How often do surges happen on the mains?

Hi,
We are going to use the V10H300P MOV to protect our 20W, 240VAC, offline Flyback.

We note that it can survive up to one thousand 10us surges.

How often do you think such a surge happens on the mains in Europe/USA (or whichever other country)?

V10H300P MOPV datasheet
https://www.littelfuse.com/~/media/...rs/littelfuse_varistor_hmov_datasheet.pdf.pdf

Thats a trick question, it depends on more than 1 thing. To begin with, how close are you to the main transformer for the city?

1. What is your system(3 phases, zero and ground or other)?
2. How often do you have them now?
3. Do you know that any attempts to fix this is dangerous and you do it on your own risk!

I can only answer from experience: 1A fuse followed by a 20mm varistor across a sports center installation in Bristol where machines/pumps/lights are constantly switching has never blown since 2003. It will be different even between neighboring buildings so 'your mileage will vary' but noise is common, serious surges are rare in most places.

If this is in one of your street lighting installations, bear in mind that lights are normally daisy chained along a single switched cable so the chances of a heavy inductive device dumping voltage on the same circuit is very low and spikes will naturally dissipate in the cable inductance and capacitance to ground.

Brian.

Thanks, I must admit I wonder how mains transients are such a problem, even where high current switching takes place.
Suppose there is 500A flowing in a mains cable and the mains cable inductance is 20uH.
Then the energy is 0.5*L*I^2*t
Now consider that this inductive current circuit gets broken.
Now also suppose that nearby on the same phase there is a 220uF capacitor bank downstream of a PFC stage (obviously also downstream of a mains rectifier etc).
Considering a 10us spike interval, then the power to be dissipated is 250kW.
Supposing the capacitor bank has 380VDC on it….
Then that equates to 658A (VIt = 0.5*L*I^2*t) flowing for 10us. That would take the capacitor bank voltage up by only 30V…….hardly a problem, and when you consider that there are other devices connected to the mains, each with their own capacitor bank, it makes you wonder how mains transients ever rear their ugly heads at all?
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The other factor you missed out is that the released energy is on the disconnected side of the switch so by virtue of it's isolation, the energy is somewhat reduced. A good device that is likely to spike energy back to the supply will normally have some kind of clamping and filtering in it's own power feed so although never perfect, it will greatly reduce outgoing spikes.

Think of spikes as a pure source of almost unlimited energy superimposed on the mains is simply not a realistic scenario. There are many other factors which all reduce the spikes so what you would see in a real life situation is never as bad as calculations suggest.

Brian.

thanks, i am thinking that many transients are actually either power system faults like "disconnected neutral", or where a mains connected motor has catastrophically failed, and its mains rectifier has blown short, and the motor ends up load dumping its winding energy back into the mains.......in such a situation, little in the way of transient protection woudl protect against this....something would have to blow up somewhere....obviously whatever blew up would conveniently clamp it so that other devices wouldnt blow up.

.obviously whatever blew up would conveniently clamp it so that other devices wouldnt blow up

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Obviously you have not seen what happens when a housing estate loses its neutral from the sub-station!
Think massive damage from strange mains waveforms and almost double peak mains voltages. I've seen fluorescent light tubes explode as well as extensive damage to just about everything electronic. Even quite large mains transformers burn out. Mega-bucks of damage and very unhappy insurance companies!

Brian.

Even if the voltage is with 20 volts higher, that is enough to damage your devices. They are built for 250VAC maximum. Also you are wrong, the current rises for a short time, but that is more than enough to damage the electronics and electrics especially since today they are made out of cardboard.

If you want to reduce the spikes, there are a few ways, thats why I asked for your system type. Adding additional grounding poles is the easiest way, it will stabilize your voltage, make sure they are not touched by anyone or you will "dog/neighbour/child barbeque. Normally your circuit braker board should have a short and over-current defense, but thats not enough as you will have to flip the switch back on often.

betwixt said:

...There are many other factors which all reduce the spikes so what you would see in a real life situation is never as bad as calculations suggest.

Brian.

Click to expand...

How many MOVs are needed if its far worse than the calculations are suggesting?

Zak28 & ArakelTheDragon, Treez is speaking of transients rather than the entire network rising by 20V RMS.

Grounding will never stop transients on the power lines and the 'circuit braker board' situation rarely if ever exists on the street lighting products he manufactures.

MOVs do not stop transients, they can only conduct excess current when the voltage across them reaches threshold. The current and power they dissipate has to go somewhere otherwise the problem could easily be solved by fitting one big MOV across the lines leaving the power station. The passage of transients across the power network is very complicated and different in every installation but the brief spikes that MOVs are intended to clamp tend to be very localized, often within a single building and when they conduct the energy is partly dissipated in the device and dissipated in the wiring impedance feeding it. Consider what would happen if you connected an infinitely large MOV across an infinitely small power impedance and fired even a brief transient into it.

From a transient point of view, don't think of copper wires carrying 115/230V AC, think of an RF transmission line with distributed inductance and capacitance. The source of the transient spike is usually a disconnecting load with the 'spike' voltage on the disconnected side of the switch so only the very brief arcing as the switch opens is conducted into the wiring. Think of that as a signal source of a few cycles of RF and analyze what happens to it down a mismatched transmission line.

There are momentary dips or peaks in the AC when for example a voltage stabilizer 'tap changes' but these are also brief and in any case are unlikely to be present in a dedicated power feed.

Brian.

thanks, yes thats right i am speaking of transients

This should have been the file on why the voltage increases more without grounding. I will look a bit more if this is not it.

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Here is a little more:

This one, page 11: **broken link removed**

Maybe this book too.
https://books.google.cz/books?id=0g...unding poles for preventing transient&f=false