Sizing fuse and MOV for UL safety

[perky]

Hi All,

I'm designing a capacitive power supply for 230V mains. I understand that UL requires a fuse and an MOV for safety (see Figure 10 of Microchip's AN954, here), however I'm having difficulty in defining the fuse breaking current specificiation and size (energy handling rating) of the MOV.

In my circuit, assuming there's double insultation between L and N, the maximum normal current that could flow is around 60mA and under single fault conditions this could rise to 2A. The inrush current is limited to 800mA normally for 0.5ms. So I think a 100mA 'T' type fuse would be OK. My concern is that the MOV would absorb transients from the mains and during that short time could take quite a few amps. What should the breaking current of the fuse be, and how big should the MOV be?

Any help appreciated.

Mark.

 

[duffy]

The pulses the MOV takes are very brief in duration and don't affect the fuse rating. I don't understand your concern over voltage spikes during the inrush current phase of startup, are you experiencing a lot of diode switching noise?

 

[perky]

The pulses the MOV takes are very brief in duration and don't affect the fuse rating. I don't understand your concern over voltage spikes during the inrush current phase of startup, are you experiencing a lot of diode switching noise?

I'm not worried about inrush currents, that's sorted now. My concern is the MOV briefly taking more current than the breaking current specification of the fuse while absorbing transients - exceeeding the breaking current could cause the fuse to destroy itself and possible fail short circuit.

Mark.

 

[duffy]

I didn't ask if you were concerned about the inrush current, I asked if you were concerned about the voltage spikes (which is what an MOV responds to) during the inrush current. Do you understand how one of these devices works? It conducts only when its voltage exceeds a certain level.

 

[duffy]

You have this across the mains, right? You aren't confusing it with an NTC thermistor or something, are you? They look the same, but have very different functions.

 

[perky]

You have this across the mains, right? You aren't confusing it with an NTC thermistor or something, are you? They look the same, but have very different functions.

The circuit is a fuse on live with an MOV on the other side of the fuse to neutral as shown in figure 10 here.

The MOV conducts and goes very low impedance when the voltage exceeds a threshold. The transient current could be very high for this short time, but if that current exceeds the 'breaking current' of the fuse then the fuse is not guaranteed to fuse open circuit (as far as I am aware).

I'm not sure why you thought I was concerned about this during the inrush phase specifically, the inrush current is far less than the clamping current and the fuse if it's a 'T' type won't blow.

Mark.

 

[duffy]

The inrush current is limited to 800mA normally for 0.5ms. So I think a 100mA 'T' type fuse would be OK. My concern is that the MOV would absorb transients from the mains and during that short time could take quite a few amps.

Afraid I still don't understand what you are asking.

 

[perky]

Uhh, because of this?
Afraid I still don't understand what you are asking.

The inrush current is limited to 800mA normally for 0.5ms. So I think a 100mA 'T' type fuse would be OK. My concern is that the MOV would absorb transients from the mains and during that short time could take quite a few amps.

OK, I see your confusion. The first and second sentences relate to inrush current, which I thought the fuse would deal with. The last sentence is concerned with transient absorbtion, which of course can occur at any time - the phrase 'during that short time' in that sentence refers to the clamping current time *not* to the inrush time. The last sentence is independent of the first two.

So, during a transient event (which can occur at any time), what maximum current do you think will pass through the fuse? Greater than its 'breaking current' spec? (Do you know what I mean by breaking current?)

Mark.

 

[duffy]

Yes, it is - but it isn't enough by itself. The fusible link has thermal mass, so it's really power x time = energy. That's why I mentioned that the pulses are typically very short in duration and don't affect the fuse value. If this situation ever does cause a problem, then it points to a bigger issue with noise that needs to be addressed separately.

 

[duffy]

(in which case you would probably want the fuse to blow anyway)

 

[perky]

Yes, it is - but it isn't enough by itself. The fusible link has thermal mass, so it's really power x time = energy. That's why I mentioned that the pulses are typically very short in duration and don't affect the fuse value. If this situation ever does cause a problem, then it points to a bigger issue with noise that needs to be addressed separately.

Yes, in fact the fuse has a 'melting integral' which is (I^2)t to determine when the fuse will blow. However, the 'breaking current' beyond which the fuse may catastrophically fail (even short circuit) is only ever quoted in Amps. So the energy at which it will fail is unknown. The only spec I've got to work with is current, not energy. So if you're saying the circuit can breach the breaking current rating, technically the fuse is operating outside of specification.

 

[duffy]

If you know about (I^2)t, you already have what you need for figuring out the rating for spikes (if that's even necessary) -

- but more importantly use it for that inrush surge to charge the caps. This is why I thought you might be wondering about NTC thermistors; they are commonly used to reduce inrush surge to caps in power supplys so the fuse doesn't blow. The fuse is not operating outside of specification, that is the specification.

"Breaking current", especially with reference to 'catastrophic failure', especially especially in reference to 'fail short' conditions, is a term I've only ever seen used with exotic high-voltage fuses (where you need to "break" an arc). This shouldn't be an issue with a bridge-and-cap supply off 220V. Could you provide a link to that fuse spec?

 

[perky]

If you know about (I^2)t, you already have what you need for figuring out the rating for spikes (if that's even necessary) -

Only true for determining whether the fuse will act as a fuse and blow open.

"Breaking current", especially with reference to 'catastrophic failure', especially especially in reference to 'fail short' conditions, is a term I've only ever seen used with exotic high-voltage fuses (where you need to "break" an arc). This shouldn't be an issue with a bridge-and-cap supply off 220V. Could you provide a link to that fuse spec?

All fuses have a 'breaking current' or 'breaking capacity' spec. Here's one:
https://www.littelfuse.com/data/en/Data_Sheets/372_LF_070606.pdf
This has a 'breaking capacity' of 35A.

Here's the definition of breaking capacity:
Breaking capacity of fuses - Wikipedia, the free encyclopedia

Note what it says: "The maximum short circuit current which can occur under fault conditions should not exceed the rated breaking capacity of the fuse. Otherwise breaking of the fault current cannot be guaranteed."

Now, an overvoltage condition would cause excessive current to flow through the MOV potentially greater than the breaking capacity of the fuse. This is current, not energy, so duration of that current is not defined. If the fuse were to be damaged by this current it might not act like a fuse in the designed way afterwards.

Mark.

 

[duffy]

Good heavens, this is really keeping you up at night, isn't it? Why not just get a fuse with a higher spec, like this one -
https://www.electro-tech-online.com/custompdfs/2009/03/201P_LF_072106.pdf

...but in reality, I suspect you won't actually get near that 35A (or 50A, according to UL) with the failure mode you are considering. Your fuse has a good 3Ω or so resistance, the MOV still has a few ohms even at the top of its rated curve, your wall outlet probably isn't rated for anything like 35A or 50A at 220V, either.


That "breaking the fault current" reference from the Wikipedia entry is talking about breaking a high-voltage arc that is conducting through the ionized plasma of something like a busbar sized "cut out" fuse (or what was left of it) at an electrical substation channeling millions of watts at hundreds of kilovolts - not household voltage popping a wire in a PCB mount Littlefuse.