# what value of capacitor to use for surge protection 240V

#### lecman

##### New Member
I was hoping someone could tell me what value of capacitor to use when protecting against spikes and surges in a 240V AC mains sockets please.
the reason being is i bought a 4-way mains adaptor with surge protection, when i took it apart all it was, was a capacitor between Live and Neutral but i could not read the value would anyone like to advise

##### Well-Known Member
Sounds more like it would be a marvelous MOV (Metal Oxide Varistor) than a capacitor. Google Metal Oxide Varistor and see if that is not what you are looking at.

Ron

#### lecman

##### New Member
You are right thank you its been 20 years since I was at college studying electrical and electronic engineering and I am sure back then we used smoothing capacitors unless I am a bit rusty. I have always enjoys electronic projects, but nowadays I am an IT/Telecoms engineer.
Could you tell me what the value of the MOV across 240V AC wouLd be please. I am googling as well

#### westom

##### New Member
Could you tell me what the value of the MOV across 240V AC wouLd be please. I am googling as well
MOVs do not do protection. A surge is on any one or every wire. The current going to ground. No MOV stops that. Best protection for any wire is a low impedance copper wire to what does protection - single point earth ground. But some wires cannot connect directly. So we use an MOV, GDT, avanche diode, etc to make that earth connection.

Critical is not the protector. Critical is how a connection to earth is made. For example, if a wire is too long (ie ' more than 10 feet'), has sharp bends, is inside metallic conduit, etc, then protection is compromised.

Two factors apply to 240 volt protection. First is the protector. A device that acts like a wire during a surge must be at least 50,000 amps to remain fucntional after many surges; after many years. That is life expectancy.

Protection for each one surge is defined by the quality of a single point earth ground. Earthing is an art. Any protector (ie MOV) is defined by how it connects to earth.

Responsible companies that provide 240 volt solutions include Siemens, ABB, Square D, Leviton, Cutler-Hammer, Protectiongroup (Polyphaser), Keison, and Intermatic to name but a few. Companies that make superior electrical equipment also provide protectors that can make a low impedance connection to earth.

#### Brevor

##### Member
Could you tell me what the value of the MOV across 240V AC wouLd be please.
There should be a number on the MOV, Google that number for a data sheet. If its a number like V260LA10A the clamping voltage is 260 Volts.

#### westom

##### New Member
There should be a number on the MOV, Google that number for a data sheet. If its a number like V260LA10A the clamping voltage is 260 Volts.
Its clamping voltage is much higher than 260 volts. If a surge current is a milliamp (considered zero), then that MOV is somewhere between 450 and 500 volts. If conducting an amp, voltage is something over 600 volts. If conducting any serious surge, then its voltage is somewhere over 1000 volts.

Fortunately existing protection inside appliances must be robust.

How much surge current does it conduct at 260 volts? According to manufacturer datasheets, a few microamps. Even at 300 volts, that MOV is clamping nothing.

#### lecman

##### New Member
View attachment 64169Very interesting Guys, i like your feed back, i have attached a picture of the 4-way mains sockets that is surge protected, across Live and Neutral i believe the MOV is the blue component

View attachment 64169

As you are from the USA your sockets are 2 pin where in the UK we are 3 pin, 3rd pin being a solid Earth. I could not get a close up of the MOV so i don't know the correct value, any ideas what the blue component please??

#### KeepItSimpleStupid

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##### Well-Known Member
Here in the US we also use three prong outlets, High, Neutral and Ground. Anyway, yes, the small blue devices look to me like Metal Oxide Varistors or MOVs. I also see what looks like neon bulbs at the outlets with series resistors. I believe in your images Yellow is ground with brown and blue being High and Neutral respectively. I see a MOV across the mains, a MOV fron High to Ground and a MOV from Neutral to Ground which would be normal.

Ron

##### Well-Known Member
KISS beat me. I think the little guy lower right is the Neutral to Ground.

Ron

#### WTP Pepper

##### Active Member
Sounds more like it would be a marvelous MOV (Metal Oxide Varistor) than a capacitor. Google Metal Oxide Varistor and see if that is not what you are looking at.

Ron
It's also a good idea to add a thermal fuse in series as a MOV can self destruct under serious overload conditions causing thermal runaway and eventually a fire.

Commercial devices here in the UK tend to parallel MOVs to disipate the energy. This is fine for our nice clean 240VAC that may have the odd spike on it, but very few devices will withstand a lightning strike directly.

For the best method you should add spark arrestors as well. Our BT phone socket have them as do most decent radio systems on the aerials.

##### Well-Known Member
It's also a good idea to add a thermal fuse in series as a MOV can self destruct under serious overload conditions causing thermal runaway and eventually a fire.

Commercial devices here in the UK tend to parallel MOVs to disipate the energy. This is fine for our nice clean 240VAC that may have the odd spike on it, but very few devices will withstand a lightning strike directly.

For the best method you should add spark arrestors as well. Our BT phone socket have them as do most decent radio systems on the aerials.
Yeah, there should be a fuse. Many surge surpressor devices are fused or have a circuit breaker in them and now that it was mentioned I didn't see one in the pictures? MOVs are fine for their intended applications. When it comes to lightening strikes in real close proximity I suggest prayer.

Ron

#### lecman

##### New Member
Ok guys what i would like to do with the 4 -ways mains socket in the picture is modify it so it gives better protection, I would add a thermal fuse and, i take it the spark arrestors could not be added between Live and neutral and also neutral and Ground.
I am just looking into getting the best values for MOV i would use ERZ-V14D431 ?, fuse would use5 or 3 Amp, and spark arrestors impossible to add ( i think), would be interest in what other people have to say, as i was a bit confused with Weston answer regarding earthing the MOVs

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##### Well-Known Member
MOV part numbers depend on manufacturer. I do not see where the linked to MOV would be a good choice for a 220 / 240 volt AC system. A Google of MOV selection guide will bring up countless hits like this one. Here are more tips. The list goes on and on.

You fuse up stream from the MOVs or other circuit protection. The idea being if the MOV breaks down on a large transient spike the fuse is the sacrificial link. You fuse based on the ratings of what you have. Depends for example what the cord rating is (wire gauge) and the outlets.

Spark arrests units are generally used at a mains entry point and not included in a simple household surge suppressor.

The unit you posted images of is an everyday off the shelf surge protector, why modify of change it? It's like trying to make a silk purse from a sows ear, it won't work. You would do just as well to build a new unit from scratch or buy a larger unit. Unless you have major power company issues I wouldn't worry much about it and run with what you have. In the off chance of a lightening strike pray.

Ron

#### westom

##### New Member
As you are from the USA your sockets are 2 pin where in the UK we are 3 pin, 3rd pin being a solid Earth.
Search for the expression NEMA15-5R. USA uses three prongs. Your ‘two prong’ claim is promoted by others who tell us what to know without first learning facts.

Also learn safety ground is completely irrelevant to protection. Safety ground and earth ground are electrically different. Even numbers were posted that said why: ie less than 10 feet. Those who know only from observation or advertising would never learn that. Learn the technology - AND numbers - before making conclusions. MOVs do not do protection. Knowledge based in 100+ years of well proven science.

What happens when a surge on the hot wire (maybe 5000 volts) is shunted to the neutral? 5000 volts on the hot wire AND 4600 volts on a neutral wire. Where is the protection?

What must be learned before making any recommendation was posted. "MOVs do not do protection. A surge is on any one or every wire. The current going to ground. ... Best protection for any wire is a low impedance copper wire to what does protection - single point earth ground." Not safety ground; earth ground.

Only those who did not learn this stuff assume a third prong in UK or US power points is earth ground. An MOV from hot to neutral means maybe 5000 volts on the hot wire and 4600 volts on the neutral. Where is protection? Not existing. Because the MOV did not make what was defined earlier – with numbers. A low impedance connection to what?

"If a wire is too long (ie ' more than 10 feet'), has sharp bends, is inside metallic conduit, etc, then protection is compromised." A 5000 volts on one AC wire. And maybe 4600 volts on other wires. No protection because MOVs are too close to an appliance and too far from earth ground.

Some UK installations are so poor as to only be earthed back at the substation. Protection all but non-existent. A country with almost no surges should see no surge damage ever. Still surge damage happens in the UK due to virtually no protection - ie MOVs not connected less than 3 meters to earth. Relevant expression even includes numbers. And what does protection: single point earth ground.

It should be obvious: safety ground and earth ground are electrically different. A wall power point is not earth ground. An MOV not earthed does not even claim that protection.

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#### westom

##### New Member
You fuse up stream from the MOVs or other circuit protection. The idea being if the MOV breaks down on a large transient spike the fuse is the sacrificial link.
A perfect example of conclusions based in subjective reasoning and wild speculation.

View the numbers. A fuse is rated for maybe 250 volts. How does that blown fuse stop something rated at thousands of volts? It doesn't. The blown fuse remains conductive - as it specification numbers state.

More numbers. A fuse takes milliseconds or longer to blow. Surges are done in microseconds. *Easily* 300 consecutive surges could pass through a fuse before it even thinks about blowing. Where is the protection? I can only exist when subjective reasoning exists by ignoring numbers.

A fuse only disconnects power AFTER surge damage has occurred. So that fire does not kill humans. Known when one first learns specification numbers. And if one ignores subjective claims based only in speculation.

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#### KeepItSimpleStupid

##### Well-Known Member
Lets keep the apples and oranges separate.

Lightning strikes vs. a washer starting up.

Let's face it electronics are sensitive. It won't hurt to put TVS suppressors in an outlet. Better surge suppressors have an RFI filter and a WARRANTY. They will cover, and have, connected equipment up to say $50,000 USD. I can't find the Warranty, but this is a Euro version of a surge suppressor I use in the US: http://www.tripplite.com/en/products/model.cfm?txtSeriesID=825&txtModelID=1346 Again, if you want to protect your dwelling against a Lightning hit, have a "Whole House Surge Suppressor" installed at the mains panel per the manufacturer's instructions. IF the manufacturer requires two ground rods for their protection, you have to follow it to get the WARRANTY. The Warranties are usually lifetime or limited lifetime and they cover connected equipment. Now that I think about it, let's call this outlet modification: "spike suppression - better than nothing" for that's what it is. Here is a blurb worth reading. It's emphasis is on telephone lines though: http://www.electro-tech-online.com/custompdfs/2012/05/surgebro.pdf Last edited: #### Reloadron ##### Well-Known Member Most Helpful Member A perfect example of conclusions based in subjective reasoning and wild speculation. View the numbers. A fuse is rated for maybe 250 volts. How does that blown fuse stop something rated at thousands of volts? It doesn't. The blown fuse remains conductive - as it specification numbers state. More numbers. A fuse takes milliseconds or longer to blow. Surges are done in microseconds. *Easily* 300 consecutive surges could pass through a fuse before it even thinks about blowing. Where is the protection? I can only exist when subjective reasoning exists by ignoring numbers. A fuse only disconnects power AFTER surge damage has occurred. So that fire does not kill humans. Known when one first learns specification numbers. And if one ignores subjective claims based only in speculation. Thank you for the enlightenment. Ron #### lecman ##### New Member So if i would to agree with what your saying (and i do ) the mains socket picture offers no protection, so how can they sell this with a logo on the front saying "surge protector" thank you Westom. KISS and Reloadron you have been very helpful and have given me alot to look at #### KeepItSimpleStupid ##### Well-Known Member Most Helpful Member So if i would to agree with what your saying (and i do ) the mains socket picture offers no protection, so how can they sell this with a logo on the front saying "surge protector" Try this one: We had a expensive multi-channel analyzer at work that kept blowing acquisition cards at like$1000 USD to repair every time. It was connected to a Scanning Electron Microscope which has high voltage supplies etc. We happened to have the schematics and I called the vendor and asked, how come there is zero surge suppression in the supply?

They told me that they specify 120 V, 60 Hz power. If it deviates, then your not supplying what the product calls for. A surge suppressor like I showed you solved the problem.

I had another issue with an X-Ray set with 100 kV supplies. It kept blowing RS-232 Driver chips. This problem, I took a different approach. I added RS-232 Isolation. Problem never occurred again and it's been 20 years.

Now, before I started working at the company, they had a electron beam evaporator with a 15 kV , 1 Amp supply in it. Every so often, probably yearly it would pop a transistor in the High Voltage regulator. I got tired of fixing it, so I attacked it. I found loose connections in the High Voltage divider and replaced a few parts like a 1 meg 200 W resistor. The problem never re-occurred.

One day, a monitor didn't work that was plugged into one of those fancy surge suppressors with a warranty didn't work. Examining the outside of the surge suppressor it was covered in black. The large expensive monitor was replaced under warranty.

In my early days of employment, we had a brand new expensive computer that just so happened to be plugged into an outlet with a bad ground and it fried. Had it been plugged into any sort of "surge suppressor", it may have survived.

Things were dying all over the new building and it was puzzeling. I discovered, not while the building was under warranty that there was a defect in some of the nearly five hundred 120 V outlets. One ground would lift when another plug was plugged into the outlet. That was a tough one.

Now, we had these aging computers (before the PC) dieing and they were REQUIRED for our daily operation. Since I was doing self-maintenance (troubleshoot to module level) I invested in to a power line conditioner and a surge suppressor. Eventually a Macintosh computer was plugged into the same system. it went 15 years before it was replaced and no issues but a floppy drive and dust. Not even a hard drive. The conditioning system was about \$1000. It was designed to handle a brown-out.

I built an amplifier with 40,000 uf of capacitance in it. I had to put MOV's across the capacitors to prevent blowing a component.

What can kill electronics is sudden power fails and the reapplication of power causes surges.