Is A PTC a circuit breaker?

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Lanzones

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I have been arguing with my boss about the PTC is a special resettable circuit breaker. But he insist that I should NOT even consider PTC as a circuit breaker.. IS this true?

I am designing a mains transformer for SMPS that will be submitted for AS certification.. the certification says that that I should have some sort of protection in a form of fuse or CIRCUIT BREAKERS that are approved by recognised standards. We both agreed that we could use fuse as our protection and then I suggested that we could use a combination of fuse and PTC will be better. and even stated that the PTC can replaced the fuse because we are encapsulating the whole electronics boards for intrinsic safety which I would say impossible to replace the fuse if this device is blown. This is when the argument starts...

I have not given up arguing with him about this... So to cut the story he ask me to show a certified document from UL/NEC/etc saying that a PTC is a special type of resettable circuit breaker and can be use in replacement of a fuse..

Could someone attach a certified documents here or provide me a link confirming that my claims are correct?... Or maybe I'm be wrong.... PLease help
 
I've only ever used PTC's on mag lock circuits for access control work. While they do serve a sort of pseudo fuse-like role, I'd never consider them as an overcurrent device that I'd stake my life or safety on.
 
A fuse or circuit breaker completely opens the protected circuit. A PTC almost opens it, a very small, but possibly lethal current still flows, more so as it cools off.

It could possibly be used for protection of equipment, but never as a disconnecting means.

I've seen PTC's commonly used in two instances, one in series with the de-gaussing (sp?) coil in color TV's that use a CRT, and the other in series with the start windings of small induction motors. In these instances the PTC is designed to provide a high current for a short time, then drastically reduce the current as long as the device is in operation. It could get tricky to use them for protection where a specified, maybe variable current is needed to flow normally, but block excess current. Heat dissipation would need to be carefully controlled, and the PTC might not heat up quick enough to protect the rest of the circuit.

If the above could be overcome, they would work well as back-up protection for the fuse. Finding documentation could be a real challenge though.
 
PTC = thermistor

PTC = thermistor

Of course. My brain was looking for a power device and not a control device.

Thank you for the reminder.
 
What some people do is to use a PTC as a thermal sensing device that then is monitored by a relay that opens up on temperature rise above a predetermined threshold, dropping out a contactor/relay. If you already have the contactor/relay there for some other reason such as isolation, it makes sense otherwise it would be much more expensive than a CB or fuse. On small motors these can even be used as motor overload devices, precluding the need for and external OL relay or a kilxon thermal cutout.
 
Additional information here, a thermistor is a semiconductor whose resistance changes with temperature.

A PTC (positive temperature coefficient) increases resistance as temperature increases.

A NTC (negative temperature coefficient) decreases resistance as temperature increases.
 
I think the original poster may have been talking about a polyswitch?
Polyswitches are often used as fuse replacements inside of equipment, but as cited not as a disconnect means.

Thermistors come in both positive (PTC) and negative (NTC) temperature coefficients. These are sensing devices and not intended for power circuits.

Polyswitches are PTC devices but have a more abrupt temperature vs. resistance curve (more like a switch).

See this for more info:
http://circuitprotection.com/07catalog/polyswitchTechFund.pdf


They are handy in that they self reset after the input power is removed. I prefer a fuse myself. In critical applications they can be a good choice.
 
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