240V Mains Surge Protection Circuit - Page 1
Forum section: electronics / beginners

author: gblades
author_rank: Contributor
timestamp: July 13, 2016, 08:59:59 am
content: Does anyone know of any good guides to practical design and construction of mains surge protection circuits?I would like to make a couple to protect a couple of sheds. One has it's own mains feed and the other is fed from a house. I have searched and found quite a bit of information but it seems to be either all theoretical or pages where people have made one themselves by just putting a few MOVs across the supply. I would like to do it properly.I am thinking of something along the lines of :-1) GDT's between L and GND, and N and GND.2) MOV between GND and N.3) Thermal fuse connected to L and MOVs connected to the other side of the fuse to N and GND.4) Neon indicator between the output of the fuse and N5) Thermal fuse sat between the MOVs with some 'heat sink plaster'.That way I have the GDT to help protect from severe surges and then the MOV protection. The MOV only have a limited life and will start to conduct and heat up at end of life so there is a thermal fuse and neon to stop this and indicate when the protection has failed.What I am mainly struggling with is real world suggestions for the current ratings of the components and the temperature rating of the fuse.Thanks

author: bktemp
author_rank: Super Contributor
timestamp: July 13, 2016, 09:18:37 am
content: Because GDTs short the voltage once triggered, they can conduct much higer currents (because of the low arc voltage), but they don't reset themselves. That's why there needs to be a fuse in front of the GDT for cutting the power. So the GDT should be designed to trigger only for very large surges, otherwise it is better to use MOVs because they will stop conducting when the surge is over.Using a GDT directly at the input is a bad idea, because it will start conduction at every spike it sees and keeps conducting until the power is cut of completely. And there are a lot of spikes on mains.I tried that many years ago and the GDT triggered and tripped the fuse every couple of days. Then I added a small LC filter in front of the GDT and all problems were gone.

author: gblades
author_rank: Contributor
timestamp: July 13, 2016, 09:31:57 am
content: Thanks. So best to just use some beefy MOVs then.

author: DutchGert
author_rank: Frequent Contributor
timestamp: July 13, 2016, 09:42:29 am
content: I did a teardown of this puppy once:I lost the photo's unfortinately but i did make a schematic so that could be useful

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 13, 2016, 09:45:20 am
content: Quote from: DutchGert on July 13, 2016, 09:42:29 amHoly crap, a website that actually uses brackets in its URL. I've never seen that before, and I've been on the internet forever!Here's a forum safe link.http://tinyurl.com/zbqjjwmTim

author: Jeroen3
author_rank: Super Contributor
timestamp: July 13, 2016, 10:04:22 am
content: Just put one in your distribution box.Like this: AL-SEC-T2-1S-350-FMThey have plenty, probably also a ready-made kit you scan wire up. They even have outdoor kits for solar panel wiring.They've made the varistor part swappable, since these are consumables.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 13, 2016, 10:05:45 am
content: Anyways,The shortage of practical information you've found, may reflect a deeper truth: is it practical at all? Do any of the people who've added a device, actually know it's doing anything at all?More to the point: how do you know you need it? Is it just for paranoia? Does your local power get an unusual rate of unusually high energy surges?The circuit shown above is about the best way to do it. I've seen MOVs straight to ground, too, but that may not be a good idea because of extra GND leakage, especially as the MOVs fail over time. All equipment is required to withstand 2.5kV (H/N to GND), which is the expected surge that the power company clamps things to; reducing this is probably not necessary at all.The choice of MOV size relates to the expected (statistical average) rate and energy of the surges to protect against. If you know this, you can make an informed selection for the size and number of MOVs to use, and estimate how long they will last before the unit needs to be replaced. (Of course, it may always get exploded sooner, say by a direct lightning strike... but, at least you'll know then.)Finally, there are standards for the size, type and position of surge protectors. The peak current and pulse energy are much higher for a mains-inlet (whole building) protector, and smaller for secondary and tertiary branches at some distance from there. Consumer surge protecting strips are tertiary, and don't need to handle as much. This may be typical of your second case, with the attached garage (assuming it doesn't experience a direct lightning strike itself, or get induced surge from a nearby strike). You'd be looking at an inlet type for the first case (mains enters the garage).Here's a brief overview; refer to the standards (or other documents about them) for more ideas.http://www.schneider-electric.com.au/documents/electrical-distribution/en/local/surge-protection-promo/surge-protection-devices-specifications.pdfTim

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: July 13, 2016, 10:14:40 am
content: from memoryMOVs Line to N, ( with internal fuses and an indicator to show if still ok)Gas Arrestor N to E.or for a much improved protectionSingle pole LC filter on Line to N. Single Inductor per phase on Line side. with a resonant freq about 1khz. L made with ferrite core big enough to withstand surge without saturating completely. (IDK how to size this. ) but was about 10 turns for a 20 amp surge protector. Mains rated poly caps. Watch they dont overheat if you have bad quality mains.Gas Arrestor N to E.MOVs Line to N, before filter. about 200kAMOVs Line to N, after filter. about 40kAyou will get some insertion loss. from the filter. hopefully not too much .Very important a very good earth very close to entry point of building.As to if you need it.Do a risk assessment, to the international standard. iso something.probably only if you live in the tropics.

author: Gyro
author_rank: Super Contributor
timestamp: July 13, 2016, 10:17:35 am
content: I notice you are talking about surge protection for yourSheds. That might have a big impact on what you can safely connect to GND (Earth) as you may be distant from the mains supply earth connection, your ground impedance will be higher relative to the surroundings (Shed structure?)You need to be careful and understand what sort of earthing scheme your electricity supply uses before proceeding. A useful guide to identification here:https://www.tlc-direct.co.uk/Book/5.2.1.htmBe safe.

author: gblades
author_rank: Contributor
timestamp: July 13, 2016, 10:28:26 am
content: Probably a good idea if I describe how it will be used. When I mentioned garage that was more of a case that I am running mains down to my parents garage soon and if I make a surge protector then I might as well put one there as well.The main use will be in a friends farm shed. The shed is some distance from the house and has it's own mains feed and earth rod. It was recently struck by lightning and in situations like that I know there is not really much you can do. As it happened it did not do much damage. The extent of the obvious damage was a light switch which must have arced and partially melted as the mechanism no longer latches in the off position.The idea of the protection is to install in additional buildings where the mains supply is branched off into with each of these having their own earth rod and surge protection mainly to protect against induced surges.

author: Gyro
author_rank: Super Contributor
timestamp: July 13, 2016, 11:45:49 am
content: Useful additional information.As far as your parents' garage goes, the cautions about earthing in my previous post would apply. You might be as well getting something CE marked off the shelf for that (a standard surge protected mains strip might be as good as anything). There's also the question of whether there's anything in the garage sensitive enough to need surge protection given that the house is likely to see the same surges anyway.With regard to the Farm shed., that's an entirely different problem. It's presumably fairly exposed if it has already been struck by lightning. As you say, in the case of a direct strike all bets are off!. Any surge protection that you put in will be 'sacrificial'. Given that the damage was in the lighting circuit (being the highest located wiring) You might be better off installing a lightning conductor, to a separate ground spike. Nothing is going to protect the internal wiring circuits otherwise. It would also give better fire protection, which is probably the greater risk. Again, you need to consider what is going to be powered in the shed and how sensitive it's likely to be.The other thing to consider (for the farm shed) is how you're going to build the protector. If you're talking incoming mains supply then you need a big metal case (maybe a DIN rail one) with terminals big enough to take the supply cable etc. solid short connection to the ground spike. It's not likely to be something you can put on a PCB in a small box. Again, localised protection for any sensitive equipment might be a better bet.

author: Curtis
author_rank: Contributor
timestamp: July 13, 2016, 12:48:33 pm
content: I have installed many of these when I worked as a electrician several years ago. They will protect your whole house. You can even buy a"circuit breaker"surge protector that fits into your electrical panel.http://www.homedepot.com/p/Eaton-Whole-House-Surge-Protector-CHSPT2ULTRA-1/204761136http://www.homedepot.com/p/Square-D-QO-Surgebreaker-Surge-Protective-Device-Takes-2-Load-Center-Spaces-QO2175SB/100202111?MERCH=REC-_-PIPHorizontal1_rr-_-204761136-_-100202111-_-N