AC switch fault current philosophy

piglet squeeled :

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1. Use a very rugged AC switch device, like a 40A triac or 60A SCR pair with the largest half cycle surge rating - like 500 to 1000A.

2. Install a fast acting magnetic breaker able to break the turn off arc in one or two milliseconds.

If there are other good ways, designers of pro-lighting dimmer don't know about them.

BTW:

Stop posting f****it questions about stuff you are clueless about.

..... Phil

If you're talking thyristors (SCR/TRIAC), you have no choice but to do 1. Fault current ramps up well within a cycle, so there's no chance to turn it off later. Make sure you use a fuse with a smaller I^2t rating than the thyristor.

For industrial applications, there are "semiconductor fuses": super fast, they clear in less than a cycle. For smaller (consumer scale) loads, a regular fast-blow may have low enough I^2t, or alternately, you can afford to use a somewhat larger thyristor.

Otherwise, with transistors, #1 is impossible. No transistor can burn a mains fuse without itself blowing far, far sooner. Current must be switched off, diverted to a snubber network (usually an RCD clamp, TVS or MOV), and then either it stays off, or it turns back on and off, controlling current (with an inductor added, to set a maximum dI/dt).

In a switching-current-limit mode of operation, fault time is limited by the energy capacity of the clamping device. If you can "stir" the energy back into the source (easier with a DC circuit -- i.e., a regular buck converter), you can run a limit condition basically forever (or at least, until something else overheats). If not, then the dissipating device will eventually overheat, and you'll need to monitor that and stop at some point.

Tim

Thanks Phil. Happy New Year

piglet

Thanks Tim. My preferred option was always a big enough SCR but I asked in case that option is considered too old fashioned now that MOSFETs are becoming available that with only a small amount of inductance and snubbing can switch off fast enough to survive.

SCR I2t is still much much cheaper than equivalent MOSFET SOA :)

piglet

You might limit the current and trip the mains breaker to cope with the short. That way you simultaneously protect the shorted device and send users a dramatic message that something's wrong.

Thank you,

Is it? I mean, can you buy the latter at all? Even IGBTs have short-circuit ratings measured in microseconds, with the values of 10us and 5us occuring most often.

Best regards, Piotr

A round-about way that I have found very useful: Sense current, blow fuse with "crowbar" circuit... amazing how much light a fast-blow fuse will make ;-) ...Jim Thompson

but you can turn of a fet in microseconds

How does that work?

If you have a shunt coil to trip the breaker separately (like a lot of UL1077 breakers do, or the guts of a GFCI receptacle), you can open the circuit without drawing fault current. Though that still won't act fast enough to save a transistor (not without a switching mechanism like I described, anyway).

Tim

You mean the breaker at the _panel_? How do you know if it's magnetic or thermal?

And then somebody plugs it into daisy-chained extension cords and your 'safety device' burns down the building. Brilliant all round. :(

Cheers

Phil Hobbs

Phil, WTF? I was thrilled to see a question from piglet. I don't see any reason for you to start off so rude... get a grip.

George H.

don't implement it stupidly. The current limiter must have overheat protection.

NT

piglet, I know nothing of AC power. But personally I'd rather have a rugged switch.

George H.

thermal?

safety

tion.

How are you going to do that if you don't know how it's connected?

Building a known fast breaker into the box, I get. Relying on the capabilit ies of a hypothetical breaker at an unknown upstream point, not so much.

Cheers

Phil Hobbs

e:

he

or thermal?

'safety

ection.

I'm not making sense of that question

ities of a hypothetical breaker at an unknown upstream point, not so much.

Mains supply breakers aren't hypothetical. In UK they're well defined. I pr esume they are in all other 1st world countries too.

And the whole point of overheat cutout is that it interrupts the current if for any reason the supply fails to. That could be as simple as a fusible r esistor.

NT

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Trolls are questions that HAVE no answer because the problem is undefined.

.... Phil

r 'safety

tection.

So I gather. See the comment about daisy-chained extension cords above. Tha t's a well-known way of burning down buildings, even without deliberately s horting the mains to ground.

ities of a hypothetical breaker at an unknown upstream point, not so much.

There are magnetic (fast) and thermal (slow) types here. A thermal breaker might very well not clear the short before the thyristor melted. Or the wir ing might be marginal, but good enough for many more years unless somebody did something idiotic like shorting the mains.

In any case, round these parts if you caused a fire by deliberately shortin g the mains to ground, you'd get sued right down to your socks, and rightly so. If you did it in France or Italy and someone died as a result, you mig ht very well go to jail.

"For any reason?"

I wouldn't want to be liable for a claim that large, personally.

Cheers

Phil Hobbs

Exactly. Fast turn off makes possible something not possible before.

So which is better, a few milli-seconds of hundreds of amperes and which causes a clearly identifiable indication of fault requiring manual intervention to reset - or - a few tens of micro-seconds of tens of amperes and a circuit that automatically resumes when the fault is cleared?

Also faults can range from a transient tungsten filament fragment falling across supports to the non-transient nail through a cable.

For example if this were a domestic light switch which mode would be preferable? Should I stick with the tried and tested method or is there any point in exploring the newer mode?

Don't mean to be a troll but I do appreciate your help settling this question.

piglet

Let's be clear here: High-current AC-line-voltage switches are better made with IGBTs than MOSFETs, because the IGBT's Vce(sat) is considerably-less than the FET's Id*Rds(on).