I have come a cross a line of MOV-based surge protector that supposedly differ by the amperage of the appliance they are designed to protect. For example one protects "LCD and plasma monitors" and the other "dishwashers and washing machines". They specifically boast different amperage ranges as the difference (as opposed to features such as timers meant to protect mechanical compressors).

However it is my understanding than MOV-based surge protectors work by clamping voltage, not amperage so I am assuming it is a marketing ploy. Am I correct in my assumption?

  • \$\begingroup\$ When you have the device at hand, just look at the differences \$\endgroup\$
    – PlasmaHH
    Mar 18, 2016 at 18:22
  • 1
    \$\begingroup\$ No, they have a max energy rating J Joules, energy peak that can accept. You'll find it in every daasheet, I never seen a guy selling MOVs on my doors, where did you see such marketing tricks? \$\endgroup\$ Mar 18, 2016 at 18:54
  • \$\begingroup\$ @PlasmaHH for the sake of discussion let's assume they are different. I'm asking whether a difference concerning amperage tolerance in an MOV-based protector even makes sense. \$\endgroup\$
    – t0x1n
    Mar 18, 2016 at 20:24
  • \$\begingroup\$ @MarkoBuršič I'm afraid that's standard practice in practically all technician appliance installation in Israel (primarily washing machines and dish washerss). The "technicians" (more like salesmen) are also trying to sell you magnets that allegedly reduce water scale. So on the topic at hand - I take it you are confirming my understanding that MOV-based protectors protect all appliances equally, regardless of amperage, and that no special per-appliance protectors are needed? \$\endgroup\$
    – t0x1n
    Mar 18, 2016 at 20:28

2 Answers 2


Movs clamp voltage, but they have a very soft knee. With rising surge current the clamping voltage will increase. As a typical example, a 100v nominal mov will start conducting 1ma at maybe 80 or 90 volts. At the peak current it maybe nearly 200v clamping, though this could be for currents up to several kA.

Movs are rated to different current levels because they must dissipate the energy as heat. A larger mov at the same voltage rating will take more energy before it is destroyed.

As always, data sheets are your friend and should show you curves for v and I. Most likely this will be the difference for this series.

Edit in response to the follow up comment below:

Choosing an actual mov is not about the down-stream load, but about the level of protection you need to meet. Depending on your industry and country there will be a spec defining the sort of transients which may occur, allowing you to size your mov accordingly. For a normally operating power delivery system, a mov should have no effect on the equipment at all. It's when problems happen (eg. lightning strike) that your kit needs to survive.

If you are talking about Black box protectors then that is a little different. I would guess that a surge protection plug socket would carry both live and neutral current, and therefore must be rated for a normal operation current. Just like an extension cord, the current carrying copper must be able to supply the load. This however has nothing to do with the mov rating.

  • \$\begingroup\$ Please forgive my ignorance, but I don't fully understand how to translate your answer to address the question. Suppose I have an LCD monitor and a laundry machine, each drawing a different amperage. Do I need two separate MOV protectors, each rated for the amperage of the specific appliance? \$\endgroup\$
    – t0x1n
    Mar 18, 2016 at 23:42
  • \$\begingroup\$ Thank you for the edit, upvoted. I'm still a bit unclear on the bottom line though - do I need an "LCD" protector for my LCD and a "dishwasher" protector for my dishwasher due to the different amperage draw of the appliances? Also, what do you mean by "black box" protectors? I'm talking about the regular protectors you buy in the store, they look like this: img.ksp.co.il/item/7295/b_1.jpg \$\endgroup\$
    – t0x1n
    Mar 19, 2016 at 0:16
  • \$\begingroup\$ That image is exactly what I meant by Black box protectors. Because you plug your device into the box and plug the box into the socket, the box needs to carry the current from the socket to the device. A washing machine probably takes considerably more current than a monitor, so you need a higher rated box. \$\endgroup\$
    – Loganf
    Mar 19, 2016 at 0:58
  • \$\begingroup\$ I think I get it now. The MOV protection has nothing to do with the current (which means the technician was wrong, or more likely lied). It is merely the current carrying copper itself inside the surge protector box that should be rated high enough for the appliance, but that's just for normal operation and has nothing to do with surge protection (just like an extension cord). \$\endgroup\$
    – t0x1n
    Mar 19, 2016 at 10:27
  • \$\begingroup\$ So seeing as how a 5$ extension cord presumably contains the copper to support any standard appliance, I don't see any reason why any run of the mill surge protector won't. Besides, I've been using "LCD" protectors for all my appliances (e.g. fridge) for months, wouldn't they have fried by now if they weren't properly rated? \$\endgroup\$
    – t0x1n
    Mar 19, 2016 at 10:27

It has nothing to do about power of appliance, rather the probability of a specific surge and their numbers in the life of the appliance. An LCD and a dishwasher could have the same MOV, bigger is always better. The manufacturer chooses the size of MOV according to the place available (a small MOV is better than nothing) and statistics about surge frequency (numbers of surges per life) and their energy. The effectivenes of MOV arrester can be further improved by adding a choke before MOV, this limits the gradient of current rise di/dt, a chocke without MOV can eliminate the majority of the surge by itself. Or in modern electronics you will see a MOV then a common mode choke and then a second MOV.
It depends also on the ratio between price of protection vs. price of protected parts, a 2k€ LCD has to be well protected meanwhile a clock for 10€ probably doesn't have any surge protection.


I see your question is related to add on surge protection devices, not for MOVs as the electronic part and use in the appliance. Then what you can do is to install a surge protector device in your home. First of all you have to check the ground of your house, because no surge arrester wil help you if you don't have a good earthing. Second thing is to know the type of connection to the mains system: TN, TT, IT,.. then choose the capabilty according your place (keraunic level, it's a probability of lightnigs and their power in your area) - https://www.google.si/search?q=keraunic+level+map&biw=1333&bih=655&tbm=isch&tbo=u&source=univ&sa=X&ved=0ahUKEwjMvfK-08zLAhUL1xQKHccuCLgQsAQIIQ#imgrc=4TR8CQgdfcnp6M%3A.

similar device: http://uk.farnell.com/abb/ovr-t2-1n-40-275-sp-ts/surge-protect-device-ac-1ph-n/dp/1895137


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