Let's Review Mains Voltage Breakdown and Min Copper Clearance - Page 1
Forum section: electronics / beginners

author: Mechatrommer
author_rank: Super Contributor
timestamp: January 25, 2013, 05:29:19 pm
content: i found a link to program to calculate capacitance of a diy plate or butterfly variable capacitor. the dielectric is air.site:http://www.alexloop.com/artigo21.htmlthe program:http://www.alexloop.com/capcalc.exenow as the topic said, its not about capacitance, but about trace/conductor clearance before voltage breakdown. if you run and fill the app, the mains 240V breakdown clearance is only around 82µm. and its for air dielectric, not solder masked (i believe solder mask should have higher resistance to breakdown correct?) i read last time for mains rated safety, we need at least 3mm clearance between conductor, thats 36X safety factor or nearly 3600% margin! am i missing something? or the app is flawed?

author: SeanB
author_rank: Super Contributor
timestamp: January 25, 2013, 06:51:27 pm
content: Mains isolation is based on the peak spikes expected on the mains. 1kV pulses as switches etc open and close, and the spacing is based on these. Your mains surge clamps typically have a clamping voltage of around 800V at peak, so the clearance needs to be big enough to handle this when humid, having a thin film of dust and such on it. Capacitors normally are used with a power limited supply, not with something capable of delivering megawatt energy pulses.

author: Mechatrommer
author_rank: Super Contributor
timestamp: January 25, 2013, 07:45:57 pm
content: ok thanks for clarification, the main point is "spike". so i think just maybe, with safety factor of 2X, 3mm clearance, the standard makers expect 4KV of spike in the mains system, according to the program (1.5mm clearance breakdown). is that common or possible to get such 4KV spike? in 240V mains. how much wattage of a system thats capable of doing that?

author: Kremmen
author_rank: Super Contributor
timestamp: January 25, 2013, 07:57:16 pm
content: Quote from: Mechatrommer on January 25, 2013, 07:45:57 pmThink what will emerge from the wall socket when the lightning hits an air feeder cable leading to a house. And no surge protection in the meter panel...

author: M. András
author_rank: Super Contributor
timestamp: January 25, 2013, 09:16:29 pm
content: i dont think too many household distribution system has lighting and such surge protection devices fitted, atleast here, i looked up some prices for surge protectors for din rails to mount on the panel but i quickly decided i dont need it. btw those devices which are meant to be inside the distribution panels. are 1 time devices like those extension cords etc or can survive multiple events?

author: Mechatrommer
author_rank: Super Contributor
timestamp: January 25, 2013, 09:27:19 pm
content: Quote from: Kremmen on January 25, 2013, 07:57:16 pmok an idea popped up in my mind (maybe a stupid one). how about i put a "spark jumper" just at the mains line incoming to my circuit? ie where both life and neutral line soldered to pcb, just after that i make two very close and very thin traces between them. the idea is, when lightning hit, it will spark there, not at the other traces or components later in the pcb, so they will be safeor just use the standard MOV, NTC or something to limit the surge? by doing so i dont have to comply with clearance standard since i got protection at the input already? is it brilliant or stupid?

author: SeanB
author_rank: Super Contributor
timestamp: January 25, 2013, 09:28:49 pm
content: Distribution panel devices are multiuse, but are going to clamp at a very high voltage ( and an insane current to ground as well) and generally incorporate a breaker, so that they trip off in the event of a long pulse. Some have a built in fuse to indicate it no longer works by turning off an internal indicator lamp. Bigger ones have a voltage free relay contact to integrate into a warning system to alert you to them failing. You pay for those though......Most are very robust gas discharge devices, and you will find them throughout the distribution system to limit overvoltages. Often you will see them on a pole as an extra insulator with an earth wire or a steel bar connecting all 3 then a thick wire down the pole to an earth mat buried next to the pole.

author: Mechatrommer
author_rank: Super Contributor
timestamp: January 25, 2013, 09:43:51 pm
content: ok i think i got it now. the idea of the standard is to be safe everywhere. at distribution panel stage, and at the device/product itself. if all system are in compliance, there will be redundant safety, correct? thats about engineering standard right? human safety first. now i remember the other thread about standard compliance approval. its not that critical or necessary for "electronics kit" unless perharps its directly connected to mains supply. but today's smps wallwart mostly didnt comply to this 3mm clearance (traces are solder masked mostly but still not all), some are about 1mm or less clearance. these devices are sold worldwide and not get banned. from china of course. i dont know how big the wallwart will be if its designed by some EU or US standard.

author: Kremmen
author_rank: Super Contributor
timestamp: January 26, 2013, 09:22:19 am
content: Quote from: Mechatrommer on January 25, 2013, 09:27:19 pmI think it was said already, but anyway: Not a stupid idea at all since it is regularly done exactly that way. The only thing where your thinking falls short is the implementation. You should understand that in the distribution grid the impedances are low enough that surge currents of tens or hundreds of kiloamperes flow. The protection devices must be physically robust to withstand that kind of energy. More the thickness of your thumb that a PCB track. Such a track would evaporate at the first strike. So air gaps yes, but between heavy, solidly supported electrodes and not next to the end device but outside the house to prevent the fireworks from entering in the first place. Inside the house, MOVs and TVSes and possibly smaller PCB mounted gas discharge tubes. Depends on your application.An anecdote: many years ago when i was a kid, we were in my uncle's house during a heavy thunderstorm. In those days it was enough to have electricity in the first place, never mind protection. An enormous lightning struck somewhere close to the old fashioned open air feeder and phone wires that got a righteous jolt from it. By chance i was looking at just in the direction where a wall socket spit out a foot long bright flame, blowing the socket cover to bits and instantly igniting the curtain hanging in front of it. Had the house been empty, it would have been a goner. As it was we put out the fire. Luckily the phone was disconnected, since the surface mounted phone socket was standing half a meter out the wall, having ripped the wiring right out of its fixings due to the pressure of the arc in the housing.Quote from: M. András on January 25, 2013, 09:16:29 pmMOVs are tricky because while they are multi-use, there is no easy way to see if they are OK or not. I have in my house the medium level protectors done with very heavy MOVs and the rail mounted units do have an indicator that turns from green to red when the unit is done for. How that works, i have no idea.Air gaps and gas discharge protectors are multi-use as long as they are still there. And they don't get destroyed that easily since when active, unlike MOVs they don'd dissipate very much themselves.

author: SeanB
author_rank: Super Contributor
timestamp: January 26, 2013, 09:38:30 am
content: The way you do the indicator is simple. Just one MOV with a 5A fuse ( or any rating in that area, just a standard fast blow one) in line with it. Then a neon indicator across the fuse to show it is blown, along with the green indicator and a 47k resistor across the MOV to show power. The resistor drops the voltage across the neon to below it's striking voltage when it is in series with the other one when the fuse pops, so it will not light. Better units have a single use thermal switch in series with the MOV's so that if one has a sustained overload it will heat up and go open circuit, cutting off the output of the unit. Thermal fuse is mounted in a sleeve so it is in good contact with the MOV it is protecting.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: January 26, 2013, 09:50:52 am
content: The issue with HV spikes on mains is not so much the spike, as the energy is typically low, but that it can initate an arc, which forms a conductive path for the high energy lower voltage, which then causes the damage, e.g. carbonising a PCB making it permanently conductive.Creepage/clearance distances in product standards also take into account the possible presence of moisture, dust etc. that can reduce the effectiveness of air or solid insulation.

author: Kremmen
author_rank: Super Contributor
timestamp: January 26, 2013, 01:31:22 pm
content: Quote from: SeanB on January 26, 2013, 09:38:30 amWhat you describe could be the way for small scale protection devices, but mine are not like that. As i said, they are rail mounted in the distribution panel. Firstly, they will interrupt max 50 to 100 kiloamp surges so a 5 amp fuse in series is just going to go away. The recommended max back-up fuse is 160 A. Secondly, the indication is not electronic but mechanical/chemical. The green is non-electrical shown through a small window (see pic).

author: SeanB
author_rank: Super Contributor
timestamp: January 26, 2013, 01:54:44 pm
content: Those use a thermal trip, when the MOV's heat up from a surge they cause a bimetal strip to trip the flag. They also have a big disc inside, and when it fails and cracks from age or old age then the flag is also tripped. The 160A upstream fuse is not optional, though you can use a 160A breaker instead, with the fuses as further upstream protection and final protection.The fuse is used in series with a small MOV that drives the indicator in smaller devices, the rest of the MOV's are in parallel with this indicator, and the thermal fuse is on the input for safety. Often a regular fuse or breaker is in there as well for overload protection.

author: Kremmen
author_rank: Super Contributor
timestamp: January 26, 2013, 03:04:34 pm
content: Thanks Sean, never took the effort to find out but now i knowI didn't mean that the back-up fuse was optional; certainly not. Only that with these particular devices the recommendation is that it be at most 160A.

author: Neilm
author_rank: Super Contributor
timestamp: January 26, 2013, 07:05:23 pm
content: Quote from: mikeselectricstuff on January 26, 2013, 09:50:52 amIt is not only HV spikes on the mains, but if you have an issue and lots of current starts to flow and you are relying on a fuse the inductance of the lead will cause a large voltage to appear as the fuse blows. In poorly designed applications, this can lead to an arc flash explosion. A MOV before the fuse will clamp this voltage and keep it manageable.Neil

author: Mechatrommer
author_rank: Super Contributor
timestamp: January 27, 2013, 07:41:30 am
content: Quote from: Neilm on January 26, 2013, 07:05:23 pmi guess you mean a MOV "after" the fuse when looking from the mains supply side? we usually put the fuse as close as possible to mains supply right? in case of blown fuse, arching on the supply side should give less harm to the circuit (high density traces side) since on that (mains supply) side we can put lot of "in compliance" traces clearance with not much space penalty since there shold be only two thick but short copper connectors. putting the MOV on the circuit side, should protect the device, am i correct?

author: Neilm
author_rank: Super Contributor
timestamp: January 27, 2013, 10:51:09 am
content: Quote from: Mechatrommer on January 27, 2013, 07:41:30 amI mean a MOV BEFORE the fuse - if there is enough current flow and enough inductance to cause a the voltage to rise it will rise on the supply side of the rupturing fuse. However, if there is a lot of clearance on the supply side this will also work, providing any wires / plastic are rated to several kV.I should also say that I have not seen this occur on domestic supplies - we were investigating why a meter on a 600V (CAT IV) system would spectacularly fail. The arc was contained within the meter (so by strict definition was safe) but it shouldn't happen.Neil

author: SeanB
author_rank: Super Contributor
timestamp: January 27, 2013, 11:17:29 am
content: You never want a MOV before a fuse, unless you have a protective device before it to clear any fault conditions that make the MOV fail short circuit. Fuse must be rated to break the voltage rating, which means in mains use you need a ceramic fuse capable of breaking mains to a standard. A glass fuse will not do here, as it cannot handle a dead short with mains applied, you need a glass filled ceramic one, which will handle the transient. It has to be capable of handling the surge, and must be before any filter caps, MOV's and such that are parallel, though you can have a series inductor ( not a common mode one) before it to limit transients, though it will need clearance to several kV to handle the spikes it will get from the mains.

author: Mechatrommer
author_rank: Super Contributor
timestamp: January 27, 2013, 11:47:23 am
content: Quote from: SeanB on January 27, 2013, 11:17:29 ami always think what matters is the amperage rating of a fuse. at 12V or 240V, if its written 1A on it, then it will fail at 1A regardless of the voltage applied.

author: SeanB
author_rank: Super Contributor
timestamp: January 27, 2013, 11:53:43 am
content: True, but what about when it is breaking. A 36V blade fuse will arc over quite well if you use it at 220V, and it will do so until it has melted enough to break the arc. Conversely a fuse rated at 1000V will break cleanly with no arcing after the breaking time.