Doubt varistor

I have a voltage surge protector 220v, it uses 3 x 20mm varistors and does not have a internal thermal fuse. When a varistor bursts or explodes or at the end of its useful life, the protection is lost but the surge protector sockets continue to operate without protection?



Varistors that have failed no longer provide protection against over-voltages. However, it is typical for them to fail to a short-circuit condition, causing a local fuse such as the one in the image provided to open, indicating a fault of some kind.

If such a fuse is simply replaced, the remains of the varistor may not trip the fuse again, allowing continued operation without voltage protection. Failures to an open-circuit condition are also possible, especially in the case of more modern varistors that have built-in thermal protection as a safety feature.

If you doubt the protection provided by a power distribution device such as the one shown, testing or replacement is recommended.


I have never changed any of the fuses on this voltage surge protector, I have been using this device for years without grounding on the ground, but recently I put grounding on it

This surge protector does not have a thermal fuse, just the main fuse.

If one of the 3 varistors burst explodes some signal shows that I will know that it is necessary to change the varistor???

Hi @cloudff7 ,

Unfortunately, this is a product we do not currently carry and therefore are unable to support this. You may have to look in the owners manual, seek help from the distributor you got the item from, or reach out to the original manufacturer. By looking at it however, it appears it does not give any indication whether or not the device is protecting properly. As a guess, if the fuse has never opened on it, more then likely it should be protecting properly. If the fuse ever opens on it, then it would be more questionable if the protection feature is working.

TL1150-ND is an example of a surge protector that actively shows whether or not the outputs are being protected, I do not see this on the device you have.

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Surge protectors don’t need a thermal fuse unless they are being operated in an oven.


I doubt if one of the 3 varistors explodes from my voltage surge protector, will the protector continue to work without surge protection? I don’t know if a varistor exploded and I’ve been using the protector for years, I don’t have its electrical diagram
3 varistors F-N, F-T, N-T without thermal fuse

If a varistor fails on a surge protector, then as Rick said the protector will still likely provide power to its outlets, but its function as a surge protector will no longer apply. At that point it’s little more than an extension bar, I would not consider it to be “working” as its primary task is compromised. If you suspect there may be damage to the device, I would recommend replacement. It’s always better to be sure when it comes to circuit protection, replacing your surge box now for thirty or forty dollars is far easier than replacing blown equipment later for hundreds or thousands.

So in my case the fuse will never alert that a varistor has exploded? I never changed the fuse in this protector but if one of the 3 varistors blows, will the protector continue supplying power to the sockets?

Hi @cloudff7 ,

Correct, the fuse in your device will not alert that a varistor has exploded or isn’t functioning properly. If one varistor has damage, you will not have any way of knowing it, and it will provide power to the powered devices but not be able to provide the full protection to them for a few reasons.

For future reference, it may be a good idea to get a surge protector with an external indicator, and if the device you choose (like this one) does not have an external indicator, you may be able to get a less expensive unit with external indicator to plug in before the unit you choose, that way you will have external indication, and still be able to use something like what you have here for its other benefits such as its noise/emi filtering, number of ports etc.

I don’t see any harm in plugging one protector like this one into another indicated protector, it would most likely provide even more protection. But if you have doubts with this one, as Matt_Roen had stated, it may be best to consider purchasing a new one.

The fuse doesn’t blow when the varistor blows?

A varistor is a surge protecting component. The fuse should have been place before they are cheaper to replace.

Robert is correct, the fuse should be placed before the varistors. The fuse may or may not blow when varistors are bad.

Over 99.9% of the time varistors fail shorted due to power surges, that’s precisely what they are meant to do. A shorted thermistor will short the AC line blowing the fuse.

If the varistors of a surge protector are failing open circuit then there is either a very bad design flaw in the surge protector circuit, or the varistors were defective. In either case I would not buy that brand of surge protector again.

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Is the fuse you are referring to a thermal fuse or a common fuse?

I never needed to change the fuse in my upsai fht1200 220v surge protector

Common fuse.


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Hi @cloudff7
A fuse is there to protect against a varistor short, or short circuit in general. The fuse itself does not contain anything to limit any voltage spikes. As PaulHutch had mentioned, the varistors typically fail in a shorted state, which then would cause the fuse to open. Usually, it is the case if the fuse never needed to be replaced that the varistors are working properly, but of course we can’t state that definitively.

Hopefully the responses have helped with your question, as we don’t have any more information to provide you on this since unfortunately it is not a product we currently distribute for. :slight_smile:

I never changed any fuse in my protector and surges, is that good or bad? Is it related to the health status of the varistors? Are the varistors connected to the glass fuse? If a varistor explodes, will the glass fuse blow?

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