Risks of placing Varistor before Fuse for Mains Voltage

Question

I have seen many questions on this and other forums asking about the best option to place the fuse relative to the varistor to protect it from mains voltage spikes and the answer was unanimous: The fuse should be placed before the Varistor. The only reason stated is that the Varistor may not be able to withstand the current until the breaker trips and need something to open the circuit before the breaker: the fuse.

I understand that it is the best for safety.

However, I've seen many appliances that are sold in Europe with the Varistor before the fuse. I started to wonder why and I could definitely think of one possible advantage.

If the fuse is placed before the Varistor (best practise), in the event of overvoltage, the Varistor will short, draw a lot of current and ALWAYS blow the fuse before the breaker trips. Therefore, in any overvoltage event, the appliance would be rendered useless until the appliance is disassembled and the fuse is replaced. On the other hand, if the fuse is placed after the Varistor (not best practise), and the Varistor is the first component after mains voltage, in the event of overvoltage, there are two possibilities:

1. The Varistor shorts and gets overwhelmed by the current and fails open, before the breaker trips, leaving the circuit unprotected and exposed to the voltage spike. In this case, it would definitely have been better to have the fuse before the Varistor.
2. The Varistor shorts and the breaker trips before the Varistor gets overwhelmed by the current. In this case, it would have been definitely better than having the Fuse before the Varistor because as soon as the breaker is reset, the appliance will be working again without any problem. If the fuse was before the Varistor, the fuse would be blown and the appliance would not work until the fuse is replaced.

In short, if the first case is verified, it would be better to have placed the fuse before the Varistor. However, if the second case is verified, it would have been better to have the fuse after the Varistor so that the appliance is ready to use after the flip of a breaker lever.

So I decided to run some tests in attempt to determine which case is more likely.

I tested a 7D181K Varistor, that is, a 7 diameter Varistor that starts conducting 1 mA at aproximately 181 V across it, with a maximum clamping voltage of 300V. I connected it to 230V AC mains voltage, causing it to be conducting for some part of the sinewave. The instant I connected it to mains, it tripped not only the 16A breaker but also the main 30A breaker, in series with the entire house. I kept subjecting it to 230V AC mains. It has tripped the breaker a total of 5 times without getting overwhelmed by the mains voltage. Only after 5 times, did it fail compeletely open, actually getting split in half.

Then I tested 14D181K Varistor, that is, a 14 diameter Varistor that starts conducting 1 mA at aproximately 181 V across it, with a maximum clamping voltage of 300V. I repeated the test I found out that it could withstand more than 10 times being subjected to 230V AC mains voltage. Since it was not quitting, I got tired and connected it between two phases, to 400V AC, leveraging the properties of 3-phase AC. It was only after being subject to 400V AC for 10 times that it finally failed, also being split in half.

I understand that the results of my test are not a silver bullet, but I confess I'm now definitely very confident that a Varistor doesn't need a fuse before it to ensure it doesn't get overwhelmed by mains voltage, especially if the Varistor has a significantly high diameter, at least for 5 cycles of activation and that a 16A or even 30A breaker will do the job.

Therefore, I conclude that the only reason that is mostly stated for placing the fuse before the Varistor is not really that much of a concern.

So my final question is: What is the risk of placing a Varistor before the Fuse beyond the Varistor not being able to handle the surge?

One risk I can think of is the Varistor failing, drawing a current less that trip current of a breaker and dissipating a lot of heat, possibly starting a fire. How likely is this to happen? Is it low probability enough to justify the usability of not having to disassemble an appliance to replace the fuse, considering the appliance takes like 2 hours to disassemble and reassemble?

Are there possibly any other risks I'm not thinking of?

Answer 1

Incidentally, it's not exclusive that the breaker or varistor opens -- both can happen, as the breaker might absorb enough current/time/charge to operate, while at the same time, the varistor is busy turning into a highly conductive ball of rapidly expanding plasma. For thermo-magnetic breakers, this can take on the order of 100µs, a plausible time for arc flash to persist. I might guess it's unreliable which ones happen, though (i.e. you aren't guaranteed the breaker opens in a varistor-open scenario).

Your testing isn't exactly extensive (fault current isn't measured or well-defined, there may be multiple failure modes, and there are dozens of brands), but shows some consistency, which is encouraging.

Offhand, I don't see mention of the relative location of fuse and SPD (surge protective device) in IEC 62368-1 for example, though I only scanned it; these standards are quite lengthy and detailed, and it's easy to miss things. If the relevant wiring and safety standards that a product conforms to, don't specify this detail, it would seem free to wire it either way. To confirm whether a product has done so, you would have to know this -- which standards, and then the text of them.

Keep in mind, there are also internally-fused MOVs, usually identifiable by a bump on one side; sometimes these are available as three-terminal devices (the fuse is a pass-through element, breaking the rest of the circuit when the MOV overheats). These can be used to provide surge protection until failure, and fail softly/silently without hazard of their own destruction. (Subsequently exposing the rest of the equipment to surge/OV should be considered product end-of-life.) It's not clear if you have encountered these, so adding this just in case.

Answer 2

Good work.

You have basically proven what was well-known; when it fails, a varistor will melt into a solid lump and form a near-short-circuit. Always fuse the feed to a power varistor to avoid destruction of the device and risk of fire.

One risk I can think of is the Varistor failing drawing a current less that trip current of a breaker and dissipating a lot of heat, possibly starting a fire. How likely is this to happen?

It's a risk but, far less of a risk than wiring it the wrong side of a fuse. Varistors generally fail short circuit (like many diodes and other power components) but, there will be exceptions.

DigiKey: What Is the Failure Mode of a Varistor?

Wiki on varistor electrical characteristic

Littelfuse on metal oxide varistors

I understand that the results of my test are not a silver bullet, but I confess I'm now definitely very confident that a Varistor doesn't need a fuse before it to ensure it doesn't get overwhelmed by mains voltage

But, effectively a breaker is acting like a fuse. I mean they even have similar current-time characteristics (trying to emulate a fuse).

I'm now definitely very confident that a Varistor doesn't need a fuse before it to ensure it doesn't get overwhelmed by mains voltage

It would have failed to a short after the first application of 230 volts. Subsequently it remained a short circuit and protected the circuit that follows and, after too much over-stress from subsequent exposure to 230 volts, it finally went from a failed short-circuit to an open circuit by exploding. This shouldn't be allowed to happen as it risks a fire and risks the expensive circuitry it's trying to protect from being damaged.

What is the risk of placing a Varistor before the Fuse beyond the Varistor not being able to handle the surge?

If the varistor can handle the first surge, it may be able to handle three or four more surges but, on the fifth (sixth, seventh etc.) it will go pop. Varistors get weaker the more surges they handle i.e. internal material is destroyed within the device and, pretty much all data sheets I've read tell you that.

Good work though.

Answer 3

I'm disappointed by these results because I thought that 7mm and 14mm would withstand dozen of times, if not a hundred of times such test.

I think, but I'm not sure, that it's because the clamping voltage, 300V, was too high for experimenting on 240 VAC. Because as long as the voltage is under the clamping voltage but higher than working voltage, the varistor conducts until the fuse or the circuit breaker decides to trip.

In this case it tripped after conducting 30A. I'm surprised that both the 30A and 16A tripped at the same time. It's possible but it means it took 30A. I don't know what you mean by "instant", but half a second at 30A*220V/2=3300J. Let's say 1500J counting the part of the sine wave it's not conducting. That's a lot for a varistor. That's a minimum because it may have conducting more than 30A the time it waited for the breaker to trip.

Ideally the varistor should be conducting only above it's clamping voltage. Because in this case, the resistance of the varistor is much less (thought still very significant) and the current it conducts is much higher. Instead of conducting just enough to trip the breaker or the fuse after a few hundreds of milliseconds, say 31A, it conducts 200A (or whatever is available on the line) and the circuit breaker or the fuse trips much faster.

It would be good to experiment with a new varistor only at 400VAC. It may last longer. (again, I'm not sure about it, because varistors are not take or break things).

About the question: Yes the varistor should always be placed after the fuse. But the fuse in question should also be easily replaceable. It shouldn't take 2h to replace it. It should be done easily by non experienced users and marking and instructions should be clear about this eventuality. Unfortunately almost no appliance maker do this. They sometimes add a varistor. They do add a fuse. But they never think that the user could replace the fuse him or herself. They use screws for screwdriver heads that you will never find, not even in the most complete kits. And if you find one, or if by miracle it's a standard one, your extra-long screwdriver will be too short or it will be fastened to hard. And stuffs like that. Even worse: They often use soldered glass fuses.

They put these fuse, not to protect the varistor but to avoid hasard when the device is damaged and a short circuit appears. It makes sens, but it's stupid when you have a 16A circuit breaker which can do the job.

The fuse after the varistor has the advantage of being possibly of much less amperage than the circuit breaker. Like 0.5A instead of 16A. Then this fuse (if it's a fast one) will trip much faster, hence better protecting the varistor. And in case of short circuit inside the appliance, before the whole house goes dark.

The second thing to be very careful about, is not to put a varistor between a phase and earth when the earth can't be confirmed, verified and permanently connected. There can be a varistor between a phase and neutral but not with earth if earth can't be guaranteed. (wall outlets are not considered as earth guaranteed. So it excludes all plugable devices)

It's allowed when a fail-open device is in series with the varistor. But this fail-open device (GDT, thyristor...) will either decrease the effectiveness of the varistor by doubling the clamping voltage or defeat the very use of a varistor by adding a much slower device in series with it (GDT).

In case a varistor fails short, but didn't explode, current will go to the chassis and then to the first person touching it, if there is no earth connection. And if this current is bellow the circuit breaker amperage, it will be a permanent hasard, almost sure to hurt someone.

This is a second reason to put the fuse before the varistor. If the damaged varistor conducts 1A permanently, the 16A breaker won't trip but the 0.5A will.

It's also a good idea to use circuit breakers with less amperage in the main cabinet, when 16A is not necessary.

HTH

Answer 4

You have the technical answer, but another critical factor is legal. If you were a manufacturer, strayed from best safe practices, you would be defenseless if anyone were injured or have property damage like a fire. Such a design would never pass UL certifications in the USA.