PTC resettable fuses

[anobium]

As some of you will know I had some problems sorting out a new control panel, (sorted now ,fingers crossed)
During the course of the installation the question was raised about fuses failing.
On checking the board I was only able to locate one which was for the main power supply.
All the manuals I read showed 5 replaceable fuses but my PCB only had one.
Eventually I noticed a reference to the above and wondered is this better or worse? and whilst from the description they dont need replacing what happens if they blow?
Just curious

 

[sparkymarka]

they don't...

 

[SimonH2]

A PTC "fuse" doesn't "blow".

PTC means Positive Temperature Coefficient, and refers to a device that changes resistance with temperature. NTC is similar but is Negative Temperature Coefficient and just works the other way round - one place you'll find them is in engine temperature gauge senders on older cars where high temp -> lower resistance -> more current through gauge -> higher reading.
With a PTC, at the temperature rises, so does the resistance. Different types vary in what the temperature-resistance graph looks like, but the ones of interest here have a curve with a pronounced "step" in it - so there is a rapid change over a relatively narrow temperature range.

Under normal operation, the current through the device warms it, but not enough to reach the step. Should you overload the circuit, the heating goes up, the device reaches the step in the curve, and it's resistance goes up considerably - enough to limit the current to a low value (much lower than the fault threshold). Now the PTC device is dropping most of the supply voltage, so even at the reduced current it still stays hot enough to stay in the tripped state.
Simply stop the current for a while - either by turning off the supply, or disconnecting the load - and the device will cool, resetting once it drops below the step.

There's more at https://en.wikipedia.org/wiki/Resettable_fuse