike. Your equipment may not be protected if you get more than one high voltage spike before replacing the surge protector.

A single high voltage spike may destroy surge protectors that have a low joule rating. You'll need to replace these cheap protectors often. You'll end up saving more money in the long run by getting a good surge protector with the proper joule rating in the first place.

What Is a Good Recommended Surge Protector?

I use the APC Back-UPS XS 1000 Uninterruptible Power Supply (UPS) on my main office computer where I need to avoid losing power. Many UPS's have surge protectors built in, such as this one.

If you don't require guaranteed power without interruption, then a good surge protector is all you need, such as the Tripp Lite Isobar that I use to protect my expensive equipment.

Belkin and APC are also good manufacturers of surge protectors. Some have LED lights to indicate that they are working and that you are protected.

The most important thing to remember is to pay attention to the joules (the higher, the better) and the clamping voltage (basically around 330 volts).

3570 Joules of surge protection for power and phone lines:

Tripp Lite Isobar 10 Outlet Audio/Video Surge Protector Tel/Modem/Coax/Network 8ft Cord Right Angle Plug, & $500,000 Insurance (HT10DBS)
Tripp Lite Isobar 10 Outlet Audio/Video Surge Protector Tel/Modem/Coax/Network 8ft Cord Right Angle Plug, & $500,000 Insurance (HT10DBS)
You can also find power strips with surge protection. I use the Tripp Lite HT10DBS Isobar. It has 10 Outlets and enough room to plug in a few large transformer adapters, as shown in the image. You will also want to include surge protection on your phone lines if they are connected to your equipment. This Tripp Lite Isobar includes protected phone jacks. Another solution is to have the phone company install surge protection on each phone line coming into your premises.

Whole-House Surge Protection for All Electronics

All your electronic devices should be protected. If you have a lot of different equipment throughout your home, you may want to consider a whole house surge protector. That needs to be installed by an electrician.

It could help save everything else that wouldn't have been protected otherwise. Besides damage from lightning strikes, voltage surges from other sources can come through your electrical system too. You never know where it strikes, and protecting your entire home is the best choice when you have a lot of expensive equipment.

As a final thought, it's not just power surges that can cause damage. Computers can experience problems not related to a power surge, such as a hard disk crash. So don't forget to back up your data.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

Questions & Answers

  • How many joules of protection are needed for a clothes iron?

    Surge protectors are mainly used to protect electronic equipment. I wouldn’t consider using one for ironing clothes. There is no need for that. You wouldn’t be using your iron during a thunderstorm anyway, since you should stay away from anything connected to the electric wiring that might receive a high voltage spark from a lightning strike.

    Nevertheless, just to answer your question, remember that higher the joule rating, the more energy can be absorbed before failure. This is why you need a high joule rating for expensive equipment as I explain in my article.

    It’s more meaningful to protect electronic equipment that’s expensive to replace. This especially includes components that store important, irreplaceable data such as family pictures, financial information, business data, and personal recordings. So I’m talking about your computer and backup equipment.

    A clothes iron and other inexpensive equipment (lamps and kitchen items such as a food blender) don’t need a high level of protection. If you’re serious about using a surge protector for these types of household items, you don’t need anything over 1000 joules of surge protection. Replacing these items is cheaper than the cost of a surge protector.

    In addition, the best thing is to avoid ironing during a lightning storm. If your iron gets zapped by lightning while you’re ironing your clothes, you may have more serious problems than to worry about your iron.

© 2010 Glenn Stok

Reader Comments

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    • Glenn Stok profile imageAUTHOR

      Glenn Stok 

      2 years ago from Long Island, NY

      Jordan, When you turn on a heavy-usage device there is an initial spike that can reach close to 330 volts. If the clamping voltage were any lower, the surge protector would erroneously trip every time you turn on the device.

    • profile image


      2 years ago

      Thanks for your response Glenn, but I admit I am still in the dark with the 330v UL Let thru limit. I know about the lesser the better, so why so high ? I would have thought something like 110 volts would be ideal / best let-thru voltage vs 330v. Plus I think the clamping voltage should ideally be like 130v max. What do you think ?

    • Glenn Stok profile imageAUTHOR

      Glenn Stok 

      2 years ago from Long Island, NY

      Jordan - You need to compensate for normal spikes when a unit is turned on. The lowest UL rating for clamping voltage is 330 volts. The lower the rating, the better the protection.

    • profile image


      2 years ago

      A UL rating of 330 volt ? Isn't that too high for let thru ? Clearly I am missing something here with UL standard.

    • Glenn Stok profile imageAUTHOR

      Glenn Stok 

      8 years ago from Long Island, NY

      Frank, If they don't provide the information you need to make a purchase decision, then don't buy the product. That's their lose if they don't have complete specs, or they are hiding something about an inferior product. There is no way I can determine that.

    • profile image

      Frank in Miami 

      8 years ago

      Good site, thanks for helpful info so far.

      I wanted to purchase a new surge suppressor for my HTIB setup, but have had difficulty determining anything except protection in joules -manufacturers seem to not -or not WANT to -include either clamping voltage or response time in the specs, either on the box or online. I went with a Dynex DX-S114241 11-outlet model, with a 3750 joule protection rating. Know anything about clamping voltage or response time for this model?

      Thanks in advance.

    • Glenn Stok profile imageAUTHOR

      Glenn Stok 

      8 years ago from Long Island, NY

      LH Fernando, Clamping voltage is the level where electricity will be conducted to ground in order to protect your device. So you want to have a clamping voltage as low as possible which is still over the level of you normal highest voltage.

      The lowest UL protection rating is 330 volts. Your highest voltage during ringing is 150, so get a 330 rated clamping voltage.

    • profile image


      8 years ago

      Our (Sri Lanka) telephone system has 48 V DC supply when "on hook" and 130-150 V (rms)during ringing. What would you suggest for the clamping voltage of the SPD for the telephone line?


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