Polyfuses and flame safety - Page 1
Forum section: electronics / projects

author: poorchava
author_rank: Super Contributor
timestamp: September 26, 2013, 06:07:41 am
content: Hi,can polyswitches release gases or catch on fire in case of gross overload? I mean I know that theoretically they should not, but I'd like to make sure. Brand name ones have toe UL 94-V0 rating, but that doesn;t say anything about cathing on fire, just that it will burn for less than 10s after being set on fire from external source.I need protection levels of 1A, 2A and 4A - would it be better to use a few with lower current rating in parallel instead of a big one? Since they are PTC's, then I dun't expect problems with runaway or things like that, but maybe I'm missing something.To be honest I don't have much experience with polyfuses, as I usually prefer electronic circuit, but in this case this is part of customer's specification.Any advice appreciated

author: Mike Warren
author_rank: Unknown
timestamp: September 26, 2013, 06:22:59 am
content: I've seen burnt polyswitches many times.

author: Bored@Work
author_rank: Super Contributor
timestamp: September 26, 2013, 06:26:38 am
content: I would try to contact some fuse suppliers, and ask their application engineers for advice.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: September 26, 2013, 08:05:43 am
content: Anything will catch fire with sufficient overloadPolyfuses have voltage ratings, which will generally prevent combustion if you keep within them.

author: peter.mitchell
author_rank: Super Contributor
timestamp: September 26, 2013, 08:16:00 am
content: I was hoping to find a resistance vs temperature graph in a datasheet but apparently not;If this was available, you could just look at say... resistance between 60 and 250 degrees and then work out how much power would be dissipated in a short circuit at your input voltage.

author: nctnico
author_rank: Super Contributor
timestamp: September 26, 2013, 11:54:41 am
content: Quote from: poorchava on September 26, 2013, 06:07:41 amThey do go up in flames when the load is a short especially the SMT ones. They are also sensitive to higher temperatures. If you need to choose a polyfuse you need to select one with a trip current twice the current they need to carry. A lot of people try to use a polyfuse as a current limiting device. Try to resist that idea. A fuse is a deliberate weak point to protect against fire; this also holds true for polyfuses.

author: ovnr
author_rank: Frequent Contributor
timestamp: September 26, 2013, 12:38:08 pm
content: I'd also expect PTH polyfuses to be more resistant to catching on fire as compared to their SMD counterparts.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: September 26, 2013, 01:20:27 pm
content: Quote from: peter.mitchell on September 26, 2013, 08:16:00 amA graph would be pretty useless as any current flow would immediately change the temperature and hence the resistance.

author: Wytnucls
author_rank: Super Contributor
timestamp: September 26, 2013, 01:42:42 pm
content: Polyswitches don't really breakdown or catch fire, unless they operate outside of specified limits (voltage and current). Just like HRC fuses.Remember that, after activation, they can take hours, if not days, to return to their initial low resistance state.I can't see how setting them in parallel would be beneficial, as it would probably increase the time required to reach high resistance and protect the downstream circuit.

author: c4757p
author_rank: Super Contributor
timestamp: September 26, 2013, 01:51:20 pm
content: Quote from: mikeselectricstuff on September 26, 2013, 01:20:27 pmGive me a figure on thermal resistance and that can be calculated as well!

author: madsci
author_rank: Regular Contributor
timestamp: September 26, 2013, 08:22:47 pm
content: I use a coordinated protection approach.Observe the polyfuse datasheet for the trip curve and select a fuse that will normally never blow unless the polyfuse fails. Observe also the maximum impulse current for the poly fuse and make sure the selected upstream protector and feed line impedance will not allow this to flow. Add a small value resistor if needed....a current sense shunt perhaps.We do the same for circuit breakers in large installations where the fault current at the feeder is some obscene level that would turn a small plug on or DIN rail circuit breaker into a small frag grenade.

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 27, 2013, 02:03:18 am
content: PTC motor starters have been known to melt down. That often happens if some fault condition (like low on refrigerant for a compressor motor) causes the motor to operate at unusually low load and put out more back EMF than usual.http://hvac-talk.com/vbb/showthread.php?1340951-When-the-cheapest-price-becomes-the-most-expensive

author: ignator
author_rank: Regular Contributor
timestamp: September 27, 2013, 03:35:29 am
content: I used Raychem brand polyswitches starting back in 1989. These had a spec. max short circuit current of 40amps. I would use inductors with milliohm resistance to limit the current within this spec.As these were used for input power (avionics +28VDC power), the open (tripped) circuit voltage of 60VDC was not violated. During qualification testing, and lightning test, I did see one release it's magical smoke. I needed a bigger MOV to protect these fuses.My point being, you can exceed the trip current up to the max allowed fault current of their specs. Watch out for max tripped voltage across the part.Amazing how these protected the autopilot low impedance servo amp and discrete outputs. Customers new aircraft harness would use our products as fuses.

author: Harvs
author_rank: Super Contributor
timestamp: September 27, 2013, 03:41:36 am
content: I've seen them where an logic level input was protected by a poly fuse and clamp diode, the wiring guys applied 240ACrms across the input and it happily burn a hole in the FR4 where the diode and poly fuse once were.So it depends on what you mean by "gross overload"

author: poorchava
author_rank: Super Contributor
timestamp: September 27, 2013, 06:43:48 am
content: Hmm, that seems risky. The device is supposed to protect the circuitry from anything from simple overload to dead short. I do not know what is the short-circuit current in this power line.I also thought about "sharpening" the transition between holding current and trip current wit "crowbar protection", but I guess this could result in fire as well.Device is supposed to be used in high air pressure and different gas mix than normal air (read it: underwater installation). And any release of gasses or fire is a big no-no. Whole circuit will be encased in thick heavy duty epoxy potting but I'd like to be on the safe side.

author: nctnico
author_rank: Super Contributor
timestamp: September 27, 2013, 06:08:31 pm
content: A protection device needing protection devices kinda defeats its purposetherefore I stopped using polyfuses a while ago. In my designs I use current limiting, temperature protection, etc to keep a circuit healthy. When a fuse blows there really is something wrong with the circuit and it needs repairs anyway.