The Moisture Sensitivity Level (MSL) is a measure of how much, how fast, and how sensitive a part is to moisture before being attached. It has nothing to do with moisture sensitivity after the board is assembled, for such information you need to consult the datasheet for storage and operating conditions.
If too moisture much enters the part and it vaporizes during reflow, the high pressure can cause sufficient stress to cause the part to explode, causing a "popcorn" noise. Such failures are usually visible with careful inspection as the package will be fractured, usually near the leadframe. It may be cracked on the bottom with DFN-type packages, however, though the device may appear to be sitting at an odd angle, canted up from the PCB.
Depending on the device, other, more subtle failures may occur. Internal wire bonds could be broken causing intermittent or no connections, or in the case of a PPTC, an effectively reduced current limit. Such errors are probably rare, and of course very dependent on the construction of the device, which you don't know. For prototypes, usually people can deal with it, but for production it's important if you are concerned about reliability and/or manufacturing rejects.
You can keep moisture-sensitive parts in resealable containers if you control the humidity with desiccants or dehumidifiers, and you can use indicator cards to monitor that the humidity did not exceed some defined level. 3M makes them, and there may have been one in the bag from your supplier.
If you do end up with a big production batch of moisture-sensitive parts that have been exposed, you can "bake" them to drive off the moisture. Check with the manufacturer, but bakes can be anywhere from 40 °C at 0% humidity for 1-4 weeks (gentle, but complicated), or 125 °C for some number of hours (more aggressive, but your parts may or may not like it). If you're just making prototypes for development...just make some spares (as one does anyways) and fuggedaboutit.
For a common glass type fuse the voltage is less relevant. It's only important when the fuse has failed, that a too high voltage won't bridge the gap. As long as your fuse is intact there's hardly a voltage across it.
PTC fuses work differently.
They don't break instantly, like a glass type fuse. When there's a short, the current will rise to a high level, but there will still be a voltage drop, so combined that may give a high power. It takes some time for the fuse to heat up, so that resistance increases, and current decreases. In the graph, for the 0.35A PTC it takes a full second for the current to drop from 10A to 1A. Some of these, even in a small package, can dissipate hundreds of Watts during a very short time.
Most PTC series, like this one (just an example), will allow a higher voltage for the lower current ratings, so that the power rating is somewhat constant.
The lower voltage for the bigger devices may be explained by the device's reaction time. A slower device will have to dissipate a lot more power, even at a lower voltage. I think it's hard to compare between two different series.
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From your comment to Tony's answer I understand that you want to use this to protect your power supply. Keep in mind that these work different from ordinary fuses. A glass fuse will blow rather fast if the rated current is exceeded. A PTH protects mainly against short-circuits. It's then that they get a high current peak which heats them so that they go to a higher resistance, limiting the current. This takes some time, even at 10A for a 0.35A PTC fuse, as you can see in the graph. It also means that it doesn't protect well against a mild overcurrent. The PTC will heat up even slower and not properly protect your power supply.
Best Answer
The hold current is what it's guarranteed to carry, while supplying the load as a fuse should, so with most of the supply voltage across the load. If your load takes 1A for normal operation, then your hold current must be higher than 1A.
In the normal untripped state, the resistance is low, and the heat being dissipated in the PTC is low enough that its temperature and so resistance remains low.
When it reaches trip current, the temperature exceeds a threshhold, and the PTC becomes high resistance. The current drops due its high resistance, which removes most of the voltage from the load. As long as the supply voltage remains connected, the PTC stays hot enough to stay in that state.
You should choose a PTC with a trip current
a) Higher than your load will ever take, to avoid nuisance tripping
b) Lower than your power supply can deliver, otherwise your PTC may never trip.
A common mistake is to use a PTC with a trip current that's too high for the power supply. Under fault conditions, the PTC is then unable to trip, and the power supply cooks.
'Tripped current' is not specified for the PTC, it's the Pd, the typical power dissipated in the tripped state that's defined. The tripped current can then be estimated as Pd divided by the power supply voltage.