Power Distribution and Management

Lightning surge protection: Two is better than one!

Lightning surge protection: two is better than one

What everyone knows. To protect low-voltage installations against power surges caused by lightning surge protection devices (SPDs) should be installed as far upstream as possible. Why? To drive the awesome power from lightning strikes to the ground and stop it from spreading down through the installation.

What everybody doesn’t know is that such protection may not be enough to protect sensitive electrical and electronic equipment – especially if it’s some distance away from the primary upstream surge protector.

How do long cables affect voltage in equipment?

If the length of the cable between the primary surge protection (in the incoming switchboard) and the electrical equipment you want to protect is too long, oscillations and wave reflections may lead to sharp voltage rises in the equipment. In fact, such rises may exceed the SPD’s voltage protection level (Up) and rise to levels that are twice as high as Up.
The figure below shows how the maximum voltage curve at the end of a cable is determined by cable length for an incident wave-front voltage of 4kV/µs.

Max voltage at cable end

Why does voltage double?

Lightning emits electromagnetic waves in frequencies measured in hundreds of kHz and MHz. The voltage variations are so fast there is no way you can assume voltage is the same at any point in the cable. And the incident surge front may further excite oscillations caused by the cable’s own resonant frequencies (e.g., parasitic capacitances or filtering).

What is taking place is known as total internal reflection. This is when a wave traveling through a medium hits the medium’s boundary at an angle that is greater than its particular critical angle.

A useful way to picture total internal reflection is a wave that crashes against a wall. The wave represents the voltage wave and its height the wave’s magnitude. When it crashes against the wall, it doubles in height and flows back whence it came.

The wave represents the voltage wave

What is the best solution for failsafe lightning protection?

When reflection occurs over cable lengths shorter than 10 meters, voltage surge fronts in buildings may be disregarded.

But as recommended by the standards (IEC 61643-12 and IEC 60364-5-53), when the loads are located more than 10 meters away from the incoming-end protection device, it is necessary to add a second SPD for additional fine protection as close as possible to sensitive loads.

The second device doesn’t need to have as high a discharge capacity as the primary SPD. Imax <8kA (8 / 20) is enough. That’s because the primary surge protection device has already diverted the lightning current to the ground.

lignthing protection cascadingVisit our SPD site to learn more about solutions to better protect your power.

66 Responses
  1. Igor Brzeżek

    Yes, I’m using two protection in my computer network: one outside building near radio equpiment and second in data room. Before that every storm i had problems but now nothing happens. But we have remember about very good grounding!

    Best regards

  2. R Sukumaran

    I have been using the protection connected to most of the units. Hope i have good protection most of the time.


    Thanks for the info, we just bought a number of 2000 & 3000 watt UPS units from APC

  4. Janice Temple

    Learned I knew more than I thought I knew and learned something I wasn’t sure of.

  5. John Payton

    Could probably do better, have a couple including APC back up

  6. Chuck Sihler

    Thanks for confirming my protocol – a surge protector at each site where electronics are in use!

  7. Gary Gissiner

    Remember surge protection is only as good as the ground it is connected to. If you have more than 1 ground rod on your building they should be strapped together. Many time a house will have several ground rods, 1 for the electric service, 1 for the phone system and 1 for the cable tv/internet, lighten will seek the path of least resistance and will travel across a building taking out everything in it path to get to the best ground, so make sure all grounds are strapped together. Clean and tighten your outside ground yearly. Finally all surge protection is not created equally, we have been using APC for 10 years without loss of equipment. This should speak for its self.

  8. jim mc

    no power surge problems. either the surge protector is protecting or i am that one person in a million that does not incur any power surges.

  9. Marybeth Webster

    Our company purchases our equipment from APC. APC is a Great company !!!

  10. Theodore Wirth

    IMHO battery backup solutions provide the ultimate in clean power and I use APC battery backup solutions on our cable modem, switches, routers, NASs and PCs. In fact, during recent power outages, battery backup provided sufficient power to the cable modem, switches and WiFi router for hours so that we could continue to use our IP phones and access the Internet from our laptops, tablets and smartphones and get some work done.

  11. Michael Gonzales

    My IQ score was 105. I got one wrong answer and I knew it was wrong as I clicked! Darn!

    Thanks for the useful info tho!

    I WIN! YaY!

  12. Andrei Koribanics II

    I own an APC Back-Ups RS 1500 and am very happy with its performance.

  13. Larry Wadsworth

    I have protection in my house, but not outside. I do remember from years ago that it was better to have protection both inside and out.

  14. Julie Limburg

    Thank you for the info on power surge protection.

  15. Julie Limburg

    Thank you for the information on power surge protection.

  16. Dennis

    We have minimal probability of lighting here in Southern California. Voltage variations are the principal concern, as well as static discharges in the desert regions. I use reliable UPS systems with significant voltage variation protection and additional surge protection as needed.. Today we are at 123VAC, with recent brief drops to 93VAC.

  17. Denzel Nonhof

    Good information. Thanks. I have had protection with surge and battery backup for many years. It is a necessity for any serious person using a computer.

  18. Robert Gallant

    Great reminder about why power surge protection is so important!

  19. William George Wardino

    I live in Florida, the lightning capitol of the world! Moreover, spikes in the current are as common as dogs and cats. I wouldn’t dare risk not have quality surge protection in my home and home theater. I’ve been using APC protection for many years and it has never failed me.

  20. Anna Kleinhaus

    Good information . Do not have any installation to protect entire electrical supply to the house. But voltage variations are frequently the cause of near crashes that are prevented thNks to the APC Peotectors with Backup batteries allowing a safe and orderly shutdown of the computers. Lightning however did once burn the router and shut down everything in the network.

  21. ABugeye

    Thanks for the enlightment and the reminders about the importance of surge protection.

  22. Jeff

    my new back ups pro 1000 doesn’t seem to work correctly. I have my computer plugged into the master ooutlet. On two occasions when the power went off, my Mac G5 went off too. I have the ups so I have time to shut my mac down ccorrectly. Good chance my mac will suffer damage at some point. Maybe I have it plugged in wrong.
    I betcha I won’t receive a response to this.

  23. Robert Lotito

    I have your surge protector on my home/office equipment which includes my laptop.

  24. BCS

    Many folks buy a powered backup surge protector and then with limited outlets incorrectly plug a power strip into one of the surged/battery outlets and while the power supply may be large enough they really need a power distribution strip. Never explained well enough or shown as accessories by the 1300 and 1500 units.

  25. Larry Reiff

    I have 2 APC Smart UPS’s on each of my servers since they are internally mirrored . and I use the same APC Smart UPS’s at all the local stations . Along with surge protection on the panel . That is grounded very well . I did not know that the voltage will increase as the wires grow longer . Thanks for the information . Along with offsite backup of all files . ( not with a cloud service , but in my own home )

    • Jennifer Wendt

      We love to hear that Rich! It’s always heart warming to hear our customer stories.. after nearly two decades in this company, our faithful followers continue to amaze me. Please reach out to me at jennifer.wendt@apc.com if you would like to be featured in a future mailing!

  26. Ralph Clouser

    I have an ES 650 on each of my computers. They have done a great job
    with surges over their life. Replaced the Battery in one and it was as good as new.

    • Jennifer Wendt

      Hi Ralph,
      We never get tired of hearing these great stories. If you would like to be featured in one of our future emailings, please let me know at jennifer.wendt@apc.com and I will have someone reach out to you. Thanks for such a great post!

  27. Theodore Olson

    I have 3 Dell computors and 1 Dell Lap Top – all but the Laptop is protected by 3 APC Back UPS units – have had plenty of surges – power outages – close lightening strikes and plenty of electrical home equipment going on and off pulling lots of juice and have never had a problem with my computers – I have 2 networked and the same goes for them – and when I use my laptop I do plug in into one of my UPS units

    • Jennifer Wendt

      We love hearing these success stories! Thanks Theodore for sharing. If you would like to be reached by one of the members on my team, we may possibly feature your story in the next edition of our magazine or email distribution : there is nothing like hearing the ‘feet on the street’ and sharing these types of great stories with your peers! You can reach me at jennifer.wendt@apc.com.

  28. Steve Busciglio

    I wouldn’t be without dual level protection…meter based whole house protection as well as APC Back UPS for all of my critical electronics. Knock on wood, I haven’t lost anything to surges in over 15 years.

  29. Larry Mays

    All our venues have APC Backups. Florida has bad stormes with a lot of
    lightning. APC protects all our electronics.

  30. APEX

    This was interesting we recently had a surge and had to replace at least 9 different computers and we thought it was interesting that we had some of the systems had “surge protection” built in…. it did not work….

    Anyways thanks for the great post and keep up the good work!