Relay under short circuit arc suppression - Page 1
Forum section: electronics / projects

author: Sgt123
author_rank: Newbie
timestamp: October 06, 2017, 02:26:15 pm
content: Hi all, I'm trying to design some sort of arc suppression for a relay under short circuit when tripped. So i'm trying to understand how these arcs form. From my understanding, at the instant of tripping, the relay contacts instantly changes its voltage to the supply voltage. Since the switch takes some time to fully open, E = V/d, and we have a very small distance (at instant of tripping) with a very large voltage (230 V AC rms in my case). The electric field is so high that it causes breakdown of the air and a conduction arc forms.Correct ?So my main goal is to reduce the dv/dt across the switch which will reduce the electric field and prevent breakdown ?How can I prevent or suppress the arc from forming ?, will a snubber across the contacts work ?

author: Ghydda
author_rank: Regular Contributor
timestamp: October 06, 2017, 05:47:24 pm
content: Quote from: Sgt123 on October 06, 2017, 02:26:15 pmThe key to solving this problem is to realize, that the current flowing in the contacts at the instant the contacts open up is driven by a current generator. Thus it is not the low 230 volt AC alone that ionizes the air, but rather a lot higher voltage, in fact as high as needed to ionize the gap forming between the contacts.The solution is to provide an alternate current path, while the contact sets opens up, redirecting the current away from the contacts. A snubber circuit across the contact sets would be the first thing I would try.Cheers!

author: jbb
author_rank: Super Contributor
timestamp: October 06, 2017, 06:55:03 pm
content: A MOV (rated generously higher than the line voltage) is a classic countermeasure.However, if you're talking about serious currents, you'll need a serious relay, maybe even a circuit breaker.EDIT: I didn't read the first post properly and gave some bad advice.Breaking 230V AC short circuits is really hard. The fault currents in a normal house range from 1kA to 5kA, which will blow your MOV and relay apart.See my next post down below.

author: duak
author_rank: Super Contributor
timestamp: October 06, 2017, 09:35:05 pm
content: Is current already flowing through the contacts? If it's above a certain amount, determined by the contact size and material, an arc forms instantly while the contacts are close together (millionths of a meter) and has a voltage drop on the order of 10 - 20 V. If the current is high enough, the contact material ionizes to sustain the arc and the only ways to stop it are to reduce the current or increase the length of the arc to where it cools down. Switching power line AC is relatively easy as the current goes to zero every few milliseconds. DC is far more difficult especially if there is significant inductance in the circuit. I have one relay that tried to switch 50 VDC at about 2 A off and but the arc persisted and completely melted the arm carrying the contact.There are various descriptions of the phenomonon on the 'net but because of the variety of circuits and conditions it's hard to suggest a single solution.Cheers,

author: jbb
author_rank: Super Contributor
timestamp: October 07, 2017, 01:37:19 am
content: Quote from: duak on October 06, 2017, 09:35:05 pmOh yeah, DC breaking can be amonster. It can be done if you have to, but there will be more engineering effort involved.In addition, if you look at DC circuit breakers for V > 50V or so, you may find they have aminimumbreaking current. This is because some of them need the electromagnetic forces to 'blow' the arc through the device into an arc chute structure. If there isn't enough current, the arc will persist. We tested that and it's definitely true.

author: Sgt123
author_rank: Newbie
timestamp: October 07, 2017, 06:19:10 pm
content: Thanks for the replies, I am making a circuit breaker as a project. This is my schematic so far (see attachment),So basically its a pretty simple design. The main circuit current is stepped down by the CT. The bridge rectifier makes it all positive so the MCU can measure it through the 16 ohm resistor. The current is sampled and true rms is calculated and if its greater than the reference current (breaker rating), it trips the relay via the logic level MOSFET.The short circuit part is where im stuck. Simulating it, this is the actual current before it trips (see attachment). Trips in under 2ms, though in reality it might be longerHow would a TVS diode do across the contacts ?, thinking it should work much better than a snubber which can only be designed for a fixed load

author: jbb
author_rank: Super Contributor
timestamp: October 07, 2017, 10:28:49 pm
content: I'm sorry, I didn't read your first post properly and see that you wanted to break a short. Sorry.I should have said this earlier:breaking 230V AC short circuits is really hard. The fault currents in a normal house range from 1kA to 5kA, which will blow your MOV and relay apart.I am not kidding. If you weren't wearing safety glasses you could have eye injuries. If you weren't wearing ear protection you might risk hearing damage (unlikely but possible from one bang).Obviously a domestic wall socket won't deliver this much juice but you get the general idea.(With marketing up front...) You can see that there's a lot of power to deal with. The blue plasma would typically be up around 20,000 Kelvin.It seems like you want to have a precision trip point on a breaker. I suggest that the best way to do this is to buy a proper industrial circuit breaker with an external trip coil input. You can then have your ATmega signal to the breaker 'trip now', and let the carefully designed breaker do the rest.Alternatively, you could put a normal circuit breaker in series with your proposed circuit, and add a small delay (say 50ms) to your code between detecting the overcurrent and tripping your relay. That way the normal circuit breaker would handle short circuits and strong overcurrents for you.If you really want to try out making your own breaker which can interrupt a short circuit, you totally can. But you should be prepared for minor explosions (and possibly small fires). You should get some electrical safety training first if you don't already have it. Once your test articles start ripping themselves apart, you need to very deliberate about the safety of your test stand or you can end up with very dangerous situations.

author: Sgt123
author_rank: Newbie
timestamp: October 08, 2017, 01:31:05 am
content: Thanks for that bit of information, I have a few more questions. So basically I need a sort of relay that has inbuilt arc suppression, so I can control it with my atmega safely.I did some research and apparently its called a contactor,https://en.wikipedia.org/wiki/ContactorWhere can I get one thats not too expensive ?, just looking for a single phase oneAlso, how would a solid state relay or triac work instead of using a mechanical relay ?, since it trips on a zero crossing of the current and there is no arcing

author: duak
author_rank: Super Contributor
timestamp: October 08, 2017, 04:41:06 am
content: A contactor is a big relay designed for switching greater voltage and current. Contactors often have two contacts in series and quite often have arc chutes to collect and guide the arc away from the contacts to safely cool down.I doubt that the relay or contactor will be able release and try to open the circuit within 2 ms because there is a reverse diode across the coil. Because the coil is an inductor and there is a current flowing through it there is energy stored in the magnetic field that must be extracted or dissipated. The time constant of a simple inductive circuit is T = L/R. When the diode is conducting, ie. when it is trying to protect the MOSFET from the high voltage that the inductor would otherwise generate, it will have a low resistance (R). This means that the current will take a much longer time to fall to where the contacts open. Note also that because the contacts open slowly, they will not interrupt the current and will be subjected to the heat of any arc formed. One solution to this is to use a MOSFET with a high drain to source breakdown voltage and limit the voltage generated by the coil with a parallel resistor.I have seen circuit breakers that can be tripped by an external signal used in CNC machine tool servo drives. Here the stored mechanical energy in the breaker's spring is used to open the contacts as quickly as possible once it has been triggered. I believe Mitsubishi was one of the manufacturers.Alternately, a circuit using thyristors controlled by a microprocessor could be implemented such that the next AC line cycle will only be passed through if the current level in the previous cycle is below the desired threshold. Note that thyristors can fail short circuited so they cannot be considered to be fail-safe. Conceivably thyristors with short term current ratings could be found that won't be damaged by a single short circuit current cycle or event.Cheers,

author: SeanB
author_rank: Super Contributor
timestamp: October 08, 2017, 07:36:41 am
content: As you are in South Africa I think CBI still makes those triggered breakers, as they were used in the early Conlog prepayment meter units, and they still use a variant in the new ones to trip the indoor coil when your money is up. The are simply a standard CBI RCD unit with the trip wires brought out, and not connected to the internal current difference sense coil, and basically trip with 30mA of current through the coil. Otherwise the outdoor unit contactor will work there as well, as it also has remote trip ability, and both are SANAS approved for short circuit clearing as well.

author: Sgt123
author_rank: Newbie
timestamp: October 08, 2017, 02:23:04 pm
content: "One solution to this is to use a MOSFET with a high drain to source breakdown voltage and limit the voltage generated by the coil with a parallel resistor."R = V/I, so I is just the relay operating current ? and voltage is my choosing as long as its below the FETS breakdown ?

author: Sgt123
author_rank: Newbie
timestamp: October 08, 2017, 02:32:22 pm
content: Quote from: SeanB on October 08, 2017, 07:36:41 amYes, I'm from SA. Where do they sell those ?Was looking through mantech and found thesehttp://www.mantech.co.za/Categories.aspxMost of them seem to be three phase. I'm just looking for single phase. The one at the top for R120, not sure if that will doEdit:Its under components,electromechanical,contactor in the link

author: SeanB
author_rank: Super Contributor
timestamp: October 08, 2017, 04:41:02 pm
content: Not sure if it will survive opening into a short circuit. From experience with them they do tend to detonate and weld themselves into a permanent on quite easily. They do not have enough arc suppression inside the case for anything over 20A resistive load. You need something that uses a circuit breaker contact style with the arc suppressor block and the gas vent to break up the arc properly, and this really does not fit into that style case easy.Probably best to phone Conlog factory itself, ask to speak to one of the electrical engineering people and see if they still have the trip units around, or if CBI still has some in stock, or just grab a prepayment meter and use the internal contactor of it. eghttps://www.gumtree.co.za/a-car-accessories-styling/other/maitland-electrical-wholesalers/1002118249310910038030709For R350 it will be cheap enough, and will at least be capable of breaking 63A with 5kA short circuit current. Don't know where in SA you are, I live in Durban, and the Conlog factory is over the hill from me, but i do not know anybody there any more.

author: Nauris
author_rank: Regular Contributor
timestamp: October 08, 2017, 04:50:30 pm
content: Quote from: duak on October 08, 2017, 04:41:06 amTrip coils (and auxiliary contact blocks) are also available for that ABB S260 series breaker seen in video above. They are actually available from most circuit breaker makers. Tend to be a bit pricey, tought.

author: duak
author_rank: Super Contributor
timestamp: October 10, 2017, 03:50:53 pm
content: Sgt123 asked:"One solution to this is to use a MOSFET with a high drain to source breakdown voltage and limit the voltage generated by the coil with a parallel resistor."R = V/I, so I is just the relay operating current ? and voltage is my choosing as long as its below the FETS breakdown ?Yes, I is the coil current. The inductance of the coil will develop a compliance voltage high enough to drive current through the circuit for as long as it can. A higher resistance means a shorter current decay time but also a higher compliance voltage. The absolute quickest way is to discharge the current to a very high voltage but also make sure the coil current goes to zero but does not reverse. Almost as good is to open circuit the coil but it will be difficult to prevent an arc from forming.I built an actuator test jig once where the coils were driven by 24 V while the inductive discharge resistor bank operated at over 100 V while dissipating well over half of the power applied to the actuator.Cheers,

author: oldway
author_rank: Super Contributor
timestamp: October 11, 2017, 04:36:17 am
content: Using a circuit breaker with shunt trip, this change totally your initial project.It became an adjustable overcurrent protection.It is also a lot safer because short circuit condition is clear by the simple electro-mecanical device from the circuit breaker.But there is a problem you did not think about : selectivity.In a well designed electric circuit, in case of short circuit or overcurrent, the only circuit breaker or fuse wich clear the fault must be the nearest to the fault.How could you garantee selectivity if you use an adjustable overcurrent protection ?You should also ask you some other questions : your µc need an auxiliary power supply, what about if this power supply fails ?The overcurrent protection will stop to work ?What's your goal?To replace a safe and well working technology of electro-mechanical circuit breaker by an unsafe, less reliable and more sofisticated technology using a µc ?In my opinion, your project is nonsense and dangerous.

author: oldway
author_rank: Super Contributor
timestamp: October 11, 2017, 08:35:28 am
content: In 1982, I had to solve a similar problem but with direct current.These were high power (12V 600A) boost converters with fast thyristors for telecomunications powered by enormous 48V batteries.The 48V input was protected by a contactor, there was no fuse nor circuit breaker.The specification required that in the event of a commutation fault, the converter had to be protected by the contactor and that it is then able to restart without fault or damage.This kind of protection had never worked.In the event of a switching fault, the contacts were completely melted, the contactor was damaged, the thyristors in short circuit and one of the converters had even caught fire.I was asked to solve the problem.The first thing I have done is to install a current limiting resistor at the input to limit the short circuit current to acceptable values for the contactor.Then, I wired the 3 contacts of the contactor in parallel instead of in series .... With these two modifications, we could cause commutation faults voluntarily without any problem.I then upgraded the trigger circuits of the thyristors to avoid commutation faults as far as possible .... We did not have any problems afterwards.